Broadband colloidal quantum dot LED for active plasmonics ...lib.ugent.be/fulltxt/RUG01/002/007/097/RUG01-002007097_2013_000… · ration and resulting in multiple pads of about 1

Floris Tallieu

plasmonicsBroadband colloidal quantum dot LED for active

Academiejaar 2011-2012Faculteit Ingenieurswetenschappen en Architectuur

Voorzitter: prof. dr. Isabel Van DriesscheVakgroep Anorganische en Fysische Chemie

Voorzitter: prof. dr. ir. Daniël De ZutterVakgroep Informatietechnologie

Master in de ingenieurswetenschappen: toegepaste natuurkundeMasterproef ingediend tot het behalen van de academische graad van

Begeleider: Pieter GeiregatPromotoren: prof. dr. ir. Dries Van Thourhout, prof. dr. Zeger Hens

Preface

Five years of education in engineering physics at Ghent University, five years of gathering intel-

lectual knowledge. This master thesis is a crown of hard labour, perseverance and a continuous

search towards optimal solutions. It is the practical realization of theoretical approaches and

experimental hunches. Those are the reasons why this thesis would not have been as it is now

without the help, insight and support from other people.

Most of my acknowledgment goes to my supervisor, Pieter Geiregat, who fulfilled his role beyond

my expectations. Next to his technical support and already acquired knowledge, he provided

help with practical issues and even prepared some samples.

Next, gratitude should be offered to the joint between the Photonics Research Group (which is

part of the Department of Information Technology INTEC) and the Physics and Chemistry of

Nanostructures Group (PCN). The cooperation allowed to access more resources, more knowl-

edge and more interaction with the researchers. For this and for their guidance, I would like to

thank both of my promotors, prof. dr. ir. Zeger Hens and prof. dr. ir. Dries Van Thourhout.

However, not only the help of the people to whom you are accountable has to be mentioned,

also they who daily work next to you and help you with smaller or bigger issues deserve a word

of acknowledgment. Therefore, I would first like to thank everybody in both research groups for

the pleasant time I had, especially Antti, Stijn and Marco for providing quantum dots, Parvathi,

Kasia, Abdoulghafar and Tangi for their help during measurements and depositions and Bram

for his interest and helpful insights in my work.

In addition, people outside those research groups were of invaluable help. Not in the least prof.

dr. Philippe Smet and prof. dr. Diederik Depla who provided their material at my disposal,

combined with their experience and insights. Special thanks goes to Kilian for performing nu-

merous ALD depositions, Aimi for his experience with admittance measurements, Olivier for

XRD spectra and SEM images and to Francis and Wouter who helped me with the smallest

problems in the S1.

Of course, writing a thesis cannot be done individually, support from the people closest to

you is of crucial importance. First, my brother Steffen deserves to be mentioned for his support

and interest in my thesis subject. Next, special thanks goes to my girlfriend Tilde, who sup-

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Preface

ported me to deliver a solid piece of work in a pleasant environment.

Finally, I would like to thank my parents for making me who I am, raising me in my best interest

and supporting me in my decisions. Without them, this master thesis would never have seen

the light.

Floris Tallieu

May 2012

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Copyright statement

The author gives permission to make this master dissertation available for consultation and to

copy parts of this master dissertation for personal use.

In the case of any other use, the limitations of the copyright have to be respected, in particular

with regard to the obligation to state expressly the source when quoting results from this master

dissertation.

Floris Tallieu

May 2012

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Broadband colloidal quantum dotLED for active plasmonics

by Floris Tallieu

Master thesis submitted to obtain the academic degree of

Master in engineering: applied physics

Academic year 2011-2012

Promotors: prof. dr. ir. Zeger Hens, prof. dr. ir. Dries Van Thourhout

Supervisor: ir. Pieter Geiregat

Faculty of Engineering and Architecture

Ghent University

Department of Information Technology (INTEC)

President: prof. dr. ir. Daniel De Zutter

Department of Inorganic and Physical Chemistry

President: prof. dr. Isabel Van Driessche

Summary

In this work, two types of LEDs are designed, fabricated and characterized. Their structure is

based on a combination of colloidal nanocrystals and inorganic layers. The first method uses

different layers to improve the transport of carriers when a DC voltage is applied, while the

second operation mechanism consists of two insulating layers which, when an AC voltage is

applied, imply the emergence of an electric field over the quantum dots, resulting in transport

of carriers, allowing radiative recombination of electron-hole pairs.

The characteristics of the layers are investigated, mainly focusing on their resistivity, mobility,

surface morphology and influence on the luminescence of the quantum dots. Furthermore,

electroluminescence experiments have been performed on both types, which showed most success

for the AC stacks.

Integration in a plasmonic structure is shortly discussed by analyzing a dipole emittor in a

metal-dielectric-metal structure, while stacks, based on the AC operation mechanism, have been

integrated on silicon waveguides.

Keywords

colloidal nanocrystals, LED, electroluminescence, plasmonics, integrated photonics

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Broadband colloidal quantum dot LED for active plasmonics

Floris Tallieu

Ghent University, Faculty of Engineering and Architecture,

Department of Information Technology, Department of Inorganic and Physical Chemistry

Abstract In this work, two types of LEDs are designed, fabricated and characterized. Their structure is based on a combi-nation of colloidal nanocrystals and inorganic layers. The first method is based on the transport of carriers with a DC voltageapplied, while the second operation mechanism is based on an AC voltage-driven system with insulating layers. Characteriza-tion of the layers and electroluminescence experiments have been performed. Integration in a plasmonic structure is shortlydiscussed by analyzing a dipole emittor in a metal-dielectric-metal structure, while experimental stacks have been integratedon silicon waveguides.

Keywords colloidal nanocrystals, LED, electroluminescence, plasmonics, integrated photonics

I. Introduction

Integration of light-emitting devices in plasmonic structuresor on silicon is a promising field. The applications of inte-grated plasmonics are ranging from x-ray devices to sensing,light trapping and ICs, while the advantages of CMOS com-patible silicon with the advanced functionalities of photoniccomponents are already widely tested [1]. Indeed, applicationsof silicon in telecommunications, biosensing, etc. are alreadyknown for their passive functionalities (signal routing, filtering,. . . ), yet the active photonics such as on-chip light generationor detection proves to be more challenging, mainly due to theindirect band gap of silicon. Therefore, different light-emittingdevices with quantum dots have been developed. Their opera-tion mechanism, device structure and results are discussed andfollowed by a short discussion on their integration. The quan-tum dots are solution-based colloidal nanocrystals, obtainedthrough a wet chemicable synthesis procedure and tunableto emit from the UV to the mid-infrared with high quantumyield [2].

II. Operation mechanisms

Two types of devices have been produced, each based on adifferent operation mechanism.

A. Direct charge injection - DC

The first electroluminescence method is based on direct chargeinjection. A schematic of the stack is shown in fig. 1.

Figure 1: Schematic of the stack based on direct charge injectionto induce electron-hole pairs in the quantum dot layer.

The electron transport layer (ETL) and the hole transport layer(HTL) help improve the injection of carriers into the quantumdots by reducing the energy barrier between the electrodes andthe energy level of the QD energy bands. Therefore, the ETL ismostly n-type, since the majority of carriers are electrons, whilethe HTL is mostly p-type [3]. The stacks are deposited on anITO covered glass substrate, which also serves as cathode. Theanode is always aluminium, deposited by electron beam evapo-ration and resulting in multiple pads of about 1.8 mm×1.8 mm.

B. Field-driven ionization - AC

The AC stacks by field-driven ionization are built with insu-lating layers. A schematic is shown in fig. 2.

Figure 2: Schematic of the stack based on field-driven ionizationto induce electron-hole pairs in the quantum dot layer.

Here, an AC voltage up to 100 V peak-to-peak is applied be-tween the ITO and the aluminium, resulting in the emergenceof a high electric field due to the capacitance series, induced byboth insulating layers and the quantum dot layer. Indeed, ifthe voltage drop per quantum dot exceeds the band gap, elec-trons in the valence band will tunnel to the conduction bandof the neighboring dot, while holes will make the exact oppo-site movement. This induces the possibility of simultaneouslyhaving an electron in the conduction band and a hole in thevalence band of the same quantum dot. This might lead toradiative recombination and photon emission. Since electronswill propagate to one side and holes to the other, an inter-nal field will build up, opposing the externally applied field.This diminishes the voltage drop per QD (or equivalently thelocal field strength). When this drop is no longer sufficientlyhigh, carrier transport from CB to VB or vice versa does nolonger take place. However, when the polarity is changed, theidentical process will occur in the opposite direction, since theelectric field of course reversed polarity as well [4].

III. Device structures and optimal layer composition

The choice of the materials for the transport layers in the DCstacks and the insulating layers in the AC stacks is of coursecrucial. Since there is strived towards the use of inorganic com-pounds to avoid fast degradation, nickel oxide and copper oxidehave been investigated as HTL. The latter however showed nop-type properties when DC magnetron sputtered. Nickel oxidehas been deposited by the same deposition method with a fixedoxygen pressure to yield a resistivity of 0.07 − 0.8 Ωcm for a30 to 60 nm thin film. For the ETL, zinc oxide is used, or inquantum dots, or deposited by ALD. Although they seemedviable options from literature [5], most research went to ro-tatable DC magnetron sputtered ZnO:Al (2 wt% Al), sincemultiple parameters can be changed, including the argon andoxygen pressure, the target-substrate distance and the thick-ness of the layer. They have been chosen to yield a resistivityof 0.4 Ωcm for a thickness of 75 nm.The material choice for the insulating layers in the AC stackswas pretty straightforward, since deposition of alumina couldbe done by electron beam evaporation or by ALD. Investiga-tion of the quality of both layers clearly showed the inferiorityof the ones deposited by e-beam. Therefore, 40 nm layers havebeen deposited by atomic layer deposition, both for the topand bottom layer. Admittance measurements also yielded a

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value of ∼ 7.5 for the relative permittivity which is in goodagreement with literature [6].

IV. Results

Next to the extensive characterization of the ZnO:Al layer, theeffect of adding this or an alumina layer on top of the quantumdots have been investigated. Therefore, emission spectra, life-time measurements, transmission spectra and excitation spec-tra have been measured, which showed that the decrease inintensity of the photoluminescence spectra is due to the trans-mission characteristic of the extra layers and extra channelsfor non-radiative recombination, which reduces the lifetime ofthe photons. For the alumina layer, photoluminescence is onlyconserved for core-shell quantum dots/rods. Current-voltagemeasurements on the total stacks were quite similar to liter-ature results, but their irreproducibility, probably due to theaccumulation of charges at interfaces and electrodes, are anindication why no electroluminescence has been measured forthe DC stacks.Admittance measurements on the AC stacks and their indi-vidual layers showed that the voltage drop per quantum dothad to be at least the band gap energy to show electrolumi-nescence, which is predicted by the model of field-driven ion-ization. Electroluminescence have been measured for core-shelldots/rods emitting in the visible and in the infrared. An ELand PL spectrum are shown in fig. 3 for a CdSe/CdS quantumdot layer emitting in the red part of the visible spectrum.

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Figure 3: EL and PL spectrum for an AC stack with CdSe/CdSquantum dots emitting in the red part of the visible spec-trum.

The electroluminescence spectrum is in good agreement withthe PL spectrum, indicating the same emission principle. Toshow the operation mechanism, the intensity of the peak dur-ing one period of the applied block wave of 20 kHz is depictedin fig. 4. Indeed, the high electric field at the beginning of thepositive pulse induces a sharp intensity increase, followed by arather linear decrease due to the emerging internal field.

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Figure 4: Light intensity in function of time.

The same mechanism happens in the opposite direction whenthe negative block pulse is initiated. The peak at exactly oneperiod might be caused by the internal field itself, which isstrong enough to induce light emission when the polarity ischanged.

The power in function of the applied voltage (at a frequency of20 kHz) is depicted in fig. 5 for the same stack, together withits linear fit. The power also shows a linear increase with thefrequency in the 1 − 15 kHz range.

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Figure 5: Output power of the AC stacks with CdSe/CdS dots(70 nm QD layer) in function of the applied voltage.

A maximum power of 2 µW has been measured. Based on theintercept, the underlying theory of the operation mechanism isconfirmed.

V. Integration

A model where a quantum dot, modeled as a dipole source,emits in the dielectric layer of an MDM structure, has beenconstructed to analytically calculate the spontaneous emissionenhancement factor. The resonant peak in the spectral resultis caused by the resonance of the surface plasmon polariton, asexpected.Next to this plasmonic model, an integrated structure has beendesigned to couple the light into a waveguide. A conceptualschematic is shown in fig. 6.

Figure 6: Threedimensional schematic of the integrated structureon silicon waveguides.

To check the influence an AC voltage over the quantum dotlayer, passive loss measurements have been performed forchanging voltage and changing area of deposited dots on thewaveguide. On top of the silicon waveguide, a monolayer ofquantum dots was deposited, then a 50 nm ALD layer of alu-mina and an ITO layer of 20 nm for top contacting. The siliconwaveguide layer itself is used as the bottom contact. An aver-age value of 2008 dB/cm is obtained for the loss caused by thequantum dot layer.

VI. Conclusion and future plans

The electroluminescence of the bulk AC devices with quan-tum dots emitting in the visible and near-infrared region showsthat this path of light generation looks promising, further effortshould be put into its characterization and further integrationfor plasmonic and silicon photonics purposes. Important as-pects supporting the latter are its compatibility with standardprocessing technologies, as well as the versatility of the plat-form, e.g. the easy combination with classic resonators suchas nitride microdisks. Furthermore, the characterization of theZnO:Al layer can be used to conduct different modulation ex-periments.

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VII. References

[1] Simply silicon. Nature Photonics, 4(8):491, 2010.

[2] Y. Yin and A. P. Alivisatos. Colloidal nanocrystal synthesisand the organic-inorganic interface. Nature, 437(7059):664–670, September 2005.

[3] J. M. Caruge, J. E. Halpert, V. Wood, V. Bulovic, andM. G. Bawendi. Colloidal quantum-dot light-emittingdiodes with metal-oxide charge transport layers. NaturePhotonics, 2(4):247–250, March 2008.

[4] V. Wood, M. J. Panzer, D. Bozyigit, Y. Shirasaki,

I. Rousseau, S. Geyer, M. G. Bawendi, and V. Bulovic.Electroluminescence from nanoscale materials via field-driven ionization. Nano letters, 11(7):2927–32, July 2011.

[5] J. Heo, Z. Jiang, J. Xu, and P. Bhattacharya. Coherentand directional emission at 1.55 µm from PbSe colloidalquantum dot electroluminescent device on silicon. Opticsexpress, 19(27):26394–8, December 2011.

[6] M. D. Groner, F. H. Fabreguette, J. W. Elam, and S. M.George. Low-temperature Al2O3 atomic layer deposition.Thin Solid Films, (16):639–645, 2004.

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Broadband colloidal quantum dot LED for active plasmonics

Floris Tallieu

Ghent University, Faculty of Engineering and Architecture,Department of Information Technology, Department of Inorganic and Physical Chemistry

Abstract In dit werk worden twee types LED’s ontworpen, gefabriceerd en gekarakteriseerd. Hun structuur is gebaseerdop een combinatie van colloıdale nanokristallen en anorganische lagen. De eerste methode is gebaseerd op het transport vanladingsdragers door het aanleggen van een DC spanning, terwijl het tweede werkingsmechanisme gebaseerd is op een AC spanningmet isolerende lagen. Karakterisatie van de lagen en elektroluminescentie-experimenten werden uitgevoerd. De integratie ineen plasmonische structuur wordt kort besproken door analyse van een dipoolbron in een metaal-dielektricum-metaal-structuur,terwijl experimentele devices werden geıntegreerd op een silicium golfgeleider.

Keywords colloıdale nanokristallen, LED, elektroluminescentie, plasmonics, geıntegreerde fotonica

I. Inleiding

Integratie van lichtemitterende devices in plasmonische struc-turen of op silicium is een veelbelovend researchdomein. Detoepassingen van geıntegreerde plasmonische structuren reikenvan x-ray devices tot sensoren, lichtvangst en geıntegreerde cir-cuits, terwijl de voordelen van CMOS compatibel silicium metde geavanceerde functionaliteiten van fotonische componentenreeds uitgebreid getest zijn [1]. Immers, toepassingen met si-licium in telecommunicatie, biosensoren, etc. zijn reeds ge-kend voor hun passieve functies (signaalsturing, filtering, . . . ).Desalniettemin zijn de actieve fotonische toepassingen zoalson-chip lichtgeneratie of -detectie een grotere uitdaging, gro-tendeels door de indirecte bandkloof van silicium. Daarvoorwerden verschillende lichtemitterende devices ontwikkeld metkwantum dots. Hun werkingsmechanisme, devicestructuur enresultaten worden besproken en gevolgd door een korte dis-cussie over hun integratie. De kwantum dots zijn colloıdalenanokristallen, geproduceerd in oplossing door een natte che-mische synthese en afstembaar om te emitteren in het UV totin het midden-infrarood met een hoge quantum yield [2].

II. Werkingsmechanismes

Twee devicetypes zijn geproduceerd, elk van hen gebaseerd opeen verschillend werkingsmechanisme.

A. Directe ladingsinjectie - DC

De eerste elektroluminescentiemethode is gebaseerd op directeladingsinjectie. Een schema van het device is weergegeven infig. 1.

Figuur 1: Schema van het device gebaseerd op directe ladingsin-jectie om elektron-gat-paren te creeren in de QD laag.

De elektronentransportlaag (ETL) en de gatentransportlaag(HTL) helpen de injectie van ladingsdragers in de kwantumdots te verbeteren door de energiebarriere tussen de elektro-des en het energieniveau van de kwantum dots te verlagen.Om die reden is de ETL meestal n-type, aangezien de meer-derheidsladingsdragers elektronen zijn, terwijl de HTL meestalp-type is [3]. De lagen zijn gedeponeerd op een glassubstraatbedekt met ITO, die ook meteen als de kathode gebruikt wordt.De anode is altijd aluminium, gedeponeerd via het opdampenm.b.v. een elektronenbundel en resulterend in meerdere vier-kante devices van ongeveer 1.8 mm × 1.8 mm.

B. Veld-gedreven ionisatie - AC

De AC devices door veld-gedreven ionisatie zijn opgebouwdmet isolerende lagen. Een schematische voorstelling is weerge-geven in fig. 2.

Figuur 2: Schema van het device gebaseerd op veld-gedreven ioni-satie om elektron-gat-paren te creeren in de QD laag.

Hier wordt een AC spanning tot 100 V peak-to-peak aange-legd tussen de ITO-laag en het aluminium, wat een hoog elek-trisch veld opwekt door de serie condensatoren, gevormd doorbeide isolerende lagen en de kwantum dot laag. Immers, als despanningsval per kwantum dot groter wordt dan de band gap,zullen de elektronen in de valentieband naar de conductiebandvan de naburige dot tunnelen, terwijl de gaten de exact tegen-gestelde beweging zullen maken. Dit creeert de mogelijkheidom terzelfdertijd een elektron in de conductieband en een gatin de valentieband van dezelfde kwantum dot te krijgen. Ditkan vervolgens tot radiatieve recombinatie leiden en bijhorendeemissie van een foton. Aangezien elektronen zullen propagerennaar de ene kant en gaten naar de andere, zal een intern veld op-gebouwd worden, tegengesteld aan het extern aangelegde veld.Dit verlaagt de spanningsval per kwantum dot (of equivalentde sterkte van het lokale veld). Wanneer deze spanningsvalniet langer hoog genoeg is, wordt het ladingstransport van deconductieband naar de valentieband of vice versa onderdrukt.Wanneer de polariteit wordt omgewisseld, vindt het identiekeproces plaats in de tegengestelde richting, aangezien het elek-trisch veld natuurlijk ook van polariteit veranderd is [4].

III. Devicestructuren en optimale laagsamenstelling

De keuze van de materialen voor de transportlagen in de DCdevices en de isolerende lagen in de AC devices is natuur-lijk cruciaal. Omdat er gestreefd wordt naar het gebruikvan anorganische componenten om snelle degradatie te ver-mijden, werden vooral nikkeloxide en koperoxide onderzochtals HTL. Deze laatste bleek echter geen p-type eigenschappente vertonen wanneer ze gedeponeerd werd via DC magnetronsputtering. Nikkeloxide werd op dezelfde manier gedeponeerdmet een vaste zuurstofdruk, resulterend in een resistiviteit van0.07 − 0.8 Ωcm voor een 30 tot 60 nm dunne film. Voor deETL werd zinkoxide gebruikt, ofwel als kwantum dots, ofwelgedeponeerd via ALD. Hoewel deze goede opties bleken uit deliteratuur [5], werd de meeste research gedaan naar ZnO:Al(2 wt% Al), gedeponeerd via roterende DC magnetron sputte-ring, aangezien veel parameters gewijzigd kunnen worden, o.a.de argon- en zuurstofdruk, de target-substraatafstand en dedikte van de laag. Deze werden zo gekozen om een resistiviteitte bekomen van 0.4 Ωcm voor een dikte van 75 nm.De materiaalkeuze voor de isolerende lagen van de AC deviceswas behoorlijk eenvoudig, aangezien de depositie van aluminakon gedaan worden door zowel opdamping met een elektronen-bundel of atomaire laag depositie. Analyseren van de kwaliteitvan de beide lagen toonde echter duidelijk de minderwaardig-heid van degene gedeponeerd via de elektronenbundel. Daarom

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werden 40 nm lagen gedeponeerd via ALD, zowel voor de bo-venste als de onderste laag. Admittantiemetingen leverden eenwaarde van ∼ 7.5 op voor de relatieve permittiviteit, wat ingoede overeenkomst is met de literatuur [6].

IV. Resultaten

Naast de uitgebreide karakterisatie van de ZnO:Al-laag, werdhet effect van het deponeren van deze of een aluminalaag bo-venop de kwantum dots onderzocht. Daarvoor werden emissie-,transmissie-, excitatiespectra en de levensduur opgemeten,waaruit werd besloten dat de afname van de intensiteit in defotoluminescentiespectra te wijten is aan de transmissiekarak-teristiek van de extra lagen en aan extra kanalen voor niet-radiatieve recombinatie, wat de levensduur van de fotonen sterkinkort. Stroom-spanningskarakteristieken op de complete de-vices waren in goede overeenkomst met literatuurresultaten,maar het gebrek aan reproduceerbaarheid, waarschijnlijk doorde opstapeling van ladingen aan interfaces en elektroden, is eenaanwijzing waarom geen elektroluminescentie opgemeten werdvoor de DC devices.Admittantiemetingen op de AC devices en hun individuele la-gen toonden aan dat de spanningsval per kwantum dot min-stens de bandkloofenergie diende te zijn om elektrolumines-centie te bekomen, wat ook voorspeld is door het model vande veld-gedreven ionisatie. Elektroluminescentie werd geme-ten voor core-shell dots/rods, emitterend in het visuele en IRspectrum. Een elektro- en fotoluminescentiespectrum wordtweergegeven in fig. 3 voor een kwantum dot laag, die emitteertin het rode gedeelte van het zichtbare spectrum.

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Figuur 3: EL en PL spectra voor een AC device met kwantumdots die emitteren in het rode gedeelte van het zichtbarespectrum.

Het elektroluminescentiespectrum is in goede overeenkomstmet het PL spectrum, wat aantoont dat hetzelfde emissieprin-cipe plaatsvindt. Om het werkingsmechanisme aan te tonen,wordt de intensiteit van de piek gedurende een periode van deaangelegde blokgolf van 20 kHz weergegeven in fig. 4. Zoalsverwacht veroorzaakt het hoge elektrisch veld aan het beginvan de positieve puls een scherpe intensiteitstoename, gevolgddoor een eerder lineaire afname door het opkomende interneveld.

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Figuur 4: Emissie-intensiteit in functie van de tijd.

Hetzelfde mechanisme gebeurt in de tegengestelde richtingwanneer de negatieve blokpuls bereikt wordt. De piek zicht-baar op exact een periode is mogelijks veroorzaakt door hetinterne veld zelf, maar hier werd geen bevestiging voor gevon-den.Het vermogen in functie van de aangelegde spanning (bij een

frequentie van 20 kHz) is weergegeven in fig. 5 voor dezelfdestack, samen met een lineaire fit.

