*Corresponding author. Fax: #49 641 99 33119; e-mail:michael.k.topf@expl.physik.uni-giessen.de.

Journal of Crystal Growth 189/190 (1998) 330—334

Low-pressure chemical vapor deposition of GaN epitaxial films

M. Topf*, G. Steude, S. Fischer, W. Kriegseis, I. Dirnstorfer, D. Meister, B.K. MeyerI. Physics Institute, University of Giessen, Heinrich-Buff-Ring 16, D 35392 Giessen, Germany

Abstract

GaN films were grown epitaxially on sapphire and 6H-SiC substrates by low-pressure chemical vapor deposition(LPCVD) using gallium(III) chloride and ammonia as gallium and nitrogen precursor, respectively. The properties ofthese samples were examined by X-ray diffraction, Hall effect measurements, secondary-ion mass spectroscopy (SIMS)and photoluminescence (PL). All GaN layers exhibit high free carrier concentrations between n"2]1018 and5]1019 cm~3 caused by unintentional n-type doping. We provide evidence that this high unintentionally doping islinked to the oxygen content in the films. The correlation between the optical properties with respect to position and lineshape of the band-edge luminescence and the electrical properties, i.e. the free carrier concentration, is dis-cussed. ( 1998 Elsevier Science B.V. All rights reserved.

PACS: 73.61.Ey; 78.55.Cr; 81.10.Bk; 81.15.Gh

Keywords: Gallium nitride; Chemical vapor deposition; Luminescence

1. Introduction

GaN epitaxial films are commonly grown fromthe gas phase by metalorganic chemical vapor de-position (MOCVD). It is the method of choice togrow high-quality AlGaN/GaN and InGaN/GaNheterostructures for device fabrication [1]. How-ever, because of the metalorganic Ga precursor andthe use of ammonia the films show significant in-corporation of carbon (C) and hydrogen (H). H isknown to passivate the Mg-acceptor, the role of

C is still undetermined. There is some evidence thatC may induce a shallow acceptor [2,3], but therealso exist predictions that self-compensation mightoccur [4]. Therefore, the growth of GaN withC free precursors and intentional C doping wouldbe desirable to resolve such ambiguities.

A carbon-free gas-phase growth technique is thehydride vapor-phase epitaxy, i.e. the synthesis ofGaN from GaCl and NH

3. GaCl is synthesized in

situ from HCl and metallic Ga. To save this addi-tional synthesis, we used chemically stable GaCl

3together with NH

3in a low-pressure CVD system.

In this paper we present details of the set up anddescribe the process of the chloride transportLPCVD method for the C-free growth of GaN

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 8 5 - 1

Fig. 1. Set up of the chloride transport LPCVD system for the growth of GaN.

layers. Furthermore, we discuss the potential of thismethod with respect to structural, optical and elec-trical properties of the deposited GaN films.

2. Experimental procedure

We employed a horizontal quartz reactor, whichis resistance heated by a ten-zone oven. The tem-perature profile in the reactor was chosen such thatthe deposition in front of the substrates was mini-mized by lowering the supersaturation in the gasphase. GaCl

3and NH

3were used as Ga and N pre-

cursors, respectively. GaCl3

is sublimated andtransported by N

2as a carrier gas into the reactor.

The overall reaction of the growth process is givenby the following equation:

GaCl3#NH

3¢ GaN#3HCl. (1)

Additionally, the N2

transport gas can be mixedto NH

3to control the total pressure independently

from the partial pressures of the precursors. N2and

NH3

gas flows were controlled by mass flow con-trollers (MFCs). The purity of N

2and NH

3is 6.0

and 5.0, respectively. According to the manu-facturer the purity of GaCl

3is 99.999%. The

films were deposited either on (0 0 0 1)Al2O

3or on (0 0 0 1) n- or p-type 6H-SiC substrates. A

schematic drawing of the set up is shown in Fig. 1,and in Table 1 the typical growth parameters arelisted.

3. Results and discussion

The epitaxial GaN films were investigated bystandard X-ray diffraction. The X-ray rockingcurves typically found had a half-width of 10arcmin (note that the films were grown withouta buffer layer). The films appeared transparent andexhibited mirror-like surfaces. The sample proper-ties were independent of the growth time, i.e. a filmwith a thickness of 14 lm exhibited the same char-acteristics as a film of 1 lm thickness. For a thick-ness above 0.8 lm cracks appeared in the GaNlayers. More details about the structural propertiesof the GaN layers grown on Al

2O

3can be found in

Ref. [5].The electrical properties are summarized in

Fig. 2. All films show high n-type conduction.From Hall effect measurements free carrier concen-trations (n) between 1018 and 5]1019 cm~3 andmobilities from 70 to 220 cm2/V s are usually found(see Fig. 2). The mobility as a function of the freecarrier concentration n, which is the difference be-tween the donor and acceptor densities N

D!N

A,

M. Topf et al. / Journal of Crystal Growth 189/190 (1998) 330–334 331

Table 1Typical growth parameters

Growth temperature (°C) 980Total pressure (mbar) 0.5Gas flow N

2(sccm) 200

Gas flow NH3

(sccm) 400Gas flow GaCl

3(sccm) 1.5

Growth time (h) 1Growth rate (lm/h) 2—4

Fig. 2. Mobility and free carrier concentration for GaN filmsgrown by LPCVD. The data are taken at room temperature.

