Sidewall GaAs tunnel junctions fabricated using molecular

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Sidewall GaAs tunnel junctions fabricated usingmolecular layer epitaxyTo cite this article: Takeo Ohno and Yutaka Oyama 2012 Sci. Technol. Adv. Mater. 13 013002

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Sci. Technol. Adv. Mater. 13 (2012) 013002 (16pp) doi:10.1088/1468-6996/13/1/013002

TOPICAL REVIEW

Sidewall GaAs tunnel junctionsfabricated using molecular layer epitaxyTakeo Ohno1 and Yutaka Oyama

Department of Materials Science, Graduate School of Engineering, Tohoku University,Aramaki Aza Aoba 6-6-11-1020, Sendai 980-8579, Japan

E-mail: [email protected] and [email protected]

Received 16 June 2011Accepted for publication 10 November 2011Published 2 February 2012Online at stacks.iop.org/STAM/13/013002

AbstractIn this article we review the fundamental properties and applications of sidewall GaAs tunneljunctions. Heavily impurity-doped GaAs epitaxial layers were prepared using molecular layerepitaxy (MLE), in which intermittent injections of precursors in ultrahigh vacuum wereapplied, and sidewall tunnel junctions were fabricated using a combination of device mesa wetetching of the GaAs MLE layer and low-temperature area-selective regrowth. The fabricatedtunnel junctions on the GaAs sidewall with normal mesa orientation showed a record peakcurrent density of 35 000 A cm−2. They can potentially be used as terahertz devices such as atunnel injection transit time effect diode or an ideal static induction transistor.

Keywords: molecular layer epitaxy of gallium arsenide, impurity doping, sidewall tunneljunction, thin film, deep level, quantum-confined tunnelling, terahertz devices

1. Introduction

A tunnel junction (Esaki tunnel diode) consists of joined,heavily doped p- and n-type semiconductor layers. The dopingresults in a depletion layer with a width of the order of 10 nmor less. Owing to quantum tunnelling through this layer, anegative differential resistance (NDR) region appears in thecurrent–voltage characteristic of the diode.

The first semiconductor tunnel junction was realizedby Esaki using germanium [1]. Later, tunnel junctions havebeen fabricated using Si [2, 3], Si/SiGe [4, 5], GaAs [6–15],InAs/Si [16], GaInAs/GaInNAs [17], InP/InGaAs [18],GaAsSb/InGaAs [19] and InAsP/GaAsP [20]. Thesetunnel junctions found numerous applications. Forexample, a tunnel junction can reduce the resistance ofa semiconductor laser [21, 22]. Photovoltaic conversionefficiencies of up to 40% have been demonstrated by solarcells containing tunnel junctions of III–V semiconductors

1 Present address: International Center for Materials Nanoarchitectonics,National Institute for Materials Science, Namiki 1-1, Tsukuba 305-0044,Japan.

[23, 24]. Structures such as a spin-injection light-emittingdiode [25] and a tunnel-injection transit time effect diode(TUNNETT) [26–29] have also been reported. Tunneljunctions have been used in transistor structures, such astunnel transistors [30], tunnel-source field-effect transistors(FETs) [31], δ-doped tunnel FETs [32], heterojunction bipolartransistors (HBTs) with a degenerately doped emitter [33],and ideal static-induction transistors (ideal SITs) [34–36].

Recently, terahertz (THz) devices have attractedsignificant attention. Their (0.1–10) × 1012 Hz frequencyrange is suitable for a wide variety of applications, suchas wireless communications, spectroscopy, biomedicaland molecular imaging and high-speed signal processing.Consequently, Gunn diodes [37, 38], resonant tunnellingdiodes [39, 40], quantum cascade lasers [41, 42] andemitters based on transistors [43, 44] have been investigatedfor use in THz oscillators, and a high electron mobilitytransistor [45–47], an HBT [48, 49] operated at severalhundred GHz and a metal–insulator–metal THz detector[50, 51] have been developed.

In the case of a semiconductor device employing an Esakitunnel junction, device performance is determined by the

1468-6996/12/013002+16$33.00 1 © 2012 National Institute for Materials Science Printed in the UK

Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

quality of the tunnel junctions. Peak current density (Jp) isone of the most important factors that contribute to the outputpower of the device. An increase in Jp effectively improvesthe output signal. The peak-to-valley current ratio PVCR =

Jp/Jv, where the valley current density Jv corresponds tothe excess current, also indicates the quality of the tunneljunction—the larger the PVCR, the higher the quality.The tunnel junction demonstrates the importance of lownegative resistance, because the resistance per junction areadetermines the maximum oscillation frequency. In addition,low zero-bias specific resistivity is required to reduce parasiticJoule heating in the device. In general, these characteristicsaffect the performance of semiconductor devices, includingtunnel junctions. A high-quality tunnel junction should beheavily doped (a higher degeneracy level) in the p- andn-type semiconductor layers and have very steep profiles ofimpurities at the tunnel junction interface.

Here, we show the importance of high concentrations andabrupt profiles of impurities in the TUNNETT device andideal SIT that we developed. The TUNNETT consists of GaAsp+n+in+ multilayers with a p+n+ tunnel junction. Under areverse bias, electrons are injected through tunnelling fromthe p+ anode to the i drift layer. The oscillation frequencyand power are determined by the thickness of the i layerand the tunnelling current density (i.e. doping concentration),respectively. We reported that the TUNNETT can workin the continuous-wave mode with fundamental oscillationfrequencies from 60 to 700 GHz [28]. Importantly, the n+

layer at the p+n+ tunnel junction must become a completedepletion layer that requires abrupt impurity profiles. Theideal SIT is also based on GaAs n+ p+n+ multilayers witha potential barrier induced electrostatically between the n+

source and the n+ drain. The barrier height formed by thecompletely depleted p+ layer is controlled by the appliedvoltages of the gate and the drain, which indicates thetunnelling and ballistic transport without carrier scattering inthe channel. Our group reported a tunnelling and ballisticoperation with an estimated electron transit time shorter than10−13 s, which is suitable for THz operation [35]. Both heavydoping and a very steep concentration profile are required forhigh barrier and short channel in this device.

