Łukasz Wachnicki Strukturalna, optyczna i elektryczna charakteryzacja warstw monokrystalicznych oraz nanostruktur tlenku cynku otrzymywanych metodą osadzania warstw atomowych Rozprawa doktorska wykonana w Oddziale Fizyki i Technologii Nanostruktur Półprzewodnikowych Szerokoprzerwowych ON-4 Promotor: prof. nadzw. dr hab. Elżbieta Guziewicz Warszawa, Instytut Fizyki PAN 2014
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1
Łukasz Wachnicki
Strukturalna, optyczna i elektryczna
charakteryzacja warstw
monokrystalicznych oraz nanostruktur
tlenku cynku otrzymywanych metodą
osadzania warstw atomowych
Rozprawa doktorska wykonana w Oddziale Fizyki i Technologii Nanostruktur
Półprzewodnikowych Szerokoprzerwowych ON-4
Promotor: prof. nadzw. dr hab. Elżbieta Guziewicz
Warszawa, Instytut Fizyki PAN 2014
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Podziękowania Chciałbym podziękować mojej promotor prof. nadzw. dr hab. Elżbiecie Guziewicz za opiekę
nad przebiegiem doktoratu i przygotowaniem rozprawy doktorskiej.
Dziękuję również wszystkim pracownikom zespołu ON-4.2 za wspólną pracę i okazaną pomoc
w przygotowywaniu niniejszej rozprawy.
W szczególności chciałbym podziękować prof. dr. hab. Markowi Godlewskiemu za
umożliwienie pracy w jego zespole, za okazaną życzliwość i nieocenioną pomoc merytoryczną.
Dziękuję również mgr Sylwii Gierałtowskiej za życzliwą atmosferę, nieocenioną pomoc
w zmaganiach w laboratorium ALD, za wykonane pomiary AFM oraz pomiary charakterystyk
napięciowo-prądowych.
Dziękuję dr. Bartłomiejowi Witkowskiemu za wszelką pomoc oraz za wykonane pomiary SEM
i CL wykorzystane w niniejszej pracy.
Dziękuję dr Aleksandrze Wierzbickiej, dr. Jarosławowi Domagale i prof. dr. hab. Wojciechowi
Paszkowiczowi za pomoc w pomiarach dyfrakcji rentgenowskiej.
Dziękuję dr Ewie Przeździeckiej i mgr Annie Dużyńskiej za pomiary fotoluminescencji
niskotemperaturowej oraz dr Piotrowi Dłużewskiemu za pomiary TEM.
Szczególne podziękowania składam dr Elżbiecie Janik i całemu zespołowi SL3.1 za miłą
i owocną współpracę dotyczącą struktur „core-shell”.
Dziękuję również mgr. Krzysztofowi Kopalko za ogromną pomoc i wsparcie w stawianiu
pierwszych kroków w laboratorium ALD.
Bardzo dziękuję mojej żonie Agnieszce, synkowi Szymonowi i córeczce Kindze za wsparcie
w trudnych chwilach oraz mobilizację do napisania tej pracy.
Szczególne, pragnę podziękować moim kochanym Rodzicom, których miłość, wsparcie
i nieustająca wiara we mnie zawsze pomagały mi przezwyciężać wszelkie trudności i dodawały
sił, niezbędnych do realizacji życiowych planów.
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Realizacja rozprawy doktorskiej była współfinansowana przez Unię Europejską
w ramach Europejskiego Funduszu Rozwoju Regionalnego, w ramach dotacji
Innowacyjna Gospodarka (POIG.01.01.02-00-008/08).
Realizacja rozprawy doktorskiej była również współfinansowana przez program
„Potencjał naukowy wsparciem dla gospodarki Mazowsza - stypendia dla doktorantów”
- projekt systemowy Samorządu Województwa Mazowieckiego, realizowany w ramach
DOI: 10.1016/j.mee.2008.07.016 Published: DEC 2008
48. Hybrid Organic/ZnO p-n Junctions with n-Type ZnO Grown by Atomic
Layer Deposition
Luka, G.; Krajewski, T.; Wachnicki, L.; et al.
ACTA PHYSICA POLONICA a Volume: 114 Issue: 5 Pages: 1229-1234
Published: NOV 2008
49. Characterization of ZnO Films Grown at Low Temperature
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Przezdziecka, E.; Krajewski, T.; Wachnicki, L.; et al.
ACTA PHYSICA POLONICA a Volume: 114 Issue: 5 Pages: 1303-1310
Published: NOV 2008
50. ZnCoO Films Obtained at Low Temperature by Atomic Layer Deposition
Using Organic Zinc and Cobalt Precursors
Lukasiewicz, M.; Wojcik-Glodowska, A.; Godlewski, M.; et al.
ACTA PHYSICA POLONICA a Volume: 114 Issue: 5 Pages: 1235-1240
Published: NOV 2008
100
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Monocrystalline zinc oxide films grown by atomic layer deposition
Ł. Wachnicki a,⁎, T. Krajewski a, G. Łuka a, B. Witkowski c, B. Kowalski a, K. Kopalko a, J.Z. Domagala a,M. Guziewicz b, M. Godlewski a,c, E. Guziewicz a
a Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Polandb Institute of Electron Technology (ITE), al. Lotników 32/46, Warsaw 02-668, Polandc Cardinal Stefan Wyszynski University, College of Science, Department of Mathematics and Natural Sciences, Warszawa, Poland
In the present work we report on the monocrystalline growth of (00.1) ZnO films on GaN template by theAtomic Layer Deposition technique. The ZnO films were obtained at temperature of 300 °C using dietylzinc(DEZn) as a zinc precursor and deionized water as an oxygen precursor. High resolution X-ray diffractionanalysis proves that ZnO layers are monocrystalline with rocking curve FWHM of the 00.2 peak equals to0.07°. Low temperature photoluminescence shows a sharp and bright excitonic line with FWHM of 13 meV.
Zinc oxide (ZnO) is extensively studied as a prospective materialfor electronic and optoelectronic devices. This material is a II–IVsemiconductor with a direct energy gap of 3.37 eV at roomtemperature [1,2]. This large energy gap makes ZnO a transparentmaterial, which predestinates it for application as a transparentelectrode in solar cells [3]. A high exciton binding energy (60 meV)strongly influences the optical properties of ZnO and makes itattractive material for optoelectronic applications.
In our recent works we demonstrated good crystallographic andelectrical parameters of ZnO films grown by the Atomic LayerDeposition (ALD) method [4,5] and their advantageous propertiesfor application in novel electronic devices like a new generation ofmemories [6–8]. Those ZnO films were polycrystalline when grown attemperature between 60 °C and 240 °C using diethylzinc as an organiczinc precursor. In this communication we report on the firstmonocrystalline growth of (00.1) ZnO films on GaN templates byatomic layer deposition using diethylzinc as a zinc precursor.
A monocrystalline growth of semiconductor materials is of greatinterest in semiconductor technology, since monocrystalline filmsusually have higher mobility of carriers, which positively influenceselectrical characteristics of the devices. This is related to lowerelectron scattering at grain boundaries [9], which usually areabundant in polycrystalline and amorphous films. Moreover, weexpect a lower number of defects and more efficient light emissionfrom epitaxial layers than in case of polycrystalline and amorphousones. Therefore monocrystalline growth is desired for most of
application in electronics and optoelectronics. It is important thatnew application in memory devices require low thermal budged ofthe growth with a growth temperature limited to 300 °C.
There are fewmethods of obtaining epitaxial semiconductor layerslike molecular beam epitaxy (MBE), pulsed laser deposition (PLD),metal organic chemical vapor deposition (MOCVD) or hydrothermalgrowth. One of them is also ALD, which was originally invented bySuntola [10] for an epitaxial growth. In the 80's this method was evennamed Atomic Layer Epitaxy (ALE) and several papers reportedmonocrystalline growth of II–VI compounds using ALE [11–13]. Lastyear this technique is rather being used for deposition of polycrys-talline or amorphous layers, therefore now it is more generally calledAtomic Layer Deposition (ALD). Recently we observed a boominginterest in ALD, since in 2007 it was applied by Intel for deposition ofhigh-k oxides in 45 nm generation processors [14].
In our previous work we reported the first monocrystalline ZnOlayers obtained by ALE using ZnCl2 as an inorganic zinc precursor [13].This epitaxial ZnO growth was achieved at 480 °C and FWHM of the00.2 peak was 0.18°. However, a relatively high growth temperatureas well as a very low growth rate (0.5 Å per cycle) is not acceptable fornovel electronic applications, which require a low temperaturegrowth. In this paper we describe an epitaxial growth of zinc oxidethat was performed at 300 °C with diethylzinc, which nowadays iscommonly used for both the ALD and theMOCVD growth of zinc oxidefilms for electronic applications.
2. Experimental
The main advantage of the ALD method is a self limitation andsequential growth process, which enables using very reactiveprecursors and reducing growth temperature, while keeping goodcrystallographic and optical parameters. Such a precursor is a
diethylzinc (Zn(C2H6)2, DEZn), which we used in the presented work.The zinc oxide film at the surface was created as a product of thedouble-exchange chemical reaction of DEZn with water:
C2H5–Zn–C2H5þH2O→ZnOþ2C2H6
The epitaxial ZnO films were formed on a GaN/Al2O3 template inthe Savannah-100 reactor from Cambridge Nanotech. Before thegrowth templates were etched in a solution of hydrochloric acid anddigested in solvents. ZnO films were deposited at temperature of300 °C which is relatively low when compared to temperature usedfor a ZnO growth in other deposition methods like CVD, MOCVD orMBE (for review see [15]).
The ALD process consists of repeating of four deposition steps:deposition of the first precursor; purging the reaction chamber withan inert gas, when non-reacted precursors’ and by-products'molecules are removed; deposition of the second precursor; purgingof the reaction chamber. In our experiments we varied pulse time (e.g.a dose) of an oxygen precursor and the purging time after water andobserved changes in crystallographic and optical properties in orderto find out conditions for an epitaxial growth. The structure and thecrystallographic orientation of ZnO layers were measured by X-raydiffraction using the X'Pert MPD diffractometer in a full angular range.Quality of the layers were investigated by a high resolution X'PertMRD diffractometer equipped with the X-ray mirror, a four-bouncemonochromator at the incident beam and a three bounce analyzer atthe diffracted beam. The surface morphology and films cross-sectionimages were obtained by scanning electron microscopy (SEM, HitachiSU-70) with the operation voltage of 15 kV. Room temperaturephotoluminescence (RT PL) studies were performed with a xenonlamp and the CM 2203 spectrometer. LT PL measurements wereperformed with He-Cd 325 nm laser line and CCD camera. Theelectrical properties of ZnO samples were obtained from the Halleffect measurement at room temperature in the van der Pauwconfiguration using the RH2035 PhysTech GmbH system equippedwith a permanent magnet giving a field of 0.426 T.
