Single crystal growth of GaN using a Ga melt in Na vapor

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0022-0248/$ - se

doi:10.1016/j.jcr

�CorrespondiE-mail addre

(T. Yamada).

Journal of Crystal Growth 281 (2005) 242–248

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Single crystal growth of GaN using a Ga melt in Na vapor

Takahiro Yamadaa,�, Hisanori Yamaneb, Hirokazu Iwatac, Seiji Sarayamac

aInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, JapanbCenter for Interdisciplinary Research, Tohoku University, 6-3 Aramaki, Aoba-ku, Sendai 980-8578, Japan

cDepartment of R & D Center, Research and Development Group, Ricoh Company, Ltd., 5-10 Yokarakami, Kumanodo,

Takadate, Natori 981-1241, Japan

Received 28 February 2005; accepted 8 April 2005

Available online 26 May 2005

Communicated by K.W. Benz

Abstract

GaN single crystals were grown by heating a Ga melt in Na vapor at 720–800 1C and 5MPa of N2 for 200 h. The Ga

melt absorbed Na from the vapor and formed a Na–Ga melt. Transparent prismatic GaN single crystals grew from the

wall of a boron nitride crucible in the melt. Seventy five percent of Ga reacted with nitrogen and changed into GaN

crystals at 720 1C when the initial amount of a Ga melt was 0.15 g. With a Ga melt of 0.75 g, 11% and 57% of Ga

changed into GaN single crystals at 720 1C and at 800 1C, respectively. The rest of the Ga crystallized as a Na–Ga

intermetallic compound after cooling. The size of the prismatic GaN single crystals obtained at 800 1C was 1.0–2.5mm

long and 0.3–1.0mm wide.

r 2005 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

Gallium nitride (GaN)-based semiconductors,which have been used to fabricate blue light-emitting diodes [1] and injection laser diodes [2],

e front matter r 2005 Elsevier B.V. All rights reserve

ysgro.2005.04.022

ng author. Tel./fax: +81 22 795 4402.

ss: [email protected]

are highly promising materials for optoelectronicdevices in the violet and ultraviolet light regionsand for high-speed electronic devices [3,4]. Most ofthese devices are currently fabricated on hetero-substrates of sapphire or silicon carbide. Manydefects and dislocations are introduced into thedevices due to the large mismatches of the latticeconstants and thermal expansion coefficients be-tween GaN and the substrate materials. In order to

d.

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Ga

Na

electric furnace

BN crucible

stainless-steel container

stainless-steel rod

valve

N2 gas

thermocouple

Fig. 1. Schematic drawing of the apparatus for GaN single

crystal growth. Ga and Na were separately charged in the upper

and lower BN crucibles.

T. Yamada et al. / Journal of Crystal Growth 281 (2005) 242–248 243

develop more reliable and high performancedevices, high-quality GaN bulk single crystalsubstrates are desired for homoepitaxial growthof the device films with low dislocation and defectdensities.

The growth of GaN bulk single crystals hasbeen attempted by various methods, for instance,hydride vapor phase epitaxy [5,6], high N2

pressure solution growth [7–9], sublimationgrowth [10,11], ammonothermal growth [12,13],flux growth [14–24] and melt slow cooling [25]. Wehave studied the crystal growth of GaN by the Naflux method [14–20]. GaN single crystals weregrown by using a Na–Ga melt at relatively lowtemperatures (600–800 1C) and low N2 pressures(below 10MPa). These conditions are close to thegrowth condition of commercially available InPbulk single crystals. The largest single crystal ofGaN so far produced by Na flux method is aplatelet of 10mm in the longest direction and0.1mm thick [17].

In the present study, we show that GaN singlecrystals can be grown by heating a Ga melt in Navapor at 720–800 1C and a N2 pressure of 5MPa.The Na vapor is absorbed into the Ga melt and aNa–Ga melt was formed during the heating.Colorless or very slightly brown-colored transpar-ent GaN single crystals started growing even at theinitial stage of crystal formation, in contrast towhat is usually observed in growth from premixedmelts, where the initial GaN always crystallizes onthe crucible wall as black and minute crystals[16,20].

