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Journal of Crystal Growth 264 (2004) 13–16

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+8610826

E-mail

0022-0248/

doi:10.101

Assessment of Li–Ga–N ternary system and GaN single crystalgrowth

W.J. Wanga, X.L. Chena,*, Y.T. Songa, W.X. Yuanb, Y.G. Caoa, X. Wua

aNanoscale Physics & Devices Laboratory, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080,

People’s Republic of ChinabDepartment of Chemistry, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China

Received 18 November 2003; accepted 15 December 2003

Communicated by M. Schieber

Abstract

Phase diagram of the system Li–Ga–N was constructed and assessed by using the calculation of the phase diagram

(CALPHAD) method. The phase diagram suggests that GaN crystal can be grown from the Ga-rich Li–Ga–N ternary

solution under 1–2 atmospheric pressure of N2 and moderate temperature. Using starting materials Li3N and Ga with

molar ratio 1:3, colorless transparent GaN platelets with sizes 3–4mm were obtained at 800�C and under 1 atm

nitrogen atmosphere.

r 2003 Elsevier B.V. All rights reserved.

PACS: 81.05.Ea; 81.10.Dn

Keywords: A2. Growth from solutions; A2. Single crystal growth; B1. Gallium compounds; B1. Nitrides; B2. Semiconducting III–V

materials

1. Introduction

GaN-based optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs)from the blue to ultraviolet wavelength have beendeveloped [1,2]. Performances of the LEDs andLDs depend on, to some extent, the quality ofGaN epitaxy film, which is closely related to thesubstrate material. Up to now, substrate materialsused for GaN film include sapphire, Si, 6H-SiC,

onding author. Tel.: +861082649036; fax:

49531.

address: xlchen@aphy.iphy.ac.cn (X.L. Chen).

$ - see front matter r 2003 Elsevier B.V. All rights reserve

6/j.jcrysgro.2003.12.017

MgAl2O4, LiAlO2, LiGaO2 etc. But all substratematerials except GaN may introduce stresses anddislocations into GaN epitaxy film due to latticemismatch and different thermal expansion coeffi-cients. Therefore, a homoepitaxial substrate, i.e.GaN bulk crystal, is strongly desired.

Despite of the big efforts, no method for thegrowth of real bulk GaN has been developed up tonow. Among the reported growth techniques, fourmethods are available to obtain bulk GaNcrystals: the sublimation method [3], the hydridevapor-phase epitaxy (HVPE) method [4], the high-pressure solution growth (HPSG) method [5] andthe flux method using Na [6]. Crystal growth from

d.

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W.J. Wang et al. / Journal of Crystal Growth 264 (2004) 13–1614

a melt or melt-solution is a widely acceptedtechnique for industrial-scale production of im-portant semiconductor materials. To grow GaNcrystal with industrially relevant diameter undermoderate pressure and temperature, we developeda solution growth process using Li3N as a solvent[7,8] recently. High quality GaN crystals with sizesof several millimeters were grown from Li–Ga–Nsystem. However, no information about the Li–Ga–N ternary phase diagrams was available. Sinceknowledge of the phase diagram and thermoche-mical properties of the ternary system can provideguidance in optimizing the crystal growth para-meters, we calculated Li–Ga–N phase diagramusing the CALPHAD method. In this paper, theternary phase diagram and bulk GaN crystalgrown from Li–Ga–N system were reported.

2. Li–Ga–N ternary phase diagram

Phase relations in the ternary system Li–Ga–Nare completely unknown in the literature. Relevantbinary sub-systems, Ga–N, Ga–Li and Li–Nsystem were assessed by Davydov et al. [9], Yuanet al. [10] and Wang et al. [11], respectively. Theunique ternary nitride Li3GaN2 was reported byJuza and Hund [12,13] and Kamler et al. [14].Li3GaN2 was found to be cubic, space group I a -3.It decomposes and transfers into GaN and Li3N at900�C.

The Gibbs energy function GFi ðTÞ ¼

�GFi ðTÞ2HSER

i (298.15K) for the element i

(i=Ga, Li, N) in the phase is described by thefollowing equation:

GFi ðTÞ ¼ a þ bT þ cT ln T þ dT2 þ eT3

þ f =T þ gT7 þ hT�9; ð1Þ

where different sets of coefficients (a through h)may be used in different temperature ranges, andHSER

i (298.15K), the molar enthalpy of pure solidelement i at 298.15K, is taken as standard elementreference (SER) state; bcc for elemental Li andorth for elemental Ga. In present work, the Gibbsenergy functions of Ga, Li and N are taken fromthe SGTE data for pure elements compiled byDinsdale [15].

For substitutional solutions, like the liquid, themolar Gibb’s energy is equal to

�GFm � HSER ¼ refGF

m þ idGFm þ exGF

m; ð2Þ

where

HSER ¼ xLiHSERLi ð298:15 KÞ þ xGaHSER

Ga ð298:15 KÞ

þ xNHSERN ð298:15 KÞ;

refGFm ¼ xGa½�GF

GaðTÞ � HSERGa ð298:15 KÞ�

þ xLi½�GFLiðTÞ � HSER

Li ð298:15 KÞ�

þ xN½�GFNðTÞ � HSER

N ð298:15 KÞ�;idGF

m ¼RTðxGa ln xGa þ xLi ln xLi þ xN ln xNÞexGm

F is the excess Gibbs energy, expressed by theRedlich–Kister polynomial: exGm

F=binary contri-butions+ternary contributions, binary: ¼xixj

Pmn¼1

nLFðxi � xjÞn�1 ternary: ¼ xixjxkðLixi þ

Ljxj þ LkxkÞ; where nLF are the interaction para-meters between i and j elements, whose generalform is

nLF ¼ a þ bT þ cT ln T þ dT2 þ eT3 þ fT�1: ð3Þ

The calculation is carried out by means of theChemSage software [16]. The principle is tominimize the Gibbs energy of phases in equili-brium. For each of the selected data, a certainweight is given based on personal experience and ischanged by ‘‘trial and error’’ method during theprogram run, until most of the selected experi-mental information is reproduced within theexpected uncertainty limits.