We used molecular layer epitaxy (MLE) as a methodof GaAs growth to fabricate the GaAs TUNNETT andGaAs ideal SIT. Nishizawa et al achieved the growthof a monomolecular GaAs epitaxial layer by applyingintermittent injection of arsine (AsH3) and trimethylgallium(TMG) precursors in an ultrahigh vacuum [52, 53]. Inthis method, the GaAs layer is epitaxially grown in amonomolecular unit by molecular reactions on the surface,which is why it is called MLE. The MLE was inspiredby the Suntola’s idea of an atomic layer epitaxy forII–VI compound semiconductor polycrystals [54, 55], whichprovides a self-limiting mechanism and has been widely usedfor growth with an atomic-level thickness [56–58]. Similarself-limiting GaAs growth methods have been reportedthat use a combination of a carrier gas and a specialtechnique, such as a rotating susceptor [59], a dual growthchamber [60], laser irradiation [61], a short-residence-time

reactor vessel [62], a pulse jet [63] or an atmospheric pressurereactor [64].

Numerous GaAs p+n+ tunnel junctions have beenreported, as shown in table 1. They had the Jp values of only10–1000 A cm−2 or less for the tunnel junctions fabricated bymolecular beam epitaxy (MBE) [6, 7, 10, 13], metal–organicvapour phase epitaxy (MOVPE) [8, 9] and metal–organicchemical vapour deposition (MOCVD) [14, 15]. We haveachieved the fabrication of a sidewall GaAs tunnel junctionwith a record Jp of 35 000 A cm−2 [11, 12]. The advantagesof MLE growth which contribute to a high J p include lowtemperature and the possibilities of heavy doping and controlof the film thickness with atomic accuracy.

In this paper, we review the fabrication of sidewall GaAstunnel junctions by MLE.

2. Sidewall GaAs tunnel junctions fabricated byarea-selective regrowth with molecular layer epitaxy

The sidewall GaAs tunnel junction [11, 65] consists of ann+- and p+-GaAs epitaxial layer with a metal pad on a(001)-oriented semi-insulating GaAs substrate, as shown infigure 1(a). The junction has a 100 µm width, 50 nm depthand 6 × 10−8 cm2 area. The area can be easily lowereddown to 10−8 cm2, and the wide metal contact pad on thesemiconductor surface can reduce the parasitic resistance.Figure 1(b) shows a cross-sectional image of the fabricatedsidewall tunnel junction observed by scanning electronmicroscopy.

Sidewall tunnel junctions were fabricated by MLEas outlined in figure 2. Initially, a tellurium and sulphurco-doped n+-GaAs layer was grown at 360 ◦C. This epitaxiallayer had a thickness of 50 nm and electron concentrationof 2 × 1019 cm−3. Arsine and triethylgallium (TEG) wereused as precursors for the epitaxial growth of GaAs,and diethyltelluride (DETe) and diethylsulphur (DES) wereused for donor doping. AsH3, TEG, DETe and DES wereintroduced separately into ultrahigh vacuum. To achievea high impurity concentration, DETe was exposed onthe gallium-stabilized surface (AG mode), and DES wasintroduced on the arsenic-stabilized surface (AA mode)during the MLE growth cycles [66]. Following the initialprocedure, a silicon nitride (SiNx ) layer was deposited byremote-plasma CVD at 275 ◦C, and SiNx windows wereopened using conventional photolithography and reactive-ionetching (RIE), followed by the final slight wet etching ofthe remaining thin SiNx . Through these windows, the GaAssidewall mesa was formed to a depth of 60 or 120 nm usinga H2SO4-based solution. To obtain a high-quality junctioninterface, an AsH3 surface treatment [12, 67] was performedin the growth chamber right before the epitaxial regrowth. Inthis process, the SiNx -patterned GaAs substrate was heated to350 ◦C for 30 min under an AsH3 pressure of 8 × 10−2 Pa.

After the AsH3 surface treatment, area-selective epitaxialregrowth of a beryllium-doped p+-GaAs layer was carriedout at ∼290 ◦C using bismethylcyclopentadienyl-beryllium[Be(MeCp)2]. Be(MeCp)2 was introduced by the AG modeof MLE doping to achieve a high hole concentration of

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Table 1. Comparison of GaAs tunnel junctions operated at room temperature.

Growth p-type n-type Jp Vp (V) PVCR Negative Zero-bias Note Referencemethod dopant dopant (A cm−2) resistance (k� µm2) specific

resistivity (� cm2)

MBE C Si 400 – – – – [6]MBE Be Si 1824 0.19 28.2 25.1 7.8 × 10−5 Containing excess [7]

arsenic [7]MOVPE C Si 17 0.08 17 – – [8]MOVPE C Si 57 0.1 23 – – [9]MBE Mn – 28 0.25 ∼1 – – Magnetic [10]

semiconductorMLE Be Te&S 31 000 0.21 2.1 1.2 5.3 × 10−6 Sidewall structure [11]MLE Be Te&S 35 000 0.25 3.2 – – Longer AsH3 [12]

treatmentMLE Be Te&S 23 500 0.57 1.7 0.09 2.2 × 10−5 Z-type negative [12]

resistanceMBE Be Si 16 000 3.4 22 0.226 ∼1 × 10−3 pin structure [13]MOCVD C Si 500 0.17 4.5 92 9.6 × 10−5 Autocompensated [14]

impuritiesMOCVD C Si 1530 0.63 6.9 2.7 4.1 × 10−4 [15]

Top viewCross-sectional view

[110]{001}

RF

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S.I. GaAs

Ti/Au nonalloyedcontact

Sidewall tunnel junction

Sidewall tunnel junction

RF F

Regrown p+-GaAs:Be1st n+-GaAs:Te&Sepitaxy

1st n+-GaAs:Te&Sepitaxy

Top viewCross-sectional view

[110]{001}

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F

FF

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]

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Regrown p+-GaAs:Be1st n+-GaAs:Te&Sepitaxy

1st n+-GaAs:Te&Sepitaxy

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Ti/Au nonalloyedcontact

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1st n+-GaAs:Te&Sepitaxy

(a)

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n+-GaAs:Te&S layer(50 nm) Regrown p+-GaAs:Be layer

SI GaAs substrate

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100 nm

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n+-GaAs:Te&S layer(50 nm) Regrown p+-GaAs:Be layer

SI GaAs substrate

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100 nm

1 µm

n+-GaAs:Te&S

p+-GaAs:Be

(b)

Figure 1. (a) Schematic of the cross-sectional and top views of the fabricated sidewall GaAs tunnel junctions. The Ti/Au nonalloyed metalcontact pad is 100 µm in length. In the top view, the arrows indicate the positions of the sidewall tunnel junctions, and ‘F’ and ‘R’ denotethe first epitaxial layer and the regrowth layer, respectively. (b) Cross-sectional views of the sidewall GaAs tunnel junctions in the normalmesa orientation observed by scanning electron microscopy. (Reproduced with permission from [11] ©2002 AIP and [12] ©2004 ECS.)

