Journal of Crystal Growth 211 (2000) 184}188

Structural studies on synthesised gallium nitride

M. Senthil Kumar, P. Ramasamy, J. Kumar*

Crystal Growth Centre, Anna University, Chennai 600 025, India

Abstract

Gallium nitride (GaN) powder has been synthesised through reaction between metal gallium and ammonia (NH3) in

a resistively heated quartz reactor. Experiments have been performed for various reaction temperatures (range900}9503C) and reaction periods (4}12 h). The optimised reaction temperature and period are 9503C and 8 h, respective-ly. X-ray powder di!raction (XRD) and scanning electron microscopy (SEM) studies have been carried out on thesynthesised GaN powder for di!erent growth conditions and the results have been correlated. XRD pattern reveals thatthe synthesised GaN is of a single-phase wurtzite structure. The calculated lattice parameter values are a"3.186 As andc"5.174 As . XRD pattern for the samples prepared for the reaction period of less than 8 h exhibit GaN peaks along withgallium oxide (Ga

2O

3) peaks. The change in the surface features with respect to the reaction period has been investigated

using SEM. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 81.05; 81.70; 81.10

Keywords: Gallium nitride; Synthesis; XRD; SEM

1. Introduction

Recent technological advances in the group III-nitride semiconductor research have created a rev-olution in the realisation of short-wavelengthopto-electronic devices. Gallium nitride (GaN) isused for the fabrication of light emitting diodes(LEDs) and laser diodes (LDs) which emits light inthe blue/UV wavelength region of the electromag-netic spectrum due to its signi"cant forbidden gapenergy of 3.45 eV at room temperature. E$cientlight emitting LEDs and LDs and various elec-

*Corresponding author. Fax: #91-044-2352774.E-mail address: marsjk@annauniv.edu (J. Kumar)

tronic devices such as Schottky diodes, photodetec-tors, bipolar junction transistors have already beenfabricated based on this material [1}4]. Sapphireand silicon carbide are the most commonly usedsubstrate materials for the heteroepitaxial growthof GaN "lms but they exhibit high dislocationdensities in the grown "lms because of their largelattice mismatch with GaN. GaN homoepitaxialgrowth activities on GaN substrate are compara-tively very less because of the limited availability ofGaN substrates and also due to the di$culties inthe bulk growth of GaN.

Currently, sublimation and high-pressure andhigh-temperature growth are the two major tech-niques adopted for the bulk growth of GaN [5,6].Synthesised GaN powder is essential as a startingmaterial for the GaN bulk growth by sublimation

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 7 9 7 - 6

method. Thick "lm of GaN has been grown bysublimation sandwich technique using the syn-thesised GaN powder as the source material [7].Growth of thick "lm of polycrystalline GaN at lowpressures by direct reaction of gallium (Ga) andatomic nitrogen obtained by electron}cyclotronresonance (ECR) plasma has also been reported[8]. Chu et al. have synthesised GaN usingammonolysis of gallium suboxide technique [9].Several reports are available on the various aspectsof GaN synthesis by reacting molten Ga withammonia (NH

3) [10,11]. This technique involves

some critical experimental parameters such asthe reaction temperature and the reactionperiod. In this paper, we present the synthesis ofGaN by the reaction between Ga and NH

3as

well as by X-ray di!raction and scanning electronmicroscopy studies on the synthesised GaNpowder.

2. Experimental procedure

The experimental set-up consists of a long hori-zontal-type quartz reactor of length 100 cm withsuitable gas inlet and outlet ori"ce and the reactoris kept inside a resistively heated furnace. In theexperiment, gallium (Ga) metal of purity 5 N wastaken in a quartz or alumina reaction boats. Am-monia was allowed to #ow over the Ga surfacefrom the beginning of the experiment. Nitrogen gaswas used as a carrier gas to bring the ammoniamolecules into the reaction zone. Ga was slowlyheated to the desired experimental temperature.The synthesis was carried out in the temperaturerange of 900}9503C for various reaction periods of4}12 h to determine the optimum growth condi-tions for obtaining well-synthesised GaN material.After the completion of each experiment, thesystem was gradually cooled to room temper-ature by a continuous #ow of ammonia. The sam-ples were etched in HF : HNO

3mixture to remove

the unreacted Ga at the end of the experiment andthey were washed with deionised water and thendried.

The structural properties of GaN powder wereanalysed using X-ray di!raction (XRD) and scan-ning electron microscopy (SEM) studies.

3. Results and discussion

3.1. Synthesis of GaN

A series of experiments were performed for vari-ous reaction temperature and time. The real prob-lem in the synthesis of GaN is to react atomicnitrogen with the Ga melt which is not possiblebecause of the stable nature of the nitrogen molecu-le. In most of the GaN growth techniques, ammoniais used as the nitrogen source since it is thermallyunstable at relatively high temperatures and decom-poses into atomic nitrogen and hydrogen. Detailedthermodynamic calculations on ammonia decompo-sition have been reported by Heon Lee et al. [12].

The formation of GaN is given by the reaction

2Ga#2NH3P2GaN#3H

2.