For the ternary calculations, the binary data setswere combined and extrapolated into the ternaries.Calculations were performed including the gasphase at 1 bar total gas pressure. The calculatedisothermal section of the Li–Ga–N system at800�C is shown in Fig. 1.

3. Bulk GaN single crystal growth from Li–Ga–N

system

The starting materials for the growth of GaNsingle crystals are pure Ga metal (99.999%) andLi3N (synthesized using Li metal (99.9%) and N2

(99.999%) in our lab). These starting materialswere weighed as predetermined molar ratio and

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Fig. 1. Isothermal section of the Li–Ga–N system at 1073K.

Fig. 2. Optical micrograph of GaN crystals grown using

starting materials Li3N and Ga with molar ratio 1:3 (a, b),

1:6 (c) and 1:12 (d).

10 20 30 40 50 60 70 80 90

0

500

1000

1500

2000

2500

3000

104

202004

201

112

200

103

110

102

101

2θ (deg.)100

Inte

nsity

(a.u

.)

002

100

100

c=5.186Å

a=3.190Å

Fig. 3. XRD pattern for the powder obtained by milling the

bulk crystals.

W.J. Wang et al. / Journal of Crystal Growth 264 (2004) 13–16 15

put into a Tungsten crucible (50mm innerdiameter, 60mm depth), then the crucible wasplaced inside an induction heating apparatus.After removing the air by a vaccum pump, theapparatus was filled with nitrogen and the pressurewas increased up to 1 atm. The sample was heatedto 800�C, then was slowly cooled at a rate of 2–3�C/d. The grow process lasts 120–168 h. GaNsingle crystals were obtained and separated fromresidual substances by soaking in HCl solution.Optical micrograph of GaN crystals grown usingstarting materials Li3N and Ga with molar ratio1:3, 1:6 and 1:12, respectively, are shown in Fig. 2.X-ray powder diffraction of the crystals indicatesthat they are wurtzite structure see Fig. 3. All thecrystals obtained are colorless and transparent.Moreover, the results of growth with ratio 1:3 arebetter than that with ratio 1:6 and 1:12. The relationbetween crystal growth and molar ratio of startingmaterials is discussed combined with the Li–Ga–Nternary phase diagram in following section.

4. Discussion

In Li–Ga–N system, at 800�C and 1 bar totalgas pressure (see Fig. 1), there are a liquid phase L,

a two-phase region with liquid phase and GaN anda three-phase region with liquid phase, GaN andLi3GaN2. On the tie line between Li3N and GaN,there exists a ternary compound Li3GaN2. Ac-cording to the Li–Ga–N phase diagram, GaNsingle crystal can be grown from the liquid phaseand justify the diagram is reasonable. The crystalgrowth mechanism has been reported in theprevious paper [7,8]. The Li–Ga–N phase diagramsuggests us that the richer the Li in the Li–Ga–Nmolten solution, the higher the concentration of N

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W.J. Wang et al. / Journal of Crystal Growth 264 (2004) 13–1616

in the solution. However, we couldnot grow GaNcrystal from the Li-rich region, and GaN crystalonly can be grown from the Ga-rich solutionbecause the two-phase region of GaN and liquidphase exists in the Ga-rich region. Our experi-mental results agree well with the phase diagram.In our work [8], using starting materials Li3N andGa with molar ratio 2:1 (Li/Ga=6:1), we did notobtain GaN, but obtained Li3GaN2. While in theGa-rich region, with molar ratio 1:3, 1:6 and 1:12,colorless transparent platelet GaN were obtained,see Fig. 2. Here, we think the growth using startingmaterials Li3N and Ga with molar ratio 1:3 (Li/Ga=1:1) is belong to GaN growth from the Ga-rich solution, because a little of the Li3N alwaysvolatilized during heating process and in the Li–Ga–N ternary solution the molar ratio of Ga andLi is more than 1:1. Obviously, the sizes of GaNsingle crystals grown with molar ratio 1:3 arelarger than that with molar ratios 1:6 and 1:12. Itis because the concentration of N in the Li–Ga–Nsolution is decreased with the concentration of Lidecreased, see Fig. 1. In the Ga-rich Li–Ga–Nternary solution, the higher the concentration ofLi, the higher the concentration of N is. With thetemperature of the solution down slowly, GaNcrystals slowly grow from the supersaturatedsolution. In the growth condition using startingmaterials Li3N and Ga with molar ratio 1:3, theamount of N in the solution is more, so the size ofcrystal grown out from the solution is larger.

5. Conclusions

Li–Ga–N ternary phase diagram was assessedusing CALPHAD method. The phase diagramsuggests that GaN crystal can be grown from theGa-rich Li–Ga–N ternary solution under moder-ate pressure and temperature. Considering thevolatilization loss of Li3N, using starting materialsLi3N and Ga with molar ratio 1:3 (Li/Ga=1:1),

colorless transparent GaN platelets with sizes 3–4mm were obtained.

Acknowledgements

The authors gratefully acknowledge GTT forsupplying the ChemSage software. This work isfinancially supported by the National NaturalScience Foundation of China under the grantnumbers:59925206 and 59972040, and by theMinistry of Science and Technology of Chinaunder the grant number: 2002AA311210.

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