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Figuur 5: Lichtvermogen van de AC stacks met CdSe/CdS dots(70 nm QD laag) in functie van de aangelegde spanning.

Het maximaal opgemeten vermogen bedraagt 2 µW. Op basisvan het intercept, werd de onderliggende theorie van het wer-kingsmechanisme bevestigd. Het lichtvermogen vertoont ookeen lineaire toename bij toenemende frequenties in een bereiktussen 1 en 15 kHz.

V. Integratie

Er werd een model gemaakt waar een kwantum dot, gemodel-leerd als een dipoolbron, emitteert in de dielektrische laag vaneen MDM structuur, om analytisch de spontane emissie en-hancement factor te berekenen. Dit levert, voor een voldoendedunne dielektrische laag, een resonante piek in het spectraleresultaat, veroorzaakt door de gap SPP mode, zoals verwacht.Naast dit plasmonisch model werd ook een geıntegreerde struc-tuur ontwikkeld om licht te koppelen in een silicium golfgelei-der. Een conceptueel schema wordt weergegeven in fig. 6.

Figuur 6: Driedimensionaal schema van de geıntegreerde structuurop een silicium golfgeleider.

Om de invloed van een AC spanning over de kwantum dotlaag te onderzoeken, werden passieve verliesmetingen uitge-voerd voor een veranderende spanning en varierende opper-vlakte van gedeponeerde dots op de golfgeleider. Bovenop desilicium golfgeleider werden een monolaag van kwantum dotsgedeponeerd, vervolgens een 50 nm ALD alumina laag en eenITO-laag van 20 nm, die als bovencontact fungeert. De silici-umlaag zelf wordt gebruikt als het andere contact. Een gemid-delde waarde van 2008 dB/cm werd bekomen voor het verliesveroorzaakt door de kwantum dot laag, wanneer geen spanningwerd aangelegd.

VI. Conclusie en toekomstplannen

De elektroluminescentie van bulk AC devices met kwantumdots die emitteren in het zichtbare en nabije infrarood spec-trum toont aan dat dit pad van lichtgeneratie veelbelovendlijkt, waardoor inspanningen zouden moeten gedaan wordenom verdere karakterisatie en integratie in plasmonische struc-turen en op silicium golfgeleiders door te voeren. De compa-tibiliteit met standaard processingtechnologieen en de veelzij-digheid van het platform, zoals de eenvoudige combinatie metklassieke resonatoren zoals nitride microdisks, zijn enkele be-langrijke aspecten die dit verder ondersteunen. Bovendien kande karakterisatie van de ZnO:Al laag gebruikt worden om ver-schillende modulatie-experimenten uit te voeren.

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VII. Referenties

[1] Simply silicon. Nature Photonics, 4(8):491, 2010.

[2] Y. Yin and A. P. Alivisatos. Colloidal nanocrystal synthesisand the organic-inorganic interface. Nature, 437(7059):664–670, September 2005.

[3] J. M. Caruge, J. E. Halpert, V. Wood, V. Bulovic, andM. G. Bawendi. Colloidal quantum-dot light-emitting dio-des with metal-oxide charge transport layers. Nature Pho-tonics, 2(4):247–250, March 2008.

[4] V. Wood, M. J. Panzer, D. Bozyigit, Y. Shirasaki, I. Rous-seau, S. Geyer, M. G. Bawendi, and V. Bulovic. Electrolu-minescence from nanoscale materials via field-driven ioni-zation. Nano letters, 11(7):2927–32, July 2011.

[5] J. Heo, Z. Jiang, J. Xu, and P. Bhattacharya. Coherentand directional emission at 1.55 µm from PbSe colloidalquantum dot electroluminescent device on silicon. Opticsexpress, 19(27):26394–8, December 2011.

[6] M. D. Groner, F. H. Fabreguette, J. W. Elam, and S. M.George. Low-temperature Al2O3 atomic layer deposition.Thin Solid Films, (16):639–645, 2004.

x

Contents

1 Introduction 1

1.1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Thesis report structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Colloidal nanocrystal quantum dots 4

2.1 Quantum dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.2 Theoretical approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Colloidal nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Synthesis of colloidal nanocrystals . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1.1 Nucleation phase . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1.2 Growth phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.2 Types of colloidal nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3 Optical and electrical properties of colloidal quantum dots . . . . . . . . . . . . . 10

2.3.1 Light absorption and emission . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3.2 Radiative and non-radiative recombination . . . . . . . . . . . . . . . . . 12

2.3.3 Dielectric function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3 Composition of the quantum dot LED 14

3.1 DC quantum dot LED based on direct charge injection . . . . . . . . . . . . . . . 14

3.1.1 Basic structure: theoretical approach . . . . . . . . . . . . . . . . . . . . . 14

3.1.2 Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1.3 General structure and layer description . . . . . . . . . . . . . . . . . . . 17

3.1.3.1 Substrate and ITO layer (cathode) . . . . . . . . . . . . . . . . . 17

3.1.3.2 Hole transport layer . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1.3.2.1 Organic TPD . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1.3.2.2 Nickel oxide . . . . . . . . . . . . . . . . . . . . . . . . 18

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Contents

3.1.3.2.3 Copper oxide . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1.3.3 Colloidal nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.3.4 Electron transport layer . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.3.4.1 Zinc oxide - quantum dots . . . . . . . . . . . . . . . . 19

3.1.3.4.2 Zinc oxide - atomic layer deposition . . . . . . . . . . . 19

3.1.3.4.3 ZnO:Al - rotatable magnetron sputtering . . . . . . . . 19

3.1.3.5 Aluminium contacts (anode) . . . . . . . . . . . . . . . . . . . . 20

3.1.4 Improvement of the LED stack . . . . . . . . . . . . . . . . . . . . . . . . 20

3.2 AC quantum dot LED by field-driven ionization . . . . . . . . . . . . . . . . . . . 20

4 Deposition and measurement methods 24

4.1 Deposition methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.1.1 Ion-beam reactive magnetron sputtering . . . . . . . . . . . . . . . . . . . 24

4.1.2 Cylindrical rotating magnetron sputtering . . . . . . . . . . . . . . . . . . 25

4.1.3 Atomic layer deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.1.4 Electron beam evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.1.5 Spincoating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2 Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2.1 Current-voltage measurements . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2.2 Electroluminescence measurements . . . . . . . . . . . . . . . . . . . . . . 28

4.2.2.1 EL measurements by fibre coupling . . . . . . . . . . . . . . . . 28

4.2.2.2 EL measurements by direct coupling into spectrometer . . . . . 29

4.2.2.3 Power measurements . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.3 Photoluminescence measurements . . . . . . . . . . . . . . . . . . . . . . 29

4.2.3.1 Emission spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.3.2 Excitation spectrum . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.3.3 Lifetime measurements . . . . . . . . . . . . . . . . . . . . . . . 30

4.2.4 Atomic force microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2.5 Scanning electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2.6 Absorption measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2.7 XRD measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2.8 Admittance measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.9 Thickness measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.9.1 Optical profilometer . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.2.9.2 Piezoelectric contact profilometry . . . . . . . . . . . . . . . . . 33

xii

Contents

5 Characterization of the layers 34

5.1 DC LED stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1.1 Hole transport layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1.1.1 TPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1.1.2 Copper oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.1.1.3 Nickel oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5.1.2 Electron transport layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.1.2.1 ZnO - quantum dots . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.1.2.2 ZnO - atomic layer deposition . . . . . . . . . . . . . . . . . . . 39

5.1.2.3 ZnO:Al - rotatable DC magnetron sputtering . . . . . . . . . . . 39

5.1.3 Surface and interface characterization . . . . . . . . . . . . . . . . . . . . 39

5.1.3.1 Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.1.3.1.1 Quantum dot layer . . . . . . . . . . . . . . . . . . . . 40

5.1.3.1.2 ZnO:Al layer . . . . . . . . . . . . . . . . . . . . . . . . 41

5.1.3.2 Cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.2 ZnO:Al - rotatable DC magnetron sputtering . . . . . . . . . . . . . . . . . . . . 43

5.2.1 Influence of the argon pressure . . . . . . . . . . . . . . . . . . . . . . . . 43

5.2.2 Influence of the target-substrate distance . . . . . . . . . . . . . . . . . . 47

5.2.3 Influence of the layer thickness . . . . . . . . . . . . . . . . . . . . . . . . 48

5.2.4 Influence of the oxygen pressure . . . . . . . . . . . . . . . . . . . . . . . 50

5.2.5 Standard parameters for n-type ZnO:Al in LED stacks . . . . . . . . . . . 52

5.3 AC LED stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6 Stack results 56

6.1 Photoluminescence measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.1.1 Basic emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.1.2 Influence of ZnO:Al layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.1.2.1 Emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

6.1.2.2 Lifetime measurements . . . . . . . . . . . . . . . . . . . . . . . 58

6.1.2.3 Excitation spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.1.3 Influence of an Al2O3 layer . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.2 Current-voltage characteristics of the DC LED stack . . . . . . . . . . . . . . . . 60

6.2.1 TPD-QDs-ZnO QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.2.2 NiO-QDs-ZnO:Al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.2.3 Reproducibility of the IV measurements . . . . . . . . . . . . . . . . . . . 62

6.3 Admittance measurements of the DC LED stack . . . . . . . . . . . . . . . . . . 63

6.4 Admittance measurements of the AC stack . . . . . . . . . . . . . . . . . . . . . 64

xiii

Contents

6.4.1 Simplified model calculations . . . . . . . . . . . . . . . . . . . . . . . . . 65

6.4.2 Extended model calculations . . . . . . . . . . . . . . . . . . . . . . . . . 66

6.5 Electroluminescence of the AC stacks . . . . . . . . . . . . . . . . . . . . . . . . . 69

6.6 Modeling of the EL of the AC stacks . . . . . . . . . . . . . . . . . . . . . . . . . 74

6.7 Comparison between both methods and commercialization considerations . . . . 75

7 Integrated light source under AC field excitation 77

7.1 Plasmonic integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

7.1.1 Surface plasmon concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

7.1.2 Modeling of a dipole light source emitting in an MDM structure . . . . . 79

7.2 Integration of the AC stack on a silicon waveguide . . . . . . . . . . . . . . . . . 82

7.2.1 Description of the structure . . . . . . . . . . . . . . . . . . . . . . . . . . 82

7.2.2 Integration considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

7.2.3 Building steps of the integrated stack . . . . . . . . . . . . . . . . . . . . 84

7.2.3.1 Silicon waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . 84

7.2.3.2 Lithography steps and deposition methods . . . . . . . . . . . . 84

7.2.4 Loss measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

8 Conclusion 89

A Python code 92

A.1 Single sweep for current-voltage characteristic . . . . . . . . . . . . . . . . . . . . 92

A.2 Hysteresis characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

xiv

List of Figures

1.1 CIE chromaticity diagram for quantum dots and a standard HDTV color triangle. 1

1.2 Examples of applied plasmonics. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Comparison of the band diagram of a bulk semiconductor and a quantum dot. . 5

2.2 Injection method of the precursor in a hot solvent for colloidal nanocrystal synthesis. 7

2.3 Energy barrier of the nucleation reaction. . . . . . . . . . . . . . . . . . . . . . . 8

2.4 Equilibrium solubility of a particle as a function of the particle radius. . . . . . . 8

2.5 Absorbance spectra of cadmium and lead chalcogenides having different sizes. . . 10

2.6 Absorption (solid line) and photoluminescence spectrum (dashed line) for a CdSe

quantum dot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.7 Different absorption and emission wavelength due to valence band splitting. . . . 12

2.8 Main recombination mechanisms in quantum dots. . . . . . . . . . . . . . . . . . 12

2.9 Calculated dielectric functions of (a) bulk PbS and (b) PbS QDs. . . . . . . . . . 13

3.1 Electron and hole injection into the quantum dot, resulting in radiative recombi-

nation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.2 Schematic depiction of the energy band diagram for a DC quantum dot LED

structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3 Basic DC quantum dot LED structure. . . . . . . . . . . . . . . . . . . . . . . . . 17

3.4 Chemical structure of TPD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.5 Complete stack, showing all individual pads. . . . . . . . . . . . . . . . . . . . . 20

3.6 Schematic of stacks to reduce carrier concentration and charging effect. . . . . . 21

3.7 Conceptual circuit to acquire EL by field-driven ionization. . . . . . . . . . . . . 21

3.8 Band diagram when applying a positive voltage pulse, explaining the principle of

field-driven ionization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.9 Experimental stack and used circuit for the AC quantum dot LED. . . . . . . . . 23

4.1 Schematic and picture of the DC magnetron sputtering setup. . . . . . . . . . . . 24

4.2 Planar nickel target for DC magnetron sputtering. . . . . . . . . . . . . . . . . . 25

4.3 Picture of the setup and a ZnO:Al target for rotatable magnetron sputtering. . . 25

xv

List of Figures

4.4 Schematic and picture of the electron beam evaporation setup. . . . . . . . . . . 27

4.5 Setup to measure IV and EL on DC stacks. . . . . . . . . . . . . . . . . . . . . . 28

4.6 Acceptance cone of the multimodal fibre. . . . . . . . . . . . . . . . . . . . . . . 28

4.7 EL setup by direct coupling into the spectrometer, showing the input slit, the

monochromator and the CCD camera. . . . . . . . . . . . . . . . . . . . . . . . . 29

4.8 Transmission spectrum of a ZnO:Al thin film. . . . . . . . . . . . . . . . . . . . . 31

4.9 Schematic and resulting graph of XRD setup. . . . . . . . . . . . . . . . . . . . . 32

4.10 Schematic of the principle of the Veeco profilometer using vertical scanning inter-

ferometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.1 Transmission spectrum of TPD after annealing. . . . . . . . . . . . . . . . . . . . 35

5.2 XRD spectra of a copper oxide layer with varying oxygen pressure. . . . . . . . . 35

5.3 Hysteresis curve of the voltage in function of the oxygen level for copper oxide at

constant current and argon pressure. . . . . . . . . . . . . . . . . . . . . . . . . . 36

5.4 Hysteresis curve of the voltage in function of the oxygen level for nickel oxide at

constant current and argon pressure. . . . . . . . . . . . . . . . . . . . . . . . . . 37

5.5 Transmission spectrum of a thin nickel oxide film of 115 nm. . . . . . . . . . . . . 38

5.6 Transmission spectra of ZnO nanocrystals, annealed at different temperatures

and durations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.7 AFM image of a cracked PbS quantum dot layer. . . . . . . . . . . . . . . . . . . 40

5.8 AFM image of a 70 nm spincoated quantum dot layer. . . . . . . . . . . . . . . . 40

5.9 AFM topography images of ZnO:Al layers. . . . . . . . . . . . . . . . . . . . . . . 41

5.10 Cross section SEM image of TPD-QD-ZnO nanocrystals stack. . . . . . . . . . . 41

5.11 Cross sections of the NiO-QD-ZnO nanocrystal stack. . . . . . . . . . . . . . . . 42

5.12 Cross section SEM image of a ZnO:Al layer under standard parameters with a

thickness of 869 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.13 Resistivity of ZnO:Al sputtered thin films in function of the argon pressure. . . . 44

5.14 XRD data of the ZnO:Al thin films with changing argon pressure. . . . . . . . . 44

5.15 Detail of the XRD data plots for changing argon pressure. . . . . . . . . . . . . . 45

5.16 Band diagram and electron distribution function showing the filled electron states,

explaining the Burstein-Moss shift. . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.17 Mobility (squares), electron density (circles) and resistivity (triangles) in function

of the argon pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48


of target-substrate distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48


of layer thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

xvi

List of Figures


of the oxygen pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.21 Resistivity of ZnO:Al sputtered thin films in function of the oxygen pressure,

before and after annealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.22 XRD spectra before and after annealing of the first and last data point of the

oxygen pressure set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.23 Cross section SEM image of the AC stack with the alumina layers deposited by

electron beam evaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.24 Cross section SEM images of the AC stack with alumina layers deposited by ALD. 54

5.25 Cross section SEM image a Si-QD-Al2O3 (e-beam)-aluminium stack. . . . . . . . 54

5.26 Capacitance-frequency measurements of an alumina layer of 38 nm on two differ-

ent pads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.1 Emission spectra of PbS quantum dots. . . . . . . . . . . . . . . . . . . . . . . . 56

6.2 Emission spectra of the glass-QRs and glass-QRs-ZnO:Al samples. . . . . . . . . 57

6.3 Transmission spectrum of an 86 nm ZnO:Al layer. . . . . . . . . . . . . . . . . . 57

6.4 Lifetime measurements of the glass-QRs and glass-QRs-ZnO:Al samples and their

corresponding exponential fits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.5 Excitation spectra of the glass-QRs and glass-QRs-ZnO:Al samples. . . . . . . . 59

6.6 Emission spectra of the glass-alloy QDs and glass-alloy QDs-Al2O3 samples. . . . 59

6.7 Influence of an alumina layer on the emission spectra and lifetime measurements

of the glass-CdSe/CdS core-shell QDs and glass-CdSe/CdS core-shell QDs-Al2O3

samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.8 Emission spectrum of the glass-PbS/CdS core-shell QDs-Al2O3 sample. . . . . . 60

6.9 Current-voltage characteristics for TPD-QDs-ZnO QDs stack. . . . . . . . . . . . 61

6.10 Comparison of current-voltage characteristics of the DC LED stacks with literature. 61

6.11 Current-voltage characteristics for NiO-QDs-ZnO:Al stacks. . . . . . . . . . . . . 62

6.12 Repeated current-voltage measurements on a NiO-QDs-ZnO(ALD) stack. . . . . 63

6.13 Repeated current-voltage measurements on a NiO-QDs-ZnO:Al stack, including

a negative voltage measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.14 Procedure to determine trap level density in the NiO-QD-ZnO:Al stack. . . . . . 65

6.15 Capacitance-frequency measurements of the total AC stack. . . . . . . . . . . . . 66

6.16 Capacitance density-frequency plot of alumina layers and total stack, where the

QD layer is spincoated once or twice. . . . . . . . . . . . . . . . . . . . . . . . . . 67

6.17 Conductance-frequency plot and fitting curves of the ITO-Al2O3-Al, where the

alumina layer is the bottom oxide layer of the total stack. . . . . . . . . . . . . . 67

6.18 Electrical circuit proposal for the ITO-Al2O3-Al stack. . . . . . . . . . . . . . . . 67

xvii

List of Figures

6.19 Capacitance-frequency plot of the quantum dot layer (spincoated once or twice),

based on the extended model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

6.20 Electroluminescence spectrum of the stack ITO-Al2O3 (38 nm)-QRCdSe/CdS (1x

SC)-Al2O3 (50 nm)-Al contacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

6.21 Applied waveforms to measure electroluminescence. . . . . . . . . . . . . . . . . . 70

6.22 Light intensity in function of time. . . . . . . . . . . . . . . . . . . . . . . . . . . 71

6.23 Output power of the AC stacks with CdSe/CdS dots (70 nm QD layer) in function

of the applied voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

6.24 Output power of the AC stacks with giant CdSe/ZnS dots in function of the

applied voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.25 Output power of the AC stacks with giant CdSe/ZnS dots in function of the

applied frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

6.26 On and off state of yellow-orange emitting CdSe/CdS rods. . . . . . . . . . . . . 73

6.27 Number of recombining electron-hole pairs in the central layer in function of time

and different frequencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

7.1 Important parameters describing the gap surface plasmon polariton mode in func-

tion of the central layer thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . 79

7.2 MDM structure with a dipole light source in the dielectric layer. . . . . . . . . . 79

7.3 Real and imaginary part of the relative permittivity of silver, based on the Drude-

Lorentz model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

7.4 Spontaneous enhancement factor in function of the wavelength for different thick-

nesses of the dielectric layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

7.5 Influence on the spontaneous emission enhancement factor of the location of the

dipole source in the dielectric layer of the MDM structure. . . . . . . . . . . . . . 82

7.6 Schematic of the integrated structure. . . . . . . . . . . . . . . . . . . . . . . . . 82

7.7 Microscopic picture of the created structures. . . . . . . . . . . . . . . . . . . . . 83

7.8 Different lithography masks for integrated stack. . . . . . . . . . . . . . . . . . . 85

7.9 Power loss difference between the waveguides with and without quantum dots

(first design), measured by the OSA. . . . . . . . . . . . . . . . . . . . . . . . . . 86

7.10 Power output difference, measured by the OSA and the power meter, for varying

interaction length and for all waveguide sets (first design) at a wavelength of

1540 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

7.11 Power output difference between the waveguides with and without quantum dots

(second design), measured by the power meter at a wavelength of 1518.6 nm. . . 87

xviii

List of Figures

7.12 Power output difference between the waveguides with and without quantum dots

and only the alumina layer, measured by the power meter at a wavelength of

1518.6 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

xix

List of Tables

2.1 Density of states of quantum confined semiconductors with a parabolic dispersion

relation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Overview of the properties of chalcogenide nanocrystals. . . . . . . . . . . . . . . 9

5.1 XRD results of magnetron sputtered copper oxide at varying oxygen pressure. . . 35

5.2 Hall-Vanderpauw measurements on NiO thin films. . . . . . . . . . . . . . . . . . 37

5.3 XRD results and calculation of grain size with Scherrer’s formula in function of

the argon pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46


the target-substance distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49


the layer thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50


the oxygen pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.1 Summary of both operation mechanisms, their advantages and disadvantages. . . 76

xx

List of abbreviationsAlq3

AZO

DEZ

DOS

ETL

EQE

HDA

HOMO

HTL

ITO

IV

LED

MDM

NA

NC

ODE

OLED

OSA

PL

QD

QR

QY

SC

SEM

SILAR

SPP

TDPA

TM

TOP

TPD

ZTO

Tris(8-hydroxyquinolinato aluminium)

Aluminium-doped Zinc Oxide

Diethyl Zinc (Zn(CH2CH3)2)

Density Of States

Electron Transport Layer

External Quantum Efficiency

Hydroxydopamine

Highest Occupied Molecular Orbital

Hole Transport Layer

Indium Tin Oxide

Current-Voltage

Light Emitting Diode

Metal-Dielectric-Metal

Numerical Aperture

Nanocrystal

1-octadecene

Organic Light Emitting Diode

Optical Spectrum Analyzer

Photoluminescence

Quantum Dot

Quantum Rod

Quantum Yield

Spincoated

Scanning Electron Microscopy

Sequential Ion Layer Addition and Reaction

Surface Plasmon Polariton

Thiodipropionic Acid

Transverse Magnetic

Trioctylphosphine

N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine

Zinc Tin Oxide

xxi

Chapter 1

Introduction

1.1 Context

Quantum dots are a promising research area in a lot of domains. Solar cells, photovoltaics,

medical imaging and disease detection, anti-counterfeiting capabilities, even counter-espionage

and defense applications are current areas of investigation [1–8]. Mainly however, quantum dots

might have its most promising applications in light emitting diodes. They can be tuned to emit

in a wide range of wavelengths, ranging from blue to infrared, meaning that they can be used

for display applications or even in telecommunication. To show the possibilities of quantum dot

LEDs, a comparison of the color gamut is made with classical liquid crystal displays. This is

shown in fig. 1.1, where we clearly see that the spectral purity of quantum dots is higher than

a standard HDTV color triangle.

Figure 1.1: CIE chromaticity diagram for quantum dots and a standard HDTV color triangle. [9]

Of course, air-stable components are necessary to be able to produce this in an inexpensive way.

Therefore, in this thesis, we strived to work with inorganic components as surrounding layers

for the QDs, since they show significantly slower degradation in time, caused by the negative

1

Chapter 1. Introduction

influence of air and moist on organic compounds.

The second part of the thesis title refers to plasmonics. This term comprises the optical prop-

erties of metal structures at the nanoscale. More specific, nowadays lots of applications are

under investigation involving the use of surface plasmons. Some examples are shown in fig. 1.2,

while the theoretical approach is explained in chapter 7. As should be clear, the applications

(a) Creation of soft

x-rays by a hot spot.

(b) Efficient sensing due

to resonance shifting mea-

surement.

(c) Enhanced light trap-

ping in a solar cell.

(d) Plasmonic integrated cir-

cuit.