For all the experiments, a quantity of 3}5 g ofgallium was taken for the synthesis. During syn-thesis, Ga reacts with available atomic nitrogenand forms GaN which is dark grey in colour. Thecrust formed on the Ga surface decreases furtherreaction of Ga with nitrogen and ends up witha little amount of unreacted Ga at the bottom of thereaction boat. GaN powder of 2 g was obtainedfrom the starting charge of 5 g gallium. Shibata etal. have synthesised 104.6 g of GaN by injectingammonia into 3400 g of Ga [14].

The position of the Ga source kept inside the hotzone with respect to ammonia inlet is also a veryimportant parameter. In our system, the length ofthe furnace is 60 cm and the Ga was kept in themiddle of the furnace. When the length of the fur-nace was reduced, no synthesised material wasobtained.

3.2. X-ray diwraction studies

XRD spectra were recorded for the GaN syn-thesised under di!erent experimental conditions.XRD spectra for the samples prepared at the tem-perature of 9003C exhibit gallium oxide (Ga

2O

3)

peaks only and no peaks corresponding to GaNwere observed. This might be due to the oxidationof Ga during the heating period and the oxygenprobably came from the liquid ammonia. There-fore, the reaction temperature was further increased

M. Senthil Kumar et al. / Journal of Crystal Growth 211 (2000) 184}188 185

Fig. 1. XRD pattern of GaN prepared at 9503C for 4 h.

to enhance the atomic nitrogen for the reaction. Allthe experiments were carried out at 9503C by in-creasing the amount of atomic nitrogen species.Fig. 1 shows the X-ray di!raction spectrum of thesample synthesised at 9503C for a duration of 4 h.The spectrum contains gallium oxide (Ga

2O

3)

peaks as a major phase in addition to the minorGaN peaks. Ga

2O

3formed during the heating

process reacts with atomic nitrogen at higher tem-peratures and starts to convert into GaN. Ga

2O

3has been partially converted into GaN during thisreaction period. Fig. 2 shows the XRD pattern ofthe GaN powder prepared within a period of 6 h.Relevant GaN-related peaks have been obtainedwith additional peaks of Ga

2O

3. The broadened

peaks reveal the poor crystalline nature of the ma-terial.

When the reaction period is extended to 8 h,X-ray di!raction exhibits the GaN peaks only andis shown in Fig. 3. The very sharp peaks with highintensity show the "ne crystalline nature of thesynthesised material. This indicates that for thereaction period of 8 h, all the Ga

2O

3species

are converted into gallium nitride. The conversionof gallium oxide into GaN has been reported in Ref.

[13]. GaN has crystallised in the hexagonal struc-ture and the lattice parameters have been cal-culated as a"3.186 As and c"5.174 As which arein very good agreement with the reported values[10].

Reaction period was further increased to 12 hwhile the reaction temperature was still at 9503C.Then, all the synthesised GaN material gets evap-orated and deposited on the walls of quartz reactorbecause of its low sublimation temperature.

3.3. Scanning electron microscopy studies

Scanning electron microscopy studies were per-formed on the samples prepared for di!erent reac-tion periods (the reaction temperature was keptconstant at 9503C). SEM photograph of the sampleprepared for 4 h is shown in Fig. 4. It was observedthat the sample consists of a mixture of Ga

2O

3and

GaN. The XRD result corresponding to thissample showed Ga

2O

3peak as a major phase. Fig.

5 shows the SEM picture of the sample synthesisedfor a duration of 6 h. The formation of Ga

2O

3on

the GaN surface due to the oxidation of Gahas recently been reported by Shibata et al. [14].

186 M. Senthil Kumar et al. / Journal of Crystal Growth 211 (2000) 184}188

Fig. 2. XRD pattern of GaN prepared at 9503C for 6 h.

Fig. 3. XRD pattern of GaN prepared at 9503C for 8 h.

Fig. 6 shows the surface features of GaN preparedin a reaction period of 8 h. This picture shows the"nely crystallised morphology of GaN without anyinclusion of Ga

2O

3particles. The XRD pattern

also reveals the absence of Ga2O

3for this particu-

lar sample. The reaction temperature and reactionperiod play a very crucial role in the formation ofGaN.

M. Senthil Kumar et al. / Journal of Crystal Growth 211 (2000) 184}188 187

Fig. 4. SEM photograph of GaN prepared at 9503C for 4 h.

Fig. 5. SEM photograph of GaN prepared at 9503C for 6 h.

Fig. 6. SEM photograph of GaN prepared at 9503C for 8 h.

4. Conclusions

GaN has been synthesised through a reactionbetween metal Ga and NH

3in the temperature

range of 900}9503C. The reaction period has beenvaried from 4 to 12 h. Sythesised GaN powder hasbeen characterised by XRD, which reveals the hex-agonal structure. Detailed SEM analysis has beencarried out on the samples prepared under variousexperimental conditions. The SEM analysis re#ectsthe XRD results for the samples prepared for vari-ous growth conditions. Important growth para-meters such as reaction temperature and reactionperiod for the synthesis of good quality GaNhave been optimised. The synthesised material is tobe used as the source material for the growth ofbulk crystals using sublimation and also for thickepitaxial "lms using sublimation sandwich tech-nique.

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