Fig. 3. Depth profile of positive and negative ions seen by SIMSmeasurements for a 1.5 lm GaN layer. Positive argon ions witha current of I"2]10~7 A and a voltage of º"6 kV have beenused as a sputter gas.

agrees with the data of Ilegems et al. using hydridevapor-phase epitaxy [6]. For uncompensated films(N

A/N

D"0) the mobility should be higher by a fac-

tor of four [7]. We estimated a compensation ratioof N

A/N

D"0.8 for our layers. Thus, in addition

to shallow donors there must be an appreciableamount of acceptors in the GaN layers. Therefore,we investigated our layers by secondary-ion massspectroscopy (SIMS). With respect to the trace im-purities we took special care for the group IVelements and the group VI elements. Although onewould expect the incorporation of Si due to thehigh growth temperature the concentration of Siwas well below 1018 cm~3, therefore the Si donorcannot account for the n-type conductivity of ourlayers. There is certain evidence that the high freecarrier concentration is linked to a high oxygencontent in the layers. SIMS usually detects oxygenin the range of 1019 cm~3 [8]. With a compensa-tion ratio of N

A/N

D"0.8 the donor concentration

is five times the free carrier concentration which fits

to the oxygen content we usually find in the films.Whether all the oxygen detected by SIMS is electri-cally active remains to be investigated. A typicaldepth profile of impurity ions (Si`, O~, Cl~) andmolecular fragments (CN~) detected by SIMSmeasurements is shown in Fig. 3.

The oxygen contamination could originate fromthe gallium chloride, which is hygroscopic, or theammonia. With respect to compensating acceptorsonly carbon was found in concentrations around5]1018 cm~3. In principle, the LPCVD processshould be a method, free of C in the sense that theprecursors are free of C. The high content of C inthe layers can be explained by the use of quartzvalves sealed by organic Apiezon vacuum grease,which is partially transported into the reactionzone. In the near future these valves will be ex-changed by valves sealed by Teflon sealings, whichresist the attack from GaCl

3and from NH

3.

Luminescence is a standard characterizationtechnique to judge the quality of the GaN films.The CVD films grown without a low-temperaturebuffer layer showed strong band-edge emissioncaused by the neutral donor bound exciton. Fromthe position as well as the full-width at half-max-imum (FWHM) of the band edge luminescence weobtained important information about the proper-ties of the films (see Fig. 4). Due to the differences inthermal expansion coefficients between 6H-SiC andGaN, on the one hand, and between sapphire andGaN on the other, the films were either under

332 M. Topf et al. / Journal of Crystal Growth 189/190 (1998) 330–334

Fig. 4. Low-temperature photoluminescence: samples (a) and(b) have a free carrier concentration of n"2]1018 cm~3 and(c) n"5]1019 cm~3.

tensile (6H-SiC) or compressive strain (sapphire).The exciton line position is therefore red-shifted forthe films on SiC and blue-shifted for the films onsapphire [8,9]. This observation also demonstratedthat two-dimensional growth was achieved.

GaN films on sapphire had a FWHM of thedonor bound exciton line (see Fig. 4) of 6.6 meV,whereas on 6H-SiC it was 5 meV (measurementtemperature: 4.2 K, free carrier concentration lessthan 3]1018 cm~3). At room temperature theband-edge luminescence is still dominating, where-as the notorious yellow luminescence band at2.2 eV is not detectable for films on 6H-SiC dueto the spectral overlap with the donor acceptorpair recombination in the SiC substrate. For filmsgrown on sapphire this deep-level recombination isa factor of hundred lower in intensity than theband-edge luminescence. With increasing carrierconcentration the bound exciton line broadens sig-nificantly and eventually reaches a linewidth of70 meV for n"5]1019 cm~3. There is a goodcorrelation between the FWHM of the boundexciton and the free carrier concentration and one

obtains the following relationship [10]:

FWHM (n)"1]10~10n0.57 meV (2)

which is valid for 1018(n(5]1019 cm~3. Be-sides the FWHM, the position of the band-edgeluminescence also depends on the free carrier con-centration n. With increasing n a blue shift is ob-served which can be explained by screening effects.One example how the line shape evolves is shownin Fig. 4c. One notes the asymmetric line shape,a steep high-energy increase and a pronouncedlow-energy tail (see Fig. 4c). Hanamura explainedthis peculiar line shape for highly doped semicon-ductors as a consequence of the Stark effect due tocharged impurities [11]. Under the assumptionthat only the nearest ionized impurity, whichshould follow a Poisson distribution, contributes tothis Stark effect, he derived the following equation:

I(E) dE"CA(4E*0)3@4D*ED~7@4

]expA!4

3A*E3@4B dE,

A"pN#*a30(4E

*0)3@4. (3)

Here C is a proportionality constant, N#*

is theconcentration of charged impurities, and a

0and

E*0

are the effective Bohr radius and binding energyof the intrinsic exciton, respectively. From a fit tothe band-edge luminescence one obtains N

#*which

is equal to the density of acceptors. Again one endsup with compensation ratios of N

A/N

D"0.8 con-

firming the conclusions reached from the mobilitydata.

For even higher n, luminescence takes place fromthe Fermi energy in the conduction band withk-nonconserving transitions [3].

4. Conclusions

In conclusion, we presented our results on thegrowth and characterization of GaN epitaxialfilms. We used 6H-SiC and sapphire as substratematerials and a chloride transport LPCVD growthtechnique with gallium(III) chloride and ammoniaas precursors. The films had still high background

M. Topf et al. / Journal of Crystal Growth 189/190 (1998) 330–334 333

carrier concentrations which were attributed tooxygen incorporation either from the moisture ofthe salt or from the ammonia source. A correlationbetween optical properties such as the bound ex-citon line position and half-width with electricaldata was achieved.

Acknowledgements

We are indebted to the Deutsche Forschungs-gemeinschaft and the Volkswagen Foundation,within the “Photonik” program, for partial sup-port. S.F. gratefully acknowledges the Hanns-Seidel-Foundation for a scholarship funded by theBMBF.

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