Figure 1.2: Examples of applied plasmonics. [10]

of plasmonic structures are very wide, ranging from the creation of soft x-rays1 by a hot spot2

to sensing, light trapping and integrated circuits [10, 11]. Furthermore, research projects have

already been initiated to use the silicon-plasmonic platform for optical interconnection [12].

Here, an active plasmonic application will be discussed by using the quantum dot LED structure

in between two metal plates, resulting in an optical source in an MDM structure. This might

link an electronic circuit with the photonic one in an easy and inexpensive way.

Finally, integration of the light source on silicon remains a field which cannot be ignored, because

of its compatibility with standard processing techniques, as well as the versatility of the plat-

form, e.g. the easy combination with classic resonators such as nitride microdisks. That’s why

in this thesis, one type of the LEDs will be integrated on a silicon waveguide and subsequently

be analyzed.

1.2 Motivation

Choosing a master thesis was one of the most difficult things to do as student civil engineering

in applied physics. Not only you want it to be fun, you also want it to be rewarding for you and

everybody around you. Therefore I looked first of all to my own interests, which lie in photonic

applications and in quantum mechanics. Of course, finding something which combines both is not

easy. However, intrigued by the word quantum dot of which I barely knew anything, this master

thesis appealed to me because of multiple reasons, in first instance due to its variety of work,

1X-rays with an energy between 12 and 120 keV.2Spot where the intensity of an incoming light beam can be concentrated by more than four orders of magnitude.

2

Chapter 1. Introduction

from practical experimental to theoretical simulations. Furthermore it attracted me because it

is a collaboration between two research groups, both important in the science community. All

of these mentioned elements made me consider this thesis as a challenge, as I preferred it.

1.3 Goals

The title can clearly be split into two parts. First of all, a working quantum dot LED should be

made, where attention should be put to its air-stable operation. Therefore, different inorganic

materials should be characterized in order to make layers with optimal electronic and optical

properties. Also emphasis should be put on the broadband operation, where the same LED

stack could be used with a wide wavelength range of different quantum dots.

Next to the quantum dot LED, its integration into plasmonic materials should be optimized.

To achieve this, simulations considering the quantum dots in the LED stack as optical source

of photons propagating along metal-dielectric(-metal) structures are done. Finally, one type of

the LEDs has been integrated on a silicon waveguide and associated measurements have been

performed.

1.4 Thesis report structure

In chapter 2, an extensive introduction of colloidal nanocrystals is given, where the theoretical

and the practical part are explained, as well as different types of nanocrystals. In chapter 3,

a closer look is taken to the composition of the quantum dot LED, elaborating on the general

structure, on all different layers individually and on the operation mechanisms. Next, all used

deposition, measurement and characterization methods are discussed in chapter 4, ranging from

IV measurements to SEM pictures. Based on these, characteristics of all layers are determined

and optimized for the LED stack in the next chapter. Chapter 6 shows various kinds of mea-

surement results on this total stack. Finally, in chapter 7, we arrive, on the one hand, at the

integration into a plasmonic structure, where an MDM with optical source is analyzed. On the

other hand, loss measurements of an integrated structure on a silicon waveguide are performed.

3

Chapter 2

Colloidal nanocrystal quantum dots

To be able to go into detail on colloidal nanocrystal quantum dots, all phrase parts need a

profound understanding. Therefore, an elaborate explanation about quantum dots is given,

followed by the limitation to colloidal nanocrystals.

2.1 Quantum dots

2.1.1 Definition

According to the Encyclopedia of Laser Physics and Technology, quantum dots are defined as

microscopic structures confining charge carriers in three dimensions. This charge confinement

is in fact a quantum effect, where the part ‘dot’ points out that this happens in all three spatial

dimensions. A first appearance of the term occurred in 1988 by Reed [13]. However, it was

already in the early 80’s that materials have been discovered which behave according to the

above definition [14].

2.1.2 Theoretical approach

As explained in the previous subsubsection, three-dimensional spatial confinement occurs. This

reflects itself in the dimensions of the structure, which are in every direction of the order of

nanometers. This physical range is of importance because it has the same order of magnitude as

the exciton Bohr radius, which indicates the most probable distance between the electron and

the hole for an electron-hole pair in a bulk material. Conclusively, in the length scale regime

below some tens of nanometers, strong confinement occurs which quantum mechanically only

yields discrete energy levels inside the quantum well [15]. A pictorial representation of what

happens with the energy levels is shown in fig. 2.1.

This can quantum mechanically be explained based on the Schrodinger equation and the Pauli

exclusion principle. The time-independent Schrodinger equation for the potential well is defined

4

Chapter 2. Colloidal nanocrystal quantum dots

(a) Bulk semiconductor. (b) Quantum dot.

Figure 2.1: Comparison of the band diagram of a bulk semiconductor and a quantum dot.

as:

− ~2

2m∇2Ψ(r) + V (r)Ψ(r) = EΨ(r) (2.1)

where m denotes the effective mass of the particle and ∇2 is the Laplace operator.

For an infinite square potential well in one dimension, this results in easy solutions for the bound

energy states:

En = n2h2

8mL2(2.2)

This indeed shows that the smaller the well, the further the energy levels separate.

Since an infinite square potential well in one dimension is not a good representation for the

zero-dimensional case of the spherical quantum dot, a better approximation can be made by

considering the bound energy states of a sphere with infinite square potential well. These are

given analytically (even for the finite case) by [16]:

Ekl =h2

8πm

(aklR

)2(2.3)

In this equation, akl is the kth zero of the lth spherical Bessel function of the first kind. It

should be clear that the same inverse squared dependence on the well radius is found as in eq.

(2.2).

To refer back to the original depiction of the quantum dot band diagram in fig. 2.1(b), we can

easily calculate the new band gap, taking into account the original band gap Eg, the effective

masses of the holes mh and electrons me and the fact that the first zero to be considered is a00:

Eg,QD = Eg +h2

8π

(a00R

)2( 1

mh+

1

me

)= Eg +

h2

8πµ

(a00R

)2(2.4)

As expected, the band gap becomes larger due to the quantum confinement. The change also

scales with 1/R2, showing that smaller particles shift the absorption to higher energies (or

shorter wavelengths). This simplified expression can be expanded with two terms [17]:

A negative term scaling with 1/R which includes the Coulombic electron-hole interaction,

A position-dependent solvation energy term, arising from dielectric screening.

5


However, these additional terms do not undo the increasing band gap for decreasing diameter.

A last important subject in the discussion of the quantum confinement of quantum dots is

the density of states of zero-dimensional systems. To illustrate what happens, in table 2.1, a

graphical depiction and corresponding expressions of the DOS are presented when a subsequent

confinement is performed from 3D to 0D. All dispersion relations are assumed parabolic. As

Dim Image DOS Graph

3 12π2

(2m∗

~2)3/2√

E − Ec

2∑n

m∗

π~2H(E − En)

1∑n

1π~

√m∗

2(E−En)

0∑n

2δ(E − En)

Table 2.1: Density of states of quantum confined semiconductors with a parabolic dispersion relation.

predicted in the discretization of the energy levels, only delta peaks arise in the 0D DOS. When

measured in reality, they are peaks width a certain width, caused by both homogeneous and

inhomogeneous broadening.

2.2 Colloidal nanocrystals

Nanocrystals are defined as a portion of matter where at least in one dimension the length scale

is smaller than 100 nm and the material is monocrystalline [18]. The creation of nanoscale

particles can be done in several ways: epitaxial, sputtering, ion implantation, precipitation in

molten glasses, chemical synthesis. . . [15]. The latter can be divided into different methods,

such as reverse micelles technique, sonochemical synthesis and colloidal synthesis [19]. Since in

this thesis only colloidal nanocrystals are used, an elaborate explanation of their synthesis and

the different types are given.

6


2.2.1 Synthesis of colloidal nanocrystals

The creation of colloidal nanocrystals is a bottom-up method, where the reaction of the precur-

sor(s) in a solution gives rise to the nucleation of small particles, which grow bigger the longer

the reaction is maintained. The nucleation phase was first described by La Mer and Dinegar,

who succeeded in synthesizing monodispersed sulphur colloids [20]. The growth phase, which

in fact consists of a rapid size increase and finally a slower process, known as Ostwald ripening,

where the larger particles grow and the smaller particles dissolve back to monomers.

2.2.1.1 Nucleation phase

As stated before, when the concentration of the precursor, which is injected in a hot solvent as

shown in fig. 2.2, reaches a sufficiently high level called the nucleation threshold, super-saturation

occurs. This results in the decomposition of the precursor in monomers and subsequent agglom-

Figure 2.2: Injection method of the precursor in a hot solvent for colloidal nanocrystal synthesis. [15]

eration of the latter. If no more precursor material is added to the solvent, the concentration

level will return below the nucleation threshold and the nucleation process will be put on hold.

Thermodynamically, the formation of the nanocrystal particles is a consequence of overcoming

an energy barrier, as depicted in fig. 2.3. The free energy difference between the solution and

the (n-monomer) nanocrystal is given by:

∆G = n(µc − µs) + 4πr2γ (2.5)

where µs and µc are the chemical potentials of the solution and the nanocrystal (per monomer)

respectively, r the particle radius and γ the surface tension. This formula can be rewritten and

approximated in function of the particle radius as single variable. The surface term dominates

the negative first term until the critical radius rc is reached as shown in the graph.

7


Figure 2.3: Energy barrier of the nucleation reaction. [21]

2.2.1.2 Growth phase

After nucleation, it is energetically favorable for the aggregates to grow. When there is a large

excess of monomers, the growth rate is determined by the rate of the monomer-surface reaction

and only affects the average radius, not the size distribution. This fast process is followed

by a diffusion-limited growth where the transport of monomers becomes the limited factor [21].

When the monomer supply is completely depleted, Ostwald ripening occurs. This phenomenon is

characterized by the detachment of monomers from the smaller particles, which are subsequently

used for the growth of the larger ones. A schematic representation of the equilibrium solubility

of a particle as a function of the particle radius is depicted in fig. 2.4, which shows that particles

smaller than the mean particle size will dissolve, whereas the particles larger than the mean

particle size will grow.

Figure 2.4: Equilibrium solubility of a particle as a function of the particle radius. [22]

Size dispersion is not easy to describe but is mainly determined by three parameters: the

monomer concentration, the surface tension and the temperature. Controlling these is not

evident, but literature and experience yield adequate results [21,23–26].

8


2.2.2 Types of colloidal nanocrystals

The basic principles of nucleation and growth of colloidal nanocrystals apply for all of them, yet

from thermodynamic point of view, a solution of nanoparticles is not a stable state. They have

to be stabilized by ligands, or by charge. Here, a small overview of attainable nanoparticles is

given.

The first syntheses of nanocrystals were performed in a nitrogen environment, and were mainly

cadmium and lead chalcogenides (sulfides, selenides and tellurides). Most of the syntheses are

already described by standardized methods including multiple parameters, such as size, size

dispersion, PL quantum yield, etc. In table 2.2, a small overview is given of the most common

chalcogenide nanocrystals, including alloys, which are cadmium with different ratios of selenium

and sulphur ODE precursors.

Material Size [nm] Size dispersion [%] PL QY [%] Ligands

CdSe 2− 4 6− 8 1− 5 Stearic acid & HDA

CdSe (blue) 1.6− 2.2 1− 6 1− 5 Stearic acid & HDA

CdS 6 5 Trap emission Fatty acid

CdTe 3− 11 4− 8 Unknown TDPA

Alloys 2.4− 3 5 10 Fatty acid

PbSe 3− 8 5− 9 40 Oleic acid

PbS 3− 10 5− 14 20− 90 Oleic acid

Table 2.2: Overview of the properties of chalcogenide nanocrystals.

The PL quantum yield (on which will be elaborated in the next section) can often be improved,

for example by adding a ZnS layer, by ligand exchange or by changing the core/shell ratios.

Three types of adapted syntheses deserve special attention:

Cation exchange reaction: Outer layers of the core are replaced by an other chalcogenide

(e.g. CdSe):

– PbSe: PL QY: 25− 40%

– PbS: PL QY: 10− 90%

SILAR: Layer by layer addition: PL QY: 1− 40%

Flash: Flush at lower temperatures, adding TOP-S and CdSe seeds at higher temperature:

PL QY: up to 60%

Not only quantum dots can be created in chemical synthesis, also rods, such as CdSe and

CdSe/CdS rods which have cylinder radii from 2 to 20 nm and a length smaller than 100 nm.

9


They can show even higher photoluminescence quantum yield than normal dots.

A last important synthesis of nanocrystals is the creation of ZnO quantum dots, which are not

stabilized by any ligands, but by charge. While the London-interaction between the colloids is

usually lowered by the adding of capping molecules, they can also be countered by the Coulomb

repulsion if the colloids carry an electric charge [22,27].

2.3 Optical and electrical properties of colloidal quantum dots

2.3.1 Light absorption and emission

As should be clear up to now, both the absorption and the emission of photons of the quantum

dots are strongly dependent on its size and its bulk band gap. This can be easily seen if we

look at an example of an absorbance spectrum of different dispersed quantum dots. For some

cadmium and lead chalcogenides, this is shown in fig. 2.5.

Figure 2.5: Absorbance spectra of cadmium and lead chalcogenides having different sizes. [27]

Some important conclusions can be drawn from these spectra:

Since they are all different semiconductors, they have a different band gap energy, resulting

in shifted spectra accordingly.

For example, CdS has a bulk band gap of 2.42 eV at 300 K, while CdSe has a band gap of

1.74 eV [28,29]. CdS will thus have absorption spectra at shorter wavelengths than CdSe.

The presence of peaks in each of the spectra refers to the discretization of the energy levels,

as was also clear in the density of states. The first absorption peak (agreeing with the light

with the longest wavelength/smallest energy to be absorbed) is of course most visible. On

10


the broadening of the peaks is elaborated further in this subsection.

Absorption spectroscopy also yields information about the size dispersion and the concen-

tration.

The size dispersion is easily measured by fitting a Gaussian curve to the first peak, while the

calculation of the concentration of the quantum dots is based on following formula:

I

I0= 100.0277d

3c0l (2.6)

where II0

denotes the fraction of the transmitted light measured at the peak, d the diameter of

the particles (immediately determined by the peak position), c0 the unknown concentration and

l the propagation length of the light through the cuvette (1 cm) [30].

Since we are mainly interested in the emission rather than the absorption, a plot of both spectra

might yield more information about the band diagram. An example for a CdSe quantum dot

is given in fig. 2.6. First of all, we see that the first absorption peak does not coincide with

Figure 2.6: Absorption (solid line) and photoluminescence spectrum (dashed line) for a CdSe quantum

dot. [31]

the emission peak. This is due to the splitting of the highest valence band, as depicted in fig.

2.7, caused by electron-hole exchange interaction [32]. In the absorption spectrum, only the

transition from the lowest one of both states is visible, since this one has the highest oscillator

strength. The energy transfer during absorption will thus be bigger than when the electron

relaxes to the highest energy state in the valence band. This Stokes shift results in a red-shift

of the PL spectrum.

Second, multiple reasons can be cited to cause the broadening of the peaks. Mainly, since the

spectra are measured for all of the illuminated dots (in solution or deposited on a substrate), a

broadening of the PL peak and the absorption peak occurs due to the size dispersion. Moreover,

coupling between phonons and electrons will cause an additional broadening. The occurence of

11


Figure 2.7: Different absorption and emission wavelength due to valence band splitting.

the absorption peaks at higher energy levels are less visible, because the energy eigenvalues de-

rived from the Schrodinger equation get closer together, resulting in a smaller energy separation

between the discretized levels, ending up in a continuum.

2.3.2 Radiative and non-radiative recombination

As we know, absorption of a photon results in the creation of an electron-hole pair where the

electron gets excited to the conduction band. There are however multiple ways of recombining.

They can be divided into two categories, radiative and non-radiative. The former is of course

discussed in the previous subsection, because it describes the phenomenon of emitting a photon

when recombination occurs. A schematical depiction is shown in fig. 2.8(a).

When non-radiative recombination takes place on the other hand, the energy is not transferred to

an emitting photon, but consumed in other ways. In quantum dots, the relative influence of the

different processes remains undetermined. It is however known that Auger recombination has a

significant contribution [32–34]. During this process, the relaxation energy of the recombining

(a) Radiative recombination. (b) Non-radiative Auger recombi-

nation.

Figure 2.8: Main recombination mechanisms in quantum dots.

electron is transferred to an other electron in the conduction band, resulting in the state depicted

in fig. 2.8(b). Next to this Auger process, surface recombination can also take place.

Of course, the internal quantum efficiency is thus given by:

ηint =γR

γR + γNR(2.7)

12


where γR and γNR are the radiative and non-radiative recombination rates respectively. To

calculate the photoluminescence quantum yield however, also the reabsorption by other quantum

dots and by the surrounding matter must be taken into account. The PL QY can thus only be

defined as:

PL QY =# photons emitted

# photons absorbed(2.8)

2.3.3 Dielectric function

A last item which is important for both electrical and optical properties is the dielectric function.

It is useful to know for multiple applications, e.g. determining quantum efficiencies of QD

solar cells and lasing applications where the refractive index of the QDs is of importance when

present in the cavity [35, 36]. Optical constants of bulk materials can be determined from a

Kramers-Kronig analysis of the bulk absorption coefficient α, which implies the knowledge of

the extinction coefficient k. This value allows the calculation of both εR and εI . The absorption

coefficient µ of the QDs is however not that easy to determine, since it is dependent on the

local field factor fLF (which has dependencies on both εR and εI , the wavelength, the solvent

refractive index ns and εI itself (Maxwell-Garnett effective medium theory)) [37]:

µ =2π

λns|fLF |2εI =

2π

λns

9n4s(εR + 2n2s)

2 + ε2IεI (2.9)

Rewriting the Kramers-Kronig relations in a discrete summation and transforming this into a

matrix formalism, allows the calculation of εI and thus εR. Results for different lead chalcogenide

dots are shown in fig. 2.9 [37].

Figure 2.9: Calculated dielectric functions of (a) bulk PbS and (b) PbS QDs. [36]

It should be kept in mind that the PbS quantum dots have an εR between 10 and 20 in the

near-infrared region. Other lead chalcogenide dots have even higher values.

13

Chapter 3

Composition of the quantum dot

LED

In this chapter, we’ll discuss the general structure of our two types of quantum dot LEDs.

First, the quantum dot LED based on direct charge injection and operating under a DC voltage

is discussed, where the focus is mainly put on a description of the individual layers. This is

followed by a description and theoretical approach of the AC stack and its operation mechanism

of field-driven ionization. In chapter 5, characterization is performed to optimize the layers.

3.1 DC quantum dot LED based on direct charge injection

The tunability of quantum dots offers great possibilities in display technology. This is however

only possible thanks to the recent progress made in the efficiency and thus the brightness of the

electroluminescent structure. Therefore, a small overview will be given from the first devices

made to devices currently under investigation. To explain the context, a theoretic approach of

the direct charge injection by applying a DC voltage is given first.

3.1.1 Basic structure: theoretical approach

As explained in the previous chapter, emission of a photon from a quantum dot is only possible

when radiative recombination of an electron-hole pair occurs. Multiple things should be taken

into account:

Electrons should be injected into the conduction band of the quantum dots,

At the same time, holes should be injected into the valence band,

When recombination of an injected electron and a hole occurs, it should happen radiatively,

The emitted photons should not be absorbed too much by surrounding layers, especially

when they are propagating towards the desired side.

14

Chapter 3. Composition of the quantum dot LED

This process is pictorially shown in fig. 3.1, where the bands are drawn symmetric (both vertical

and horizontal), which is not the case for a realistic band diagram.

Figure 3.1: Electron and hole injection into the quantum dot, resulting in radiative recombination.

The general structure of the first type of quantum dot LEDs includes a DC voltage over the

structure, which results in a current of electrons and holes. As is clear from the above picture,

electrons (flowing out of the anode) should arrive at the quantum dot layer at the same time as

the holes (coming from the cathode). Since the type (electrons or holes) is of great importance,

this charge injection should be improved by adding extra layers. They are called electron or

hole transport layers, accordingly to the type which is predominantly present when a current

flows. A schematical energy band diagram to show their purpose is drawn in fig. 3.2. We indeed

Figure 3.2: Schematic depiction of the energy band diagram for a DC quantum dot LED structure.

conclude that the majority of the carriers coming from the cathode are holes and the hole energy

barriers between the cathode and the quantum dots have been made smaller. Similar barriers

are present for the electrons coming from the anode. The main issue of these stacks involves

the imbalance of carrier injection and corresponding charging at interfaces or electrodes. More

detailed information will be provided in the next subsubsection.

15


3.1.2 Progress

The emergence of quantum dot LEDs with a basic DC voltage happened in 1994 when a multi-

layer of quantum dots mixed into hole-transporting polymers was sandwiched between a trans-

parent anode and a metallic cathode [38,39]. Next, the quantum dots were deposited on top of

the same polymers, still showing only poor results of external quantum efficiencies smaller than

0.1 % [40]. The use of organic materials as charge injection layers, similar to those in OLEDs,

has increased the EQE significantly (up to 2.1 %) [41, 42]. However, these stacks had some

major disadvantages:

The use of organic materials leads to fast degradation when exposed to atmospheric con-

ditions (especially oxygen and moist),

The PL QY of the QDs was not higher than 10 %, which allowed much room for improve-

ment.

Indeed, the increase of the QY for quantum dots in specific ranges of the visible spectrum led

to better devices, still with organic materials. The latter are chosen to align good with the

band structure of the quantum dots, such as TPD and Alq3 as hole and electron transport layer

respectively [43].

One of the layers can be replaced by an inorganic layer, such as the hole transport layer NiO, but

the first all-inorganic devices were produced in 2007 [44]. Here, the ETL was also inorganic, e.g.

ZnO:SnO2. However, this device still showed small EQE in comparison to the optimized organic

ones. Therefore, different layers have been tried, such as ZnO nanocrystals as ETL [45]. Most

improvements have been made by adding small layers to solve free carrier and charge issues,

resulting in much higher EQEs [9]. This is discussed in further detail in subsection 3.1.4.

Recently, also other designs using quantum dots as an electroluminescent source have been

produced. One example is the use of giant CdSe/CdS core/shell nanocrystal quantum dots,

where the CdS acts both as a good hole and electron injection layer for the CdSe core. To

improve the injection of holes even more, organic PEDOT:PSS has been used, while a layer of

insulating LiF is used as an electron blocking layer to prevent recombination at the cathode [46].

Next to this concept, also all-QD multilayer films have been used to control the transport

of the carriers [47]. A more remarkable design is the inverted stack structure, where ITO

is used as electron injection layer, while aluminium operates as cathode. Longer operating

lifetimes and high EQEs up to 7.3 % have been measured [48]. Next to this inverted structure,

a recent publication (May 2012) has shown the efficient use of ZnO quantum dots as electron

injection layer, while organic PEDOT:PSS is the HTL. Furthermore, the radiance and the

EQE have been improved by controlling the inter-dot spacing between the luminescent dots,

affecting two processes. Indeed, increasing the spacing decreases the efficiency of inter-dot

charge transfer, while the balance has to be kept between the charge injection and the efficient

16


radiative recombination within the quantum dot layer. Using linker molecules (which replaced

the (mostly oleate) ligands) between three and eight CH2 groups, the quantum efficiency has

been shown to vary over two orders of magnitude between the worst and optimal point [49].

3.1.3 General structure and layer description

Our focus is put to the development of an LED structure where only inorganic compounds are

used. This offers possibilities towards easy processing and atmospheric stability. The basic stack

is depicted in fig. 3.3. Multiple transport layers have been investigated and are discussed here.

As hole transport layers, organic TPD has been used in first instance, but fast replaced by the

inorganic materials nickel oxide and copper oxide. Zinc oxide has been used as electron hole

layer, although in many different shapes. First, zinc oxide quantum dots have been investigated,

which can be easily deposited by spincoating. Atomic layer deposition of zinc oxide has been

used as well, but only on rotatable magnetron sputtered ZnO:Al an extensive characterization

has been done. Each of the layers of the stack will now be explained shortly.

Figure 3.3: Basic DC quantum dot LED structure.