3. Results and discussions
Fig. 1 shows a typical XRD graph of the monocrystalline ZnO filmdeposited at 300 °C on gallium nitride template. The presence ofexclusively the 00.2 peak originating from the wurzite-type ZnO phaseaccompanied by those from monocrystalline GaN and Al2O3 (a veryweak peak at 41.5) shows that ZnO film is 00.1 oriented. No trace of areflection caused by another layer orientation or contribution of a
foreign phase was found. High-resolution X-ray diffraction (HRXRD)confirmed that obtained ZnO layers were monocrystalline with a fullwidth at half maximum (FWHM) rocking curve of 00.2 reflectionmeasured with analyzer equals to 0.07 (Fig. 2). This value of rockingcurve is identical as measured for gallium nitride template, whichmeans that the ZnO film quality is limited by imperfections from thesubstrate GaN layer. The perpendicular lattice constant c was deter-mined fromHRXRDmeasurements of the symmetrical 00.2 reflection as(5.2022±0.0001) Å,while the parallel constant awas determined fromasymmetrical reflection −1–1.4 as (3.2536±0.0005) Å (Fig. 3). Theseresults correlate well with the lattice constants in bulk ZnO (at roomtemperature) which are 5.2069 Å and 3.2495 Å [1], respectively. Fromthe lattice parameter results we can conclude that obtained layers aretensile strained which is due to the lattice mismatch. It is interesting tostress that structural quality of samples were comparable for ZnO filmsgrown with the same pulsing time of both precursors and the samethickness. Purging time does not influences the structural parameters ofmonocrystalline films, though it influences strongly optical andelectrical parameters as will be shown below.
Deposited films have a columnar microstructure as it is shown inFig. 4. The two-dimension cross-section SEM image presented in theinset shows that this type of surface morphology derives from acolumnar growth, which is typical for ZnO and other wurzitematerials.
The optical properties of the ZnO filmswere characterized by roomtemperature (RT) and low temperature photoluminescence (LT PL)which was measured with excitation energy above the ZnO energygap. Fig. 5 shows RT PL spectra measured for ZnO layers grown withdifferent purging time after an oxygen precursor. The spectra weretaken for “as-grown” filmswithout any post-growth annealing. Strongluminescence in the band-edge region is observed even roomtemperature. Its intensity strongly depends on the ALD growthparameters as pulsing and purging time of an oxygen precursor.Presented results were taken for samples grown with the same andshort pulsing time of water (15 ms), while purging time varied from8 s to 20 s. The most intensive band edge emission we observed forthe shortest purging time. However, in this case a defect-relatedemission observed in the green region (about 2.4 eV) was also veryintensive and exceeds that excitonic one. The highest ratio of excitonicto defect-related PL is observed for ZnO layer grownwith 20 s purgingafter water. Moreover, for this layer the energy of band-edge PL isblue-shifted, which indicates a better optical quality.
Fig. 5 shows LT PL spectra obtained for ZnO layers grown with 8 sand 14 s purging time after water. The defect-related photolumines-cence in both spectra is very weak, while band-edge PL is sharp with
Fig. 1. The XRD data of the ZnO film grown on the GaN substrate at temperature of300 °C.
Fig. 2. 00.2 reflection, 2θ/ω scan, inset: the rocking curve of ZnO/GaN collected withanalyzer, FWHM=250″ for both curves.
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FWHM of 13 meV as shown in the inset. The high energy peak islocated at 3.360 eV and corresponds to a neutral donor bound excitonrecombination [16] (Fig. 6). The next LTPL peak located at 3.33 eV
might be related to deep acceptor or free-to-band transition [17,18].In Fig. 7 we present temperature dependence of the edge emission.The logarithmic scale enables observing bound exciton, free excitonand phonon repetitions, which is a fingerprint of a high optical quality.
The electrical properties of ZnO films were examined by Hall effectmeasurements. The I–V characteristics of Au/Ti contacts on ZnO layerswere linear. For ZnO films grown with 8 s and 14 s purging time afteran oxygen precursor free carrier concentration was at the level of5*1018 cm−3, while carrier mobility for both samples was determinedas about 40 cm2/Vs. For ZnO layer obtained with 20 s purging timeafter water we observed enhanced carrier concentration(1.3*1019 cm−3) and lower mobility (31.5 cm2/Vs). This resultshows that electrical parameters of monocrystalline ZnO films areextremely sensitive for any change in growth conditions.
4. Conclusions
Monocrystalline zinc oxide thin films were grown on GaNsubstrate by Atomic Layer Deposition method at 300 °C. The X-raydiffraction showed that the films exhibit wurtzite symmetry and areoriented along the c-axis with 00.2 rocking curve equal to 0.07°. Thelow temperature PL shows sharp bright excitonic line with FWHM of13 meV. Hall measurements showed n-type behavior with a carrierconcentration of 1.3*1019 cm−3 and a carrier mobility of 31.5 cm2/Vs.These results confirm that ALD method is very prospective fordeposition of ZnO films with very good electronic and optoelectronic
Fig. 3. −1–14 reflection, Reciprocal Space Map collected with antiscattered slit1/8°before detector.
Fig. 4. Surface morphology and the cross-sectional SEM image of the ZnO (500 nmthick) on GaN (3 μm thick) thin film grown at temperature 300 °C.
Fig. 5. Room temperature PL spectra of ZnO films with different purging time after anoxygen precursor.
Fig. 6. Low temperature PL spectra of ZnO films with different purging time after anoxygen precursor; inset: the band-edge range PL region.
Fig. 7. Low temperature PL spectra of ZnO/GaN-ALD film in the logarithmic scaleshowing three phonon repetitions.
4558 Ł Wachnicki et al. / Thin Solid Films 518 (2010) 4556–4559
properties applicable in various devices. This method is moreprospective than any other one, as it is a self-limiting process thatcan be performed at relatively low temperature. We can use thismethod to obtain cross-bar memory devices where the requirementof low thermal budget of the growth must be fulfilled.
Acknowledgments
The research was partially supported by: European Union withinEuropean Regional Development Fund, through grant InnovativeEconomy (POIG.01.01.02-00-008/08) and Polish Ministry of Scienceand Higher Education — project no. 0663/B/T02/2008/35.
References
[1] C. Klingshirn, Phys. Status Solidi, B 244 (9) (2007) 3027.[2] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct. 34
(2003).[3] M. Godlewski, E. Guziewicz, G. Łuka, T. Krajewski, M. Łukasiewicz, Ł. Wachnicki,, A.
Wachnicka, K. Kopalko, A. Sarem, B. Dalami, Thin Solid Films 518 (2009) 1145.[4] I.A. Kowalik, E. Guziewicz, K. Kopalko, S. Yatsunenko, A. Wójcik-Głodowska, M.
Godlewski, P. Dłużewski, E. Łusakowska, W. Paszkowicz, J. Cryst. Growth 311(2009) 1096.
[5] E. Guziewicz, I.A. Kowalik, M. Godlewski, K. Kopalko, V. Osinniy, A. Wójcik, S.Yatsunenko, E. Łusakowska, W. Paszkowicz, M. Guziewicz, J. Appl. Phys. 103(2008) 033515.
[6] M. Godlewski, E. Guziewicz, J. Szade, A. Wójcik-Głodowska, Ł. Wachnicki, T.Krajewski, K. Kopalko, R. Jakieła, S. Yatsunenko, E. Przezdziecka, P. Kruszewski, N.Huby, G. Tallarida, S. Ferrari, Microelectron. Eng. 85 (2008) 2434.
[7] T. Krajewski, E. Guziewicz, M. Godlewski, Ł. Wachnicki, I.A. Kowalik, A. Wojcik-Glodowska, M. Łukasiewicz, K. Kopalko, V. Osinniy, M. Guziewicz, Microelectron. J.40 (2009) 293.
[8] E. Guziewicz, M. Godlewski, T. Krajewski, Ł. Wachnicki, A. Szczepanik, K. Kopalko,A. Wójcik-Głodowska, E. Przeździecka, W. Paszkowicz, E. Łusakowska, P.Kruszewski, N. Huby, G. Tallarida, S. Ferrari, J. Appl. Phys. 105 (2009) 1.
[9] M. Godlewski, E.M. Goldys, M. Philips, T. Böttcher, S. Figge, et al., Acta Phys. Pol., A102 (2002) 627.
[10] T. Suntola, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth. Part 3b—GrowthMechanisms and Dynamics, Elsevier, Amsterdam, 1994, p. 605.
[11] A. Szczerbakow, E. Dynowska, M. Godlewski, K. Swiatek, J. Cryst. Growth 183(1998) 708.
[12] A. Szczerbakow, E. Dynowska, K. Swiatek, M. Godlewski, J. Cryst. Growth 207(1999) 148.
[13] K. Kopalko, M. Godlewski, J.Z. Domagala, E. Lusakowska, R. Minikayev, W.Paszkowicz, A. Szczerbakow, Chem. Mater. 16 (2004) 1447.
[14] C. Zhao, T.Witters, P. Breimer, J. Maes, M. Caymax, S. De Gendt, Microelectron. Eng.84 (2007) 7.
[15] R. Triboulet, J. Perrière, Prog. Cryst. Growth Charact. Mater. 47 (2003) 65.[16] B.K. Meyer, J. Sann, D.M. Hofmann, C. Neumann, A. Zeuner, Semicond. Sci. Technol.
Lett. 81 (2002) 1830.[18] E. Przezdziecka, T. Krajewski, L. Wachnicki, A. Szczepanik, A.Wójcik-Głodowska, S.
Yatsunenko, E. Lusakowska, W. Paszkowicz, E. Guziewicz, M. Godlewski, ActaPhys. Pol., A 114 (2008) 1303.