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

1. n+-GaAs:Te&S growthon SI GaAs (001)

2. SiN CVD & SIN RIEfor opening window

3. GaAs mesa wet etching

4. AsH3 surface treatment

5. p+-GaAs:Bearea-selective regrowth

6. Ti/Au metal patterningby lift-off process

AsH3

1. n+-GaAs:Te&S growthon SI GaAs (001)

2. SiN CVD & SIN RIEfor opening window

3. GaAs mesa wet etching

4. AsH3 surface treatment

5. p+-GaAs:Bearea-selective regrowth

6. Ti/Au metal patterningby lift-off process

AsH3AsH3

Figure 2. Fabrication process of a sidewall GaAs tunnel junction.

8 × 1019 cm−3 [68]. The thickness of the epitaxial layer was60 nm. Finally, nonalloyed Ti/Au metal contacts were formedusing a conventional lift-off process. The resulting junctionareas were of the order of 10−8 cm2. The sidewall tunneljunctions were fabricated on the GaAs sidewall of the normalmesa [∼{111}A], a 45◦-inclined configuration [∼{110}A] andthe reverse mesa orientations [∼{111}B]. Importantly, exceptfor the sidewall mesa orientation, the fabrication conditionsfor these sidewall junctions were identical because three kindsof sidewall mesas were formed in the same substrate, and theregrowth process on the sidewall mesa was performed in thesame run.

GaAs tunnel junctions on the GaAs sidewalls of thenormal mesa orientation have exhibited a record Jp of up to31 000 A cm−2 with a PVCR of 2.1 at room temperature. TheJp of the 45◦-inclined configuration and that of the reversemesa orientation are 5200 and 2100 A cm−2, respectively.Figure 3 shows the J–V characteristics of the sidewall tunneljunctions with various sidewall mesa orientations.

Table 2 summarizes the electrical characteristics obtainedby J–V measurements of sidewall tunnel junctions, revealingstrong mesa-orientation dependences of Jp, Jv and resultantPVCR. The Jp value for the direct tunnelling [69] is criticallyaffected by many parameters, such as the trap density, theoccupation probability of traps and the deformation potential.The Jv is also dependent on the doping concentration [70].

The electron concentration of the first n+-GaAs layerand hole concentration of the regrown p+-GaAs layer onthe (001) substrate were approximately 2 × 1019 and 8 ×

1019 cm−3, respectively. However, the hole concentration atthe tunnel junction is p = 2.4 × 1020 cm−3 for the normalmesa, if calculated using the measured peak voltage (Vp) andassuming n = 2 × 1019 cm−3. Although lateral secondary-ionmass spectroscopy (lateral SIMS) [71] cannot be appliedto this sample because the length of the SIMS crater areamust be 50 nm or less, conventional SIMS measurementswere performed for the planar structure formed on the {001}surface under the identical epitaxial and regrowth process.Figure 4 shows the SIMS depth profiles for the p+n+ planartunnel junction made on the {001} surface. The Be profileshows a pile-up at the regrowth interface with a concentrationof up to 1020 cm−3, whereas the Te and S profiles appear flat.This value for Be agrees well with the hole concentrationestimated from the Vp. Thus, the actual doping profile has aδ-function-like peak at the tunnel junction interface, and thisfeature may be one of the reasons for the high Jp.

3. Impurity doping characteristics in the molecularlayer epitaxy of GaAs

3.1. Be doping characteristics for the p+-GaAs layer

A high concentration of impurities and abrupt profiles at thetunnel junction interface are the most important factors forachieving a high-quality tunnel junction. For acceptor doping,as widely reported in the field of MBE of GaAs [72–75],Be is a good candidate for heavy p-doping and for steepimpurity profiles because of its low diffusion coefficient [76].The covalent radius of Be is similar to the radii of Ga and As,and the lattice strain in Be-doped GaAs was expected to besmall. Consequently, the Be-doped GaAs epitaxial layer wasused for the sidewall GaAs tunnel junctions.

To investigate the doping characteristics, a Be-dopedGaAs epitaxial layer on the GaAs substrate was grownby intermittent injection of AsH3, TEG and Be(MeCp)2

in ultrahigh vacuum [68]. The substrate was heated by aninfrared (IR) lamp, and the temperatures were controlled by anIR pyrometer. Arsine and TEG were chosen as precursors forthe epitaxial growth of GaAs, and Be(MeCp)2 was the sourcegas for acceptor doping. As substrates, we used undopedsemi-insulating (100), (111)A, (111)B and (110) GaAs wafersprepared by the liquid encapsulated Czochralski method.

Figure 5 shows the relationship between the Hall mobilityand the hole concentration in Be-doped p+-GaAs grownon a (100) GaAs substrate and obtained from Hall-effectmeasurements using the van der Pauw method. Our MLEdata fit well into the Hilsum’s formula [77], as well asthe results reported by other groups for Be-doped GaAsgrown by MBE at 600 [78], 530 [79] and 270 ◦C [80].The Hall mobility of the MLE layer is nearly equal tothat of the MBE-grown layer, even at a very low growthtemperature (∼290 ◦C). We performed x-ray multicrystaldiffractometry (XRD) analysis using the g = 004 diffractionplane with a prefixed Ge (220) asymmetric-configuration

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Table 2. Sidewall GaAs tunnel junction characteristics at room temperature.

Sidewall mesa Jp (A cm−2) Jv (A cm−2) Vp (V) PVCR Negative resistance Zero-bias specific resistivityorientation (k� µm2) (� cm2)

Normal mesa 31 000 14 900 0.214 2.1 1.2 5.3 × 10−6

45◦-inclined configuration 5200 3110 0.130 1.7 7.3 1.0 × 10−5

Reverse mesa 2100 1530 0.136 1.4 20 2.9 × 10−5

Figure 3. J–V characteristics of sidewall GaAs tunnel junctions at room temperature (RT) in the (a) normal mesa, (b) 45◦-inclined and (c)reverse mesa sidewall orientations. (Reproduced with permission from [65] ©2004 Elsevier.)

