Heterojunctions of amorphous wide band gap nitrides and silicon

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D| OND AND RE TED h' T| R|ALS ELSEVIER

Diamond and Related Materials 7 (1998) 397 41)i

Heterojunctions of amorphous wide band gap nitrides and silicon A. Werbowy '~'*, J. Szmidt a A. Sokolowska b A. Olszyna b " Institute o./"A,lio'oeh'cirotdc.~' am/Ol, toelectronics. H'arsaw Universit.r o/Techm, logy. Kos-vkowa 75. 00-662 lt~trsaw. Pohuul t, Facultl' o./"Materials Science and Engineering. H'arsaw [#liversitr ¢~/' Technoh~Kr. Narh,tta 85. 02-524 I/~u'.~'aw. l~oland Received 23 Jub~e 1997; accepted 12 September 1997

Abstract

in situ dopc.t nanocrystailine BN. AIN and GaN films were deposited by mean:; of impulse plasma assisted CVD method on silicon substrates, As a result c-BN (n-type)/Si(p-type) as well as AIN(p-type);Si (n-type) and GaN (p-typei Si(.-type)heterojunction structures were formed. On the basis of C-1" measurements barrier heights of investigated Sinitride systems were estimated together with work function ,¢alues tp of boron and aluminum nitrides. Also a hypothetical energy band diagram for obtained AIN/Si heterostructures was proposed. ~;~ 1998 Elsevier Science S.A.

Keywords: Band structure: Doping ,-type: Doping/,-type: Nitridc

!. Introduction Wide band gap nanocrystalline compounds like diamond-like carbon (DLC), cubic boron nitride (c-BN). boron phosphide (BP), aluminum nitride (AIN) or gallium nitride (GaN) are promising materials for high temperature microelectronics and short wave optoelectronics, in their investigation properties featured by monocrystalline lbrms of these materials are taken as a reference, although synthesis of these ibrms in some cases is still difficult. This in turn is one of the main obstacles preventing their common practical application. Present work is related to our research on amorphous and nanocrystalline wide band gap compounds (DLC. c-BN ) which can be obtained through easy and inexpensive methods. "l"heir microstructure, typical for materials of unordered arrangement of atoms manifests itself in a narrowing of the band gap. low carrier mobility and difficulties in effective doping. In our previous works [!.2] we showed thai in situ doping of nanocrystailine c-BN films is possible, leading to f o r m a t i o n o f . - t y p e semiconductors. As it turned out. the essential element of such process is low temperature annealing of so obtained la'yers [I 3 ] . In tile course of furtlaer investigations it also became evident that nitrides in general (c-BN, AIN and GaN) are superior over DLC as they show lower leakage conductance. Now we

* Corresponding author. 11925-9635 98/$19.{1{I,v 1998 Elsevier Science B.V. All rights reserved. PII ,~,,~,~'"'*'~5-9635(t~7.)00227-6

would like to present results of our more recent experiments demonstrating that nanocrystalline AIN and GaN layers can also be doped during their synthesis process (with zinc atoms being /,-type dopant for AlliBV nitrides).

2. Experimental details 500 to I000 n m thick, nanocrystallinc BN, AIN and

GaN tilms were obtained by means of an impulse plasma a~,,sisted CVD method [! ] ( Fig. ! ). BN |tiros were deposited on/,-type (p --- 15 flcml whereas AIN and GaN films deposited on ,-type ( p = 0 . 0 0 5 0 . 0 2 f l c m and p = 0.20.5 f~cm, respectivelyl Si substrates. For boron nitride synthesis BH3NH3 was used. while aluminum and gallium nitrides were obtained fi'om vapors o f AI(CIt3)3 and Ga(CH3)3. In all ca,~es nitrogen served as a carrier gas [Table I ). Produced materials were also i, sit, doped by sulfur (BN) or zinc (AIN. GaN) atoms in order to grow as n-type or p-type semiconducting films and thus form helerojtmctions with silicon. Part of the so created structures was subsequently annealed in nitrogen for l h at temperatures ranging from 475 to 700 K. In ~rder to allow their electrical characterization on top of annealed as well as tmannealed layers metal, AI and Au dot contacts (of 0.75 mm diameter) were evaporated. This enabled the current voltage ( I V) and high

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Source of dopanl atom,,

Experimental ASTMGaN 4-1078 d(nm) d(nm) 0.270 0.270

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d(rim) 0.269 0. I B2 0.1.55 0.132

3. Obtained results and their analysis

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Unannealed BN films were partly amorphous and partly nanocrystalline. The amorphous phase mainly contained high pressure E-BN lbrms whereas the nanocrystalline phase was composed of h-BN and c-BN forms. After annealing at temperatures higher than 50q K the I,~vers became completely nanocrystalline. The following a'.luw.~ic forms of BN were identified in their structure: E-BN, c-BN, h-BN and I-BN [3] AIN and % GaN:,~,iil'ls, were also nanocrystailine which is sho .... , by el ctron diffraction patterns ( Fig. 2, Table 2).