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Comparison of dimethylzinc anddiethylzinc as precursors for monocrystallinezinc oxide grown by atomic layer deposition method
L. Wachnicki*,1, M. Lukasiewicz1, B. Witkowski1, T. Krajewski1, G. Luka1, K. Kopalko1, R. Minikayev1,E. Przezdziecka1, J. Z. Domagala1, M. Godlewski1,2, and E. Guziewicz1
1 Polish Academy of Sciences, Institute of Physics, al. Lotnikow 32/46, Warszawa 02-668, Poland2 Department of Mathematics and Natural Sciences, College of Science, Cardinal Stefan Wyszynski University, Warszawa, Poland
Received 21 September 2009, revised 21 September 2009, accepted 2 February 2010
The new trend in semiconductor technology is search of new
materials, which could replace silicon in specialized applica-
tions. One of them is zinc oxide (ZnO), a II–VI semiconductor.
In this work we show how to obtain at relatively low
temperature monocrystalline layers of ZnO using atomic layer
deposition (ALD) method with reactive dimethylzinc (DMZn)
and diethylzinc (DEZn) precursors. ZnO films were deposited
on gallium nitride substrates at 300 8C. We compare the results
obtained for these two organic zinc precursors. High resolution
X-ray diffraction analysis proves that our ZnO layers are
monocrystalline with FWHM of (00.2) peak 250 arcsec for
DEZn and 335 arcsec for DMZn precursors.
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Zinc oxide (ZnO) is an attractivematerial for several applications, because of its advan-tageous optical, electrical, and piezoelectric properties. Forexample, epitaxial films of ZnO are investigated foroptoelectronic applications, for short wavelength emitters,which is due to their wide band gap of 3.37 eV at roomtemperature and high exciton binding energy (60 meV) [1,2]. To compete with GaN one should solve limitations –quality of the grown material should be improved and bettercontrol of its electrical properties should be realized. Thelatter relates to a known problem of unintentional high n-typedoping of ZnO. Strong n-type doping of ZnO may howeverbe attractive property of ZnO making it suitable forphotovoltaic [2]. We demonstrated recently that ZnO filmsgrown by ALD are suitable for such application.
The problem to be solved is quality of ZnO filmsobtained by the ALD. The ALD, discovered by Suntola in1980s [3], was originally used for the growth of polycrystal-line and amorphous thin films, grown at, respectively, lowtemperature (LT; 400–450 8C) [4–6]. This was advantageousfor some new applications, such as already mentionedphotovoltaic but not in a new area in the semiconductortechnology related to the change of the device architecturefrom planar to three-dimensional [3, 7–12]. The latter
application imposes critical restrictions on temperature ofthe semiconductor processing as in 3D structures metal pathsare placed not only at the top, but also below active parts ofthe device. Typical temperature limit here is 350 8C. Thistemperature is much lower than typically used in the silicontechnology, therefore a lot of efforts is focused now on newmaterials, which can be grown inside the above temperaturelimit [3]. These materials should show good electricalproperties, i.e., good structural quality.
In this paper, we report on properties of monocrystallineZnO grown on gallium nitride substrates with dimethylzinc(DMZn) and diethylzinc (DEZn) as zinc precursors anddeionized water as oxygen precursors, films grown attemperatures below mentioned above limit. The simplicityof ALD technology combined with relatively high quality ofobtained monocrystalline ZnO films makes it attractive forfabrication of modern semiconducting devices with 3Darchitecture.
2 Experimental details ZnO films were deposited ongallium nitride substrates by the ALD method. We used twodifferent and very reactive organic zinc precursors, i.e.,DMZn and DEZn. DEZn is composed of zinc bonded to twoethyl groups, whereas DMZn is composed of zinc bonded to
Phys. Status Solidi B 247, No. 7, 1699–1701 (2010) / DOI 10.1002/pssb.200983687 p s sbasic solid state physics
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two methyl groups. As an oxygen source, deionized waterwas used. Gallium nitride substrates (commercial GaN/sapphire epilayers) were solvent-cleaned first, followed byetching in HCl and water solution, rinsed with deionizedwater, and blow dried with N2 gas before loading. ZnO filmswere deposited at 300 8C as a product of the double-exchangechemical reaction of DMZn with water:
CH3 � Zn� CH3 þ H2O ! ZnOþ 2CH4
or chemical reaction of DEZn with water:
C2H5 � Zn� C2H5 þ H2O ! ZnOþ 2C2H6:
We used Savannah 100 reactor from the CambridgeNanoTech Company.
The structure and the crystallographic orientation ofZnO layers were measured with X-ray diffraction (XRD)using the X’Pert MPD diffractometer in a full angular range.Quality of the layers was investigated with a high resolution(HR XRD) X’Pert MRD diffractometer equipped with the X-ray mirror, a four-bounce monochromator at the incidentbeam and a three bounce analyzer at the diffracted beam. Thesurface morphology and films cross-section images wereobtained by scanning electron microscopy (SEM, HitachiSU-70) at the operation voltage of 15 kV. Room temperaturephotoluminescence (RT PL) studies were performed with theCM 2203 spectrofluorymeter. LT PL measurements wereperformed with a He–Cd 325 nm laser line and a CCDcamera. The RT electrical properties of ZnO samples wereobtained from the Hall effect measurements in the Van derPauw configuration, using the RH2035 PhysTech GmbHsystem equipped with a permanent magnet giving a field of0.426 T.
3 Result and discussion Figure 1 shows the XRDspectra of the films grown at 300 8C using DEZN and DMZn.The ZnO diffraction peak matches that of the bulk wurtziteZnO and has similar full width at half maximum (FWHM) as
the one from GaN substrate. ZnO films grown using DEZnand DMZn have a hexagonal wurtzite structure and show oneorientation, resulting in dominant (00.2) XRD peak.
High-resolution X-ray diffractometry in a double crystalconfiguration shows that ZnO films grown on GaN substrateare crystalline, with a FWHM of their associated rockingcurves of approximately 335 arcsec for DMZn and 250 arc-sec for DEZn. These values are close to the one we measuredfor GaN/sapphire substrate indicating similar concentrationsof screw dislocations in a substrate and ZnO layers.
Table 1 shows comparison of c and a ZnO latticeconstants, as determined from XRD investigations. Fromcomparison with bulk ZnO values we conclude that forDEZn, lattice constant a is more relaxed than for DMZn. Inturn, the lattice constant c for DMZn is very close to thevalues reported for the bulk ZnO crystals. Moreover, forDEZn we observed larger strains in c lattice constant. HRXRD measurements (Fig. 2) show that the rocking curve forthe asymmetrical reflection (-1-1.4) peak is larger than forthe GaN substrate and for DEZn is equal to 2015 arcsec(760 arcsec for GaN substrate) and for DMZn is equal to1680 arcsec (500 arcsec for the GaN substrate). Thisindicates worse in-plane orientation of ZnO layers andhigher concentration of edge dislocations, as compared toGaN substrates.
The PL of the ZnO films, studied at RT and LT, is shownin Figs. 3 and 4. Both RT and LT PL spectra are dominated bystrong emission of excitonic origin (‘‘edge’’ PL). At RT thisemission is due to free excitons and at LT donor boundexcitonic (D0X) dominates.
1700 L. Wachnicki et al.: Comparison of dimethylzinc and diethylzinc as precursors for monocrystalline ZnOp
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Figure 1 (online color at: www.pss-b.com) The XRD data of theZnO films grown by the ALD on GaN substrates at temperature of300 8C.
Table 1 Lattice constant for two different precursors.
Observation of sharp excitonic emissions is commonlytreated as indication of a good quality of the films. Thedefect-related photoluminescence in both spectra and forboth types of ZnO layers is very weak, while band edge PL atLT is sharp with FWHM of 6.1 meV for DEZn and 5.6 meVDMZn. The LT PL peak at 3.33 eV can be related either toacceptor bound excitonic transition or a free-to-boundtransition.
RT electrical properties of ZnO films were obtained fromHall measurements with the Van der Pauw configuration. ForZnO grown using DEZn free electron concentration wasequal to 1� 1019 cm�3 and carrier mobility was determinedas 32 cm2/Vs. For films grown with DMZn free electronconcentration was 5� 1018 cm�3 and carrier mobility was
equal to 40 cm2/Vs, indicating slightly better quality of filmsgrown with DMZn, as also concluded from HR XRD and LTPL investigations.
4 Conclusions Monocrystalline ZnO films weregrown on GaN substrate by the ALD method at 300 8Cusing two different organic zinc precursors: DEZn andDMZn. ZnO layers show structural quality close to that ofGaN substrates. In both types of layers we observed strongemission of excitonic origin with FWHM of excitonic peaksof about 6 meV, comparable to those reported for bulk ZnOsamples. This indicates high potential of the ALD method fordeposition at LT monocrystalline films and good optical andelectrical properties.
Acknowledgements The research was supported byEuropean Union within European Regional Development Fund,through grant Innovative Economy (POIG.01.01.02-00-008/08).
References
[1] C. Klingshirn, Phys. Status Solidi B 244(9), 3027–3073(2007).
[2] S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner,Superlattices Microstruct. 34, 3 (2003).
[3] T. Suntola, in: Handbook of Crystal Growth, Part 3b: GrowthMechanisms and Dynamics, edited by D. T. J. Hurle (Else-vier, Amsterdam, Lausanne, New York, 1994), pp. 605–663.
[4] A. Szczerbakow, E. Dynowska, M. Godlewski, and K. Swia-tek, J. Cryst. Growth 183, 708 (1998).
[5] A. Szczerbakow, E. Dynowska, K. Swiatek, and M.Godlewski, J. Cryst. Growth 207, 148 (1999).
[6] K. Kopalko, M. Godlewski, J. Z. Domagala, E. Lusakowska,R. Minikayev, W. Paszkowicz, and A. Szczerbakow, Chem.Mater. 16, 1447 (2004).
[7] M. Godlewski, E. Guziewicz, G. Łuka, T. Krajewski,M. Łukasiewicz, Ł. Wachnicki, A. Wachnicka, K. Kopalko,A. Sarem, and B. Dalami, Thin Solid Films 518, 1145–1148(2009).
[8] I. A. Kowalik, E. Guziewicz, K. Kopalko, S. Yatsunenko,A. Wojcik-Głodowska, M. Godlewski, P. Dłuzewski,E. Łusakowska, and W. Paszkowicz, J. Cryst. Growth 311,1096 (2009).