Figure 4. SIMS profiles for the p+n+ plane GaAs tunnel junctionfabricated on a {001} surface by the identical regrowth conditionsas the sidewall tunnel junction.

monochromator. Be-doped GaAs layers with concentrationsup to 1020 cm−3 exhibited no double peaks in their XRDrocking curve implying that Be doping induced insignificantlattice strain in GaAs. These results indicate that the Be-dopedp+-GaAs MLE layers grown at low temperatures have a highcarrier concentration and good crystal quality even at heavydoping.

Figure 6 shows the SIMS depth profile of Be in aBe-doped GaAs multilayer grown on a (100) GaAs substrateat 290 ◦C. The sample consists of a stack of undopedGaAs, Be-doped GaAs grown in AG mode, undoped GaAs,Be-doped GaAs grown in an AA mode, undoped GaAsand GaAs substrate. Here, the AA mode means that theBe(MeCp)2 gas was introduced after the GaAs surfacehad been As-terminated by exposure to AsH3, followedby evacuation. In contrast, the AG mode indicates thatthe impurity gas was introduced after exposure to TEG.As previously mentioned, the surface stoichiometry can becontrolled by changing the timing of the introduction of the Be

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Figure 5. Relationship between the Hall mobility and the holeconcentration in the Be-doped GaAs MLE layer at roomtemperature. The broken line corresponds to the empirical Hilsum’sformula. (Reproduced with permission from [68] ©2003 Elsevier.)

Figure 6. SIMS profile of Be in a multilayered Be-doped GaAslayer grown by MLE. (Reproduced with permission from [68]©2003 Elsevier.)

dopant. In this sample, although two Be-doping layers weregrown by changing only the doping mode, the incorporation ofBe was enhanced when a Ga-stabilized surface was exposedto Be(MeCp)2, i.e. the AG mode. A substrate-orientationdependence of Be doping in GaAs MLE was also apparent,as shown in figure 7 [81].

To achieve heavy doping of Be in GaAs, the surfacestoichiometry prior to the introduction of the Be dopant gaswas controlled by changing the AsH3 and TEG supply times.As shown in figure 8(a), the Be concentration in AA modedecreased with increasing AsH3 injection time. However, theconcentration of Be was enhanced by increasing the AsH3

injection time in AG mode. In the case of the TEG supply,incorporation of Be was enhanced with decreasing TEGexposure duration in AA and AG modes. It is possible toestimate the effects of surface stoichiometry on the surfacereaction using rate-law considerations. The rate law for Beincorporation can be expressed as

r

(≡

d [Be]

dt

)≡

[Be]

t= k

[Be(MeCp)2

]α[AsH3]β [TEG]γ ,

(1)

Figure 7. Substrate orientation dependence of the Be concentrationfor two impurity doping modes in the MLE method. (Reproducedwith permission from [81] ©2008 Wiley-VCH.)

where r is the chemical reaction rate, [Be] is the Beconcentration, t is the source-gas supply time, k is the rateconstant, and α, β and γ are the orders of the reaction.Figure 8(b) shows the AsH3 supply dependences of thereaction rate for Be incorporation for the two doping modes.The order of reaction for the AsH3 supply (1.3 for AA modeand 0.7 for AG mode) is smaller than that for the TEGsupply (1.4 for AA mode and 1.9 for AG mode). From thetheorem of monomolecular reactions [82] and unimolecularcollision [83], Be(MeCp)2 is less activated by the TEG supply,and thus, the order of reaction by the TEG supply tendsto be 2. On the contrary, Be(MeCp)2 is activated more bythe AsH3 supply, and thus, the order of reaction by AsH3

supply tends to be 1. These results, which depend on thesurface stoichiometry, are useful for obtaining high dopingconcentrations.

3.2. Alternate Te and S doping characteristics forn+-GaAs layer

Te and S can be used as n-type dopants of GaAs. Thediffusion coefficient of Te is one of the smallest amongthe n-type dopant impurities. Thus, Te atoms hardly diffuseduring growth, and a very steep impurity profile wasexpected. However, a differential strain was observed in thehomoepitaxial layers doped with 1019–1020 cm−3 of Te [84].On the contrary, differential strain was low in the heavilyS-doped GaAs epitaxial layer, although the covalent radiusof S is smaller than the radii of Ga and As. Sulphur is moresuitable for obtaining a steep impurity profile than Se or Si.In accordance with the previously discussed results, Te andS co-doping were expected to reduce the lattice strain, whichwould then enhance the incorporation of impurities becauseof the strain compensation. Thus, to investigate the Te andS co-doping, MLE growth on a GaAs (100) substrate wasperformed using AsH3, TEG, DETe and DES. Previously,co-doping was studied for InGaAs:Zn and C [85], GaAs:Yband Al [86], ZnTe:Al and N [87], ZnCrTe:I and N [88],ZnO:Al and N [89], SnO2:Co and Al [90] and ZnO:Mn andN [91].

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Figure 8. (a) Be concentration as a function of AsH3 injection time for AA and AG modes. (b) Be incorporation reaction rate versus. AsH3

supply for the two modes. (Reproduced with permission from [68] ©2003 Elsevier.)

Figure 9. Relations between the S and Te concentrations when DES is introduced prior to DETe injection (a) and vice versa (b).(Reproduced with permission from [66] ©2002 Elsevier.)

The concentration of Te increased monotonically withincreasing S coverage, whereas DETe was supplied to aGaAs surface covered with S in AA mode, as shown infigure 9(a) [66]. In contrast, the S concentration decreasedmonotonically with increasing concentration of Te whenDETe was introduced before DES was supplied in AA mode,as shown in figure 9(b). The effect of the underlying S layeron the enhanced incorporation of Te can be explained bythe large electronegativity of S with respect to Te and Asand its relationship with the charge distribution in DES andDETe molecules near the surface. Conversely, the effect of theunderlying Te layer on the reduced incorporation of S can beexplained by the small electronegativity of Te relative to thatof S.

Table 3 summarizes the electrical and crystallographiccharacteristics of Te and S co-doped GaAs MLE layersthat were investigated by Hall-effect measurements andXRD analyses. All samples were grown at the same GaAssubstrate temperature of 360 ◦C and had an electron density of∼1.7 × 1019 cm−3; this specific value is close to the effectivedensity of electron states in degenerate GaAs. When Te-dopedand alternate Te and S co-doped layers were compared, thedifferential strains (1a/a) were almost the same at 3.1–3.6 ×

10−3; however, the full-width at half-maximum (FWHM) wassmaller for the co-doped layer (137 arcsec). This result means

that the crystal quality was improved by co-doping with Teand S, presumably because of strain compensation in thecrystal structure.