Fig. 2. Electron diffraction patterns of unannealed: (a) A1N: and (bl GaN lilms.

3.1. n-l~pe doping Boron nitride tilms were in situ doped with sulfur which is a donor-type impurity for this material. In as deposited state, BN tilms did not behave like semicondueting o,es as tile I-V and C-V characteristics of the

Table 2 Experimental d(nm}

ASTM d(nm)

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tl.276 11.243

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AIN 8-262 0.268 0.177 0.153 0.132

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investigated structures did net vary fi'om those observed for samples with tmdoped layers. Desirable change was brought by their annealing at temperatures higher than 600 K. / I" curves of such specimens became diode-like with a forward to reverse current ratio ranging from 10z to 10~', though they were showing instabilities as well as a certain level of unrepeatubility. Their C V curves also had a junction-type character. Their plots in the I/C" =./~Vr) coordinate system (Fig. 3) allowed estimation of the potential barrier ~p, [4] between silicon and boron nitride (1.5 to 4.5 eV ). This made possible, in its turn, determination of the boron nitride work function ~&-nn ( - 0 . 4 - 2 . 6 e V ) . The value of the latter might suggest the existence of negative electron affinity between BN and Si, which, in a group of wide band gap materials, had been earlier observed in the case of diamond [5]. it should be also pointed out that our aoempts of sulfur duping of other nitrides were not successful as the investigated samples did not show rectifying behavior prior to, as well as after, annealing. .~..~. p-type doping Similar results were obtain :d in the case of Zn doped (acceptor-tyr? impurity) nanocrystalline AIN films. I ~.1," and C- V curves of unannealed structures were like those observed for MIS structures, with a low leakage undoped

aluminuna nitride layer. Annealing at temperatures higher than 601)K also resulted in the appearance of rectifying current voltage characteristics, but with a lower forward to reverse current ratio (10 2 l03) I]:an for BN. givitig evidence of an AIN(p-type)Si(,-typc) heterojw,~clion formation. However an exceptio11~! repeatability and stability of the measured curves ( Fig. 4) in the group of such materials should be pointed out. ( ' I," characteristics were also junction-like and linear in I/C"=f(VRI coordinate system. The potential barrier between Si and AIN was estimated to be close to l eV giving a work function value ~P,,n equal to 3.8 eV. On this basis a hypothetical band diagran~ so-called Anderson's model [6] ..........of thc AIN/Si system was proposed (Fig. 5). In the case of samples with Zn doped GaN layers ((JAN (p-type)/6i(n-type) heterostructures) we observed quite a different situation as diode-like bt,havior manifested itself only during I I" measurer,.,~nts of urtannealed specimens {forward to reverse current ratio 102-- 104), which is shown in Fig. 6. Annealing at temperatu~'es higher tha:a 600 K had, in this case, a negative impact, because ; l/ curves becam,, unrepeatable and unstable, Iqs;,ng at the same time their junction-like character (,,rowth of reverse currents). This is opposite to what w~ts observed for MIS structures with undoped GaN films, where annealing stabilized the I l' chalactcro

A. Werl,ou'y et al. / Diamond amt Rektted MateriaL~' 7 (19~6) 397-401

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characteristics of annealed heterostructttres with an undoped (B) and a Zn doped (A} AIN layer.

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Our experiments shown that effective doping of wide band gap nanocrystalline nitrides is possible as it results in the formation of" n-type (in the case of sulfur doped BN) as well as p-type (in the case of zinc doped AIN and GaN) semiconducting films. From this point of view, the unordered structure of these materials turns out not 9 be a serious problem. Sulfur, i.e. donor-type doping of investigated nitrides

A. Werbowy et al. /Diamondand Related Materials 7 (1998) 397-401

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was not successful except of BN films, however even in this case their elcctricai characteristics were not repeatable. It is possible, thal a negative electron affinity might be responsible for such behavior. Much better results were achieved with zinc i.e. acceptor-type doping of nitrides. A considerable rise in the reverse current observed for both AIN(p-type)/Si(n-type) as well as GaN(p-typej/Si(n-type) heterostructures, compared to samples with undoped films, is in accordance with theory and the dielectric properties of investigated materials. Our experiments also ,,;how the importance of low temperature annealing (with the exception of GaN) for activation of dopant atoms introduced in situ to nitride films and subsequent formation of p-n heterojunction structures. Their low temperature thermal processing in all cases resulted in structural changes of the investigated nitrides like for instance disappearance of the amorphous phase and the rise of short range matrix ordering. In general obtained results are the first step leading to fabrication of heterojunctions of wide band gap nitrides and silicon.

.Acknowledgement This work was suvported by grant no. 7TO8A 021 09 from the State Committee tbr Scientific Research. "

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