2. Experimental section

In an Ar filled glove box (MBraun, O2o1 ppm,H2Oo1 ppm), 0.15–0.75 g of Ga metal (RasaIndustries, 99.99995%) and 1.0 g of Na metal(Nippon Soda, 99.95%) were loaded separatelyinto BN crucibles (16mm inner diameter, 12mmdepth). Then the crucibles of Ga and Na wereplaced, respectively, at the upper and lowerpositions in a stainless-steel container (SUS 316,21mm inner diameter, 400mm length) as illu-strated in Fig. 1. The container was connected to aN2 gas feed line and 3–4MPa of N2 (Nippon

Sanso, 499.9999%) was introduced into thecontainer at room temperature. The sample washeated at 720–800 1C for 200 h with an electricfurnace and the N2 pressure was maintained at5MPa during the growth. The reaction tempera-ture was defined as the temperature measured witha chromel–alumel thermocouple set to the outerbottom wall of the lower Na crucible (see Fig. 1).After cooling of the sample to room tempera-

ture, the crucibles were taken out of the container.A part of the Na loaded in the lower crucible stillremained after the growth experiment. This meansthat Na vapor was always present during thecrystal growth. The equilibrium vapor pressure ofNa at 720 and 800 1C are 18 and 45 kPa,respectively [26,27]. The actual vapor pressure atthe Ga melt might be lower because the Na vaporalso condensed at the cooler part of the container,mainly on the stainless-steel rod.GaN crystals in the crucible of Ga were

separated from the solidified Na by soaking inethanol. Any Na–Ga intermetallic compound that

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BN crucible

GaN single crystals

(a)

(b)

Na-Ga inter-metallic compound

Na-Ga inter-metallic compound

GaN single crystals

Na and Na-Ga inter-metallic compound

T. Yamada et al. / Journal of Crystal Growth 281 (2005) 242–248244

crystallized was removed by dissolution in nitro-hydrochloric acid. The yield of GaN based on theinitial Ga mass was calculated by the weighing theGaN crystals prepared in the crucible. Theresulting crystals of GaN were powdered andcharacterized by X-ray diffraction (XRD). CuKaradiation was used on a diffractometer with apyrolytic graphite monochromator (RigakuRINT2000). The crystal quality was evaluated byX-ray rocking curve analysis. The diffraction peakfrom (1 0 1 0) plane (10.0 reflection) was measuredin o-scan mode using a high-resolution four-circlediffractometer (Philips X’Pert-MRD) and CuKa1radiation obtained from a Ge (2 2 0) four-crystalmonochromator. The crystals were observed withan optical microscope and a scanning electronmicroscope (SEM, Hitachi S3500N).

(c)

GaN single crystals

Fig. 3. Schematic illustration of cross-sections of the products

prepared at 720 1C and 5MPa of N2 for 200 h with 0.15 g (a),

0.45 g (b) and 0.75 g (c) of Ga.

3. Results and discussion

The yields of GaN prepared at 720 1C and5MPa of N2 pressure for 200 h are plotted inFig. 2 as a function of the Ga mass loaded in theBN crucible. The cross-sectional views of somesamples prepared are schematically illustrated in

100

80

60

40

20

0

Yei

ld (

%)

0.80.60.40.2

Ga mass / g

Fig. 2. Yields of GaN prepared at 720 1C and 5MPa of N2 for

200 h versus Ga mass loaded in the BN crucible.