4. Defect aspects in sidewall GaAs tunnel junctions

4.1. Defects at the sidewall GaAs tunnel junction interface

The identification and control of defects in semiconductorsis one of the most important research fields becausedefects determine the electrical and optical characteristics ofsemiconductors and crucially affect the material quality anddevice performance. To reveal deep levels related to the tunneljunction interface of the sidewall GaAs tunnel junctions, aphotocapacitance measurement was applied to the sidewallp+in+ junctions, and the emission spectra caused by deeplevels at the junction interface were obtained.

The measurement of photocapacitance allows evaluatingdeep levels in semiconductors and oxides. Details of thephotocapacitance equipment and the measurement system aredescribed elsewhere [92, 93]. The defects in the depletionlayer are ionized by monochromatic light at low temperature,and the ionized-level density is then determined from thechange in the depletion-layer capacitance. This measurementprovides a more precise determination of level density

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Table 3. Room-temperature electrical and crystallographic characteristics of S-doped, Te-doped and Te and S co-doped GaAs grown byMLE.

Dopant Thickness (nm) Concentration of S (cm−3) Concentration of Te (cm−3) n (cm−3) 1a/a FWHM (arcsec)

S 77 4.5 × 1019 – 1.6 × 1019 – –Te 110 – 4.2 × 1019 1.7 × 1019 3.1 × 10−3 165Te&S 118 2.3 × 1019 4.5 × 1019 1.6 × 1019 3.6 × 10−3 137

Reverse mesa0.88 eV

0.75 eVCommon

1.04 eV

EV

EC

Conduction band

Normal mesa 45 -inclinedconfiguration

0.90 eV

Valence band

Reverse mesa0.88 eV

0.75 eVCommon

1.04 eV

EV

EC

Conduction band

Normal mesa 45 -inclinedconfiguration

0.90 eV

Valence band

(b)

Figure 10. (a) Photocapacitance emission spectra at 6.6 K ofsidewall GaAs p+in+ junctions as a function of sidewall mesaorientation. 1C/C is the normalized change in junctioncapacitance, which is proportional to the deep-level density.(b) Simple flat-band energy diagram that corresponds to theobserved optical transitions. (Reproduced with permissionfrom [96] ©2006 Wiley-VCH.)

and activation energy compared with other methods suchas deep-level transient spectroscopy [94] and photoinducedcurrent transient spectroscopy [95] because the ionization ofdeep-level traps is achieved by monochromatic illumination ata low fixed temperature.

Figure 10 shows the photocapacitance emission spectraat 6.6 K of the sidewall p+in+ junctions with differentsidewall mesa orientations [96]. The increase in the junctioncapacitance (1C/C) is caused by the photoionization of theoccupied deep levels, and the decrease in 1C/C is thencaused by the occupation of the deep levels by electrons.In the various sidewall mesa orientations, the commondeep levels were detected at 0.75 eV below the conductionband (EC–0.75 eV) and might be attributed to a deepdonor level in GaAs (EL2). However, the photocapacitancespectra exhibited differences in the neutralized-level positionand density that depended on the three sidewall mesaorientations. Importantly, the deep-level densities (NT), whichare proportional to 1C/C , were 1C/C = 0.44, 0.48 and

0.63, respectively. The magnitude of NT increases in theorder normal mesa <45◦-inclined configuration<reversemesa orientation.

As shown in table 2, the PVCR of the sidewall tunneljunctions show a strong dependence on the sidewall mesaorientation. The PVCR increases in the following order:reverse mesa <45◦-inclined configuration<normal mesaorientation. Here, the NT measured by the photocapacitancemethod was the lowest on the normal mesa orientation,whereas PVCR obtained by the J–V measurement was thehighest on the normal mesa orientation. This result showsthat the deep-level densities and the PVCRs do correlate andthus the photocapacitance spectra can explain the sidewallmesa orientation dependence of the sidewall tunnel junctioncharacteristics. Deep levels at the tunnel junction interface aredependent on the sidewall mesa orientations, and these levelsseriously affect the conduction characteristics in sidewallGaAs tunnel junctions. Although the Jv for the normal mesaorientation was the highest in the present tunnel junctions, thisis because the Jv is dependent not only on NT but also on thedoping concentration [70].

4.2. AsH3 surface treatment for the improvement of sidewallGaAs tunnel junction characteristics

Semiconductor devices operated by tunnelling and ballisticconduction mechanisms require especially high-quality tunneljunctions in which defect states at the junction interface arewell controlled. To obtain a high-quality junction, the effectsof the AsH3 surface treatment just prior to regrowth are shownfor the improvement of PVCR.

Figure 11 shows the PVCR as a function of AsH3 surfacetreatment time (tFD). The PVCR increases monotonicallyfrom 1.6 to 4.2, when tFD became longer [12]. Thisincrease occurred because of the reduction in the deep-leveldensity at the junction interface. In addition, the ratio ofJv(300 K )/Jv(77 K ) was almost constant for tFD of 30and 60 min, but an AsH3 surface treatment for 120 minreduced that value. We conclude that a sufficient AsH3

surface treatment with long tFD induces shallower deeplevels, which will be followed by a less-strained atomicconfiguration around the defects. The control of surfacestoichiometry by AsH3 surface treatment has been reportedto reduce the nonstoichiometry-related defects at the regrowninterface [67]. Thus, the AsH3 surface treatment reduces thedensity of deep levels at the sidewall tunnel junction interface,and the Jv decreases, while the PVCR increases for longer tFD.

It is also shown that the longer AsH3 surface treatmentenhances the steepness of the Be profile and that the Bepile-up at the interface increases as the tFD becomes longer.

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Figure 11. Room-temperature PVCR as a function of AsH3 surfacetreatment time. (Reproduced with permission from [12] ©2004ECS.)

For instance, the Be concentrations measured using SIMSon the p+n+ tunnel junction structure on the {001} surfacewere 9 × 1019, 1.2 × 1020 and 1.5 × 1020 cm−3 for tFD = 30,60 and 120 min, respectively. The improved steepness of theBe profile and the enhanced Be interface concentration at theregrown interface are the major reasons for the high Jp ofsidewall tunnel junctions.