3.1.3.1 Substrate and ITO layer (cathode)

All of the LED structures are built on an insulating glass substrate covered with ITO (Sigma-

Aldrich). This layer always has following properties [50,51]:

Highly degenerate n-type semiconductor

Band gap ≈ 4 eV

Good hole transport layer

Surface resistivity: 30-60 Ω/sq

Thickness: 100-200 nm

17


Ionization potential: 4.76 eV

To improve contacting with the probe needle, a plaque of tungsten compound is deposited

directly on the ITO by ultrasonic soldering.

3.1.3.2 Hole transport layer

The injection of holes can be improved by adding a hole transport layer. They have the property

that the barrier height for the holes is reduced, while blocking the electrons from the cathode.

Three possibilities are discussed.

3.1.3.2.1 Organic TPD TPD is an organic material with the structure as shown in fig.

3.4. It has already been efficiently used as an HTL in OLEDs and quantum dot LEDs [44, 52].

Earlier, it was shown that TPD molecules function as a hole trap at low concentration while

being hole transporting at high concentration. This is explained by the increased possibility of

charge hopping via the additive sites of the polymers [53]. The HOMO level of TPD is situated

around 5.5 eV [54]. However, it degrades fast when exposed to air, which doesn’t make it

suitable for cheap mass production of quantum dot LEDs.

Figure 3.4: Chemical structure of TPD.

3.1.3.2.2 Nickel oxide Nickel(II) oxide (NiO) is a chemical compound which can be easily

deposited through reactive magnetron sputtering (see chapter 4). The hole transporting charac-

teristic appears due to the non-stoichiometry. Indeed, during the sputtering process, the oxygen

pressure can be changed, which induces an excess of oxygen atoms, resulting in a metal deficient

material NiOx (x > 1). This gives rise to p-type properties [55, 56]. Further characterization in

function of the sputter parameters is discussed in chapter 5.

3.1.3.2.3 Copper oxide The characterization of the copper oxide is harder than of nickel

oxide because the possible formed compounds behave differently:

Cu2O (cuprous oxide): semiconductor with a band gap of 2.137 eV

CuO (cupric oxide): p-type semiconductor with a narrow band gap of 1.2 eV

18


Attempts to make both oxides have been done with DC magnetron sputtering. Details are

discussed in chapter 5.

3.1.3.3 Colloidal nanocrystals

As already explained intensively in chapter 2, different types can be synthesized. Here, a short

description of the used nanocrystals is given:

PbS: standard NC which emits at the telecom wavelength 1330 nm (QY ≈ 30 %)

CdSe/ZnS: Core of CdSe, shell of ZnS, emitting from blue to red

CdSe/CdS dots/rods: Core of CdSe, shell of CdS, yellow and red emission

SILAR particles and alloys: multiple kinds, emitting in the visible range

Giant: CdSe/CdS nanocrystals, having a very thick (≥ 10 monolayers) shell, emitting in

the yellow range

PbS/CdS: Core of PbS, shell of CdS, emitting in the infrared

3.1.3.4 Electron transport layer

Zinc oxide is known to possess good electron transport properties. Therefore, several different

structures have been analyzed.

3.1.3.4.1 Zinc oxide - quantum dots Next to the active layer of luminescent quantum

dots, zinc oxide dots might be used as an effective hole transport layer [48, 57, 58]. As already

mentioned before, these dots are stabilized by charge.

3.1.3.4.2 Zinc oxide - atomic layer deposition Atomic layer deposition (as will be ex-

plained in chapter 4) allows very precise thickness control, because the amount of the precursor

DEZ to react with H2O can be exactly determined. Moreover, tuning the temperature changes

resistivity and mobility significantly [59].

3.1.3.4.3 ZnO:Al - rotatable magnetron sputtering A ceramic cylindrical target of zinc

oxide heavily doped with aluminium (2 wt%) can be used in a rotatable magnetron sputtering

setup. During sputtering processes, multiple parameters can be changed such as argon pressure,

oxygen pressure, deposition temperature, thickness of the layer and distance between target and

substrate. Therefore, optimal parameters can be determined.

19


3.1.3.5 Aluminium contacts (anode)

As metal anode contact, a 100 nm aluminium layer has been evaporated. In this way, stacks of

approximately 1.8 mm× 1.8 mm are created. To avoid probe needle damage to the underlying

layers, an additional dot of silver paste has been used. This has no influence on the behavior of

the stack. A picture of the complete stack is shown in fig. 3.5.

Figure 3.5: Complete stack, showing all individual pads.

3.1.4 Improvement of the LED stack

As is discussed in subsection 3.1.2, the high concentration of free carriers in the metal oxide layers

and the imbalance of injection of holes and electrons cause a strong decrease in the efficiency of

the QD LED operation due to quenching of the luminescence induced by non-radiative processes

(especially Auger) and charging of the quantum dot layer.

The influence on the QDs of the high concentrations of free carriers in the metal oxide layers

can partially be solved by adding an insulating layer between the quantum dots and the electron

transport layer. An example of such a stack is shown in fig. 3.6(a), where ZnO is the insulating

layer and ZTO the ETL.

The charging effect can be reduced if the amount of injected electrons would approximately equal

that of the injected holes. This can be tuned by adding an extra insulating layer in the ETL

which has a wide band gap and thus imposes an (additional) energy barrier for the electrons.

An experimental stack is shown in fig. 3.6(b).

Both effects can be reduced by just adding both layers. This results in the stack in fig. 3.6(c),

which increases the external quantum efficiency by at least one order of magnitude [60].

Further improvement can be achieved by displacing the ligands and using linker molecules with a

specific length, optimizing the external quantum efficiency, as described in subsection 3.1.2 [49].

3.2 AC quantum dot LED by field-driven ionization

Experiments using field-driven ionization have recently shown higher efficiencies than the DC

quantum dot LED [61]. Moreover, the reproducibility of the latter is significantly harder, not in

20


(a) Insulating layer to reduce con-

centration effect.

(b) Insulating layer to reduce

charging effect.

(c) Resulting total stack.

Figure 3.6: Schematic of stacks to reduce carrier concentration and charging effect. [60]

the less due to the effects described in the previous section, mainly considering poor injection

and transport of charges (see also chapter 6). Therefore, experimental attempts have been done

to measure stacks where locally charges are generated due to changing electric fields. First, a

theoretical explanation of the concept of field-driven ionization of quantum dots is given where

immediately our experimental stacks are also shortly discussed.

In fig. 3.7, the basic circuit is shown. As we can clearly see, the layer of quantum dots is

Figure 3.7: Conceptual circuit to acquire EL by field-driven ionization.

sandwiched between two insulating layers. Both insulators are connected with contacts, allowing

an AC voltage to be applied. A detail of the energy band diagram of the layers at different times

during a positive voltage pulse is depicted in fig. 3.8. Two peaks of electroluminescence will

occur, as will become clear when looking at the different steps. The numbers in the figure denote

the order of the process.

During the beginning of the positive pulse (fig. 3.8(a)):

1. Due to the electric field induced by the separation of charges by the insulating layers (both

oxide layers and QD layer, see also further), the quantum dot energy bands are no longer

aligned, which evokes the possibility of hopping of an electron in the valence band to the

neighboring conduction band, leaving a hole behind.

2. The charged electrons and holes will hop to the quantum dots at opposite sides under the

influence of the electric field (drift).

21


(a) At beginning of positive voltage pulse.

(b) After the positive voltage pulse.

Figure 3.8: Band diagram when applying a positive voltage pulse, explaining the principle of field-driven

ionization.

3. During this hopping, it can occur that an electron and a hole are simultaneously present

in the conduction and the valence band. In some quantum dots, these electron-hole pairs

will recombine either radiatively or non-radiatively, while the remaining created electrons

and holes stay in the conduction and valence band respectively.

Due to the transport of electrons and holes in opposite directions in the quantum dot layer, an

internal field emerges, opposed to the original one. This field will be smaller than the original

one, but still have a considerable effect in the energy diagram after the voltage pulse (fig. 3.8(b)):

1. The remaining electrons and holes will now drift in the other direction (because the re-

maining field is opposite to the original one).

2. Again, there is a probability on radiative recombination, which results in an additional

peak in electroluminescence.

Symmetric operations will occur when a negative voltage pulse is applied.

Experimentally, electron beam evaporated or ALD deposited Al2O3 have been used as insulating

material (εr = 7.55 (ALD)), resulting in the stack shown in fig. 3.9(a), where the thickness of

the layer and the type of quantum dots have been varied. Similar to the DC stacks, the contacts

are ITO and aluminium (25 nm). The total circuit of the setup is shown in fig. 3.9(b). Although

22


it is an idealized version, it already gives us some insights regarding the applied voltage and

corresponding voltage drops across the quantum dot layer.

(a) Experimental AC stack. (b) Idealized circuit in the used setup.

Figure 3.9: Experimental stack and used circuit for the AC quantum dot LED.

The resistor is added to avoid short-circuiting when the device would break down. We can

describe this circuit analytically for an applied sine voltage V sin(2πft) (where f denotes the

frequency), a resistor (with resistance R) and three capacitors (with capacitances Cox,bottom,

CQD and Cox,top respectively).

First, the total impedance can be calculated by adding all reactances:

Z = R+ jXCtot = R+ j(XCox,bottom

+XCQD+XCox,top

)(3.1)

= R− j[

1

2πf

(1

Cox,bottom+

1

CQD+

1

Cox,top

)](3.2)

By doing admittance measurements on the total stack and on the oxide layers individually, each

of the capacitances can be calculated. The applied peak-to-peak voltage can be increased up to

150 V. The resistor during the experiments has a value of 20 kΩ. The voltage over the quantum

dot layer can then be calculated:

|VQD| = VXQD

|Z|(3.3)

Dividing this voltage by the number of quantum dot layers, the voltage drop over each quantum

dot can be calculated.

The ideal situation described above can be made more realistic by introducing a number of

resistors in series and parallel. Indeed, the layers cannot be perfectly insulating, since otherwise,

no power would be dissipated, allowing light to be emitted. Further analysis of this extended

model is performed in chapter 6.

23

Chapter 4

Deposition and measurement

methods

4.1 Deposition methods

4.1.1 Ion-beam reactive magnetron sputtering

A very basic schematic of the procedure of DC magnetron sputtering and the used setup are

depicted in fig. 4.1.

(a) Schematical depiction of the principle. (b) Picture of the setup.

Figure 4.1: Schematic and picture of the DC magnetron sputtering setup.

When the chamber is evacuated, argon at the desired pressure can be injected. The high DC

voltage applied to the target (see fig. 4.2) creates high electric and magnetic fields, trapping

electrons at the surface. The latter move helically around the magnetic field lines and may collide

with the argon atoms, creating positive Ar+ atoms. These impinge on the target resulting in the

ejection of the target material (in our case nickel or copper) into the chamber and thus onto the

substrate. The composition and resistivity of the deposited thin layer can be tuned by changing

the oxygen pressure in the chamber, the target-substrate distance (which in our case is always

kept at 14 cm) or the temperature (which is also kept constant at room temperature). Also the

24

Chapter 4. Deposition and measurement methods

incident power of the released ions can be changed. In general, current, voltage or power can be

kept constant. A constant current mainly results in a constant deposition rate, while a constant

voltage induces the same acceleration and thus velocity of the particles.

Figure 4.2: Planar nickel target for DC magnetron sputtering.

4.1.2 Cylindrical rotating magnetron sputtering

Rotating magnetron sputtering behaves in the exact same way as the previously described

procedure, the only difference is that now a rotating target is used, which offers advantages

towards target utilization and deposition rates. Also higher powers (due to better cooling) and

stability can be acquired [62]. Moreover, the ‘racetrack’ phenomenon1 is suppressed. In fig. 4.3,

pictures are shown of the setup and the used target, which has a diameter of about 5 cm, while

the height of the cylinder is about 20 cm.

(a) Picture of the setup. (b) ZnO:Al target.

Figure 4.3: Picture of the setup and a ZnO:Al target for rotatable magnetron sputtering.

In our case, cylindrical rotating magnetron sputtering has been used to deposit ZnO:Al with

2 wt% (which can be bought as a ceramic rotatable target). The characteristics of the layer can

again be tuned by changing different parameters, such as argon and oxygen pressure, target-

substrate distance, temperature and thickness of the layer. Other deposition methods of ZnO:Al

1Erosion of a circular shaped path on a planar target, due to inhomogeneous ejection of target material.

25


include RF and DC magnetron sputtering (with ZnO:Al targets), pulsed laser deposition, co-

sputtering of ZnO and Al targets, chemical vapor deposition and cathodic vacuum arc technique

[63–67].

4.1.3 Atomic layer deposition

As already mentioned before, atomic layer deposition is a thin film deposition technique based

on the sequential pulsing of precursor vapors. In the case of ZnO, these precursors are DEZ and

H2O. The reactions are:

ZnOH∗ + Zn(CH2CH3)2 → ZnOZn(CH2CH3)∗ + CH3CH3 (4.1)

Zn(CH2CH3)∗ + H2O→ ZnOH∗ + CH3CH3 (4.2)

where the chemical compounds with an asterisk denote surface species. By repeating these

equations, ZnO is deposited [68].

Alumina layers have been deposited by ALD as well, with Al(CH3)3 and H2O as precursors:

AlOH∗ + Al(CH3)3 → AlOAl(CH3)3∗ + CH4 (4.3)

AlCH3∗ + H2O→ AlOH∗ + CH4 (4.4)

Some of the main advantages of ALD are:

Large area thickness uniformity

Atomically flat

Excellent repeatability

Thickness control by using measured amount of precursor

By changing the growth temperature from 100C to 200C the conductivity can be changed in

a wide range of four orders of magnitude (see chapter 5) [69].

4.1.4 Electron beam evaporation

Electron beam evaporation is used to deposit the aluminium contacts and the Al2O3 insulating

layers. A schematical depiction of the process is shown in fig. 4.4, together with a picture of the

whole setup.

After evacuating the chamber, a high voltage is applied which induces an electron beam (tunable

by the imposed current) bombarding the ingot, containing the material to be deposited. The

latter evaporates and is subsequently deposited on the substrate.

26


(a) Schematical depiction of the principle. (b) Picture of the setup.

Figure 4.4: Schematic and picture of the electron beam evaporation setup.

4.1.5 Spincoating

Spincoating is used to deposit the colloidal quantum dots in uniform layers on the substrate. A

small amount of solution is dropped on the surface, which is then rotated to spread the quantum

dots by centrifugal force. Of course, part of the solvent and quantum dots will be spun away

from the sample. The remaining layer consists of a uniform layer of quantum dots, where the

leftover of the solvent evaporates.

In the case of our 2.54 cm × 2.54 cm-samples, about 100 µl is deposited and subsequently

the substrate is rotated at 2000 rpm for 30 s. Depending on the concentration, about 1 to 4

monolayers are deposited during each spin operation.2 Thicker layers can be made by spincoating

multiple times.

4.2 Measurement methods

4.2.1 Current-voltage measurements

Measuring the current-voltage characteristics of all of the DC stacks is performed by using the

setup shown in fig. 4.5, immediately optimized to measure electroluminescence as well. The

sample is put on a threedimensionally movable stage (above the 1 in the figure). The cathode

is connected with a crocodile clip through the ultrasonic soldering (which was already visible in

fig. 3.5), while the aluminium contact (the anode) is probed by a needle. To avoid scratching

and contacting issues, a dot of silver paste is added on top. The voltage source is steered by

Python and is coded to perform both single and hysteresis measurements. A general code is

added in the appendix. The results are discussed in chapter 6.

2This is for common concentrations of quantum dots in solution. Higher concentrations can even lead to layers

with a thickness of 150 nm, although this can lead to undesired cracking.

27


Figure 4.5: Setup to measure IV and EL on DC stacks.

4.2.2 Electroluminescence measurements

4.2.2.1 EL measurements by fibre coupling

The attempts to measure electroluminescence of the DC stacks are performed by fibre coupling

into an OSA or power meter. Therefore, a fibre is placed as close as possible to the bottom of

the substrate to catch as much as possible of the incoming light, using the same setup as for the

IV measurements. A fibre of Thorlabs is used with following properties:

Multimodal fibre with a core diameter of 400 µm and a cladding of 425 µm,

NA: 0.39.

The big core diameter and the high numerical aperture results in an acceptance as depicted in

fig. 4.6. For the 1.8 mm× 1.8 mm-pads and a fibre placed at 4 mm of the light source (which is

the closest possible due to the thickness of the substrate), a significant amount of light can be

captured.

Figure 4.6: Acceptance cone of the multimodal fibre.

28


4.2.2.2 EL measurements by direct coupling into spectrometer

Spectral electroluminescence measurements of the stacks emitting in the visible spectrum due to

field-driven ionization are performed by placing the sample immediately in front of a monochro-

mator, which splits the light transversally into its spectral components. A CCD camera measures

accordingly the intensity of all spectral lines simultaneously. A picture of the setup is shown in

fig. 4.7.

Figure 4.7: EL setup by direct coupling into the spectrometer, showing the input slit, the monochro-

mator and the CCD camera.

4.2.2.3 Power measurements

The output of the LEDs can also be measured by placing a suitable detector at the bottom of

the stack. In this case, a Newport optical power meter (model 1918-R) has been used, while

the detectors for the visible and the infrared are made from silicon and germanium respectively

(Newport 818-UV/IR).

4.2.3 Photoluminescence measurements

Different kinds of photoluminescence experiments can be performed by the Edinburgh spectrom-

eter. Here, emission spectra, excitation spectra and lifetime measurements are discussed.

4.2.3.1 Emission spectrum

Emission spectra are measured by exciting the quantum dots (in solution or thin film layer) at

a certain wavelength with an energy higher than the band gap. The peak emission wavelength

can be determined out of the resulting measured spectrum and comparison of the change in PL

can be done for a set of spectra.

4.2.3.2 Excitation spectrum

An excitation spectrum measurement is performed by measuring the emission at one wavelength

while varying the excitation wavelength over a certain range. In this way, the effect on the

29


absorption of an extra layer can be studied. Furthermore, the effective index can change which

might induce a small shift of the spectrum.

4.2.3.3 Lifetime measurements

Measuring the lifetime of the emitted photons by exciting with a short pulse yields information

about the recombination mechanisms. This will be clarified in chapter 6.

4.2.4 Atomic force microscopy

AFM is used to analyze the surface morphology to resolutions of fractions of nanometers. The

principle is based on forces between the sharp tip of the cantilever and the sample, which induces

a deflection of the former according to Hooke’s law. Possible present forces are chemical forces

(short range) and Vanderwaals, electrostatic and magnetic long range forces. The deflection is

measured by using a laser spot which reflects on the top of the cantilever.

Two measurement methods exist: contact and tapping mode. In the contact mode, the surface

morphology is directly visualized by measuring the static deflection signal. In the tapping mode,

the tip doesn’t touch the sample, but is oscillating slightly above its resonance frequency with

an amplitude of a few nanometers. The acting forces decrease this frequency, which is opposed

by changing the tip-sample distance. This feedback loop system allows to measure three useful

images: phase, amplitude and topography throughout the surface.

4.2.5 Scanning electron microscopy

The scanning electron microscope is used to visualize cross sections of layers, interfaces and total

stacks. It images the sample by scanning it with a beam of electrons and measuring the signal.

The samples in this thesis are all depicted by analysis of the secondary electron beam. These

are mostly low-energy electrons which are ionization products of the incident electron beam on

the sample. In the low-energy region, the mean free path of electrons varies very strongly. By

interpreting this signal, a very fine resolution is possible.

The main disadvantage of this method is the difficult visualization of insulating layers, since

only few secondary electrons are emitted.

4.2.6 Absorption measurements

Absorption measurements on quantum dots yield information about their energy band gap and

size dispersion, as already discussed in subsection 2.3.1. Absorption or transmission spectra of

thin (semiconductor) films reveal information about the opacity of the material, the thickness

of the layer and the band gap, which might be changed due to the Burstein-Moss effect (see

chapter 5).

30


Spectra are measured by analyzing the transmission in the interested wavelength range using

the PerkinElmer Lambda 900 spectrometer. Therefore, an optical system with a holographic

grating monochromator, scans all wavelengths and measures the amount of light going through

the sample with a photomultiplier tube. If this is compared to a reference sample, the difference

exactly yields the transmission of the deposited layer(s). An example spectrum of a thin layer

of ZnO:Al is shown in fig. 4.8.

100

80

60

40

20

0

Tra

nsm

issi

on [%

]

200018001600140012001000800600400λ [nm]

Figure 4.8: Transmission spectrum of a ZnO:Al thin film.

Two fast observations can be performed. First of all, the material seems to be very transparent

in the VIS/NIR region. The visible interference pattern (caused by reflection at the interfaces

glass-ZnO:Al and ZnO:Al-air) can be used to calculate the thickness of the layer. This is however

not very precise. Further, after some manipulations of the obtained data, the energy band gap

can be found as well, based on the absorption edge.

The absorption spectrum can easily be calculated if we assume the reflection to be very small:

A [%] = 100 %− T [%]−R [%] (4.5)

Indeed, in that case, the absorption is the complement of the transmission.

4.2.7 XRD measurements

X-ray diffraction is used to characterize the crystalline properties of the deposited material.

Usually, the reflection intensity is measured in function of (two times) the angle of incidence θ

respective to the plane of the sample. In fig. 4.9(a) and 4.9(b) respectively, the schematic setup

and a resulting graph example (for copper oxide) are shown.

The advantage of the θ − θ setup is that the sample stage remains fixed, while the source and

detector move simultaneously on the same circle, centered on the sample.

From the resulting graph, different conclusions can be drawn. First, the position of the peaks

yield information about the crystalline phase of the thin film. Comparison with a crystallo-

graphic database can be made, paying attention to small deviations in the angles. These might

31


(a) Schematic of an XRD setup

140

120

100

80

60

40

20

0

Inte

nsity

[a.u

.]

807060504030202θ

(b) Example plot of an XRD measurement.

Figure 4.9: Schematic and resulting graph of XRD setup.

be a consequence of stress in the material. The peak width data allow to determine the grain

size based on the Scherrer formula, on which is further elaborated in chapter 5.

4.2.8 Admittance measurements

For the DC stacks, measurements of the capacitance in function of the frequency and varying

temperature can yield useful information regarding the defect (trap) distributions in or between

the different layers. The method consists in plotting the derivative of the capacitance to the

frequency (which is proportional to the number of trap states) in function of this frequency

on an energy scale. By varying the temperature, a changing energy range in the band gaps of

the semiconductor layers can be covered, since only states deeper than the Fermi level (which

depends on the temperature) can be observed. The graphs are generally plotted with a guess

for the attempt-to-escape frequency. For a trap level, this results into peaks at different energies

in the band gap. Although, trial and error for this unknown frequency might cause a perfect

overlap. The energy level at this common peak is the exact trap state level [70]. Extra infor-

mation might be revealed if the bias voltage over the stack is changed, since e.g. the influence

of charge injection into the quantum dots might alter the measurements.

The admittance of the different layers in the AC stacks is of course also of crucial importance.

They are measured in the operating frequency range (1− 100 kHz).

All admittance data have been gathered with an LF impedance analyzer of Hewlett-Packard.

4.2.9 Thickness measurements

Thicknesses of thin films can be measured in several ways. Here, two techniques are used, namely

optical white-light profilometry and piezoelectric contact profilometry. Of course, thicknesses

can also be measured by e.g. SEM or AFM, but these are more time-consuming.

32


4.2.9.1 Optical profilometer

The optical profilometer is based on vertical scanning interferometry. Here, a Mirau interferom-

eter is used. First, a beam of light from a single source is split into two separate beams and then

recombined. The beam reflected from the reference mirror is called the reference beam, while

the other beam undergoes reflectance of the sample. This results in dark and bright fringes

which makes it possible to determine the topography of the test surface.

Here, a white light source is used, because this solves the height ambiguity issues if the height

step is larger than a quarter of the wavelength when monochromatic light is chosen as source.

The focus of the white light source is moved vertically with reference to the sample to ensure

every point of the surface goes through the focal plane. The path length difference between the

two beams of the interferometer varies strongly, but each time the surface point is in focus, this

difference is equal to zero. Based on this principle, height characterization of the whole surface

is possible [71]. In fig. 4.10, this is schematically depicted.

Figure 4.10: Schematic of the principle of the Veeco profilometer using vertical scanning interferometry.