Fig. 3. The SEM micrographs of the GaN crystalsin the samples are shown in Fig. 4. X-ray powderdiffraction analysis showed that GaN singlecrystals obtained in the present study werewurtzite type.The 0.15 g of Ga, which was a sphere of about

3mm in diameter, reacted with nitrogen andchanged into a 6mm bowl-shaped aggregate ofGaN crystals (Fig. 3(a)). The yield of GaN was75% and the surfaces of GaN crystals werecovered with a thin layer of Na metal and Na–Gaintermetallic compound. Colorless transparentprismatic crystals having a width of 100–250 mmand a length of around 0.3mm, (0.6mm longest)grew on the crucible wall that contacted thebottom of the Ga droplet (Fig. 4(a)). As shownin Fig. 4(b), colorless transparent GaN prismaticcrystals also grew at a liquid–gas interface,approximately 0.4mm long toward the gas phaseand 0.25mm long toward the liquid phase. Over

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Fig. 4. Scanning electron micrographs of GaN single crystals grew at 720 1C and 5MPa of N2 for 200 h with 0.15 g of Ga on the

crucible wall (a) and at the gas–liquid inter face (b), with 0.45 g of Ga on the crucible wall at the central part (c) and marginal part (d),

and with 0.75 g of Ga on the crucible wall (e) and at the gas–liquid interface (f).


100 mm-wide (0 0 0 1) faces of the GaN prismaticcrystals were embedded at the outer surface of theaggregate that was opposite to the opening ofbowl-shaped aggregate.

The yields of GaN in the samples with 0.30 and0.45 g of Ga were 30% and 22%, respectively. Thesample prepared with 0.45 g of Ga had a buttonshape 8mm in diameter as shown schematically inFig. 3(b). Colorless transparent prismatic GaNsingle crystals grew on the crucible wall. Smallcrystals of a Na–Ga intermetallic compoundcovered the prismatic GaN crystals. The average

size of the GaN prismatic crystals grown on thebottom crucible wall was approximately50–100 mm wide and 0.2mm long (Fig. 4(c)). Somecrystals had a size over 200 mm wide and 0.6mmlong. The crystals that grew at the margin of theGaN crystal growth area were skeletal and had asize close to 1mm (Fig. 4(d)).The yield of GaN decreased to 11% in the

sample with 0.75 g of Ga. Most Ga metal crystal-lized as the Na–Ga intermetallic compound(Fig. 3(c)). Colorless transparent prismatic GaNcrystals with a size smaller than 10–50 mm in width

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and 0.1mm in length grew from the bottom wall ofthe BN crucible (Fig. 4(e)) and at the vapor–liquidinterface near the bottom (Fig. 4(f)).

Fig. 5 shows the yields of GaN in the samplesprepared at 720–800 1C and 5MPa of N2 for 200 hwith 0.75 g of Ga. The yield was 11% at 720 1C,and increased to 49% at 785 1C and to 57% at800 1C. The dimension of single crystal is increasedwith increasing temperature (see inset of Fig. 5).These crystals were colorless or slightly brown.Optical and scanning electron micrographs of theGaN single crystals prepared at 800 1C are shownin Fig. 6. Prismatic single crystals with a size of1.0–1.5mm long and 0.2–0.4mm wide grew on thebottom crucible wall. The 2.5mm long and 1.0mmwide hopper crystals containing Na and/or Na–Gainclusions grew at the marginal area of the GaNcrystal formation.

Fig. 7 shows the X-ray rocking curve of 10.0reflection from a prismatic crystal grown at 800 1C.The full-width at half-maximum (FWHM) of therocking curve was 24 arcsec. This value is compar-able to FWHM (18–25 arcsec) of 0.2 and 22.0reflections reported for the near defect free plateletGaN crystals prepared by the high N2 pressuresolution growth [8].

100

80

60

40

20

0

Yei

ld (

%)

820800780760740720700

Temperature /°C

1.5

1.0

0.5

0.0

Dim

ensi

on o

f lar

gest

cry

stal

/ m

m

800760720

Temperature /°C

Fig. 5. Yields of GaN prepared at 5MPa of N2 for 200 h with

0.75 g of Ga versus temperature. The inset presents the

temperature dependence of the dimension of largest GaN

crystal.