5. Tunnelling mechanism in sidewall GaAs tunneljunctions

5.1. Low-temperature current–voltage andconductance–voltage measurements of sidewall GaAs tunneljunctions

Figure 12(a) shows the current–voltage (I–V) characteristicsof the sidewall GaAs tunnel junctions fabricated on thenormal mesa sidewall as a function of temperature [97]. Thetemperature dependence of the VS1 near the peak voltage wasnot observed, where VS is a voltage position of the sharpstep (current step) in the NDR region. The tunnel currentflowing around the VS1 is expected to correspond to directtunnelling. The values of VS3 near the valley voltage exhibiteda temperature dependence ranging from 0.35 to 0.5 V andindicate indirect tunnelling. The PVCR value on the normalmesa was 4.7 at 15 K.

The sidewall GaAs tunnel junctions were evaluated byconductance–voltage measurements (G–V) at 6 K and atroom temperature, as shown in figure 12(b) [97]. In thedifferential G–V curve, the peaks at 9, 49 and 60 mV can beclearly observed only at 6 K, which corresponds to indirecttunnelling of the phonon emission type. Considering thephonon spectrum, the peaks above 37 mV can be assignedto multiphonon-assisted tunnelling. However, it is not clearwhether the 9-mV peak originates from phonon-assistedtunnelling because this peak (close to 0 V) might be causedby a zero-bias anomaly [98–101]. If the peak at 9 mV doesnot relate to a zero-bias anomaly, the peak at 7 mV mightrepresent a transverse-acoustic L phonon (TA(L)), and thepeak at 18 mV might be attributed to tunnelling assisted

by the two-phonon combination TA(L) + TA(X) (inset infigure 12(b)).

5.2. Fine structures in quantum-confined sidewall GaAstunnel junctions

To investigate the characteristics of sidewall GaAs tunneljunctions with narrower junction depths, both p+ andn+-GaAs of MLE layers were thinned using a H2SO4-basedetchant with the Ti/Au metal contact pad used as a mask. Theachieved junction depths (d) of the sidewall tunnel junctionswere d = 30, 20, 15 and 5 nm.

The number and voltage positions of sharp steps inthe NDR region change with the junction depth [102, 103].Three VS are observed when the junction depth is d = 50 nm,as shown in figure 12(a). The VS2 was generated on thehigh-voltage side of VS1. The voltage difference betweenthe peak and VS1 (= VS1–Vp) is 8 mV at 15 K. The voltagedifference between VS1 and VS2 (= VS2–VS1) is also 8 mV.Three sharp steps are observed when the junction depth is d =

30 nm, but the voltage difference between them is different, asshown in figure 13: VS1–Vp = 30 mV and VS2–VS1 = 25 mV.The number of sharp steps increased to four when the junctiondepth was reduced to d = 20 nm, and the VS1–Vp and VS2–VS1

are 54 mV and 45 mV, respectively. At a junction depth ofd = 15 nm, only two sharp steps appeared in the temperaturerange 15–300 K. The voltage differences of the sharp stepswere 100 mV for VS1–Vp and 88 mV for VS2–VS1. No sharpsteps appeared over the entire temperature range when thejunction depth was d = 5 nm.

If the GaAs layer was quantum-confined in twodimensions (2D), the change in the I–V curve appearedbecause of the sub-band formation in the n+-GaAs conductionband. Figure 14 shows the minimum sub-band separationenergy [104–107] when a 2D quantum well structure isassumed. The energy differences in the positions of sharpsteps (VS1–Vp and VS2–VS1) are also plotted and fit well withthe sub-band separation energy. Thus, 2D–2D tunnelling wasobserved with sidewall tunnel junctions of d = 50, 30, 20 and15 nm. However, sub-band formation at the junction depthd = 5 nm was not confirmed because of the surface depletionof the 5 nm n+-GaAs layer.

Although sharp steps were observed in the I–V curves,these steps may have appeared because of oscillations in theexternal measurement circuit of sidewall tunnel junctions. Ifvoltage is applied to the tunnel junction at an unstable point inthe NDR region, oscillations normally occur [108]. However,because the measured differences in the positions of sharpsteps correspond well with the calculated sub-band separationenergy, the appearance of VS2 and the positioning of VS1 maybe attributed to the sub-band quantisation.

6. Application of sidewall GaAs tunnel junctions forTHz devices

6.1. Sidewall GaAs n+ p+n+ and p+n+in+ structures

THz devices, such as an ideal SIT, require a high and narrowpotential barrier related to a high tunnelling probability

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Figure 12. (a) Temperature dependence of the I–V characteristics of a sidewall GaAs tunnel junction. The measurement temperature wasvaried from 15 to 300 K with a step of 5 K. (b) The differential G–V characteristics of the sidewall GaAs tunnel junction at 6 K and roomtemperature. The inset shows deconvolution of the 9 meV peak at 6 K. (Reproduced with permission from [97] ©2008 Wiley-VCH.)

Figure 13. Temperature dependences of the I–V characteristics of the sidewall GaAs tunnel junctions with the indicated junction depths d.(Reproduced with permission from [102] ©2006 Wiley-VCH and [103] ©2007 Elsevier.)

of the carriers. Planar-doped barrier structures such asn-i-p-i-n GaAs [109–112], n-p-n Si/SiGe/Si [113] andn-p-n InP/InGaAsP/InGaAs [114] have been reported. TheI–V characteristics of a vertical-type GaAs planar-dopedbarrier structure grown by MLE have been also investigated[115, 116], as well as the tunnelling characteristics of sidewallGaAs n+ p+n+ and p+n+in+ structures formed by MLE [117].

To fabricate the sidewall n+ p+n+ structure, area-selectiveMLE of Be-doped p+-GaAs was performed at 290 ◦C on anormal mesa sidewall. The thickness of the regrown p+-GaAslayer, which forms a potential barrier, was determined bythe number of dopant gas injection cycles (N). Althoughthe actual doping profile on the sidewall surface cannot beobtained by SIMS measurements, its thickness is proportional

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Figure 14. Relationship between the minimum sub-band separationenergy in the 0-band of n+-GaAs and the thickness of the MLElayer when a 2D quantum well structure is assumed.

to N in the MLE growth. Accordingly, a Te and S co-dopedn+-GaAs layer was regrown at 360 ◦C. The sidewall p+n+in+

structure was also fabricated by a similar device process,and the thickness of the first regrown n+-GaAs layer wascontrolled by varying N.

As shown in figure 15, the current density flowingbetween ‘F’ and ‘R’ in the sidewall GaAs n+ p+n+ structuredecreased with increasing thickness of the p+-GaAs layer,except for N = 240. The tunnelling current was dominant inthis structure because of the weak temperature dependenceof the J–V curve. Because no Esaki peak appeared in theN = 30, 60 and 120 samples, the p+-GaAs layer was expectedto be almost depleted. However, an Esaki peak can be seenwhen the p+-GaAs layer is thick (N = 240) because thethicker p+-GaAs layer was not completely depleted. Thisresult indicates that the remains of the nondepleted layercaused by the thickness of the p+-GaAs layer are the reasonfor the Esaki peak.