[72]

4.2.9.2 Piezoelectric contact profilometry

If optical profilometry is not possible due to e.g. high transmission in the visible range, step

profiles can also easily be determined by piezoelectric contact profilometry. Here, a stylus is

brought in contact with the surface, which generates a current due to the piezoelectric element.

Dependent on the pressure, this current changes significantly, allowing a very high tunability in

precision of the measurement. However, the characterization of soft surfaces, e.g. quantum dot

layers, does not work very well.

33

Chapter 5

Characterization of the layers

Since attention was first paid to the DC stack with direct charge injection, characterization has

been mainly performed of the layers of this stack. Furthermore, the tunability of the insulating

layers in the AC stack with field-driven ionization is only limited.

5.1 DC LED stacks

5.1.1 Hole transport layer

As already mentioned, main investigation of the hole transport layer goes to TPD, copper oxide

and nickel oxide. Each of them is individually discussed.

5.1.1.1 TPD

TPD (dissolved in toluene) is spincoated once or twice on the ITO substrate. In itself, nothing

can be changed, only the thickness of the layer can be varied. To show the usability of TPD to

catch the emitted light at the bottom of the stack, a transmission spectrum is plotted in fig. 5.1

after annealing a one time spincoated TPD layer at about 70 C in an atmospheric environment.

The transmission in the visible and infrared region ranges from 90 % to almost 100 %. However,

since the focus is to avoid organic materials, no further characterization has been performed.

5.1.1.2 Copper oxide

The use of copper oxide as hole transport layer implies that the majority of the carriers should

be holes. Since this material is magnetron sputtered from a copper target in an argon-oxygen

vacuum chamber, different compositions can be formed, as already mentioned before. Therefore,

multiple depositions at different oxygen level have been performed. The pressure of the argon is

kept constant at 0.4 Pa, while the oxygen pressure is varied from 0.13 Pa to 0.3 Pa. The current

is kept limited at 0.2 A, while the voltage varies between 400 and 500 V.

The resulting XRD spectra are shown in fig. 5.2, while the derived peak data are summarized

34

Chapter 5. Characterization of the layers

100

90

80

70

60

50

40

Tra

nsm

issi

on [%

]

200018001600140012001000800600400λ [nm]

Figure 5.1: Transmission spectrum of TPD after annealing.

in table 5.1.

140

120

100

80

60

40

20

0

Inte

nsity

[a.u

.]

807060504030202θ [°]

pO2 [Pa]

0.13 0.23 0.3

Figure 5.2: XRD spectra of a copper oxide layer with varying oxygen pressure.

pO2 [Pa] θmax,1 [] θmax,2 [] θmax,3 []

0.13 18.0 31.9 38.0

0.23 17.6 19.0 −

0.3 17.6 19.0 −

Table 5.1: XRD results of magnetron sputtered copper oxide at varying oxygen pressure.

It is obvious from the obtained data that two different compounds are formed. They seem to

correspond with following crystal structures:

At pO2 = 0.13 Pa: Cu3O4, an intermediate compound composed out of Cu(I)O2 layers

and Cu(II) sublattices [73],

At pO2 = 0.23− 0.3 Pa: CuO, cupric oxide.

35


Despite the latter is a p-type semiconductor, no evidence of hole transport has been shown.

To complete the analysis of the magnetron sputtering of copper oxide, a hysteresis curve of the

voltage has been measured where the oxygen is first increased and then subsequently decreased

to zero while the current and the argon pressure are kept constant. The result is shown in fig.

5.3.

500

480

460

440

420

Vol

tage

[V]

2015105oxygen level [sccm]

oxygen addition oxygen removal

Figure 5.3: Hysteresis curve of the voltage in function of the oxygen level for copper oxide at constant

current and argon pressure.

Two regions can clearly be distinguished:

1− 9 sccm: same voltage level during addition or removal of oxygen,

10− 20 sccm: different voltage level.

They are respectively called the ‘metal mode’ and the ‘compound’ mode [74]. Indeed, the

compounds we analyzed through XRD are in correspondence with this naming, since the Cu3O4

has been deposited at 8 sccm and indeed has pure metal copper sublattices, while the other

compounds (at 15 and 20 sccm) are non-metallic. The lower voltage to sustain the plasma

during oxygen removal is due to the presence of copper oxide on the target, which has a higher

emission coefficient than pure copper. This also explains the nomenclature.

5.1.1.3 Nickel oxide

Nickel oxides with various oxidation states of nickel such as nickelous oxide (NiO), nickel dioxide

(NiO2), nickel sesquioxide (Ni2O3), nickelosic oxide (Ni3O4) and nickel peroxide (NiO4) have

been reported [55]. Previous experiments on the same setup have revealed that only NiO and

Ni2O3 are formed under the optimized deposition parameters, together with Ni(OH)2, where

the H-atoms are present due to contamination [75].

First, we perform the same hysteresis method as with copper oxide. The result is shown in fig.

5.4. We again keep the current and the argon pressure fixed at 0.2 A and 0.4 Pa respectively.

36


460

440

420

400

380

Vol

tage

[V]

654321oxygen level [sccm]

oxygen addition oxygen removal

Figure 5.4: Hysteresis curve of the voltage in function of the oxygen level for nickel oxide at constant

current and argon pressure.

In the region between 2 and 3 sccm oxygen addition, we see a similar difference in the measured

voltage, due to oxidation of the upper layers of the target.

The oxygen level obviously also has a big influence on the resistivity of the layer. Therefore,

Hall-Vanderpauw measurements have been performed on thin nickel oxide films. However, the

Hall measurements are of very poor quality due to multiple setup problems, such as impossibility

to connect on the top of the layer, severe read-out voltage fluctuations, suboptimal placement of

the sample(holder) between the magnets, etc. Still some results have been achieved, where we

are mainly interested in resistivities from 0.01 Ωcm to 10 Ωcm. They are summarized in table

5.2.

Oxygen level [sccm] ρ [Ωcm] µ [cm2/Vs] Thickness [nm]

2.8 0.8 − 365

3.6 0.07 14 115

Table 5.2: Hall-Vanderpauw measurements on NiO thin films.

The main problem with this acquired data is that all measurements have been done on layers

with varying thickness (115− 365 nm) which are also much thicker than the used layers in the

LED stacks (30 − 60 nm). This important parameter has been neglected, which is clearly not

allowed by analyzing the results of rotatable magnetron sputtered ZnO:Al.

Since we want to collect the light at the bottom of the stack, the nickel oxide layer should be

relatively transparent in the visible-infrared region. For a 115 nm thick nickel oxide layer, the

transmission characteristic is plotted in fig. 5.5.

The transmission ranges from 5 to 25 %, which is really small, implying that the thickness

should be strongly reduced. Indeed, the total DC stacks have been processed with layers in the

30− 60 nm range.

37


30

25

20

15

10

5

0

Tra

nsm

issi

on [%

]

200018001600140012001000800600400λ [nm]

Figure 5.5: Transmission spectrum of a thin nickel oxide film of 115 nm.

Finally, the nickel oxide layers are used as hole transport layers which demands the majority of

carriers to be holes. Indeed, it is confirmed in literature by the hot probe technique1 that the

deposited films show p-type properties [55]. This can be explained by the superstoichiometry of

the deposited material, having an excess of oxygen atoms [44,75].

5.1.2 Electron transport layer

As mentioned before, different ZnO types have been deposited as n-type layer. Mainly, char-

acterization of the layers formed by rotatable magnetron sputtering of ZnO:Al has been done,

since wide tunability is possible.

5.1.2.1 ZnO - quantum dots

As discussed in section 3.1, ZnO quantum dots have been shown to be an efficient electron

transport layer. Since it is hard to perform resistivity or mobility experiments on these layers,

characterization has been limited to study the effect of annealing ZnO quantum dots at different

temperatures and durations when spincoated on a substrate. The results are shown in fig. 5.6.

In both cases, a similar trend regarding the shift of the absorption edge to longer wavelengths

is observed. This redshift can be explained by a change of the dielectric function, probably

caused by a decreased separation between the nanocrystals [76], yielding a denser layer and thus

an increased permittivity. When annealing occurs at 120 C, the total transmission curve is

lowered. With increasing annealing temperature, the ZnO QDs continually cluster and grow

bigger, while also a change in the main native defects might occur, such as oxygen atoms

entering Zn vacancies. This increased number of OZn can possibly explain the total decrease in

1A hot probe at one end and a cold probe at the other are respectively connected to the positive and negative

terminals of a current meter. If the hot probe touches the film (for a short period of time), a current is observed,

which shows the nature of the film to be p- or n-type, dependent if the current is positive or negative. [55]

38


100

95

90

85

80

Tra

nsm

issi

on [%

]

380370360350340330λ [nm]

tannealing [min] 10 30 60 120 165

(a) Annealing temperature: 70 C.

100

95

90

85

80

Tra

nsm

issi

on [%

]

380370360350340330λ [nm]

tannealing [min] 10 30 60 120 165

(b) Annealing temperature: 120 C.

Figure 5.6: Transmission spectra of ZnO nanocrystals, annealed at different temperatures and dura-

tions.

the transmission spectrum [77].

5.1.2.2 ZnO - atomic layer deposition

Only two major properties can be changed during atomic layer deposition of ZnO, the tem-

perature and the thickness. However, parameters have been standardized to a temperature of

100−120 C and a thickness between 50 and 100 nm. The resistivity is determined to be between

1 and 10 Ωcm [75]. This temperature range is on the edge of the optimal ALD window, which

makes reproducibility more difficult, but higher temperatures would influence the resistivity too

much. Furthermore, the thickness of the measured sample is crucial, which has been confirmed

by lower resistivity results for thicker layers [78,79].

Regarding photoluminescence, previous experiments showed that the luminescence of underlying

quantum dot layers is conserved [75].

5.1.2.3 ZnO:Al - rotatable DC magnetron sputtering

An elaborate characterization of the ZnO:Al thin films is discussed in the separate section 5.2.

Although, it should be noted already that this deposition technique allows the biggest change in

parameters and thus in resistivity and mobility, which makes it easy to determine an optimized

value.

5.1.3 Surface and interface characterization

Investigation of the total stack is done by analyzing the surface of the layers and the interfaces

in between. This is respectively done by AFM and SEM.

39


5.1.3.1 Surfaces

The morphology of the surfaces is of great importance to allow growth or uniform deposition

of the next layer. This is especially the case for the quantum dots, which should be uniformly

deposited, without cracks or pinholes.

5.1.3.1.1 Quantum dot layer During the first depositions of spincoated quantum dots and

subsequent layers, they did not look smooth. Therefore, AFM of PbS quantum dots emitting

in the infrared has been performed. The resulting height image is shown in fig. 5.7.

Figure 5.7: AFM image of a cracked PbS quantum dot layer.

This layer has been spincoated from a 30 µM solution of toluene. As we can clearly see, multiple

cracks are visible in the closely packed layer of quantum dots, caused by contamination of the

original solution. This issue has been solved by filtering the solution through a 0.45 µm filter.

A morphologically better example is shown in fig. 5.8, where quantum dots have been spincoated

twice on a glass substrate, yielding a layer of 70 nm, while the deviation from the average is

maximum 5 nm.

Figure 5.8: AFM image of a 70 nm spincoated quantum dot layer.

40


5.1.3.1.2 ZnO:Al layer AFM has also been performed on the ZnO:Al layer, yielding the

images in fig. 5.9. One sees the same deviation of maximum 13 nm from the average value for

both the thin (167 nm) as for the thick (869 nm) layer.

(a) 167 nm. (b) 869 nm.

Figure 5.9: AFM topography images of ZnO:Al layers.

5.1.3.2 Cross section

Since we are mainly interested in the interfaces between and thickness uniformity of the different

layers, individual SEM pictures have been made. A first one is depicted in fig. 5.10, where

TPD, quantum dots and zinc oxide nanocrystals are spincoated above each other. As is clear,

Figure 5.10: Cross section SEM image of TPD-QD-ZnO nanocrystals stack.

distinguishing the different layers is quasi-impossible. Furthermore, the total stack has a varying

thickness, probably due to the non-uniform spincoating of the TPD and the ZnO nanoparticles.

To improve the stack towards the inorganic LED we want to build, the NiO-QD interface is

41


shown in fig. 5.11(a). Again, ZnO nanocrystals are deposited on top, yet the interface is

invisible.

(a) SEM image of the NiO-QD interface (and ZnO

nanocrystals on top).

(b) SEM image of NiO+nanoparticles, clearly showing

the skew growth of the nickel oxide.

Figure 5.11: Cross sections of the NiO-QD-ZnO nanocrystal stack.

The gap between the NiO and the QDs is caused by the cleaving. However, it looks fine, which

was to be expected because the magnetron sputtered nickel oxide has been checked to have a

flat surface morphology, allowing the quantum dots to uniformly spread over the surface. In fig.

5.11(b), the growth of the NiO is indicated. Unlike expectations that the exact orientation of

the substrate didn’t matter, it does if the impinging particles do not have sufficient energy. For

an evaporation flux with an angle of α with the normal of the substrate, the impinging particles

want to conserve their atomic momentum parallel to the film, but they do not have enough

energy to fully migrate (by surface diffusion) to form perpendicular columns, resulting in the

skew growth under an angle β [80]. Their relation is known as the tangent rule:

tanα = 2 tanβ (5.1)

Contrary to the example of this extreme behavior, attention has been paid to reduce the angle

α as much as possible.

The sputtered copper oxide layer has also been checked by SEM. Here, the growth is clearly

perpendicular to the substrate.

Of course we also checked the electron transport layer, especially the ZnO:Al one. In fig. 5.12, a

SEM image of a ZnO:Al layer deposited under the standard parameters which will be discussed

in the next section, is shown in cross section. We can confirm some important characteristics:

The bottom of the ZnO:Al layer grows differently than the rest,

42


Figure 5.12: Cross section SEM image of a ZnO:Al layer under standard parameters with a thickness

of 869 nm.

The vertical growth columns are clearly visible, indicating that horizontally and vertically

electrical properties are difficult to compare. Indeed, the grains are now longitudinally

stretched in the vertical direction, which lowers the resistivity and increases the mobility

perpendicularly to the layer.

The reason for the two growth regimes has to be searched in the lattice mismatch of the silicon

and the ZnO:Al crystal. Indeed, the first growth regime of about 150 nm can be attributed to

competing growth directions of the deposited film, resulting in smaller grain sizes [81, 82]. The

further growth is clearly directional, where the columnar structure confirms that the c-axis is

perpendicular to the substrate.

5.2 ZnO:Al - rotatable DC magnetron sputtering

An extensive study of resistivity behavior and composition of ZnO:Al thin films has been per-

formed. Four parameters can be considered as important determinants: the argon pressure, the

distance between target and substrate, the thickness of the layer, the oxygen pressure in the

vacuum chamber and the substrate temperature. The influence of all these (except of the tem-

perature) is individually discussed in the next subsections based on four-point measurements,

X-ray diffraction and absorption spectra.

5.2.1 Influence of the argon pressure

One of the easiest parameters to change is the argon pressure in the vacuum chamber. Investiga-

tion in a range from 0.4 Pa to 1.5 Pa leads to the graph depicted in fig. 5.13. It should be noted

that the thickness of the films is in the range of 400 to 500 nm, unless otherwise mentioned.

43


3

4

5

678

0.01

2

3

ρ [Ω

cm]

14x10-31210864

pAr [mbar]

Figure 5.13: Resistivity of ZnO:Al sputtered thin films in function of the argon pressure. The error

bars denote a 50 % lower and higher resistivity, which corresponds with the fluctuations

in the measured data.

The resistivity curve is clearly not monotonically rising, but has a minimum. This might be

caused by the change of the energy flux impinging on the substrate.

To check any correspondence with the crystallinity of the layer, we can also plot the XRD data

of the different samples. They are shown in fig. 5.14. A plot over the full 2θ range might give

60x103

40

20

0

Inte

nsity

[a.u

.]

807060504030202θ [°]

pAr [mbar] 0.004 0.005 0.01 0.013 0.015

Figure 5.14: XRD data of the ZnO:Al thin films with changing argon pressure.

us a first indication of the formed crystalline material and the way of growing on the substrate.

Indeed, two peaks are present for every parameter value, lying at approximately 34.2 and 72.

Comparing these to the XRD database of ZnO yields the closest peaks at 34.422 and 72.562.

The respective (hkl) planes parallel to the substrate are (002) and (004), signifying that the

44


hexagonal lattice grows vertically.

Further, the graph zoom in fig. 5.15 clearly shows that the peaks shift to higher angles for

increasing argon pressure. This can be related with the lattice constant of the crystal. Since

maxima in the XRD spectrum occur at angles where the Bragg condition is satisfied,

nλ = 2d sin θ, (5.2)

increasing θ induces a decrease in the spacing between subsequent diffraction planes. Thus,

decreasing the argon pressure leads to an increase of the lattice constant. This is probably

explained due to a higher energy flux impinging on the substrate, which induces more stress

in the layer, yielding (in this case) a compression in the parallel direction of the substrate and

subsequently an increase of the vertical lattice constant c.

To gain more insight into the grain size of the formed ZnO:Al layer, we take a closer look at the

34 peak in fig. 5.15. Indeed, we notice that for the different samples the maxima are slightly

60x103

40

20

0

Inte

nsity

[a.u

.]

35.034.534.033.52θ [°]

pAr [mbar] 0.004 0.005 0.01 0.013 0.015

Figure 5.15: Detail of the XRD data plots for changing argon pressure.

shifted and the width of the peak changes as well. Analyzing these and using the Scherrer

formula, a lower bound for the grain size can be found:

D =Kλ

β cos (2θ)max

2

(5.3)

where K is the shape factor, specific to the shape of grain, but with a typical value around 0.92.

λ is the wavelength of the x-ray, while β is the full width at half maximum of the occurring peak

(expressed in radians) and θ is the corresponding Bragg angle [83]. This leads to the results in

table 5.3.

2The real values for the grain size might differ due to this factor. However, only interest is put in the relative

differences and their trends.

45


pAr [Pa] θmax [] β [rad] D [nm]

0.4 17.1 0.002685 49

0.5 17.1 0.002570 52

1.0 17.1 0.002706 49

1.3 17.1 0.002764 48

1.5 17.2 0.003829 35

Table 5.3: XRD results and calculation of grain size with Scherrer’s formula in function of the argon

pressure.

As the last column of the table gives us a lower bound of the particle size, we see that for most

of the pressures, the grain size is approximately 50 nm. The low value of 35 nm for 1.5 Pa is

probably due to its low intensity in the XRD spectrum.

Next to XRD data to determine the crystallinity of the layer, absorption spectra can give us

more insight into the electron density, based on the shift of the absorption edge. This is called

the Burstein-Moss shift. It is caused by presence of free carriers, which change the absorption

due to a change of the distribution function or by many-body effects [84]. The former one is

discussed here, since for a degenerate electron distribution all states close to the conduction band

edge are populated. This obviously shifts the absorption edge to higher energies. Based on the

theoretical approach, it is possible to determine the carrier concentration. Indeed, using the

schematical depiction of the band diagram in fig. 5.16, we can adapt the minimum absorption

energy by taking into account that the lowest unpopulated electron states lie about 4kT under

the Fermi level:

EF − 4kT − Ec =~2k2

2m∗e(5.4)

Solving for the wavevector and inserting this in the parabolic approximation of the band diagram

yields:

∆E = E − Eg (5.5)

=~2k2

2

(1

m∗e+

1

m∗h

)(5.6)

= (EF − 4kT − Ec)(

1 +m∗em∗h

)(5.7)

Making use of the Fermi integral expression for EF − Ec 4kT and the expression of the

free-carrier density n, yields:

∆E = n2/3h2

8m∗e

(3

π

)2/3(1 +

m∗em∗h

)(5.8)

The only unknowns in this equation are the electron density and the effective masses. The latter

can be deduced from Hall measurements, but they seem not to change significantly, so based on

46


Figure 5.16: Band diagram and electron distribution function showing the filled electron states, ex-

plaining the Burstein-Moss shift.

literature, we set them [85]: m∗e = 0.38 m0

m∗h = 1.8 m0

(5.9)

Furthermore, the band gap of undoped ZnO is fixed at 3.28 eV [85]. Thus, using the energy

difference between the undoped band gap and the experimental obtained energy at the absorp-

tion edge, we can determine the electron density and subsequently the mobility of the electrons

through the thin film, since we know the resistivity from the four-point measurement:

µ =1

neρ(5.10)

For the changing argon pressure, all obtained data are shown in the graph in fig. 5.17.

It reveals that the increasing resistivity (for the region from 0.8 to 1.5 Pa) is mainly due to a

decrease of the mobility while the electron density remains almost the same (5 · 1019cm−3).

5.2.2 Influence of the target-substrate distance

A second parameter is the distance between the ZnO:Al target and the substrate. Here, it is

changed from 3 to 11 cm. We can perform the same measurements as in the previous sub-

subsection, which yields the graph in fig. 5.18, showing resistivity, electron density and mo-

bility through the thin films. The almost 100-fold increase of the resistivity (from 3 · 10−3 to

2 · 10−1 Ωcm) is mainly caused by a big decrease of the mobility of the electrons and a small

drop of the carrier density. A reason for this decrease in mobility might partly be found in the

grain size, which we can deduce from the XRD spectra. The result of this calculation is shown

in table 5.4. Indeed, we see that the grain size decreases by one fifth, which might intuitively

47


1019

2

3

4

5

6

78910

20n [1/cm

3]

14x10-3

121086pAr [mbar]

1

2

3

4

56

10

2

3

µ [cm2/Vs]

0.001

2

4

68

0.01

2

4

68

0.1

ρ [Ω

cm]

Figure 5.17: Mobility (squares), electron density (circles) and resistivity (triangles) in function of the

argon pressure.

1019

2

3

4

5

6

78910

20n [1/cm

3]

11109876543dTS [cm]

1

2

4

6

810

2

4

µ [cm2/Vs]

0.001

2

46

0.01

2

46

0.1

2

46

1

ρ [Ω

cm]

Figure 5.18: Mobility (squares), electron density (circles) and resistivity (triangles) in function of target-

substrate distance.

explain a decrease of the mobility, since electrons have to take more ‘jumps’ from grain to grain

to travel the same distance.

5.2.3 Influence of the layer thickness

In this subsubsection, we change the layer thickness from approximately 80 nm to 1 µm (while

for the other experiments it is between 400 and 500 nm as earlier stated). Again, a plot related

48


dTS [cm] θmax [] β [rad] D [nm]

3 17.1 0.002686 49

5 17.1 0.002824 47

6 17.1 0.002764 48

7 17.1 0.002993 44

9 17.1 0.003385 39

11 17.1 0.003377 39

Table 5.4: XRD results and calculation of grain size with Scherrer’s formula in function of the target-

substance distance.

to the electrical properties is depicted in fig. 5.19. Here, the trend in the electron density is

1019

2

3

4

5

6

78910

20n [1/cm

3]

800x10-9

700600500400300200100thickness [m]

681

2

4

6810

2

4

6

µ [cm2/Vs]

0.001

2

46

0.01

2

46

0.1

2

46

1

ρ [Ω

cm]

Figure 5.19: Mobility (squares), electron density (circles) and resistivity (triangles) in function of layer

thickness.

rather unclear, yet we see a clear decrease of the resistivity with increasing layer thickness (from

4 · 10−1 to 6 · 10−3 Ωcm). This effect has to be taken into account when considering our LED

stack, where only layer thicknesses smaller than 100 nm are used. Again, we can check the

relation between the mobility and the grain size. The latter are given in table 5.5. The same

effect as in the previous subsubsection is observed: increasing mobility with increasing grain

size. The only exception is the 86 nm layer, but is probably caused by the low intensity of the

peak in the XRD plot.

49


thickness [nm] θmax [] β [rad] D [nm]

86 17.0 0.00328 40

167 17.1 0.003776 35

286 17.1 0.003382 39

358 17.1 0.003315 40

550 17.1 0.002764 48

746 17.1 0.00277 48

869 17.1 0.002715 49

Table 5.5: XRD results and calculation of grain size with Scherrer’s formula in function of the layer

thickness.