[9] C. Zhao, T. Witters, P. Breimer, J. Maes, M. Caymax, andS. De Gendt, Microelectron. Eng. 84, 7–10 (2007).
[10] M. Godlewski, E. Guziewicz, J. Szade, A. Wojcik-Głodowska, Ł. Wachnicki, T. Krajewski, K. Kopalko,R. Jakieła, S. Yatsunenko, E. Przezdziecka, P. Kruszewski,N. Huby, G. Tallarida, and S. Ferrari, Microelectron. Eng. 85,2434–2438 (2008).
[11] T. Krajewski, E. Guziewicz, M. Godlewski, Ł. Wachnicki,I. A. Kowalik, A. Wojcik-Glodowska, M. Łukasiewicz,K. Kopalko, V. Osinniy, and M. Guziewicz, Microelectron.J. 40, 293–295 (2009).
[12] E. Guziewicz, M. Godlewski, T. Krajewski, Ł. Wachnicki,A. Szczepanik, K. Kopalko, A. Wojcik-Głodowska, E. Przezd-ziecka, W. Paszkowicz, E. Łusakowska, P. Kruszewski,N. Huby, G. Tallarida, and S. Ferrari, J. Appl. Phys. 105,122413 (2009).
Phys. Status Solidi B 247, No. 7 (2010) 1701
Original
Paper
Figure 3 Room temperature PL spectra of ZnO films obtainedusing two different zinc precursors.
Figure 4 (online color at: www.pss-b.com) Low temperature PLspectra of monocrystalline ZnO films obtained using two differentzinc precursors.
Proceedings of the E-MRS Fall Meeting, Symposium H: Warsaw, Poland, September 19�23, 2011
Epitaxial ZnO Films Grown at Low Temperature
for Novel Electronic Application
�. Wachnickia,∗, A. Du»y«skaa, J.Z. Domagalaa, B.S. Witkowskia, T.A. Krajewskia,
E. Prze¹dzieckaa, M. Guziewiczb, A. Wierzbickaa, K. Kopalkoa, S. Figgec,
D. Hommelc, M. Godlewskia,d and E. Guziewicza
aInstitute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, PolandbInstitute of Electron Technology, al. Lotników 32/46, 02-668 Warsaw, Poland
cInstitute of Solid State Physics, University of Bremen, Kufsteiner Str. 1, Bremen 28359, GermanydCollege of Science, Department of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski University
Warsaw, Poland
Monocrystalline �lms of zinc oxide were grown at 300 ◦C by atomic layer deposition. ZnO layers weregrown on various substrates like ZnO bulk crystal, GaN, SiC and Al2O3. Electrical properties of the �lmsdepend on structural quality. Structural quality, surface morphology and optical properties of ZnO �lms werecharacterized using X-ray di�raction, scanning electron microscopy, and photoluminescence, respectively. Highresolution X-ray di�raction spectra show that the rocking curve FWHM of the symmetrical 00.2 re�ection equalsto 0.058◦ and 0.009◦ for ZnO deposited on a gallium nitride template and a zinc oxide substrate, respectively.In low temperature photoluminescence sharp excitonic lines in the band-edge region with a FWHM equal to4 meV, 5 meV and 6 meV, for zinc oxide deposited on gallium nitride, zinc oxide and sapphire substrate, respectively.
PACS: 81.15.Aa, 61.05.cp, 81.05.Dz
1. Introduction
Atomic layer deposition (ALD) introduced by Sun-tola in 1980 [1] was originally used to obtain monocrys-talline �lms on monocrystalline substrates. At presentALD is widely used to deposit both polycrystalline andmonocrystalline layers, including semiconductors andamorphous high-k oxide �lms [2, 3]. One of the mainadvantages of this method is the possibility of low tem-perature growth. This is also possible on surfaces withhighly developed morphology. Characteristic features ofthe ALD are self limitation and a sequential growth pro-cess. This enables the use of very reactive precursors andreduction of a growth temperature, while keeping goodcrystallographic and optical parameters.ZnO, which is a II�VI semiconductor with a 3.37 eV
direct band gap at room temperature, may be applied inmany devices. This includes light emitters, piezoelectrictransducers or sensors. ZnO is also a very prospectivematerial for three-dimensional memories [4] and trans-parent electronics [5], including the ones with an organicmaterial used as an active part of the device [6�8]. Forthe latter applications low temperature of ZnO deposi-
tion is essential [3, 6] and 300 ◦C is an applier limit here.The ZnO material grown within the above temperaturelimit is typically polycrystalline. For other applicationsmonocrystallinity of ZnO �lms is important. Growthmethods that are able to produce such ZnO �lms arepulsed laser deposition (PLD), chemical vapor deposition(CVD) with its modi�cations (like MOCVD) and molec-ular beam epitaxy (MBE), but temperatures used formonocrystalline ZnO growth are commonly much higherthan 300 ◦C. They often exceed 600 ◦C (for reviews see[10] and references therein). However, already it was of-ten claimed that the ALD method is not suitable for themonocrystalline growth. In a previous paper [6] we re-ported on a epitaxial ZnO growth by the ALD, for �lmsgrown on a GaN template. This observation motivatedthe presented study. We demonstrate that crystallineZnO can be grown on di�erent substrates like ZnO, GaN,SiC and sapphire at restrictive temperature limit. We an-alyze structural characteristics of these �lms and relatelayer quality with electrical parameters.
2. Experimental
ZnO �lms studied here were obtained at 300 ◦C by theALD method on GaN, SiC, Al2O3 and ZnO templatesin the Savannah-100 reactor from Cambridge Nanotech.
(A-7)
A-8 �. Wachnicki et al.
We used diethylzinc (DEZn, (C2H5)2Zn) as a zinc pre-cursor and deionized water as an oxygen precursor. Thezinc oxide �lm at the surface was created as a productof the double-exchange chemical reaction of DEZn withdeionized water
C2H5 − Zn− C2H5 +H2O → ZnO + 2C2H6 .
The structure and the crystallographic orientation ofZnO layers were determined by X-ray di�raction (XRD).The quality of the layers was investigated by a high reso-lution X'Pert MRD di�ractometer equipped with a X-raymirror and a two-bounce monochromator at the inci-dent beam. The di�racted beam was measured with a2-dimensional solid-state X-ray detector � PIXcel. Thesurface morphology and �lms cross-section images wereobtained by scanning electron microscope (SEM, HitachiSU-70) with an operation voltage of 15 kV. Room tem-perature photoluminescence (RT PL) studies were per-formed with a xenon lamp and the CM 2203 spectro-meter. Low temperature (LT) PL measurements wereobtained with He�Cd 325 nm laser line using the CCDcamera.
3. Results and discussion
One micrometer thick �lms were deposited on sub-strates with di�erent lattice mismatch to the ZnO lattice,i.e. gallium nitride template (GaN/Al2O3), sapphire, sil-icon carbide and zinc oxide single crystal. The mismatchbetween ZnO and GaN is only 1.9%. Therefore, we ex-pected lower stress and dislocations density. The latticemismatch for SiC and Al2O3 substrates is higher andequals 5.4% and 31.7%, respectively. In the latter casethe actual lattice mismatch is reduced to 18.4% by ro-tation of the ZnO unit cell of growing ZnO layers withrespect to the substrate unit cell by 30◦ [11].The ALD process consists of repeating four deposition
steps: deposition of a �rst precursor; purging the reactionchamber with an inert gas, when non-reacted precursors'and by-products' molecules are removed; deposition of asecond precursor; and �nal purging of the reaction cham-ber. Thus several parameters a�ect �nal quality of ZnO�lms. These are lengths of pulses, growth temperature,purging times, etc. Moreover, the crystallographic qual-ity of the ZnO-ALD �lm as well the surface morphologydepends on the kind of precursors used. We found thatwhen using DEZn and water for the ZnO deposition along purging time and higher growth temperature areadvantageous. The growth with c axis perpendicular tothe surface is achieved at these conditions. In this caseonly the 00.2 peak is observed in the XRD spectrum. Inthe following experiments we used ALD parameters thatare feverous for ZnO growth with c axis perpendicularto the surface. Deposition temperature was 300 ◦C. Thesame conditions were used for the ALD processes in allinvestigated substrates.In Fig. 1 SEM images are shown of ZnO/SiC, ZnO/
ZnO, ZnO/Si and ZnO/GaN interfaces. For the silicon
substrate, where the lattice mismatch is very high (about40%) the monocrystalline growth was not achieved. Onecan see here a columnar growth with a column widthabout 100 nm. A columnar growth was less evident incase of ZnO/GaN and ZnO/SiC �lms, which are of muchhigher structural quality. We observed that some dislo-cations coming from the gallium nitride and silicon car-bide substrates are spreading into the zinc oxide layer.From the SEM images and epitaxy software calculations(PANAlytical software used for plotting and analyzingrocking curves, 2-axes scans, reciprocal space) one canconclude that zinc oxide �lms are fully relaxed. An idealinterface and the best ZnO layer quality are obtained forthe homoepitaxial growth i.e., for ZnO/ZnO process. Inthis case the zinc oxide �lm is fully uniform and neitherdislocations nor grain boundaries are seen in the SEMimage.
Fig. 1. SEM studies show good structural quality ofZnO �lms deposited on di�erent substrates. The bestmonocrystalline quality was obtained for zinc oxide andgallium nitride substrates.
Figure 2 shows reciprocal space maps (RSM) of the00.2 re�ection from the epitaxial ZnO �lms depositedon GaN, SiC, Al2O3 and ZnO. The RSM maps con�rmthat ZnO layers obtained are monocrystalline, with a fullwidth at half maximum (FWHM) of the rocking curve(00.2 re�ection) equal to 0.009◦ and 0.058◦ for ZnO/ZnO and ZnO/GaN, respectively. The reason of ratherhigh di�use scattering (elliptical shape isocontures with3 ordered smaller intensity than for the Bragg peak) isdi�erent for these two substrates. Films grown on GaNtemplates re�ect the structural imperfection of the tem-plates (GaN/sapphire; dislocation density in order of the109 cm−2). Ellipses visible in Fig. 2b are twice moreelongated in Qx direction than for the ZnO/ZnO case(Fig. 2c). We suppose that di�use scattering for ho-moepitaxial growth of ZnO is correlated with the surfaceimperfection. For ZnO/Al2O3 and ZnO/SiC the FWHM
Epitaxial ZnO Films Grown at Low Temperature . . . A-9
Fig. 2. Reciprocal space maps of the 00.2 re�ectionfrom zinc oxide deposited on (a) sapphire, (b) galliumnitride, (c) zinc oxide bulk crystal, (d) silicon carbide.
of the rocking curve is equal to 0.991◦ and 0.345◦, respec-tively.