Fig. 6. Optical micrograph (a) and the scanning electron

micrograph (b) of the GaN single crystals prepared at 800 1C

and 5MPa of N2 for 200 h with 0.75 g of Ga.

Black small grains of GaN were crystallized onthe crucible wall by the Na flux method using apremixed Na–Ga melt as a starting material[16,20]. Colorless transparent GaN crystals startedto grow after the formation of the back crystals. Itwas surmised that the black color was derivedfrom nitrogen deficiency in the crystals. Thus thenitrogen content in the Na–Ga melt must be low atthe beginning of the crystal growth. The blacksmall GaN grains adhere tenaciously to thecrucible wall. In the present study, the colorlesstransparent GaN crystals directly grew on thecrucible wall at all experimental conditions andthe single crystals could be easily detached fromthe crucible wall. These results suggested that thecontent of nitrogen in the melt was enough for

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tens

ity (

a.u.

)

−100 −50 0 50 100

angle (arcsec.)

Fig. 7. X-ray rocking curve of 10.0 reflection from a GaN

prismatic crystal prepared at 800 1C and 5MPa of N2.


the growth of transparent crystals at the beginningof crystal growth by using a pure Ga melt as astarting material. Perhaps the low initial Nacontent in the melt reduces the initial growth rate,so that the nitrogen can be fully incorporated. Theexact mechanism that produced only transparentcrystals is not clear at this point.

By weighing a sample before and after soakingin ethanol and in nitrohydrocholic acid, we coulddetermine the mass of GaN single crystals and thecontent of Na in the Na–Ga melt. For the sampleprepared with 0.45 g of Ga, 22% of Ga reactedwith nitrogen and formed GaN (GaN yield 22%).The rest of Ga formed Na–Ga crystals aftercooling. The mole fraction (rNa ¼ Na/(Na+Ga))of the Na–Ga crystals was approximately 0.40. Inthe Na–Ga binary system, two Na–Ga interme-tallic compounds, NaGa4 (rNa ¼ 0:2) andNa22Ga39 (rNa ¼ 0:36), were reported [28,29].Thus, the Na–Ga crystals which covered theGaN crystals were probably Na22Ga39. In con-trast, in the sample started with 0.15 g of Ga andwhere the 75% of Ga changed into GaN, the GaNsingle crystals were covered with the Na–Gaintermetallic compound and Na (rNa ¼ 0:58).

The high yield of GaN in the sample with 0.15 gof Ga is presumably related to the higher content

of Na in the melt. In the previous studies on theNa flux method, GaN single crystals with a sizeover 1mm were prepared using a Na-rich Na–Gamelt (rNa ¼ 0:620:67). The growth rate of GaNwas extremely slow when the rNa was 0.3–0.4 [15].The yields of GaN are increased with increasingreaction temperature. Na vapor pressure andreaction rate increase with increasing temperatureand sodium could be introduced into the meltfaster. Further study on the growth process isneeded to explain the dependence of the GaN yieldon the initial amount of Ga metal and on thereaction temperature.

4. Summary

We found that a Ga melt in Na vapor and N2

atmosphere absorbed Na, and probably N at thesame time, from the gas phase and formed GaNsingle crystals at 720–800 1C and 5MPa of N2. Theyields of GaN increased with decreasing Ga massand increasing reaction temperature. Colorless orslightly brown-colored transparent prismatic crys-tals of over 1.5mm in length were grown. Thetransparent GaN crystals grew at the initial stageof crystal formation in contrast with the blackcrystal deposition on the crucible walls when usinga premixed Na–Ga melt.

Acknowledgments

The authors would like to thank Prof. F.J.Disalvo (Cornell University) for reading themanuscript of this paper. This work was supportedin part by Special Coordination Funds from theMinistry of Education, Culture, Sports, Scienceand Technology.

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Single crystal growth of GaN using a Ga melt in Na vapor