5.2.4 Influence of the oxygen pressure

Next to the argon pressure, we can also inject oxygen in the vacuum chamber. Keeping the

pressure of argon at 1.3 Pa, a variation of the oxygen pressure from 0 to 0.25 Pa has been

performed. The results on the electrical properties are visible in fig. 5.20. Here, both the

1018

2

4

6810

19

2

4

6810

20n [1/cm

3]

240x10-6

20016012080400pO2

[mbar]

0.1

2

4

681

2

4

6810

2

µ [cm2/Vs]

0.01

0.1

1

10

ρ [Ω

cm]

Figure 5.20: Mobility (squares), electron density (circles) and resistivity (triangles) in function of the

oxygen pressure.

decrease of the electron density and mobility with rising oxygen pressure lead to a strong increase

of the resistivity from 1 · 10−2 to 1 · 101 Ωcm. Regarding the grain size in table 5.6, we recognize

the same trend as in the previous subsubsection.

Contrary to the previous tables to calculate the grain size, θmax is shifting to higher values for

higher oxygen pressure. The same reasoning followed for the changing argon pressure can be

50


pO2 [Pa] θmax [] β [rad] D [nm]

0 17.1 0.00328 48

0.05 17.3 0.003776 46

0.1 17.3 0.003382 44

0.15 17.3 0.003315 43

0.20 17.3 0.002764 40

0.25 17.4 0.00277 40

Table 5.6: XRD results and calculation of grain size with Scherrer’s formula in function of the oxygen

pressure.

pursued, yet the effect is more distinct now. Next to the possible induced stress in the layer,

the composition of the crystal might change as well. To check the influence, the resistivity is

remeasured after annealing from 20 C to 360 C at 5 C/min. The resulting graph is depicted

in fig. 5.21.

0.01

0.1

1

10

ρ [Ω

cm]

250x10-6200150100500

pO2 [mbar]

Before annealing After annealing

Figure 5.21: Resistivity of ZnO:Al sputtered thin films in function of the oxygen pressure, before and

after annealing.

It is clear that for the lowest pressures of oxygen the resistivity increases, while for the highest

ones, the resistivity decreases. The resulting resistivity is approximately equal for all of the

different samples. These effects might be appointed to two speculations:

At lower oxygen pressure, lots of oxygen vacancies are induced in the ZnO:Al layer. When

annealed, oxygen atoms will fill these until stoichiometrically crystalline ZnO:Al is ob-

tained. This results into the same higher resistivity.

At higher oxygen pressure, defects are removed, improving the alignment of the lattice

parameter and thus decreasing the stress in the layer. This induces a smaller resistivity,

equal to the one without oxygen addition.

51


To confirm these findings, XRD spectra are remeasured for the first and last data point. They

are compared with their original spectrum in fig. 5.22(a) and 5.22(b) respectively.

15x103

10

5

Inte

nsity

[a.u

.]

807060504030202θ [°]


(a) Without oxygen addition.

15x103

10

5

0

Inte

nsity

[a.u

.]

807060504030202θ [°]

1510

50

x103

35.034.0


(b) pO2 = 0.25 Pa

Figure 5.22: XRD spectra before and after annealing of the first and last data point of the oxygen

pressure set.

The XRD spectrum of the sample, made without oxygen addition, does not change, while the

only change for the last data point is a shift of the peaks, without influencing their width. This

might indeed cause a change in the stress of the layer, leading to a decrease of the resistivity.

5.2.5 Standard parameters for n-type ZnO:Al in LED stacks

Based on the characterization of the rotatable magnetron sputtered ZnO:Al, we can determine

the ideal specifics for the n-type ZnO:Al layer in our LED stack. Since we are interested in

a resistivity range between 0.1 and 1 Ωcm and a layer thickness between 50 and 100 nm, we

choose our parameter set: pAr = 1.3 Pa

pO2 = 0 Pa

dTS = 6 cm

(5.11)

This corresponds to the first data point of the thickness series. Although the optimal resistivity,

the mobility of the electrons remains low, but since electrons are experimentally in excess [9],

this does not lead to electron-hole recombination issues. It should also be noted that, in all

measurements, resistivities and mobilities are calculated horizontally (so parallel to the sample),

which is contrary to the current direction in our LED stacks.

Furthermore, searching a trade-off between the temperature of the substrate and the oxygen level

in the chamber might yield lower resistivities and higher mobilities [86, 87]. This temperature

dependence has however not been investigated.

52


5.3 AC LED stacks

Characterization has only be performed on the alumina layers. The main concerns of its depo-

sition are two-fold:

Electron beam evaporation of alumina layers might yield low-quality layers, causing leaking

current and fast degradation of the layers.

ALD Al2O3 might penetrate into the quantum dot layer, causing lower efficiencies of the

stack, but better passivation.

The total stack is shown in fig. 5.23 for the electron beam evaporated alumina layers.

Figure 5.23: Cross section SEM image of the AC stack with the alumina layers deposited by electron

beam evaporation.

The insulating alumina layers deteriorate the contrast and thus the quality of the image. Never-

theless, we can still appoint the different layers and see that both alumina layers have a spacing

in between, showing that e-beam evaporated alumina does not penetrate totally into the quan-

tum dot layer.

A SEM picture of the ALD stack is depicted in fig. 5.24(a). We can distinguish the two different

layers (and an aluminium layer on top). The bottom alumina layer should have a thickness of

44 nm, while the top should be 54.3 nm. The quantum dots are invisible, and the upper layer

is smaller than the lower one, which can be appointed to two possible reasons:

Penetration of the materials in the quantum dot layer, resulting in an initially smaller

growth rate,

Difficult nucleation start on top of the quantum dots, yielding a time delay before growth

starts.

53


(a) General cross section. (b) Different layer visualization.

Figure 5.24: Cross section SEM images of the AC stack with alumina layers deposited by ALD.

Promising experiments on ZnO growth on quantum dots are being conducted to confirm this

slower initial growth. Anyhow, the flat surface morphology of the bottom layer can be confirmed

in fig. 5.24(b).

To check the growth mechanism of electron beam evaporated alumina, they have been deposited

on a quantum dot layer of 35 nm. The alumina layer should be 80 nm with a layer of aluminium

of 65 nm on top. The cross section SEM image is shown in fig. 5.25.

Figure 5.25: Cross section SEM image a Si-QD-Al2O3 (e-beam)-aluminium stack.

The picture has been taken where the cleaving hasn’t been done perfectly. We can see a remain-

der of quantum dots on the silicon substrate. This and the total thickness of the layer on top

(123 nm) show that the electron beam evaporated alumina layer partially penetrates into the

54


quantum dot layer. Anyhow, high quality SEM images should be made for both types of layers

to be able to fully characterize the growth mechanisms.

The most important parameter for an insulating layer in our stack is of course its capacitance.

Therefore, admittance measurements have been performed on single layers of alumina. First

conclusions about this layers yield:

Electron beam evaporated layers are of poor quality, meaning that they lose part of their

insulating quality, resulting in fast degradation of the layers under high voltages,

Alumina layers deposited by ALD are of greater quality, since the capacitance can be

measured over a large frequency range, without inducing electrical shorts.

Since the exact value of the capacitance is of great importance to determine the needed voltage

across the stack (see section 3.2), these have been measured as accurate as possible. The resulting

capacitance for two different pads for a 38 nm layer thickness in function of the frequency is

shown in fig. 5.26. The decrease for a frequency higher than 105 Hz is caused by a parasitic

resistor and inductance of the impedance analyzer [88]. Due to the different surfaces of the pads

1

2

3

4

5

6

789

10

C [n

F]

2 3 4 5 6 7 8

104

2 3 4 5 6 7 8

105

2 3 4 5 6 7 8

106

f [Hz]

A = 3.68 mm² A = 3.31 mm²

Figure 5.26: Capacitance-frequency measurements of an alumina layer of 38 nm on two different pads.

(determined by microscopy), the capacitances differ a little. However, to check the validity, the

permittivity has been calculated. Therefore, an approximation to a parallel-plate capacitor has

been made, which is valid, since the thickness of the alumina layer is much smaller compared to

the dimensions of the aluminium contact pad. Thus, based on the formula

εr =C · dA · ε0

, (5.12)

we obtain a value of εr ≈ 7.55 in the frequency range up to 105 Hz. This is in good agreement with

values obtained in literature for ALD deposited alumina under a wide temperature range [89].

Further characterization of the total stack with these measurements is performed in the next

chapter.

55

Chapter 6

Stack results

6.1 Photoluminescence measurements

6.1.1 Basic emission spectra

To get acquainted with the photoluminescence setup, basic emission spectra have been measured.

A first example is the comparison of the spectra of PbS quantum dots in solution and spincoated

on a glass substrate. They are shown in fig. 6.1(a) and fig. 6.1(b).

25x103

20

15

10

5

0

Inte

nsity

[a.u

.]

15001450140013501300125012001150λ [nm]

(a) PbS in toluene solution.

600

500

400

300

200

100

0

Inte

nsity

[a.u

.]

15001450140013501300125012001150λ [nm]

(b) PbS spincoated on glass substrate.

Figure 6.1: Emission spectra of PbS quantum dots.

In both plots, the maximum in the near-infrared region and the peak width are equal, which is

of course to be expected. Nevertheless, in the case of the spincoated dots, the intensity almost

reaches detector noise floor at the high wavelengths, which is visible due to its longer tail.

6.1.2 Influence of ZnO:Al layer

To check the influence of a ZnO:Al deposited layer (75 nm) on a spincoated layer of nanoparticles,

three different measurements have been performed and compared with a glass sample with only

the spincoated quantum rods: basic emission spectra, lifetime measurements and excitation

spectra.

56

Chapter 6. Stack results

6.1.2.1 Emission spectra

The emission spectra for both samples are depicted in fig. 6.2. The excitation wavelength is

400 nm.

140x106

120

100

80

60

40

20

0

Inte

nsity

[a.u

.]

700650600550λ [nm]

Reference With ZnO:Al layer

Figure 6.2: Emission spectra of the glass-QRs and glass-QRs-ZnO:Al samples.

Since they have been measured under the same conditions, intensities can be compared. The

sample with the layer of ZnO:Al has a photoluminescence intensity which is about ten times

smaller than the reference at the peak. At first, the explanation could be searched in the trans-

mission characteristic of the ZnO:Al layer. Therefore, the transmission spectra of an individual

ZnO:Al layer has been measured on a sample with the approximately same thickness (86 nm)

and is shown in fig. 6.3.

100

80

60

40

20

0

Tra

nsm

issi

on [%

]

200018001600140012001000800600400λ [nm]

Figure 6.3: Transmission spectrum of an 86 nm ZnO:Al layer.

Analyzing the situation for the peak intensity yields:

Incoming beam of 400 nm: T = 92 %

Reflected beam of 636 nm: T = 86 %

This would only decrease the peak by one fifth (Ttotal = 79 %) and is thus not sufficient to

explain the large decrease. The explanation must be found by performing other measurements.

57


6.1.2.2 Lifetime measurements

On the same samples, lifetime measurements have been performed. The intensity-time plots are

depicted in fig. 6.4. They are equalized on their maxima. Furthermore, exponential fits of both

curves are shown in the figure as well. A mono-exponential fit has been done of the quantum

dot layer measurement, since the curve is almost linear on a logarithmic scale. The other curve

has been fitted with a double-exponential function.

89

100

2

3

4

5

6789

1000

Cou

nts

[-]

50403020100τ [ns]

Reference With ZnO:Al layer Mono-exp. fit to reference Double-exp. fit to ZnO:Al layer

Figure 6.4: Lifetime measurements of the glass-QRs and glass-QRs-ZnO:Al samples and their corre-

sponding exponential fits.

Analyzing the fit coefficient of the lifetime measurement of the quantum dot layer yields a

radiative lifetime τ of 32.0 ns. The radiative decay γr is proportional to 1/τ . Depositing a

ZnO:Al layer on top induces extra channels for non-radiative decay, resulting in a decay rate

γnr ∼ 1/(4.03 ns). The change in quantum yield can thus be calculated as:

QY

QY0=

γrγr + γnr

(6.1)

yielding 11.1 %. Together with the 79 % decrease due to transmission, the ten-fold decrease of

the measured photoluminescence by adding a ZnO:Al layer is explained.

6.1.2.3 Excitation spectra

Excitation spectra can provide insight into the influence of the absorption of the ZnO:Al layer

on the absorption of the quantum dots. Spectra for both samples are shown in fig. 6.5.

Since we know from the transmission spectrum that the absorption edge of ZnO:Al is situated

around 375 nm, the decrease of intensity at the shorter wavelengths can be explained. The

minimum of the reference spectra is however redshifted by 12 nm when the ZnO:Al layer is

added. This might be assigned to the higher effective dielectric constant of the quantum dot

layer, resulting in an absorption edge for the quantum dots at a slightly longer wavelength.

58


2

46

104

2

46

105

2

46

106

Inte

nsity

[a.u

.]

600550500450400350λ [nm]

Reference With ZnO:Al layer

Figure 6.5: Excitation spectra of the glass-QRs and glass-QRs-ZnO:Al samples.

6.1.3 Influence of an Al2O3 layer

Similar measurements have been performed on the Al2O3 layer. Multiple spectra are measured,

showing luminescence conservation for some types of quantum dots.

First, the photoluminescence of alloyed quantum dots (core-only) 60 % CdSe/40 % CdS has

been measured before and after deposition of an alumina layer of 50 nm. The result is depicted

in fig. 6.6.

40x103

30

20

10

0

int [

a.u.

]

580560540520500480460λ [nm]

Reference With Al2O3 layer

Figure 6.6: Emission spectra of the glass-alloy QDs and glass-alloy QDs-Al2O3 samples.

As is obvious from the figure, the luminescence is completely quenched after alumina has been

deposited on top. The same measurement has been performed on CdSe/CdS core-shell quantum

dots and is depicted in fig. 6.7(a).

The photoluminescence is conserved for this type of quantum dots. The lifetime, depicted in

fig. 6.7(b) shows a similar change as when a ZnO:Al layer is deposited on top.

Next, infrared-emitting core-shell PbS/CdS quantum dots, blue-emitting CdSe/ZnS dots and

yellow-emitting giant CdSe/CdS (14 shells) have been investigated. They all conserve their

photoluminescence properties. The emission spectrum of the PbS/CdS dots is shown in fig. 6.8.

59


1.2x106

1.0

0.8

0.6

0.4

0.2

0.0

Inte

nsity

[a.u

.]

800750700650600550λ [nm]


(a) Emission spectra.

678

100

2

3

4

5

678

1000

coun

ts [-

]

200150100500τ [ns]


(b) Lifetime measurements.

Figure 6.7: Influence of an alumina layer on the emission spectra and lifetime measurements of the

glass-CdSe/CdS core-shell QDs and glass-CdSe/CdS core-shell QDs-Al2O3 samples.

3000

2500

2000

1500

1000

500

0

Inte

nsity

[a.u

.]

150014001300120011001000900λ [nm]

Figure 6.8: Emission spectrum of the glass-PbS/CdS core-shell QDs-Al2O3 sample.

6.2 Current-voltage characteristics of the DC LED stack

An initial important step towards electroluminescence of the stacks based on direct charge

injection is of course a suitable current-voltage characteristic.

6.2.1 TPD-QDs-ZnO QDs

First measurements have been performed on the stack with organic TPD as hole transport layer

and ZnO quantum dots as ETL, while the quantum dots were infrared PbS dots emitting at

1330 nm. The resulting IV-curves are shown in fig. 6.9.

The log-log plot clearly shows two different regimes in the analyzed voltage range. In the low-

voltage region, the measured current scales with a power of about 1.3 of the voltage. This can

be explained by space-charge limited conduction of at least one of the carriers [90]. Indeed, the

dependence on the power of 1.3 is in quite good accordance with the value of 1.5 derived for the

space-charge limited current in a plane-parallel diode (Child’s law) [91]. This is however only

an approximation, since this law assumes one type of carriers and ballistic transport between

the electrodes.

60


2

46

0.001

2

46

0.01

2

46

0.1

Cur

rent

[A]

5 6 7 8 90.1

2 3 4 5 6 7 8 91

2 3 4 5

Voltage [V]

1x TPD + 1x PbS QDs + 1x ZnO QDs 1x TPD + 1x PbS QDs + 2x ZnO QDs 2x TPD + 1x PbS QDs + 1x ZnO QDs 2x TPD + 1x PbS QDs + 2x ZnO QDs

Ohmic conduction

~ V1

Space-charge limited

conduction ~ V1.3

Turn-on

Figure 6.9: Current-voltage characteristics for TPD-QDs-ZnO QDs stack.

When the voltage reaches 1 V, a steep increase in the current is observed. This can be appointed

to the injection of charge in the quantum dots. The stack shows similarities to the pn-junction

current-voltage characteristic. In the higher-voltage region, the current scales linearly with the

voltage, suggesting ohmic conduction.

Comparing these IV-characteristics to literature leads to contradictory conclusions. The current-

voltage plots of two similar structures in Caruge et al. [90] and Cho et al. [92] with as HTL NiO

and PEDOT:PSS and as ETL ZnO:SnO2 and Alq3/TiO2 respectively are plotted in fig. 6.10.

(a) Caruge et al. [90]. (b) Cho et al. [92]: red curve: reference device with

TiO2; black curve: TiO2-based LED; blue curve: Alq3-

based LED.

Figure 6.10: Comparison of current-voltage characteristics of the DC LED stacks with literature.

Comparing the experimental characteristic with the inorganic stack in [90], we notice the same

regions and approximately the same power dependence of the current on the voltage. However,

the turn-on point is shifted to a significant higher value. Checking the current-voltage char-

acteristics in [92], we see a similar behavior as the reference device, which might indicate that

our experimental LED does not show the desired conduction mechanisms through the quantum

61


dots. Furthermore, comparison of the current range with literature shows that the experimental

stacks are tunable in a significantly smaller range than those in literature. Similar measurements

have been done with different quantum dot layers (such as CdSe/ZnS visible dots) which led to

the same conclusions.

Although no confirmation has been found on the suitability of the shape of the curves, we can

still see some major differences if the thickness of the transport layers is changed. Indeed, a

thicker layer of ZnO quantum dots leads to significantly smaller currents (up to ten times) in the

low-voltage region, while the same order of magnitude is attained in the higher voltage region.

6.2.2 NiO-QDs-ZnO:Al

Current-voltage curves have been measured for all stacks, including the total inorganic stacks

such as NiO-QDs-ZnO:Al. The result for different thicknesses is shown in fig. 6.11.

10-6

10-5

10-4

10-3

10-2

Cur

rent

[A]

0.12 3 4 5 6 7 8 9

12 3 4

Voltage [V]

50 nm NiO + QDs + 60 nm ZnO:Al 100 nm NiO + QDs + 75 nm ZnO:Al

Figure 6.11: Current-voltage characteristics for NiO-QDs-ZnO:Al stacks.

Increasing the thicknesses of both the ETL and the HTL show the same effect in the low-voltage

region as in the previous subsection. In the higher voltage regime (> 2 V), both curves cross

each other, indicating higher currents for the thicker stack.

6.2.3 Reproducibility of the IV measurements

The reproducibility of the measurements is an important aspect to check. Therefore, multiple

current-voltage measurements have been performed on the same stack. An example is shown in

fig. 6.12.

We can clearly see a changing characteristic, showing that more current flows after each cycle.

This might be an indication of charging of the quantum dot layer, as described in fig. 3.1.4. Also,

performing subsequent current-voltage measurements including a measurement in the negative

voltage region (which might induce decharging), influences the characteristic significantly, as is

depicted in fig. 6.13.

Because this does not resolve the issue, no apparent way to overcome this is readily available.

62


60x10-3

50

40

30

20

10

0

Cur

rent

[A]

43210Voltage [V]

First Second Third

Figure 6.12: Repeated current-voltage measurements on a NiO-QDs-ZnO(ALD) stack.

40x10-3

20

0

-20

Cur

rent

[A]

-4 -3 -2 -1 0 1 2 3Voltage [V]

First Second Third

Figure 6.13: Repeated current-voltage measurements on a NiO-QDs-ZnO:Al stack, including a negative

voltage measurement.

Nevertheless, using an electron blocking layer, as proposed in subsection 3.1.4, might avoid the

changing characteristic, yet this has not been experimentally tested in this master thesis.

6.3 Admittance measurements of the DC LED stack

To gain more insight in the energy band diagram and trapping mechanism in the LED stacks,

capacitance-frequency measurements have been performed on different electron-only, hole-only

and electron-hole devices. They yielded however not the desired results, since the Fermi level

and the built-in potential (which are unknown) have to be included as a decisive parameter and

the deep level states are hard to observe1.

Nevertheless, attempts have been done to get some insights. By performing the measurements

at different temperatures, densities at an energy level Eω can be calculated based on formulas

1The program has originally been designed to perform measurements on solar cells, which are mostly larger in

cross section and where usually voltages are applied up to an order of magnitude smaller than the ones used here.

63


derived in literature [70]:

Nt(Eω) = −V 2B

w[qVB − (EFn∞ − Eω)]

dC

dω

ω

kT(6.2)

where Nt is the number of traps, VB the built-in potential, w the width of the intrinsic layer

of the pin-junction in the model of the simulation program, EFn∞ the quasi-Fermi level for

the electrons far from the junction, C the capacitance of the device, k the Boltzmann constant

and T the temperature. Although the first factor changes with the unknown parameters, we

still see that the trap density scales with the derivative of the capacitance to the frequency, the

frequency itself and the inverse of the temperature. These scaling implies that all peak densities

have to be aligned in an Nt vs. (Eω − EVB) plot (where EVB is the energy level of the valence

band) for all temperatures.

Two main conclusions can be drawn:

NiO-PbS QDs-NiO, NiO-PbS QDs and NiO-CdSe/ZnS core-shell QDs stacks yield the

same result for a trap density peak at about 100 meV. This is probably not caused by the

layers individually but probably due to a trapping at the interface level. Cf-measurements

on the NiO layer only were impossible due to its pure resistor behavior.

Considering the stack NiO-QD-ZnO:Al stack, the beginning of a new peak can be observed.

The whole procedure for this stack is shown in fig. 6.14.

We indeed see the appearance of two peaks, one probably resulting from an interface with NiO

while the other seems to appear for the ZnO:Al layer only. To investigate the latter, defects at

around 250− 300 meV are checked with literature [93]. Possibilities are:

VZn: Zinc vacancy

Oi(oct): Octahedral interstitial oxygen

ZnO: Substitution of an oxygen atom by a zinc atom

No further conclusion can be drawn from the Cf-measurements. Determining the carrier type

or the band alignment is not possible.

6.4 Admittance measurements of the AC stack

Cf-measurements have also been performed on the total AC stack. They can be used to calcu-

late the capacitance of the quantum rods/dots and to determine the voltage drop per quantum

dot/rod, if the applied voltage during EL measurements and the thickness of the intermediate

layer are known.

Two examples are shown in fig. 6.15 (measured for different pads). They have following prop-

erties:

64


102

103

104

105

106

107

0

500

1000

1500

2000

2500

3000

f [Hz]

C [n

F/c

m2 ]

T = 300 KT = 310 KT = 320 KT = 330 KT = 340 KT = 350 KT = 360 K

(a) Cf-measurement at different temperatures.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.3510

18

1019

1020

Eω−EVB

[eV]

Nt [c

m−

3 eV−

1 ]

T = 300 KT = 310 KT = 320 KT = 330 KT = 340 KT = 350 KT = 360 K

(b) Trap density in function of the energy level for an

arbitrary escape frequency.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.3510

18

1019

1020

Eω − EVB

[eV]

Nt [c

m−

3 eV

−1 ]

(c) Alignment and smoothing of the resulting curve.

Figure 6.14: Procedure to determine trap level density in the NiO-QD-ZnO:Al stack.

38 nm layer of alumina, deposited by ALD

Quantum dot/rod layer

Approximately 50 nm layer of alumina, deposited by ALD2

6.4.1 Simplified model calculations

Since we’ve shown in section 5.3 that the dielectric constant of ALD deposited alumina is about

7.55 and checked the validation of the parallel-plate capacitor approximation, the resulting

capacitance over the Al2O3-QDs (70 nm)-Al2O3 stack can be determined:

CQD = (C−1tot − C−1bottom − C

−1top)−1 (6.3)

CQD = (C−1tot −dbottom + dtop

εrε0A)−1 (6.4)

2This denotes the thickness grown on a SiO2 substrate. As shown in section 5.3, the growth behavior of

alumina on QDs differs significantly.