The lattice parameters of the obtained �lms were de-termined from the RSMs of symmetrical 00.2 and theasymmetrical−1−1.4 re�ections. In Table we present thelattice parameters of ZnO. The data for �lms depositedon di�erent substrates are compared with single crystallattice parameters of ZnO [12]. One can see that latticeconstants of ZnO-ALD �lms are very similar to these ofa single ZnO crystal. For ZnO/ZnO homoepitaxial �lmsthe di�erences between lattice parameters of bulk andepitaxial ZnO are within the error limit. Surprisingly,the values of lattice parameters for the ZnO layer grownon sapphire are only about 0.02% larger than these of aZnO single crystal. However, the higher FWHM of the00.2 peak for ZnO �lms deposited on Al2O3 suggests ahigher screw dislocation density. The largest di�erencesin lattice parameters are noticed for ZnO �lms depositedon GaN and SiC substrates. For ZnO/GaN a and c latticeconstants are 1.1% larger and 1.6% smaller than latticeconstants of a zinc oxide single crystal. This shows thatthe layer deposited on gallium nitride is tensile strainedin the (0001) plane. A similar situation is observed forthe ZnO/SiC �lm. The optical properties of epitaxialZnO �lms were characterized by RT and LT PL.
Figure 3 shows RT PL spectra of zinc oxide depositedon Al2O3, GaN and ZnO single crystal. We observedhere high edge luminescence in the blue spectral region.Defect-related PL usually observed as a broad band inthe visible spectral region is not present in any measured�lms. The spectral position of the edge emission is sim-ilar for all investigated ZnO layers and is 3.25 eV. Thestrongest edge luminescence is observed for ZnO �lmsgrown on a sapphire substrate. It is probably related tothe full relaxation of ZnO/Al2O3 �lms which was postu-lated above. PL spectra measured at 12 K are presentedin Fig. 4. PL shows sharp excitonic luminescence with
TABLE
Lattice constants parameters for zinc oxide thin �lmsdeposited on zinc oxide single crystal, gallium ni-tride, sapphire, silicon carbide. The lattice constantof zinc oxide single crystal are a = 3.2495 Å andc = 5.2069 Å [9].
Substrates Lattice a [Å] Lattice c [Å]
GaN 3.2534 5.1986
SiC (4H) 3.2513 5.1957
Al2O3 3.2488 5.2057
ZnO 3.2492 5.2067
FWHM values of 4 meV, 5 meV and 6 meV for ZnO/GaN,ZnO/ZnO and ZnO/Al2O3, respectively. In PL spectrumof zinc oxide deposited on sapphire we observe two peaks:higher one at 3.36 eV corresponding to neutral donor--bound exciton recombination [13], and a smaller peakat 3.33 eV likely also corresponding to a donor-boundexciton transition [14]. The same peaks are seen for zincoxide deposited on the gallium nitride substrate, but withdi�erent intensities. Donor-bound excitonic emission at3.33 eV is dominant in the ZnO/GaN spectrum. For ZnOobtained on gallium nitride we notice phonon replied ofthe PL peaks. This con�rms the good crystallographicquality of the �lms studied. Homoepitaxial zinc oxidelayers show much weaker low-temperature photolumines-cence. We attribute this to a di�usion of lithium and/orpotassium contaminations from the commercial zinc ox-ide substrate to ZnO layers. Our �lms were grown onsubstrates obtained by hydrothermal method which con-tain large Li and K concentration.
Fig. 3. Room temperature photoluminescence for zincoxide thin �lms deposited on gallium nitride, single crys-tal zinc oxide and sapphire.
We note here that structural and optical parametersshown above are comparable with the ones reported forepitaxial ZnO �lms obtained at much higher tempera-tures by CVD or MBE [15]. For example, ZnO grownon GaN substrate by CVD at 700 ◦C shows the rockingcurve of 1000 arcsec [15], which is much higher than forour ZnO/GaN �lms (250 arcsec).The electrical parameters of ZnO/GaN and ZnO/
Al2O3 have been examined by the Hall e�ect measure-ments. These investigations were performed at room
A-10 �. Wachnicki et al.
Fig. 4. Low temperature photoluminescence for zincoxide thin �lms deposited on gallium nitride substrate,single crystal zinc oxide and sapphire.
temperature in the van der Pauw con�guration using theRH2035 PhysTech GmbH system equipped with a per-manent magnet giving a �eld of 0.426 T. The free car-rier concentration was 1.8 × 1018 cm−3 for a ZnO/GaN�lm and 2.2× 1018 cm−3 for a ZnO/Al2O3 layer. Thesevalues are about one order of magnitude lower than ob-tained for a ZnO �lm deposited on a glass substrate inthe same ALD process, where the ZnO layer was poly-crystalline. For ZnO/glass �lms we obtained n concen-tration of 1.3 × 1019 cm−3. The measured mobility ofcarriers was higher for epitaxial layers than for the poly-crystalline �lms. The mobility is also strongly correlatedwith the structural quality of the �lm. The mobility µ forZnO/GaN �lm (the FWHM of the rocking curve 250 arc-sec) was 167 cm2/(V s), whereas µ for ZnO/Al2O3 layer(the FWHM of the rocking curve 3560 arcsec) equals39 cm2/(V s). For comparison, the mobility of carriersin the reference polycrystalline ZnO/glass �lm is about30 cm2/(V s).
4. Conclusions
In conclusion, we obtained monocrystalline ZnO �lmson GaN and bulk ZnO substrates by atomic layer de-position at temperature 300 ◦C. The structural and op-tical parameters �as-grown� ZnO �lms are surprisinglygood despite low growth temperature. We note here thatstructural and optical parameters shown above are com-parable with the ones reported for epitaxial ZnO �lmsobtained at much higher temperatures by CVD or MBE.For example, ZnO grown on GaN substrate by CVD at700 ◦C shows the rocking curve of 1000 arcsec, which ismuch higher than for our ZnO/GaN �lms (250 arcsec).Electrical properties of epitaxial ZnO �lms strongly cor-relate with the FWHM of the rocking curve. For ZnO/GaN �lms mobility of carriers is a 169 cm2/(V s) and freeelectron concentration at RT is 1.8×1018 cm−3. All theseparameters favorably compare with the ones obtained athigher temperature with MBE or MOCVD.
Acknowledgments
The research was partially supported by: EuropeanUnion within European Regional Development Fund,
through grant Innovative Economy (POIG.01.01.02-00--008/08) and by grant no. 0663/B/T02/2008/35 from thePolish Ministry of Science and Higher Education.
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[15] S. Lautenschlaeger, S. Eisermann, M.N. Hofmann,U. Roemer, M. Pinnisch, A. Laufer, B.K. Meyer,H. von Wenckstern, A. Lajn, F. Schmidt, M. Grund-mann, J. Blaesing, A. Krost, J. Cryst. Growth 312,2078 (2010).
Vol. 124 (2013) ACTA PHYSICA POLONICA A No. 5
Proceedings of the 42th �Jaszowiec� International School and Conference on the Physics of Semiconductors, Wisªa 2013
Characterization of n-ZnO/p-GaN Heterojunction
for Optoelectronic ApplicationsL. Wachnickia, S. Gieraltowskaa, B.S. Witkowskia, S. Figgeb, D. Hommelb,
E. Guziewicza and M. Godlewskia,c
aInstitute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warszawa, PolandbUniversity of Bremen Institute of Solid State Physics Semiconductor Epitaxy, Otto-Hahn-Allee NW1
D-28359 Bremen, GermanycCardinal Stefan Wyszy«ski University, College of Science, Department of Mathematics and Natural Sciences
Warszawa, Poland
An important feature of zinc oxide and gallium nitride materials are their similar physical properties. Thisallows to use them as a p�n junction materials for applications in optoelectronics. In earlier work we presenteduse of ZnO as a transparent contact to GaN, which may improve external e�ciency of LED devices. In thiswork we discuss properties of a n-ZnO/p-GaN heterostructure and discuss its optimization. The heterostructureis investigated by us for possible applications, e.g. in a new generation of UV LEDs or UV light detectors.
DOI: 10.12693/APhysPolA.124.869
PACS: 85.30.Kk, 61.05.cp, 81.05.Dz
1. Introduction
Zinc oxide (ZnO) and gallium nitride (GaN) are twosemiconductor materials, which are investigated for manypossible applications in electronics, optoelectronics, biol-ogy, and medicine. Physical and chemical properties ofboth GaN and ZnO mean that these materials can beused as light emitting diodes (LEDs), sensor devices, intransparent electronics, and as solar cells [1�3].In recent years the research concentrates on ZnO prop-
erties, due to many expected advantages of devices basedon this material. For example, in envisioned ZnO-basedlaser diodes, lasing would likely occur via excitonic UVtransitions. Thus, if realized, it would lead to much lowerthreshold currents, as compared to the ones in GaN--based devices. Moreover, ZnO-based diodes constructedby us and others are characterized by low reverse-leakagecurrents of 10−7 A observed at room temperature, whichis important in point of view of some electronic ap-plications. Regarding heterostructures, GaN/ZnO het-erostructures are also very interesting, since heavilydoped ZnO can be also used as a top conductive contactto GaN-based LEDs, as demonstrated recently [4, 5].Both semiconductors are characterized by a large band
gap energy of about 3.4 eV at room temperature [6, 7].Their similar physical properties allow creation of LEDsemitting in a short wavelength spectral region [6�8].ZnO/GaN heterostructures are also investigated for thisapplication [9, 10]. This is highly interesting, since bothmaterials in normal conditions (room temperature, at-mospheric pressure) are crystallized in the wurtzite struc-ture [11, 12]. Moreover, due to the relatively small latticemismatch (about 1.6%) between ZnO and GaN it is pos-sible to achieve an epitaxial growth of zinc oxide using agallium nitride template. In fact, we demonstrated thatwe can grow zinc oxide of a high crystalline quality on
top of GaN, with a reduced concentration of imperfec-tions [11, 13]. In most of the cases the imperfectionsare related to the use of a substrate with a large latticemismatch (for example silicon for ZnO or GaN epitaxy).Short-wavelength UV detectors based on n-GaN/
p-GaN homojunctions [13, 14] have been widely stud-ied. Despite the fact that GaN-based LEDs, laser diodesand detectors are commercialized, some of their proper-ties need further improvements. In particular, our �rstinvestigations show that ZnO-based UV detectors can-not only be more sensitive, but much cheaper, as well.This fact motivated us to study various versions of pos-sible UV detectors, including the ones discussed in thepresent work.In the present work we con�rmed that zinc oxide
layers grown on GaN are characterized by a relativelysmall number of defects. This is an important �nding,since such material properties are required for optoelec-tronic or sensors and detectors applications. We presentcharacterization and optimization of n-ZnO/p-GaN het-erostructures, investigated for possible application in anew generation of UV detectors. We conclude that thesimplicity and low costs of technology we use, combinedwith a relatively high quality of obtained monocrystallineZnO �lms at relatively low temperature (< 350 ◦C),makes our approach attractive for a fabrication of mod-ern semiconducting devices.