Figure 16 shows the J–V characteristics of the sidewallGaAs p+n+in+ structures for the TUNNETT diode at roomtemperature as a function of the thickness of the regrownn+-GaAs layer at the sidewall p+n+ tunnel junction. Whenthere was no growth on the first regrown n+-GaAs layer,i.e. N = 0, the reverse breakdown voltage was very highbecause of the sidewall p+in+ structure, which had an i-GaAslayer 50 nm in length. An Esaki peak was also confirmedwhen the n+-GaAs layer became thicker (N = 28) (insetin figure 16(a)). However, when the n+-GaAs layer wasregrown by N = 15, which corresponds to approximately4 nm on the {001} surface, the reverse current increasedwithout the threshold voltage, and no Esaki peak appeared.This shape of the J–V curve is similar to that of avertical-type TUNNETT. Moreover, the achievement of a highcurrent density greater than –1 × 105 A cm−2 is importantto oscillation with high output power. Therefore, amongthe results of J–V characteristics in this measurement, theN = 15 sample is important for the oscillation of aTUNNETT because it is necessary to achieve a completelydepleted n+-GaAs layer to focus the electric field at thedepletion layer.

C

C

C

C

(b)

Cross-sectional view

S.I. GaAs

Sidewall tunnel junction

Sidewall tunnel junction

F F

1st n+-GaAs 100 nm

Regrown p -GaAs30, 60, 120, 240 cycles

Regrown n -GaAs

R

Cross-sectional view

S.I. GaAs

Sidewall tunnel junction

Sidewall tunnel junction

F F

1st n+-GaAs 100 nm

Regrown p+-GaAs30, 60, 120, 240 cycles

Regrown n+-GaAs

R

Figure 15. (a) J–V characteristics of the sidewall GaAs n+ p+n+

structure at 20 K and at room temperature. (b) Schematic of thecross-sectional view of the sidewall n+ p+n+ structure. (Reproducedwith permission from [117] ©2010 Wiley-VCH.)

6.2. GaAsSb quantum well and GaSb dot growth forTHz devices

Higher tunnelling probability corresponds to the higher outputpower of the TUNNETT oscillator. From the tunnellingprobability calculated by the Wentzel–Kramers–Brillouinapproximation, a material with a narrow bandgap and asmall effective mass can have a high tunnelling probability.Therefore, the growth of an antimony-containing compoundsemiconductor, such as GaAsSb or GaSb, from some suitablematerial candidates was performed by the MLE methodbecause this type of semiconductor has almost the samelattice constant as a GaAs substrate. GaAsSb and GaSbgrowth are also used for manufacturing photoconductiveTHz emitters [118–120], THz laser [121] and double HBTstructure [122].

GaAsSb/GaAs quantum wells have attracted attention fortheir potential applications in electronic and optoelectronicdevices, and these structures have been fabricated andevaluated in several reports [123–131]. In addition, anundoped GaAsSb quantum well was grown by MLE onundoped semi-insulating (100), (111)A, (111)B and (110)GaAs substrates using trimethylantimonide [132]. Figure 17shows the SIMS results for the GaAs/GaAsSb/GaAs singlequantum well structure grown on a (100) GaAs substrateat 360 ◦C. The Sb composition that was obtained fromthe fitted XRD data was GaAs0.79Sb0.21 with a thickness

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

N

NN

C

(b)

Cross-sectional view

S.I. GaAs

Sidewall tunnel junction

Sidewall tunnel junction

F F

1st p+-GaAs 200 nm

Regrown n -GaAs0, 15, 28 cycles

Regrown n -GaAs

R

Regrown i-GaAs

Cross-sectional view

S.I. GaAs

Sidewall tunnel junction

Sidewall tunnel junction

F F

1st p+-GaAs 200 nm

Regrown n+-GaAs0, 15, 28 cycles

Regrown n+-GaAs

R

Regrown i-GaAs

Figure 16. (a) Room-temperature J–V characteristics of thesidewall GaAs p+n+in+ structure. (b) Schematic of thecross-sectional view of the sidewall p+n+in+ structure. (Reproducedwith permission from [117] ©2010 Wiley-VCH.)

Figure 17. SIMS profiles of a GaAs/GaAsSb/GaAs singlequantum well structure grown by MLE on a (100) GaAs substrate.(Reproduced with permission from [132] ©2010 Wiley-VCH.)

of 11 nm. The gradient of the Sb concentration was 1 ×

1021 cm−3 nm−1, which means an approximately 10% pernm increase in Sb composition. The maximum Sb yieldwas achieved on the (100) substrate, and the Sb yieldratio for other orientations was (100) : (111)A : (111)B : (110)= 1 : 0.1 : 0.4 : 0.1. Photoluminescence (PL) spectra wererecorded at 77 K from the GaAs/GaAsSb/GaAs quantumwell structure, and the FWHM was large (100–200 meV) forall substrates. For the (111)A and (110) substrates, a blue shift

Figure 18. PL spectra (77 K) of a GaAs/GaAsSb/GaAs singlequantum well structure on (111)A GaAs as a function of theexcitation power. (Reproduced with permission from [132] ©2010Wiley-VCH.)

(100) (111)A

(111)B (110)

50 nm

1 µm

(100) (111)A

(111)B (110)

50 nm

1 µm

Figure 19. AFM images of GaSb dots on various GaAs substrates.(Reproduced with permission from [132] ©2010 Wiley-VCH.)

appeared when the excitation power of the light source wasincreased, as shown in figure 18. The phenomenon of blueshifting, which is caused by band bending (band modulation)induced by carrier accumulation, is well known for the type-IIband structure of GaAsSb/GaAs quantum wells. The GaAsSbsingle quantum wells grown by MLE on the (111)A and (110)substrates exhibit type-II band alignment, and a very abruptheterostructure was expected.