65


3.0

2.5

2.0

1.5

1.0

0.5

0.0

C [n

F]

104

105

106

f [Hz]

(a) Electroluminescent layer: CdSe/CdS core-shell

red quantum dots.

3

4

5

678

1

2

3

4

5

C [n

F]

104

105

106

f [Hz]

(b) Electroluminescent layer: alloyed yellow quan-

tum dots.

Figure 6.15: Capacitance-frequency measurements of the total AC stack.

where dtop is assumed to be 0, since we do not expect a separate ALD alumina layer on top

of the 70 nm quantum dot layer, based on the SEM images in the previous chapter. The area

is approximated to be 3.5 mm2 and the total capacitance averaged to 1.52 nF. The resulting

capacitance over the QD layer is then calculated to be 2 nF. Since a peak-to-peak voltage of

about 72 V (so an amplitude of 36 V) has to be applied to measure light output (see further),

the voltage drop over the total quantum rod layer can be calculated based on eq. (3.1) and (3.3)

and yields:

VQD = 6.95 V (6.5)

The dots have a diameter of 14.1 nm, which implies 3 to 5 monolayers. The resulting voltage

drop per quantum dot can thus be determined between 1.4 and 2.3 V, which, based on the

electric field model, in good agreement with the energy band gap of the dots, 2.02 eV.

6.4.2 Extended model calculations

However, previous calculations cannot explain the capacitance change when the quantum rod

layer is spincoated once or twice. Based on the capacitance density-frequency plots of the

alumina layers and the total stack in fig. 6.16 and the conductance-frequency plots of the ITO-

Al2O3-Al stack in fig. 6.17, an extension of the simple model is made by considering extra

resistors, as already mentioned in 3.2. Fig. 6.16 also confirms the uniformity of the layers, since

the capacitance densities of two different pads are almost perfectly coinciding.

As is clearly visible, the conductance G of the oxide layer is not constant, but shows a dependence

on the frequency. Therefore, a power fit has been performed in the frequency range of interest

(5 − 100 kHz). This yields a power dependence of 1.94 on the frequency. Based on this result,

a first electrical circuit proposal for the alumina layers is depicted in fig. 6.18.

66


2

3

4

56

1

2

3

4

56

C [n

F/m

m²]

2 3 4 5 6 7 8

104

2 3 4 5 6 7 8

105

2 3 4 5 6 7 8

106

f [Hz]

Bottom layer 1 Bottom layer 2 Top layer 1 Top layer 2 Stack 1x red QDs 1 Stack 1x red QDs 2 Stack 2x red QDs 1 Stack 2x red QDs 2

Figure 6.16: Capacitance density-frequency plot of alumina layers and total stack, where the QD layer

is spincoated once or twice.

10-8

10-7

10-6

10-5

10-4

10-3

10-2

G [S

]

104

105

106

f [Hz]

Original data Power fit Model fit

Figure 6.17: Conductance-frequency plot and fitting curves of the ITO-Al2O3-Al, where the alumina

layer is the bottom oxide layer of the total stack.

Figure 6.18: Electrical circuit proposal for the ITO-Al2O3-Al stack.

Indeed, the admittance expression looks like:

Y = G+ jωCp (6.6)

=1 + ω2R2

2

(1 + R1

R2

)C2

R1(1 + ω2R22C

2)+ jω

C

1 + ω2R22C

2(6.7)

where Cp denotes the result of the Cf-measurements as plotted in all appropriate graphs. Indeed,

if ω2R22C

2 remains far smaller than 1 (for the frequency range of interest) and R1 R2, the

67


quadratic dependence is present, while the capacitance remains constant:G ≈1R1

+ ω2R2C2

Cp ≈ C(6.8)

The model fit to the conductance curve, together with the already obtained value of C, yields

the values: R1 = 3.32 GΩ

R2 = 36.6 Ω

C = 6.5 nF

(6.9)

showing that the parallel path can be discarded and the conditions for the model are fulfilled.

The original data, the power and model fit of G are shown in fig. 6.17. Analogous measurements

have been performed for the top layer, resulting in similar values for the parameters.

Now that the oxides are modeled, focus is put on the quantum dot/rod layer. Therefore, we

calculate its admittance by subtracting the admittances of the oxides from the total stack.

Similar to what is done in the simplified case, the top oxide admittance is discarded, since we

assume, based on the SEM images that no separate top alumina layer is grown. If we assume

that the imaginary part of the quantum dot layer admittance consists of a similar term as the

oxide layers, we obtain the capacitance of the quantum dot layer as in fig. 6.19.

1

2

3

4

5

6

789

10

C [n

F]

2 3 4 5 6 7 8

104

2 3 4 5 6 7 8

105

2 3 4 5 6 7 8

106

f [Hz]

1xSC 2xSC

Figure 6.19: Capacitance-frequency plot of the quantum dot layer (spincoated once or twice), based on

the extended model.

Contrary to what was obtained in the idealized model, the capacitance is no longer equal. The

capacitance of the thicker layer is about two-thirds of the one of the thinner layer, which is still

a deviation of the expected halving. Two possible deficiencies of the model can explain this:

The measured conductance and capacitance have been assumed to only be determined by

the layer itself, not by the interfaces and contacts,

Unknown growth rate of the ALD deposited alumina inside the quantum dot layer and on

top.

68


Further characterization of the latter is necessary to be able to correctly model all different

layers.

Based on the thick NC layer (70 nm), a relative permittivity of 4.52 is obtained. To compare this

experimental permittivity with the real value, a crude approximation can be done. Assuming

close-packing of equal spheres, built from a 7 nm radius CdS (εr,bulk = 5.7, ignoring the CdSe

seed of 2.2 nm) and a 2 nm shell of oleic acid (εr = 2.03), and the remaining 24 % filled with

alumina (εr = 7.5), an average relative permittivity of 4.7 is found [94, 95]. Both values are in

quite good agreement, showing that the model can certainly be used if an extensive study is

performed on the growth characteristics of the ALD alumina layer.

6.5 Electroluminescence of the AC stacks

The alumina layers of the stacks which should work under field-driven ionization can, as already

mentioned before, be deposited by electron beam evaporation or by ALD. The quality difference

can be confirmed by following observations during EL measurements:

Increasing the voltage precisely until light is emitted which is just visible by eye, induces

the defects of the e-beam layers to disappear very fast (blue sparks). However, their huge

amount evaporates the aluminium contact on top, resulting in failure of the device. Mostly,

this happens within the minute after the heredescribed process.

When repeating the same process for the ALD samples, only a very low number of defects

is removed when the voltage is turned on to the emitting point. The degradation of the

device occurs much slower, unless the voltage is significantly increased.

It should again be stated that light has been detected in both systems, but instability of the

e-beam samples prevents spectrometer measurements due to difficult alignment.

A first EL and PL spectrum of the stack ITO-Al2O3 (38 nm)-QDCdSe/CdS (1x SC, 35 nm)-Al2O3

(50 nm)-Al contacts is shown in fig. 6.20. The dots have a diameter of 14 nm and a QY of 22 %.

It should again be noted that the upper layer of 50 nm is in fact significantly smaller, since the

material penetrates and fills the empty spaces in the quantum dot/rod layer first before growing

uniformly on top of the dots/rods.

The EL spectrum has been plotted by subtracting the off spectrum from the on spectrum to

remove the unwanted background influence. Both spectra are clearly well aligned, signifying that

for the electroluminescence, the same radiative emission is present as for the photoluminescence.

The small blueshift of the EL spectrum might be caused by a slight calibration deviation of the

CCD camera.

The EL spectrum has been measured by applying a 20 kHz AC block voltage pulse of about

100 V peak-to-peak. The theoretical waveform is shown in fig. 6.21, together with the real

69


1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

inte

nsity

[a.u

.]

700650600550500λ [nm]

PL EL

Figure 6.20: Electroluminescence spectrum of the stack ITO-Al2O3 (38 nm)-QRCdSe/CdS (1x SC)-Al2O3

(50 nm)-Al contacts.

voltage-time diagram.

-100

-50

0

50

100

volta

ge [V

]

6050403020100time [µs]

(a) Ideal block pulse. (b) Real block pulse.

Figure 6.21: Applied waveforms to measure electroluminescence.

As we obviously see, the steep rise in voltage has been strongly modified due to the capacitance

of the device load. The speed from low to high voltage is entirely determined by the slew rate

(expressed in V/µs). Unfortunately, the capacitor at the output load has to be charged to the

desired voltage as well. Therefore, if there is a limiting current IL of the amplifier itself, the

load capacitor can only be charged with a maximum speed. Thus, the slew rate is in this case

given by:

SA =ILCload

(6.10)

This value can be significantly lower than the unloaded one, especially in this case where capac-

itances of the order of tens of nanofarad are used [96].

Returning to the electroluminescence measurements, we can plot the intensity of the light in

function of time. This is done by measuring the surface under the spectrum peak at a certain

delay time after the trigger reaches the spectrometer. The result is shown in fig. 6.22. The ideal

block pulse is also shown to make interpretation easier. Two different curves are visible, which

are up to a factor almost equal to each other. The second (blue) curve is measured a few minutes

70


inte

nsity

[a.u

.]

50403020100t [µs]

-100

0

100

voltage [V]


Figure 6.22: Light intensity in function of time.

after the first one, leaving the circuit untouched. This indicates slight degradation of the device

during longer operation. The position of the peaks can be interpreted based on the theoretical

approach in section 3.2. After the steep rise of the voltage, the electric field induces moving

electrons and holes in the conduction and valence band respectively, resulting in an increase in

the light emission when they recombine radiatively. During the peak, the internal field builds

up and opposes the external one, resulting in evanescent emission. When the pulse is reversed

to the negative, the same mechanism occurs in the opposite direction. The peak at exactly one

period might be caused by the internal field itself, but no confirmation of this has been found.

Now that the power output of the dots is confirmed to originate from the electroluminescence,

power measurements are performed at different voltages and a fixed frequency of 20 kHz. For

the quantum CdSe/CdS dots, this is depicted in fig. 6.23.

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Pow

er [µ

W]

959085807570Voltage (pk-pk) [V]


Figure 6.23: Output power of the AC stacks with CdSe/CdS dots (70 nm QD layer) in function of the

applied voltage.

The power output increases linearly with the applied peak-to-peak voltage. As already men-

tioned in subsection 6.4.1, the intercept of the linear fit crosses the horizontal axis at 72 V. The

71


slope of the fit yields 0.1 µW/V. The maximal measured power was 2 µW. Converting these

data to a power density yields:

Slope: 2.86 µW/Vcm2

Maximum power density: 57 µW/cm2

The latter value is significantly higher compared with values from literature one year ago [61].

Similar measurements have been performed on giant yellow-emitting CdSe/ZnS dots. The power-

voltage plot is shown in fig. 6.24. The same linear trend (with a different slope) and maximum

power output of 2 µW have been measured.

1.0

0.8

0.6

0.4

0.2

0.0

Pow

er [µ

W]

858075706560Voltage (pk-pk) [V]


Figure 6.24: Output power of the AC stacks with giant CdSe/ZnS dots in function of the applied

voltage.

Power has also been measured for other types of dots/rods. For a layer of quantum rods emit-

ting yellow, a maximum power has been measured of 0.5 µW and breakdown occurred fast.

Measurements on the infrared PbS/CdS dots yielded a maximum power output of 0.3 µW.

Next, the power in function of the applied frequency (and constant voltage of 85 V peak-to-peak)

is plotted in fig. 6.25 for the case of the giant CdSe/ZnS dots.

A linear fit through the origin is performed on the first four data points, showing its propor-

tional increase. The power of the 20 kHz data point is lower than expected, because, as already

discussed, the shape of the block pulse is heavily deformed at this frequency. The deviation can

also partially be appointed to the internal electric field, which, at high frequencies, does not

have time to reach its full strength. This will be confirmed in the next section.

Based on the maximum power output of 2 µW at a voltage of 92 V peak-to-peak and a fre-

quency of 20 kHz, combined with the conductance G of the total stack, the efficiency η can be

determined, yielding:

η =Pout

Pin=

2 µW

GV 2rms

= 0.02 % (6.11)

Rewriting this value in lm/W by using the standard conversion 680 lm = 1 W (for a wave-

length of 555 nm) gives a power efficiency of 0.14 lm/W. This is only slightly lower than the

72


1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Pow

er [µ

W]

20151050f [kHz]


Figure 6.25: Output power of the AC stacks with giant CdSe/ZnS dots in function of the applied

frequency.

values between 0.17 and 3.8 lm/W, obtained for DC LED stacks in literatrure [58,92]. Further-

more, the power output scales linearly with frequency and voltage, while the power input scales

quadratically with both. This results in the dependence:

η ∼ 1

ωV(6.12)

which should definitely be taken into account when considering steps towards commercialization.

To show the emitted light of the pads visually, pictures of the on and off state of one pad are

shown in fig. 6.26. The used nanoparticles are yellow-orange CdSe/CdS rods.

(a) On state. (b) Off state.

Figure 6.26: On and off state of yellow-orange emitting CdSe/CdS rods.

Two things can be concluded from the picture. On the one hand, only a major part of the surface

emits light, while the rods on the other part are degraded, probably caused by the removal of

the ligands during ALD deposition. On the other hand, white dots can be observed, which show

small breakdown paths through the device caused by the high voltage.

73


6.6 Modeling of the EL of the AC stacks

Based on the operation mechanism and the result of the intensity measurements during one

period, a Matlab model has been developed to confirm them. Therefore, a quantum dot layer

of three monolayers has been used, where only recombination of the central layer is considered

as the main contribution to the power output. Rate equations have been developed to match

the experimental behavior. For the electrons in the conduction band of the central layer, they

are modeled for the positive block pulse as:

nie,c =

ni−1e,c +

[ T1︷︸︸︷a (VQD − Eg/q) +

T2︷︸︸︷b(ni−1e,l − n

i−1e,c

)−

R︷︸︸︷cni−1e,c n

i−1h,c

]dt VQD > Eg/q

ni−1e,c +[b(ni−1e,l − n

i−1e,c

)− cni−1e,c n

i−1h,c

]dt 0 < VQD ≤ Eg/q

(6.13)

where a, b and c are constants which model the tunnel probability between the quantum dots and

the recombination rate. ni−1e,l and ni−1h,c respectively denote the electrons in the conduction band

of the left layer and the holes in the valence band of the central layer at time step (i− 1). The

term T1 models the overlap between the valence band of the left monolayer and the conduction

band of the central monolayer, which scales with (VQD − Eg/q). The factor a is dependent

on the drift mobility and tunnel probability between the two density of states. The term T2

denotes the transport between the conduction bands from the left to the central and the central

to the right layer, caused by drift, where b is naturally dependent on the drift tunneling mobility.

The final term R is the recombination term and is of course dependent on the electrons in the

conduction band and the holes in the valence band of the central layer. When VQD < Eg/q, the

overlap between the valence band of the left layer and the conduction band of the central layer

is zero. Of course, rate equations for the outer layers and for the holes in the valence band have

been developed with similar terms, both for the negative as the positive block pulse.

To show the agreement between the model and the experiment, the number of recombining

electron-hole pairs in the central layer is depicted in fig. 6.27 for three different frequencies.

The similarities of the peak and the evanescent intensity are clearly visible. Furthermore, they

confirm the linear increase of the power output in function of the frequency. Indeed, for the

frequencies f and 2f , the integral under the period of the output with frequency f is almost

exactly doubled when the frequency is doubled. However, deviations from this principle occur

when even higher frequencies are used, because the internal field does not yet reach its full

strength when the polarity of the external electric field is reversed.

Optimizing power output, the model might be expanded by considering other waveforms than a

block pulse, such as a sine or a triangle. Furthermore the time between subsequent pulses might

be changed, inducing lower frequencies, but more electric field alterations. Finally, varying the

number of layers might yield an optimal value for the power output.

74


0

x 1011

t [a.u.]

Inte

nsity

[a.u

.]

f2f4f

Figure 6.27: Number of recombining electron-hole pairs in the central layer in function of time and

different frequencies.

6.7 Comparison between both methods and commercialization

considerations

To conclude this chapter of the stack results, a summary of both operation mechanisms, their

advantages and disadvantages is given in table 6.1 and the possible commercialization is discussed

hereunder.

Quantum dots for light-emitting purposes have already been applied commercially by QD Vision.

Indeed, they have used them to enhance the emission from an LED replacement lamp and are

currently researching the possibility of commercial QD LEDs by direct charge injection [97].

Further, Samsung is investigating large-area full-color QD displays on glass and even flexible

substrates based on the same method [98]. The commercial use of the AC stacks, which require

high voltages and frequencies, is still in a research phase because of its fast degradation and

higher load on the electronics. However, polychromatic devices with phosphors operating under

voltages higher than 200 V and frequencies between 0.1 and 1 kHz, have already been designed

and commercialized by Planar [99]. Further successful research have been performed by iFire,

who developed full-color prototypes with phosphors, but only at voltages up to 60 V and a

120 Hz frequency [100, 101]. Luminescence experimentation for applications in displays has

been conducted in sulfides, ranging from rare earth materials to doped and undoped nanocrystals

[102].

75


Direct charge injection - DC Field-driven ionization - AC

Working principle

Advantages

Low DC voltages needed

Wide tunability in resistivity and mo-

bility of the layers

PL conservation for investigated lay-

ers

Big variety of layer structures to im-

prove the carrier injection, e.g. block-

ing layers, inverted device structure

Easy processing of the oxide layers

Easy characterization of the individ-

ual layers

Power output linearly tunable in

function of the applied voltage

Optimization possible by varying the

applied voltage waveform and the

number of layers, based on model

Disadvantages

Difficult optimization process due to:

– Wide tunability of a big set of

parameters

– Hard measurements e.g. band

alignment, carrier density, . . .

Lack of processing equipment to make

efficient organic injection and trans-

port layers

High AC voltage source needed at

kHz frequencies

Small tunability of the device itself

Reduced operation time due to high

voltages and frequencies

PL quenching for core-only quantum

dots

Table 6.1: Summary of both operation mechanisms, their advantages and disadvantages.

76

Chapter 7

Integrated light source under AC

field excitation

Integration of light-emitting devices in plasmonic structures or on silicon is a promising field.

The applications of integrated plasmonics are already discussed in the introduction, while the ad-

vantages of CMOS compatible silicon with the advanced functionalities of photonic components

are already widely tested. Indeed, applications of silicon in telecommunications, biosensing, etc.

are already known for their passive functionalities (signal routing, filtering, . . . ), yet the active

photonics such as on-chip light generation or detection proves to be more challenging, mainly

due to the indirect band gap of silicon.

This chapter begins with an introduction of the surface plasmon concept, followed by simula-

tions of the quantum dot, modeled as a dipole light source emitting in a metal-dielectric-metal

structure, which will be discussed and compared with literature. The focus mainly lies on the

deduction of the wavelength-dependent spontaneous emission enhancement factor. Further, the

practical integration of our bulk device on a silicon waveguide is described and experimental loss

measurements of different structures are interpreted.

7.1 Plasmonic integration

7.1.1 Surface plasmon concept

Plasmon is a term used to denote a quantized bulk plasma oscillation of electrons in a metallic

solid. When the limitation to surface plasmons is made, it refers to the interaction of the plasma

oscillations at surfaces. Therefore, its definition can be reformulated in a classical electromag-

netic model as a fundamental electromagnetic mode of an interface between a material with a

negative permittivity and a material with a positive permittivity having a well-defined frequency

and which involves electronic surface-charge interaction [103].

To arrive at a description of its propagation mechanism, the idealized situation of two materi-

77

Chapter 7. Integrated light source under AC field excitation

als with a real permittivity can be considered, where we start from the equations describing a

slab waveguide and letting the thickness of the central layer approach to zero. This yields only

appropriate solutions for the TM polarization with a propagation constant β given by:

β =ω

c

√εdεmεd + εm

(7.1)

This should be real, which is only possible, for the non-radiative surface mode, when εm < 0

and |εm| > εd. Inserting the expression for β in the appropriate equations gives us the magnetic

field: hy = A exp(−δx) x ≥ 0

hy = A exp(−γx) x ≤ 0(7.2)

where γ and δ are real, positive and function of the frequency and the permittivities of the

materials, while y is the direction parallel to the interface and perpendicular to the propagation

direction and x the direction parallel to the normal of the interface. The analysis can easily be

extended for a lossy metal. The shapes of the fields hy, ex and ez remain the same, characterized

by:

Exponentially decaying with a peak at the interface,

Much deeper penetration into the dielectric than into the metal.

The imaginary part of the permittivity of the lossy metal describes the loss of the surface plasmon

mode while propagating. If the real part is described by the Drude model, a non-radiating surface

plasmon can only exist when

ω <ωp√

1 + εd,real(7.3)

where ωp denotes the plasma frequency [104].

Now that the fields have been determined for a single metal-dielectric interface, extension can

be made towards an MDM structure. Indeed, if two interfaces are brought close together, the

resulting mode is the fundamental transverse magnetic mode, also called the gap surface plasmon

polariton mode, which is strongly confined between the two interfaces. Here, three parameters

can be considered as crucial in the surface plasmon propagation: the propagation length1, the

spatial extent2 and the confinement factor3. Therefore, all three are plotted in function of the

central layer thickness for gold-air interfaces and both the metal-dielectric-metal as the dielectric-

metal-dielectric stack in fig. 7.1. The free-space excitation wavelength is 1550 nm.

As expected, the spatial extent of the mode in the MDM structure is strongly reduced when

smaller than a critical value (12.5 µm), as is the case for the confinement factor. It is important

1Distance wherein the electric field intensity of a traveling wave at either surface decays by a factor of 1/e.2Distance between the points in the two cladding regions where the magnetic field decays to 1/e of its peak

value.3The ratio of power in the center region of the waveguide to the total power in the waveguide.

78


(a) Propagation length. (b) Spatial extent. (c) Confinement factor.

Figure 7.1: Important parameters describing the gap surface plasmon polariton mode in function of the

central layer thickness. [105]

to stress that the spatial extent can be reduced up to 100 nm, which is smaller than 1/15th

of the excitation wavelength. One disadvantage is the decrease in the propagation length for

decreasing central layer thickness [105].

7.1.2 Modeling of a dipole light source emitting in an MDM structure

To analyze the effect of a quantum dot emitting a photon, it can be modeled as a dipole source.

Therefore, MatLab calculations have been carried out to investigate the plasmonic propagation

caused by the light source in a metal-dielectric-metal structure and compared to literature.

The modeled stack is shown in fig. 7.2, where the yellow dot denotes the position of the dipole

light source. The metal plates are supposed to be semi-infinite, allowing easier calculations of

Figure 7.2: MDM structure with a dipole light source in the dielectric layer.

the necessary Fresnel coefficients. The problem can be solved by considering the upward and

downward propagating solutions to Maxwell’s equations, which demand considerable algebra.

An easier method is a derivation based on the surface response by reflection coefficients.

The normalized decay rate for a dipole above a single surface contains three factors denoting

amplitude changes of:

The ⊥ E-field of a P-polarized wave: rP12 exp(2ik1,⊥d),

79


The ‖ E-field of an S-polarized wave: rS12 exp(2ik1,⊥d),

The ‖ E-field of a P-polarized wave: −rP12 exp(2ik1,⊥d),

when a wave propagates from a location z = d to the metal and back. The r-coefficients denote

the different Fresnel coefficients, while k1,⊥ is the projection of the wavevector in the dielectric

perpendicular to the interface. Of course, the reflection coefficients are calculated based on

the wavelength-dependent permittivity of the metal. In the here-described method, the Drude-

Lorentz model is used. For silver, the real and imaginary part of the relative permittivity are

shown in fig. 7.3.

Figure 7.3: Real and imaginary part of the relative permittivity of silver, based on the Drude-Lorentz

model.

The change from the positive to the negative real part of the permittivity occurs at 324 nm.

Dependent on the permittivity of the dielectric, a plasmon can propagate at a higher wavelength

than this when |εm,real| > εd,real.

Based on these items, the expansion towards the MDM structure can be made by keeping in

mind that multiple reflections occur at both interfaces, resulting in a geometric series [106].