2. Experimental details
Undoped zinc oxide �lms with a high crystalline qual-ity were deposited by the atomic layer deposition (ALD)method. Test samples were deposited �rst on a glass,and only then after optimizing the growth parameters, ongallium nitride template and zinc oxide (bulk material).
Before a growth substrates were chemically cleaned. ZnOlayers, with thickness in the range from 0.5 to 2 µm, wereobtained by a double exchange reaction, using diethylz-inc and deionized water as precursors
Zn(C2H5)2 + H2O → ZnO + 2(C2H6).The details on the growth parameters (pulse time pre-
cursors, purging time) are given in our recent work [11].After the process optimization, ZnO �lms were de-
posited at temperature of 300 ◦C on GaN/sapphire tem-plates, with a GaN layer grown by the metalorganicchemical vapor deposition (MOCVD). On zinc oxide andgallium nitride surfaces were evaporated ohmic contacts,consisting of Au/Ti for ZnO and Au/Ni for GaN. Con-tacts were annealed at high temperature about 400 ◦C inRTP system for 5 min.The structure and the crystallographic orientation
of ZnO layers were measured with a high resolution(HRXRD) X'Pert MRD di�ractometer equipped withthe X-ray mirror, a four-bounce monochromator at theincident beam and a three bounce analyzer at thedi�racted beam. Optical properties of zinc oxide nano-structures have been characterized by the spectro�uo-rimeter CM2203, with a xenon lamp used as the exci-tation source. The surface morphology was investigatedby the atomic force microscopy (AFM, Bruker Dimen-sion Icon) using the PeakForce Tapping and silicon ni-tride probes with sharp tips (a tip radius: 2 nm). Sur-face roughness was characterized by the root mean square(RMS) value. Films cross-section images were obtainedby scanning electron microscopy (SEM, Hitachi SU-70) atthe operation voltage of 15 kV. The Hall e�ect measure-ments were performed with a RH2035 system producedby PhysTech GmbH, with a permanent magnet giving amagnetic �eld of 0.426 T. The Hall measurements weredone in the van der Pauw con�guration with four con-tacts mechanically pressed to a square sample of ZnOthin �lm in its corners. I�V electrical characterizationswere performed using a Keithley 2601A electrometer.
3. Result and discussion
In this paper we analyze properties of the ZnO/GaNdiode with a monocrystalline zinc oxide layer depositedby the ALD.Figure 1 shows a SEM cross-section image of ZnO
(0.5 µm thick)/GaN (3 µm thick) heterostructure ob-tained by us, con�rming a sharp interface between thetwo semiconductors. The SEM investigations allow usto trace how dislocations from gallium nitride templatepass to zinc oxide layer. SEM and the following XRDinvestigations indicate that the two compounds adjust toeach other.HRXRD measurements con�rm that the obtained ZnO
layers are of a high crystalline quality. A full width athalf maximum (FWHM) of the rocking curve of 00.6 re-�ection equals 0.07◦ (Fig. 2). This value of the rockingcurve is identical to the one measured for the galliumnitride template, which means that quality of the ZnO
Fig. 1. The cross-sectional SEM image of the ZnO(0.5 µm thick) on GaN (3 µm thick) thin �lm grownby the ALD at temperature of 300 ◦C.
�lm is limited by imperfections in a GaN layer. The per-pendicular lattice constant c was determined from theHRXRD measurements of the symmetrical 00.2 re�ectionas (5.1978 ± 0.0001) Å, while the parallel constant wasdetermined from the asymmetrical re�ection (−1) − 1.4as (3.2569± 0.0005) Å. These results only slightly di�erfrom the lattice constants of a bulk ZnO (at room temper-ature), which are 5.2069 Å and 3.2495 Å [4], respectively.From the lattice parameter results we can conclude thatobtained layers are slightly tensile strained, which is dueto the small lattice mismatch between ZnO and GaN.
Fig. 2. (a) 00.2 symmetrical and (b) −1�14 asymmet-rical re�ection reciprocal space maps collected with an-tiscattered slit 1/8◦ before the XRD detector.
Electrical parameters and surface morphology mea-surements of the ZnO layers were then performed. Forthese measurements ZnO thin �lms were grown at thetemperature range between 100 and 300 ◦C (see Table I)on GaN (carrier concentration ≈ 1017 cm−3 and mobility≈ 20 cm2/(V s)), ZnO and glass substrates. ZnO �lms
Characterization of n-ZnO/p-GaN Heterojunction . . . 871
(0.5�2 µm thick) are characterized by free electron con-centrations of about 1018 cm−3. One can notice that weinvestigated junctions with relatively thick ZnO layers,done on purpose to limit gallium nitride substrate on theresults of electrical investigations of ZnO.
TABLE I
Surface roughness de�ned by the RMS andcarrier concentration vs. growth temperatureof ZnO layers with thickness of 0.5 µm ob-tained in the ALD processes on GaN template.
Growthtemperature
[◦C]
RMS[nm]
Carrierconcetration
[cm−3]
300 6 3.88× 1018
250 7 4.80× 1018
200 9 6.32× 1018
150 28 8.25× 1018
100 17 3.16× 1018
For junctions (see schematic in Fig. 3) investigationswe selected ZnO layers grown at temperature of 300 ◦C.These �lms exhibited the highest carriers' mobility atroom temperature. The increase of the mobility of car-riers was observed for �lms grown at a higher growthtemperature and for thicker zinc oxide layers. Mobilityof about 150 cm−2/(V s) was obtained for the best struc-tures. ZnO �lms were monocrystalline with a very �atsurface morphology (see Tables I and II). Similar quality�lms were deposited on ZnO substrate, as discussed inthe recent paper [13]. Test samples deposited on a glasswere polycrystalline.
Fig. 3. Schematic presentation of the n-ZnO/p-GaNheterojunction with deposited ohmic contacts.
The optimized ZnO/GaN structure was characterizedby a smooth interface, but by a relatively roughness sur-face (6 nm as compared to 1 nm of GaN template). Fornot optimized �lms the RMS value of 20 nm was obtained(see Fig. 4).Current�voltage characteristic of the heterojunction,
shown in Fig. 5, was investigated to determine the rec-ti�cation ratio of the junction, de�ned as the ratio offorward to reverse currents. This ratio is mostly deter-mined by a barrier height seen by electrons and holes.To account for the deviation from the ideal structure
TABLE II
Electrical properties of ZnO layers with thicknessof about 2 µm deposited at temperature of 300 ◦Con three di�erent substrates.
SubstrateMobility
[cm2/(V s)]
Carrierconcentration
[cm−3]
GaN 150 4× 1018
ZnO 80 5× 1018
glass 50 3× 1018
Fig. 4. Surface morphology AFM image (2 × 2 µm2)of not optimized ZnO layer grown by the ALD at tem-perature 300 ◦C (2 µm thick) on a GaN template. ZnOlayer is characterized by the RMS of about 20 nm. ForGaN template RMS value is below 1 nm.
the so-called ideality factor is introduced. For mostof ZnO structures investigated by us this factor devi-ates from 1 and is quite big (equal to 5) in the presentcase. The recti�cation ratio of our test n�p junctionis Ion/Ioff = 1.8 × 103 for a voltage of 2 V. This re-sult favourably compares to e.g. 102 reported for n-GaN/p-GaN junctions based on layers or nanowires as well asto e.g. 103 and 102 obtained for n-ZnO/p-GaN junctionsbased on layers and nanowires, respectively [15�18].
Fig. 5. I�V characteristic of ZnO/GaN heterojunctionwith Ion/Ioff = 1.8 × 103 for 2 V and Ion = 1.3 ×10−2 A/cm2.
The I(V ) characteristics of the junctions are stronglya�ected by UV illumination. Importantly, no response
872 L. Wachnicki et al.
was observed (or quite weak) in case of the visible lightillumination. At present we optimize the structures forUV detector applications. These results will be a subjectof forthcoming publication.
4. Conclusions
In this work we discuss properties of n-ZnO/p-GaNheterostructures, investigated for possible application inoptoelectronic and detector devices. ZnO layers witha high crystalline quality and good electrical proper-ties (carrier concentration ≈ 1018 cm−3 and mobility≈ 150 cm2/(V s)) were deposited by the ALD method attemperature of 300 ◦C on GaN/sapphire templates. Onzinc oxide and gallium nitride surfaces there were evap-orated ohmic contacts consisting of Au/Ti for ZnO andAu/Ni for GaN. Current�voltage properties of the het-erojunction are fairly favourably, with the recti�cationfactor of Ion/Ioff = 1.8 × 103 for a voltage of 2 V. Our�rst investigations indicate a great potential for applica-tion of the heterojunction as UV detectors, in particularin �Solar Blind� detectors.
Acknowledgments
The research was supported by the European Unionwithin European Regional Development Fund throughgrant Innovative Economy (POIG.01.01.02-00-008/08).