GaSb quantum dots [133–138] and rings [139, 140]formed on GaAs or embedded in a GaAs matrix have beenreported, and the MLE growth and optical properties ofGaSb dots have been investigated [132]. Figure 19 showsatomic force microscope (AFM) images of GaSb dots fordifferent substrate orientations. The minimum average dotdiameter (20 nm) and the highest dot density (7 × 1010 cm−2)were achieved on (111)A GaAs. The dots were smaller andmore abundant on (100) and (111)A than (111)B and (110)substrates. A PL peak at 1.0 eV appeared for the GaAs/GaSbdot/GaAs structure at 77 K in all four orientations. This peakposition is close to that for the dislocation-induced quantum

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dot [141] and to the value calculated considering presenceof a dislocation [142]. Therefore, a peak at 1.0 eV indicatesthe relaxation of lattice strain because of the introduction of amisfit dislocation at the interface.

6.3. Sidewall GaAs tunnel junction with TiOx gate structure

Although Si metal–oxide–semiconductor (MOS) transistorsare widely used, the GaAs MOS transistor has attractedsubstantial interest recently. Devices based on III–Vcompound semiconductors possess great advantages overSi-based devices for high-speed and high-power applications.To improve the transconductance of a GaAs transistor,high-permittivity (high-k) gate dielectric materials arerequired for fabrication. Several types of oxide films are beingconsidered for a GaAs MOS transistor, such as Al2O3 formedby atomic layer deposition (ALD) [143], HfO2 by ALD [144],HfO2 by plasma oxidation [145], Y2O3 by MBE [146], ZrO2

by RF sputtering [147] and Y2O3 by RF sputtering [148]. Inaddition, titanium oxide (TiOx ) MOS diodes were fabricatedby sputtering [149], liquid-phase deposition [150] and sol–gelmethods [151].

The operation of an ideal SIT has been realized, bothwith GaAs homojunction and AlGaAs heterojunction gatestructures, and transistor operation at room temperature withballistic electron transport has been achieved [35]. One ofthe most promising structures for an ideal SIT incorporatesa high-k dielectric gate. Recently, electrical and structuralevaluations have been performed on TiOx dielectrics thathad been deposited on a (100) GaAs substrate at 100 ◦Cby electron-beam evaporation of a TiO2 source in a mixedoxygen/argon atmosphere [152].

From the capacitance–voltage (C–V) results of GaAsMOS diodes with TiOx dielectrics at room temperature underthe accumulation conditions, the highest permittivity wasclose to 100. This permittivity is adequate in view of thefilm composition because the permittivity of TiO2 anataseis approximately 130, and the permittivity of Ti2O wouldexhibit a lower value. Based on the results of XRD with agrazing incidence angle, the deposited TiOx film contains asmall crystalline phase of TiO2 and Ti2O in the dominant TiOx

amorphous matri0x phase.The bandgap of deposited TiOx was measured using

the photocapacitance method and was 2.82 eV, as shown infigure 20. In addition, the dominant deep donor state is locatedat 1.14 eV below the conduction band of TiOx . Althoughthe origin of this deep electronic state is not yet clear, thisdeep level should affect the low breakdown field strengthof TiOx films (0.5 MV cm−1), which was obtained by J–Vmeasurement of GaAs MOS diodes with TiOx dielectrics.

A TiOx film was applied to sidewall GaAs tunneljunctions as a gate dielectric to realize tunnel transistoroperation. A schematic diagram of the TiOx -gated p+n+

tunnel transistor is presented in figure 21. A 20-nm-thickTiOx -gated p+n+ tunnel junction can operate as atransistor in both the enhancement and depletion modesat room temperature, as shown in figure 22. Although thetransconductance was small (∼5 mS mm−1) in the present

Figure 20. Photocapacitance spectrum (6 K) of a GaAs MOS diodewith a TiOx dielectric. (Reproduced with permission from [152]©2007 Wiley-VCH.)

S.I. GaAs

Ti/Au nonalloyed contact

Sidewall p+n+

tunnel junction

AnodeCathode Cathode

Regrown p+-GaAs:Be1st n -GaAs:Te&Sepitaxy

1st n+-GaAs:Te&Sepitaxy

Gate Gate

TiOx

S.I. GaAs

Ti/Au nonalloyed contact

Sidewall p+n+

tunnel junction

AnodeCathode Cathode

Regrown p+-GaAs:Be1st n+-GaAs:Te&Sepitaxy

1st n+-GaAs:Te&Sepitaxy

Gate Gate

TiOx

Figure 21. Schematic of the cross-sectional view of a TiOx -gatedp+n+ tunnel transistor. (Reproduced with permission from [152]©2007 Wiley-VCH.)

sample because the deposition and cleaning process hadnot yet been optimized, an ideal SIT with THz operationwill be realized by applying the improved high-kTiOx gatestructure to sidewall n+ p+n+ barrier structures with a 10-nmsource/drain spacing.

7. Summary

Sidewall GaAs tunnel junctions fabricated by molecularlayer epitaxy were introduced in this review paper. Thissidewall tunnel junction with a 50-nm junction depthshowed a record peak current density of 35 000 A cm−2.To obtain the high crystal quality of p+- and n+-GaAslayers, surface-stoichiometry-controlled Be-doping and Teand S co-doping were performed. The deep levels atthe sidewall tunnel junction interface were revealed byphotocapacitance measurements, and stoichiometry-relateddefects were reduced by AsH3 surface treatments. Thetunnelling mechanism in sidewall tunnel junctions wasinvestigated in view of phonon-assisted tunnelling andquantum-confined tunnelling. In addition, we overviewedthe recent results on the device application and fabrication

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Sci. Technol. Adv. Mater. 13 (2012) 013002 Topical Review

Voltage (V)-10 -5 0

Cur

rent

(m

A)

0

-1

-2

VG

-0.5 Vstep

Voltage (V)-10 -5 0

Cur

rent

(m

A)

0

-1

-2

VG +0.5 V step

Voltage (V)-10 -5 0

Cur

rent

(m

A)

0

-1

-2

VG

-0.5 Vstep

Voltage (V)-10 -5 0

Cur

rent

(m

A)

0

-1

-2

VG

-0.5 Vstep

Voltage (V)-10 -5 0

Cur

rent

(m

A)

0

-1

-2

VG +0.5 V step

Voltage (V)-10 -5 0

Cur

rent

(m

A)

0

-1

-2

VG +0.5 V step

(a) (b)

Figure 22. Room-temperature output characteristics of a20 nm-thick TiOx -gated p+n+ tunnel transistor operated in (a)enhancement and (b) depletion mode.

technology for THz devices, such as sidewall planar-dopedbarrier structures, Sb-related materials and high-kTiOx MOSstructures.

Acknowledgment

We acknowledge fruitful discussions with Professor Jun-ichiNishizawa on his invention of MLE, TUNNETT andideal SIT.

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