Only the dipole oscillating normal to the metal surfaces is considered, since only this one will

couple to the gap SPP (fundamental TM) mode, as the electric field in the gap is primarily

directed transversally. Based on this, the spontaneous emission enhancement factor, defined as

the ratio of the obtained decay rate and free space decay rate, is given by:

FP =γ⊥γ0

= 1− η + η

∫∞

0

1

PfreedP

dk1,‖dk1,‖

η=1=

3

2Re

∫∞

0

(1− rP12 exp(2ik1,⊥d1))(1− rP13 exp(2ik1,⊥d2))

1− rP12rP13 exp(2ik1,⊥d1) exp(2ik1,⊥d2)

k31,‖

k31k1,⊥dk1,‖

(7.4)

80


Here, the dissipated power P is related to the decay rate γ of the dipole (oscillating with angular

frequency ω) via γ = P~ω . On the second line of the equation, the internal quantum efficiency η

is assumed unity for the emittor (which is of course an approximation due to the negligence of

non-radiative processes). Furthermore, k1 and k1,‖ are the wavevector and its in-plane projection

in the dielectric respectively, while k1,⊥ is defined as (k21−k21,⊥)1/2, denoting its possible complex

character [106–108].

The spontaneous emission enhancement factors in function of the wavelength are calculated in

Matlab for different thickness of the dielectric layer and shown in fig. 7.4(a). The location of the

dipole source remains fixed at the center of the layer. The dielectric has a relative permittivity of

2.55 and both the metal plates are silver. Similar results are obtained in literature and depicted

in fig. 7.4(b).

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 10−6

0

20

40

60

80

100

120

λ [m]

FP

30 nm50 nm100 nm

(a) Analytical results. (b) Literature results. [108]

Figure 7.4: Spontaneous enhancement factor in function of the wavelength for different thicknesses of

the dielectric layer. The location of the dipole is fixed at the center.

Two regions can be distinguished. A sharp peak is visible at the wavelength where |εm,real| =

εd,real. This is the resonant contribution. At higher wavelengths, the linear dependency is

due to the non-resonant enhancement, since similar linear curves are obtained when only one

dielectric-metal interface is considered [108]. The reason for both types of enhancements must

be searched in the decay rate density spectra, which have coinciding peaks with the gap surface

plasmon polariton mode dispersion curve. The decay rate density also has contributions from

broad, large wave vector components, which originate from intrinsic metal losses [106,108].

To check the influence of the location of the source, the MDM stack has been simulated when

the location is at 1/4, 1/3 and 1/2 of the thickness (30 nm) of the dielectric layer. The result in

fig. 7.5 yields that the enhancement factor shows a more than four-fold increase if the location

is changed from the center to a quarter of the dielectric layer.

81


0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

x 10−6

0

50

100

150

200

250

300

350

400

450

λ [m]

Fp

L/4L/3L/2

Figure 7.5: Influence on the spontaneous emission enhancement factor of the location of the dipole

source in the dielectric layer of the MDM structure.

7.2 Integration of the AC stack on a silicon waveguide

Since the electroluminescence of the AC stacks is a success, its integration on a silicon waveguide

should be investigated. First, a description of the integrated structure is given. Next, some in-

tegration considerations are discussed which should be taken into account. Thirdly, the building

steps of the stacks are described. Finally, preliminary loss measurements are analyzed which

provide more insight on the influence of the layers on the silicon waveguide structure.

7.2.1 Description of the structure

The integrated structure is slightly different than the bulk device, since we want the quantum

dots to couple efficiently in the silicon waveguide. A threedimensional picture of the first design

is shown in fig. 7.6(a), while a clear schematic of the cross section of the subsequent layers is

depicted in fig. 7.6(b).

(a) Threedimensional depiction. (b) Twodimensional cross section of the waveguide.

Figure 7.6: Schematic of the integrated structure.

The used quantum dots are PbS infrared dots (emitting approximately at 1550 nm), spincoated

82


once to deposit about one monolayer. On top, an ALD alumina layer of about 50 nm is grown.

Finally, an ITO layer of 20 nm is deposited by sputtering, while the gold contacts are deposited

by electron beam evaporation. The probes for contacting are connected on the gold plates and

on the silicon waveguide itself. Indeed, in this way, an AC voltage can be applied over the

quantum dots, the alumina layer and the ITO layer. The bottom alumina layer is no longer

used as in the bulk devices, resulting in the immediate deposition of the quantum dots on top

of the silicon waveguide.

In a second design, the ITO layer has been replaced by a gold layer of 100 nm4. A microscopic

picture of the created structures is shown in fig. 7.7.

Figure 7.7: Microscopic picture of the created structures.

The characteristics of the layers are explained in subsubsection 7.2.3.2.

7.2.2 Integration considerations

Some considerations during integration (and thus miniaturization) should be taken into account.

The capacitance of the alumina layer changes according to the chosen area occupied on the silicon

waveguide. Since the capacitance scales linearly with the area, the capacitance is reduced by

about five orders of magnitude. The advantage is a significant improvement of the slew rate and

thus of the performance of the amplifier. In addition to the influence of the capacitance, the use

of the silicon as bottom contact also has implications, since the conductivity of silicon is a lot

smaller than of ITO.

If we focus on the stack with an interaction length of 100 µm and assume that both the quantum

dots and the alumina layer are surrounding the waveguide at three sides, we can calculate the

4A nickel layer of about 1 nm is deposited in between the alumina and the gold to assure the latter sticks well

to the sample, when lift-off is performed.

83


resulting capacitance of the stack as:

Cint =AintAbulk

Cbulk = 2.75 · 10−5 · 2.9 nF = 80 fF (7.5)

where we assume the same capacitance for the quantum dot layer of 35 nm as in the previous

chapter. Again, the alumina is expected to penetrate in the QD layer, but verification by

SEM of the whole design is necessary without any doubt. Since the conductance scales inversely

proportional with the area, the needed voltage to obtain the same voltage drop over the quantum

dots does not change. However, since the bottom alumina is removed and the top layer is

expected not to form a separate layer, the fielddriven operation mechanism might no longer

hold.

7.2.3 Building steps of the integrated stack

In this subsection, the different steps to build the integrated stack are discussed. They mainly

concern the start from the silicon waveguides, the lithography steps by image reversal and the

subsequent depositions by different techniques.

7.2.3.1 Silicon waveguides

The dimensions of the silicon waveguides are:

Height: 220 nm

Width: 450 nm

Length: ≥ 1 mm

At the start and the end of the waveguide, grating couplers are used to efficiently couple the

light in and out.

Furthermore, there is an alteration of the waveguide width in the center of the waveguide.

Indeed, for a length of 180 µm, the width is tapered up or down to 350, 400 and 470 nm for the

subsequent sets of each 10 waveguides. Since the structures are not deposited there, this should

not influence the propagating mode and thus the absorption by the quantum dots.

7.2.3.2 Lithography steps and deposition methods

On the substrate with the silicon waveguides, a first lithography step is performed with a mask

for the quantum dots and the alumina layer. Therefore, the image reversal technique is applied.

First, a primer and the UV-sensitive photoresist are spincoated on the total structure. Next,

the contact mask is aligned on top of the sample, followed by UV exposure of the non-covered

parts. The whole sample is baked at 120 C. This promotes the cross-linking of the polymer

resin in the exposed areas of the photoresist. These parts are now insoluble in the developer

84


solution. Then, a flood exposure of UV light on the total sample induces a chemical reaction

which causes the previously unexposed areas to be soluble in the developer. Finally, the sample

is held in this developer solution, only leaving the firstly exposed areas with a photoresist layer

(which is the inverse of the planned deposition areas).

In the case of our depositions, the mask covers parts of the silicon waveguide, as you can see in

fig. 7.8(a) in the case of the lithography for the quantum dot and alumina layer.

(a) Structure of the lithography mask for the quan-

tum dot and alumina layer on the silicon wave-

guides.

(b) Structure of the lithography masks for the SiO2,

ITO and gold individually. Pink (): SiO2, blue

(): ITO, orange (): gold.

Figure 7.8: Different lithography masks for integrated stack.

The length of the quantum dot layer is varied in a range from 10 to 100 µm. Its purpose is

to check the influence on the losses caused by the modulation of the ideal waveguide structure

by the additional layers. After this first lithography step, the quantum dot layer is spincoated

and the alumina layer deposited by ALD. To complete the first deposition, the photoresist and

layers on top are removed by stirring the samples in acetone.

The next step includes the lithography part for the deposition of SiO2 (by electron plasma-

enhanced chemical vapor deposition) in between the waveguides to avoid a short circuit, since

the silicon itself is used as bottom contact.

Similar lithography steps are performed for the ITO and the gold contacts. The individual

masks for the different layers are depicted on the same schematic in fig. 7.8(b).

7.2.4 Loss measurements

A first step towards light coupling from the quantum dots in the waveguide is the analysis of the

effect of the quantum dot layer on the loss of the waveguide. Therefore, light from a tunable laser

or a superluminescent diode (broadband emission around 1550 nm) is coupled in the waveguide

by the grating coupler and coupled out at the other side. Because of the different length of the

quantum dots, the losses will be different. The output power has been measured separately by

85


both the optical spectrum analyzer (Advantest) and its built-in power meter. The loss α per

unit length (which can of course be position-dependent) is related to the power by:

PoutPin

=

∫exp(−α(x)x)dx (7.6)

with L the length of the waveguide. We are however only interested in the loss caused by the

quantum dot layer. Therefore, we subtract the total loss from the reference stack without the

quantum dot layer from the total loss of the stack with the quantum dot layer:

αQDLQD =

[− log

(PoutPin

)]with

−[− log

(PoutPin

)]without

(7.7)

where we assume that the loss per unit length of quantum dots αQD is independent of the

deposition length on the silicon waveguide. It is of course dependent on the confinement of

the propagating mode, which is influenced by the layers above. The formula hereabove can be

simplified if the power P is expressed in dBm, yielding:

αQDLQD[dB] = − ln 10

10(Pout,with[dBm]− Pout,without[dBm]) (7.8)

First, the design with the ITO layer is analyzed. The quantum dots are emitting in the infrared

with an absorption peak at 1540 nm. The waveguides are designed to allow propagation of

modes with a wavelength ranging from 1460 to 1565 nm. When a zero bias voltage is applied,

the loss difference between the waveguides with and without quantum dots is shown in fig. 7.9.

30

20

10

0

-10

-20

Pow

er d

iffer

ence

[dB

m]

1650160015501500145014001350λ [nm]

8 µm 58 µm 18 µm 68 µm 28 µm 78 µm 38 µm 88 µm 48 µm 98 µm

Figure 7.9: Power loss difference between the waveguides with and without quantum dots (first design),

measured by the OSA.

Except for the interaction length of 38 µm (which is ignored due to an artifact during processing),

there is a clear increase in loss for increasing interaction length. This is confirmed if we focus

on the absorbance peak of the quantum dots (1540 nm). The output power difference for all

86


-15

-10

-5

0

Pow

er d

iffer

ence

[dB

m]

100806040200Interaction length [µm]

OSA data Power data

Figure 7.10: Power output difference, measured by the OSA and the power meter, for varying interac-

tion length and for all waveguide sets (first design) at a wavelength of 1540 nm.

different waveguide sets, measured by the OSA and the power meter, is depicted in fig. 7.10 in

function of the interaction length.

If all power meter data are averaged and a linear increase of the loss is assumed (which is a

crude approximation), the mean output difference is 2008 dBm/cm. This corresponds with a

loss parameter αQD of 462 dB/cm.

Passive measurements have also been performed on the second design, where the top gold layer is

of course crucial, since it is a lot more lossy than ITO. Furthermore, it changes the confinement

of the propagating TE mode. In general, the absolute loss is about 50 dBm higher than the first

design. The power output difference of the second design between the samples with and and

without the quantum dot layer is plotted in fig. 7.11 for the quantum dot absorbance peak of

1518.6 nm (and for only one set of waveguides).

-10

-5

0

5

10

Pow

er d

iffer

ence

[dB

m]


Figure 7.11: Power output difference between the waveguides with and without quantum dots (second

design), measured by the power meter at a wavelength of 1518.6 nm.

87


If the data point below the horizontal axis is ignored, an average value of 4.57 dBm is obtained

for the power output difference, yielding higher losses for the structures without quantum dots

than with quantum dots. Furthermore, no correlation with the interaction length is visible. The

reason for this odd behavior must be searched in the properties of the deposited gold layer and

its influence on the propagating mode. Further simulations, measurements and visualization of

the cross section are certainly necessary.

To find an explanation for the two different trends, samples with only the QD-Al2O3 and the

Al2O3 layer have been measured. The alumina layer seems not to cause any losses, which was

expected due to its wide band gap. The power output difference is shown in fig. 7.12 for one

set of waveguides.

-4

-3

-2

-1

0

Pow

er d

iffer

ence

[dB

m]


Power meter data Linear fit power meter data

Figure 7.12: Power output difference between the waveguides with and without quantum dots and only

the alumina layer, measured by the power meter at a wavelength of 1518.6 nm.

We see similar results as in the ITO case, although the average loss αQD is here only 55 dB/cm.

This huge decrease should be explained by the further investigation of the mode confinement in

simulations. When an AC voltage up to 30 V peak-to-peak is applied to the stacks with ITO,

no change in loss is noticed. Because of these two reasons and the unexpected behavior when

gold is used as top contact, a cross section SEM should be made to confirm the exact designs,

before proceeding to more advanced usages of the stacks.

88

Chapter 8

Conclusion

This master thesis contains results of two LED stacks with a different operating mechanism.

Here, both of them are reviewed, followed by a short summary about their integration in metal-

lic and silicon structures and concluded with future prospects.

First, the DC stacks based on direct charge injection have been investigated. The main concern

regarding this stack was the deposition of the different transport layers. Therefore, the charac-

teristics of each of them are discussed. Next to the organic TPD for the hole transport layer,

the inorganic options nickel oxide and copper oxide were investigated. The p-type properties

of the latter were however not confirmed on the XRD spectra of the DC magnetron sputtered

thin films. Nevertheless, the nickel oxide thin films showed an acceptable resistivity for hole

transport operation between 0.07 and 0.8 Ωcm dependent on the partial pressure of oxygen in

the chamber during deposition. Careful attention should be paid to the measured values, since

changes in thickness and the difference between vertical and horizontal carrier transport might

cause a severe deviation of the real value. A last important property of the nickel oxide layer is

its transmission spectrum, which shows a clear increase with increasing wavelength.

For the electron transport layer, different ZnO-based materials have been used. ZnO nanocrys-

tals, which are stabilized by charge, show n-type properties according to literature. This route

has however been discarded after preliminary unsatisfying current-voltage results. Zinc oxide de-

position by ALD results in a high quality layer, but does not leave much room for optimization,

in contrary to the deposition of a ZnO:Al layer by rotatable DC magnetron sputtering. Different

trends for changing parameters can be denoted for the resistivity, the number of free carriers

and the mobility. The most remarkable are the strong exponential increase of the resistivity for

increasing target-substrate distance and oxygen pressure, while a decrease in the resistivity is

measured for increasing layer thickness. The trends in the mobility and the free carrier density

are opposite, although not as distinct. The influence of the annealing of the oxygen series is

also worth noticing, since this results in a constant resistivity, independent of the initial oxygen

89

Chapter 8. Conclusion

pressure used. Based on these series, optimal parameters have been chosen to obtain a resistivity

between 0.1 and 1 Ωcm.

Measurements of the photoluminescence spectra of the stacks, transmission spectra, excitation

measurements and photon lifetime calculations revealed two factors for the decrease in inten-

sity. The addition of a ZnO:Al layer causes on the one hand a decrease in transmission, since

the incident beam and the outgoing beam both need to penetrate through the top layer. On

the other hand, this layer opens new channels of non-radiative recombination of the emitted

photons, resulting in a strong decrease of their lifetime. Current-voltage measurements on the

total stacks were quite similar to literature results, but their irreproducibility, probably due to

the accumulation of charges at interfaces and electrodes, are an indication why no electrolumi-

nescence has been measured for the DC stacks. Except for some trapping levels in the ZnO:Al

layer, admittance measurements did not yield more insight.

The AC stacks are based on field-driven ionization of the quantum dots. Here, the choice of the

insulating oxides was quite easy, since alumina layers can be deposited both by electron beam

evaporation and by atomic layer deposition. After the first experiments, it was immediately

clear that the quality of the evaporated layers was inferior, since it was impossible to measure

the capacitance, which implied a short circuit. Therefore, only ALD alumina layers have been

characterized by performing capacitance-frequency measurements. By modeling the layers as a

parallel-plate capacitor, deduction of the relative permittivity of 7.5 has been done, which is in

good agreement with literature. Furthermore, the complete AC stacks have been modeled as an

electrical circuit of capacitances and resistors, which yielded that the voltage drop per quantum

dot had to be at least the band gap energy to show electroluminescence, which is in good agree-

ment with the theoretical operation mechanism. The photoluminescence is not conserved for

core-only dots after deposition of an alumina layer, while for quantum rods and core-shell dots, it

is. Electroluminescence has been shown for all PL-conserving nanocrystals, ranging from visible

CdSe/ZnS giant dots to CdSe/CdS rods and PbS/CdS infrared dots. The spectra agreed very

well with the photoluminescence spectra. Based on the power measurement in function of the

applied voltage, the underlying operation mechanism has been confirmed. Maximum powers up

to 2 µW have been measured, corresponding with a power density of 57 µW/cm2. Calculation

of the power efficiency yielded a value of 0.02 % or 0.14 lm/W. Further measurements showed

a linear increase of the power output in function of the frequency. Based on the operation

mechanism and the intensity measurements during one period, a rate equations model has been

developed which confirms the theoretical approach. It also explains the linear power-frequency

plot and its flattening, caused by the incomplete internal field build-up at high frequencies.

Next to the bulk devices, the integration of the quantum dots in a plasmonic structure as

90

Chapter 8. Conclusion

well as the AC stack on silicon have been investigated. The spontaneous emission enhancement

factor has been calculated for a quantum dot emitting as a dipole in a metal-dielectric-metal

structure for a wide wavelength range. It showed a sharp peak for the resonant contribution of

the gap SPP mode. The peak height increased for decreasing thickness of the dielectric layer

and for a change of the location of the dipole from the center towards one of the interfaces.

For the integration of the AC stack on silicon, the bottom alumina layer has been removed.

The modulation of the quantum dots on the silicon waveguide has been investigated by per-

forming loss measurements. Indeed, for the discussed structure with ITO, increasing the area

of deposited quantum dots increased the losses with an average value of 2008 dB/cm. However,

increasing the applied AC voltage did not yield any change in absorption.

Now that a full characterization is performed on the rotatable DC magnetron sputtered ZnO:Al,

it can serve multiple goals. In first instance, the search towards optimal layers in the DC LED

stack should be continued, where a more profound understanding of the lack of electrolumines-

cence results is needed by analyzing the precise band alignment of the layers, the charging effect

and the quantization of the carrier densities in the quantum dot layer. Furthermore, ZnO:Al

can be used as an electroabsorption modulating layer on top of silicon waveguides [109].

The successful electroluminescence of the AC stacks should further be investigated for differ-

ent oxides with higher permittivities to achieve light emission at lower voltages. To be able to

efficiently model the stack, extensive characterization of the oxide layer growth on the quan-

tum dots should be performed. Modeling the operation of the devices by applying different

waveforms and varying number of layers might yield higher power efficiencies. Next, integration

of the AC stack on a plasmonic structure should definitely be experimentally investigated and

checked if emitting photons can couple to the plasmon mode. The modulation of the light by

the AC stacks on the silicon waveguide should surely be continued by performing measurements

and cross section confirmation checks when different top contacts are used. Here, coupling of

the light from the LEDs into the waveguide should be investigated without any doubt.

91

Appendix A

Python code

A.1 Single sweep for current-voltage characteristic

from pymeasure.units.quantity import *

from pymeasure.units.unit import *

from intec.pymeasure.instruments import *

from numpy import *

from matplotlib.pyplot import *

import time

import os.path

import pylab

# Define source

source = KEITHLEY2400A()

# Initialization

source.initialize()

## Use as voltage source and current measurement

# Set machine mode

source.mode = 'Voltage'

# Set the voltage range you are interested in

source.set source range(VoltageQuantity(value = 15, unit = VOLT))

# Set the current limit protection

source.set current limit(CurrentQuantity(value = 500, unit = MILLIAMPERE))

# Set the measurement range, if you do not give a CurrentQuantity, AUTO will be on

source.set sense range()

print ("Source ON")

# Switch on

source.switch on()

print ("Start Sweep")

# Sweep parameters

vstart = 0

vstop = 5

92

Appendix A. Python code

vstep = 0.1

voltage range = arange(vstart,vstop,vstep)

# Output and plot files

samplestr = raw input("Sample number: ")

meas = raw input("Measurement number: ")

direction = raw input("Direction (F/R): ")

if os.path.exists(samplestr+' '+meas+' '+direction+'.txt'):

print 'Measurement already exists'

source.switch off()

quit()

outfile = open(samplestr+' '+meas+' '+direction+'.txt','w')

I = zeros(len(voltage range))

# Sweep voltage and collect current in txt file

for k in range(0,len(voltage range)):

V = voltage range[k]

source.voltage = VoltageQuantity(value = V, unit = VOLT)

I[k] = source.current.value

time.sleep(0.5)

print >> outfile, V, I[k]

outfile.flush()

# Plot data

plot(voltage range,I)

pylab.savefig(samplestr+' '+meas+' '+direction+'.png')

outfile.close()

# Switch off

source.switch off()

print ("Sweep done")

show()

print ("Done")

A.2 Hysteresis characteristic

from pymeasure.units.quantity import *

from pymeasure.units.unit import *

from intec.pymeasure.instruments import *

from numpy import *

from matplotlib.pyplot import *

import time

import os.path

import pylab

#Define source

source = KEITHLEY2400A()

#Initialization

93


source.initialize()

## Use as voltage source and current measurement

# Set machine mode

source.mode = 'Voltage'

# Set the voltage range you are interested in

source.set source range(VoltageQuantity(value = 15, unit = VOLT))

# Set the current limit protection

source.set current limit(CurrentQuantity(value = 500, unit = MILLIAMPERE))

# Set the measurement range, if you do not give a CurrentQuantity, AUTO will be on

source.set sense range()

print ("Source ON")

# Switch on

source.switch on()

print ("Start Sweep")

# Sweep paramaters

vstart = −5vstop = 5

vstep = 0.1

voltage range up = arange(vstart,vstop,vstep)

voltage range down = arange(vstop,vstart,−vstep)num sweeps = 10

voltage range = arange(num sweeps*(len(voltage range up)+len(voltage range down)))

# Output and plot files

samplestr = raw input("Sample number: ")

meas = raw input("Measurement number: ")

direction = raw input("Direction (F/R): ")

if os.path.exists(samplestr+' '+meas+' '+direction+'.txt'):

print 'Measurement already exists'

source.switch off()

quit()

outfile = open(samplestr+' '+meas+' '+direction+'.txt','w')

I = zeros(num sweeps*(len(voltage range up)+len(voltage range down)))

# Sweep voltage and collect current in txt file

for m in range(0,num sweeps):

for k in range(0,len(voltage range up)):

V = voltage range up[k]


voltage range[m*(len(voltage range up) + ...

len(voltage range down))+k] = V

I[m*(len(voltage range up) + len(voltage range down)) + k] = ...

source.current.value

time.sleep(0.5)

print >> outfile, V, I[m*(len(voltage range up) + ...

len(voltage range down))+k]

outfile.flush()

94


source.voltage = VoltageQuantity(value = 0, unit = VOLT)

time.sleep(0.0)

for l in range(0,len(voltage range down)):

V = voltage range down[l]


voltage range[m*(len(voltage range up) + ...

len(voltage range down)) + len(voltage range up)+l] = V

I[m*(len(voltage range up) + len(voltage range down)) + ...

len(voltage range up) + l] = source.current.value

time.sleep(0.5)

print >> outfile, V, I[m*(len(voltage range up) + ...

len(voltage range down)) + len(voltage range up)+l]

outfile.flush()

source.voltage = VoltageQuantity(value = 0, unit = VOLT)

time.sleep(3.0)

# Plot data

plot(voltage range,I)

pylab.savefig(samplestr+' '+meas+' '+direction+'.png')

outfile.close()

# Switch off

source.switch off()

print ("Sweep done")

show()

print ("Done")

95

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