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aPolish Academy of Sciences, Institute of Physics, al. Lotników 32/46, 02-668 Warsaw, PolandbCardinal Stefan Wyszyński University, College of Science, Department of Mathematics
and Natural Sciences, Warsaw, Poland
Zinc oxide is a II–VI semiconductor material which is gaining increasing interest in various fields suchas biology, medicine or electronics. This semiconductor reveals very special physical and chemical properties,which imply many applications including a transparent electrode in solar cells or LED diodes. Among manyapplications, ZnO is also a prospective material for sensor technology, where developed surface morphology isvery advantageous. In this work we present ZnO nanowires growth using atomic layer deposition method. ZnOnanowires were obtained using controlled physical properties. As a substrate we used gallium arsenide withgold-gallium eutectic droplets prepared on the surface at high temperature. To obtain the eutectic solutionthere was put a gold thin film on GaAs through the sputtering and then we annealed the sample in a nitrogengas flow. The so-prepared substrate was applied for growth of ZnO nanowires. We used deionized water andzinc chloride as oxygen and zinc precursors, respectively. The eutectic mixture serves as a catalyst for theZnO nanowires growth. Au–Ga droplets flow on the front of ZnO nanowires. Scanning electron microscopyimages show ZnO nanorods in a form of crystallites of up to 1 µm length and a 100 nm diameter. It is thefirst demonstration of the ZnO nanowires growth by atomic layer deposition using the vapour–liquid–solid approach.
PACS: 81.07.Bc, 82.47.Rs, 68.55.–a, 81.15.Aa
1. Introduction
The atomic layer deposition (ALD) method was de-veloped by Suntola and originally was used for epitax-ial growth [1]. The simplicity of this method causesthat people used ALD technique not only for the singlecrystal deposition, but also for growth of polycrystallineand amorphous layers [2–4]. In this paper we presentnanostructure of zinc oxide which is a II–VI semicon-ductor with the direct band gap energy of 3.37 eV [5]at room temperature. It has a high exciton binding en-ergy (60 meV) [6], which have a large influence on theoptical properties of the material. Simultaneously, ZnOhas a very high chemical stability, which makes it a goodcandidate as devices component.
Due to very interesting properties zinc oxide has manyelectronics applications. The wide energy gap of this ma-terial makes it transparent in the visible range of theelectromagnetic spectrum. ZnO strongly doped with alu-minum can be used as a transparent conductive oxide(TCO). Due to the high exciton binding energy, it is pos-sible to use ZnO in the light emitting diode (LED) [7].
The protection of the natural environment is very im-portant due to the rapid development of industry. The
essential key of environmental protection is to monitorchanges in the atmosphere and hydrosphere. Therefore,construction of sensors that detects the composition ofgas in the atmosphere is crucial for industry. Differentgases or liquids have a very strong influence on the con-ductivity of the ZnO surface, thus selection of ZnO forsensor applications was obvious. In this paper we presenttwo methods of preparing such sensors based on ZnO, byusing the ALD method.
2. Experimental details
Atomic layer deposition processes were performed ongallium arsenide substrates. Cleaning chemical processeslike etching and rinsing in solvents were used prior to theZnO deposition. After chemical preparation, a thin layerof gold was deposited. The substrate with a gold layerwas annealed in the rapid thermal process (RTP). In thisprocess we obtained Au–Ga eutectic in form of droplets(shown in Fig. 1). Diameter of a single droplet was about100 nm. After processing, substrates were placed in theALD reactor — Microchemistry F-120 ALD system.
Zinc oxide nanostructures were deposited in a doubleexchange reaction using zinc chloride and deionized wa-ter as precursors
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906 Ł. Wachnicki et al.
Fig. 1. Image of eutectic droplets Ga–Au observed byAFM.
ZnCl2 + H2O → ZnO + 2HCl .
Between pulses of precursors, the reaction chamber waspurged by high purity nitrogen gas (99.9999%). Opticalproperties of zinc oxide nanostructures have been charac-terized by the spectrofluorimeter CM2203, where the ex-citation source was the xenon lamp. Surface morphologywas investigated by atomic force microscope (AFM) andscanning electron microscope (SEM — Hitachi SU-70).
3. Result and discussion
As a result of optimization of the growth process, wereceived two types of zinc oxide nanostructures (techni-cal name: Nano1, see Fig. 2a, and Nano2 — Fig. 2b).The Nano1 structure was characterized by a columnargrowth of zinc oxide. The typical nanowire size wasabout 1 µm length, while the diameter was about 250 nm.Such nanostructures grew directly on Ga–Au droplets.These droplets nucleated growth of zinc oxide nanowires.Zinc oxide nanocolumns growth on a pure gallium ar-senide substrate (without Ga–Au droplets) was not ob-served. The structure of Nano2 was also characterized bya columnar growth, but in this case the growth of ZnOnanostructures was catalyzed by a Ga–Au droplet, whichfloats on the front of growth and freezes at the top af-ter the process. Thus, this is a typical vapor–liquid–solid(VLS) growth mode. The size of the single nanowire ofNano2 structure was about 1 µm length and the diameterof about 50 nm.
Fig. 2. Surface structure of (a) Nano1, (b) Nano2 char-acterized by SEM.
Figure 3 shows the photoluminescence intensity as afunction of wavelength for the two nanostructures. For
Nano1 we observed a strong “edge” luminescence (around380 nm). High intensity photoluminescence in this areaconfirms good quality of obtained zinc oxide nanostruc-tures. In the wavelength range from 450 nm to 700 nmwe observed a very weak (in comparison to “edge lumi-nescence”) defect-related luminescence. For Nano2 nano-structure “edge”, defect-related luminescence was veryweak or even not observed. Probably, the intensity ofphotoluminescence depends of zinc oxide crystal size.
Fig. 3. Room temperature photoluminescence ofNano1 and Nano2 nanostructures.
Nano1 and Nano2 structures were tested for sensor ap-plication. We deposited Ti/Au ohmic contacts on theirsurface, and performed time–resistance measurements.
Fig. 4. Dependence of resistivity of Nano1 and Nano2nanostructures after applying a drop of acetone.
Samples resistance was tested with different solventslike acetone, isopropanol, trichloroethylene and ethanoldeposited on the surfaces. Figure 4 presents a graphshowing the sensor behavior for two obtained structures(Nano1 and Nano2). After deposition of the acetonedroplet on the sample surface, resistance increased sig-nificantly. After evaporation of solvent from the surface,measured resistance returns to the state before the dropdeposition. A similar result was observed for all solventsbut magnitude of the response was solvent dependent.We observed a very advantageous property of obtainednanosensors. Sensors were reset to their initial resistivitywithin a few minutes without any surface treatment orheating. If heating was applied, the resetting time wassignificantly shorter. We observed an increase of resistiv-ity for several tested solvents and alcohols. The problemto be solved is now a selectivity of the response to differ-ent chemical compounds.
Optical and Structural Characterization of Zinc Oxide Nanostructures . . . 907
4. Conclusions
Zinc oxide nanostructures were performed by theatomic layer deposition process. The so-obtained nano-structures show high sensitivity to chemical solvents andalcohols. The sensors structure can be applied for detec-tion of hazardous substances in a natural environment.Importantly, these devices were automatically reset af-ter solvent or alcohol evaporation, without any annealingand chemical processing.
Very important is also the method of obtaining zincoxide nanostructures, which can be easily implementedin the industry. Our technology of production thenanowires are relatively a cheap method and seem to bea good alternative against other methods of growing suchnanostructures.
Acknowledgments
The research was supported by the European Unionwithin European Regional Development Fund, throughgrant Innovative Economy (POIG.01.01.02-00-008/08).
References
[1] T. Suntola, in: Handbook of Crystal Growth, Part 3b:Growth Mechanisms and Dynamics, Ed. D.T.J. Hurle,Elsevier, Amsterdam 1994, p. 605.
[2] Ł. Wachnicki, T. Krajewski, G. Łuka, B. Witkowski,B. Kowalski, K. Kopalko, J.Z. Domagala,M. Guziewicz, M. Godlewski, E. Guziewicz, ThinSolid Films 518, 4556 (2010).
[3] E. Przezdziecka, T. Krajewski, L. Wachnicki,A. Szczepanik, A. Wójcik-Głodowska, S. Yatsunenko,E. Lusakowska, W. Paszkowicz, E. Guziewicz,M. Godlewski, Acta Phys. Pol. A 114, 1303 (2008).
[4] S. Gierałtowska, D. Sztenkiel, E. Guziewicz,M. Godlewski, G. Łuka, B.S. Witkowski, Ł. Wach-nicki, E. Łusakowska, T. Dietland, M. Sawicki, ActaPhys. Pol. A 119, 692 (2011).
[5] C. Klingshirn, Phys. Status Solidi B 244, 3027(2007).
[7] M. Godlewski, E. Guziewicz, G. Łuka, T. Krajew-ski, M. Łukasiewicz, Ł. Wachnicki, A. Wachnicka,K. Kopalko, A. Sarem, B. Dalami, Thin Solid Films518, 1145 (2009).
Nanodruty ZnO otrzymywane metodą OsadzaniaWarstw Atomowych do zastosowań sensorowych
mgr ŁUKASZ WACHNlCKl1), mgr BARTŁOM|EJ S. WITKOWSK!'),mgr SYLWIA GIERAŁTOWSKA1), mgr KRZYSZTOF KOPALKO'),
prof. nzw. dr hab. ELZBIETA GUZIEWIC7l\, prof . dr. hab. MAREK GODLEWSKIt,zl1) lnstytut Fizyki Polskiej Akademii Nauk, Warszawa
2) Uniwersytet Kardynała Stefana Wyszyńskiego, Szkoła Nauk Scisłych, Warszawa
Tlenek cynku jest materiałem półprzewodnikowym typu ll-Vl,który ma wiele zastosowań w dziedzinach nauki takich jakbiologia, medycyna i nowoczesna elektronika. Tlenek cynku(ZnO) ze względu na swoje szczególne właściwości fizycznei chemiczne moze być stosowany w urządzeniach sensoro-wych [1 ], pzezroczystej elektronice, ogniwach słonecznychoraz diodach LED. lstotną cechą materiałów stosowanychw czujnikach sensorowych jest bardzo rozbudowana mońolo-gia. W pracy przedstawiony zostanie wzrost nanodrutów ZnOnową metodą - metodą osadzania warstw atomowych ALD(ang. Atomic Layer Deposition). ALD charakteryzuje się ni-skim kosztem procesów technologicznych, mozliwością osa-dzania warstw na róznych podłozach i duza powtarzalnościąprocesów To umotywowało nas do opracowania alternatyw-nej metody otrzymywania nanosłupków ZnO, otrzymywanychdotychczas wieloma metodami technologicznymi [2-3].
Metoda osadzania warstw atomowych, opracowana pżezfińskiego fizyka Tuomo Suntolę pienłotnie wykozystywana byłado wzrostu epitaksjalnego |4].Zev,zględu na prostotę Ę metodyzczasemzaczęto uzywać jej do osadzania warstw nie tylko mo-nokrystalicznych, ale także warstw polikrystalicznych i amońicz-nych [s-6]. W niniejszej pracy zostaną zaprezentowanie nono-struktury tlenku cynku, który jest materiałem półpzewodnikowymnalezącym do grupy ll-Vl układu okresowego. Materiał ten cha-rakteryzuje się prostą przenruą energetyczną która w pokojowejtemperaturze wynosi w pzyblizeniu 3,37 eV [7]. Tak duża pze-nłva energetyczna sprawia, że Zno jest materiałem przezroczy-stym. Posiada on wysokąenergię wiąania ekscytonu (60 meV)
[8], co ma duży wpływ na właściwości optyczne tego materiału.Tak wysoka wańość wiązania ekscytonu powoduje, ze ekscytonw ZnO pozostaje nierozerwalny nawet w temperatuze pokojo-wej. Jednocześnie ZnO ma bardzo wysokątnłałośó chemicznąco sprawia, ze nie ulega on degradacji.
Dzięki swoim ciekawym właściwościom ZnO ma wie-le zastosowań w elektronice. W szczególności fakt, że ZnOposiada szeroką przeruę energetyczną powoduje, ze jestprzezroczysĘ w widzialnej części widma elektromagnetycz-nego. Jeżeli silnie domieszkujemy ZnO na typ n np, poprzezdodatek aluminium, związek ten może być wykorzystywanyjako przezroczysty przewodzący tlenek TCO (ang. Transpa-rent Conductive Oxide). Jednocześnie dzięki wysokiej energiiwiązania ekscytonu, istnieje możliwość zastosowania ZnOw diodach świecących LED (ang. Light Emitting Diode) |9|.
W związku z szybkim rozwojem przemysłu ogromne zna-czenie odgrywa ochrona środowiska naturalnego. Waznymelementem ochrony środowiska jest badanie atmosfery i hy-drosfery pod względem zanieczyszczeń. Do tego celu słuząsensory, czyli urządzenia które wykrywają skład np, gazuw atmosferze i przetwarzalą te dane na sygnał elektryczny.Poniewaz rożne gazy lub ciecze mają bardzo duzy wpływ napzewod n ictwo powierzchn iowe ZnO, dlatego istn ieje szansana uzycie tego półprzewodnika jako sensora [2]. W niniejszejpracy zostanie pokazana struktura sensorowa na bazie ZnOotrzymana przy użyciu metody osadzania warstw atomowych.
ELEKTRoNlKA 8/201,1
Technologia otrzymania nanostrukturDo procesu zostały użyte podłoża arsenku galu, które przedrozpoczęciem właściwego procesu zostały wytrawionei umyte w rozpuszczalnikach, po czym na ich powierzch-ni napylono cienką warstwę złota. Podłoże z warstwą złotazostało wygrzane w odpowiedniej temperaturze w procesieszybkiego wygrzewania RTP (ang. Rapid Thermal Proces-slng). Proces ten miał na celu doprowadzenie warstwy złotado fazy ciekłej, która w takim stanie miesza się z cząstecz-kami galu zawańymi w podłożu, tworząc w ten sposób kul-ki gal-złoto o średnicy około 100 nm (patrz rys. 1). Proceswłaściwy, czyli wzrost słupków ZnO na powierzchni arsenkugalu, odbywał się w reaktorze F-120 Microchemistry pro-dukcji fińskiej. Do wzrostu warstw wykorzystywane byłydwa prekursory (związki chemiczne). Pierwszym prekurso-rem była woda dejonizowana. Drugim nieorganiczny zwią-zek - chlorek cynku o wzorze sumarycznym ZnClr. Wodadejonizowana uzyta była jako prekursor tlenowy, natomiastZnCl, 1ako prekursor cynkowy. Do płukania pomiędzy kolej-nymi pulsami prekursorów użyto azotu wysokiej czystości(99,9999%). Podczas procesu ALD na podłozu zachodziłanastępująca reakcja podwójnej wymiany:
ZnClr+ HrO - ZnO + 2HCl
Właściwości optyczne nanostruktur zostały scharaktery-zowane za pomocą spektrofluorymetru CM2203, w którymzródłem pobudzenia była lampa ksenonowa. Celem tych po-miarów było określenie jakości otrzymywanych struktur, W po-przednich naszych pracach zaobsenłowaliśmy korelacje po-m iędzy wydaj nością emisj i przykrawędziowej (ekscytonowej)a jakością strukturalną warstw. Mońologia powierzchni bada-na była mikroskopem sił atomowych AFM (ang. Atomic ForceM icro scope) oraz skan i n gowym m ikroskopem elektronowymSEM (Scanln g Electron Microscope).
Rys. 1. Obraz z mikroskopu sił atomowych kulek Ga-Au na po-wierzchni GaAsFig, 1. lmage of eutectic droplets Ga-Au observed under atomicforce microscope
B5
Dyskusja wynikówW wyniku optymalizacji procesu wzrostu zostały otrzymanedwa rodzaje nanostruktur tlenku cynku (nazwy techniczne:Nano'l patrz rys. 2a oraz Nano2 - rys. 2b), Struktura Nano1charakteryzuje się kolumnowym wzrostem tlenku cynku gdziewysokość pojedynczego słupka wynosi około 1 pm natomiastśrednica około 250 nm. Wzrost słupków odbywa się na po-wierzchni kropek Ga-Au, które przypuszczalnie zarodkująwzrost nanokolumn tlenku cynku, gdyz w przypadku czystegopodłoża GaAs (bez kropek Ga-Au) biorącego udział w tym sa-mym procesie wzrost kolumn ZnO nie został zaobserwowa-ny. Struktura Nano2 równiez charakteryzuje się kolumnowymwzrostem, lecz w tym przypadku wzrost słupków katalizowanyjest przez kroplę mieszaniny Ga-Au, która płynie na fronciewzrostu i zastyga na końcu słupka po pzeprowadzonym pro-cesie. Jest to klasyczny wzrost VLS zaproponowany oryginal-nie do wytwarzania nanosłupków krzemowych [10]. Wysokośćsłupków w strukturze Nano2 podobnie jak w pierwszej struktu-rze wynosi około ,t pm natomiast średnica około 20 nm.
Rysunek 3 przedstawia zalezność intensywności foto-luminescencji w funkcji długości fali dla dwóch struktur. Dlapróbki Nano1 mozna zaobserwowaó silne świecenie w okoli-cach 380 nm. Region ten jest miejscem występowania emisjikrawędziowej w ZnO, Wysoka intensywność świecenia w tymrejonie oznacza dobrą jakość otrzymanego tlenku cynku [10].W zakresie długości fali od 450 nm do 700 mn występujeświecenie pochodzące od defektów, które jest niewielkiew porównaniu do świecenia krawędziowego i oznacza wy-soką jakość otrzymanych słupków tlenku cynku. W struktu-rze Nano2 nie zostało zaobserwowane zarówno świeceniekrawędziowe jak i świecenie defektowe. Prawdopodobniebrak tego świecenia związany jest z wielkością krystalitóww strukturze Nano2, które w porównaniu ze strukturą Nano1są duzo mniejsze.
Rys, 2. Powierzchnia struktury: a) Nano1 i b) Nano2 charaktery-zowana skaningowym mikroskopem elektronowymFig, 2. Suńace structure of: a) Nanol b) Nano2 characterized byscanning electron microscope
- Nano 2
- Nano 1
400 500 600 700 800
Wavelenglh, nm
Rys. 3. Wynik fotoluminescencji w temperaturze pokojowej dlastruktury Nano1 i Nano2Fig, 3. Room temperature photoluminescence for Nano'l andNano2 structures
B6
0 20 40 60 80 100 120 140 160 o
cżas lśl
50 10! l50
cżas ts]
Rys,4. Zależność oporu od czasu dla próbek Nano1 i Nano2 ponałożeniu kropli etanoluFig. 4. Dependence of resistance for Nano1 and Nano2 samplesafter applying a drop of ethanol
Struktury Nano1 i Nano2 zostały poddane badaniom senso-rycznym. Na powierzchni zostały naparowane kontakty TilAui tak przygotowany struktura została umieszczona w układziedo pomiaru zależności oporności od czasu.
Na powiezchnię struktury Nano1 oraz Nano2 zostały zakra-plane rozpuszczalniki aceton, izopropanol, trichloroetylen jak rów-niez alkohol etylowy. Na 47s. 4 przedstawiony zostałprzykładowywykres zachowania układu sensorycznego dla struktur Nano1i Nano2, Pomiar został rozpoczęty na czystej próbce następniepo wpuszczeniu kropli alkoholu został zaobserwowany znacznywzrost oporności, Po odparowaniu alkoholu z powierzchni próbkimierzony opór wraca do stanu sprzed nałozenia kropli. Podobnezjawisko zostało zaobsenłowane dla wszystkich rozpuszczalni-ków. Nalezy tu podkreślić, ze ,,resetowanie" detektora nie wyma-gało podgzewania sensora, który wykazywał wyjściową opor-ność w kilka minut po nałozeniu kropli rozpuszczalnika,
podsumowanie
W procesie osadzania warstw atomowych zostały otrzymanenanostruktury tlenku cynku, które wykazują silne właściwościsensoryczne nazwiązki rozpuszczalników i alkoholi, co mozemieć zastosowanie w wykrywaniu niebezpiecznych substan-cji w środowisku, Na szczególną uwagę zasługuje prostotaresetowania urządzenia - brak konieczności wygrzewaniaczy czyszczenia związkami chemicznymi. Urządzenie samo-czynnie się,,zeruje" po odparowaniu rozpuszczalnika. lstotnajest równiez metoda otrzymywania nanostruktur tlenku cynku.Metoda ta jest łatwo implementowana w przemyśle i stanowistosunkowo tanią alternatywę dla podobnych struktur wytwa-rzanych innymi metodami wzrostu.
Badania wspierane przez Unię Europejską w ramach Europej-skiego Funduszu Rozwoju Regionalnego, w ramach dotacji ln-nowacyjna Gospodarka (POlG.01.01.02-00-008/08).
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