ARTICLE IN PRESS

0022-0248/$ - se

doi:10.1016/j.jc

�CorrespondE-mail addr

Journal of Crystal Growth 286 (2006) 494–497

www.elsevier.com/locate/jcrysgro

The process of GaN single crystal growth by the Na fluxmethod with 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 8 August 2005; received in revised form 2 October 2005; accepted 11 October 2005

Available online 28 November 2005

Communicated by K.W. Benz

Abstract

Ga melts were heated in a boron nitride crucible at 800 1C and 5MPa of N2 for 8–200 h with Na vapor. Colorless and transparent

prismatic GaN single crystals grew from a Na–Ga melt which was formed by dissolution of Na from the gas phase. Nitrogen was

probably introduced into the melt with Na. The time dependence of the Na fraction (rNa ¼ Na/(Na+Ga)) in the melts and the yields of

GaN were investigated. rNa increased to 0.39–0.43 within 100 h, and then became almost constant at this value. The yield of GaN was less

than 2% at 50 h. The yield increased linearly with heating time after 75 h, and reached 57% at 200 h. GaN single crystals with a size of

1.5mm long were obtained on the bottom of the crucible wall. The largest crystals (3.0mm-long and 1.2mm-wide) grew at the edges of

the melt and of the GaN crystal formation area near the bottom wall of the crucible.

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

The development of gallium nitride (GaN)-based semi-conductors for high-efficiency and high-performance de-vices is being intensively pursued in many industrial R&Dlaboratories. GaN bulk single crystal substrates are desiredfor homoepitaxial growth of the device films with lowdislocation and defect densities. GaN single crystals can begrown by the Na flux method at temperatures 600–800 1Cand N2 pressures below 10MPa [1–6]. These conditions arerelatively manageable compared to those of other methods,for instance, hydride vapor phase epitaxy [7,8], high N2

pressure solution growth [9–11], sublimation growth[12,13], ammonothermal growth [14,15] and melt slowcooling [16].

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

rysgro.2005.10.073

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

ess: yamataka@tagen.tohoku.ac.jp (T. Yamada).

In the conventional Na flux method, a premixed Na–Gamelt is used as a starting material. The Na–Ga meltis heated in an N2 atmosphere to grow GaN single cry-stals. In our recent research, we found that a Ga meltheated in Na vapor at 720–800 1C and a N2 pressureof 5MPa absorbs Na from the vapor and changes intoa Na–Ga melt [17]. Colorless transparent GaN singlecrystals grew from this melt and on the wall of a boronnitride crucible, in contrast to what is usually observed ingrowth from premixed melts, where the GaN alwayscrystallizes on and near the crucible wall as black andminute crystals [4].The results of the crystal growth by heating for 200 h

with the Na vapor showed that the GaN yields increasedwith decreasing Ga mass charged initially and increasedwith increasing temperature. The size of the largest crystalwas 3mm long and that was obtained at 800 1C. In thepresent study, we studied the time dependence of the GaNyields, growth morphology and Na mol fraction in the

ARTICLE IN PRESS

Fig. 1. Yield of GaN in the sample prepared at 800 1C and 5MPa of N2

with 0.75 g of Ga (left axis) and the Na mol fraction (rNa) in the Na–Ga

melt (right axis) versus heating time.

T. Yamada et al. / Journal of Crystal Growth 286 (2006) 494–497 495

Na–Ga melts prepared by heating the Ga melt in Na vaporfor 8–200 h at 800 1C and a N2 pressure of 5MPa.

2. Experimental section

In an Ar filled glove box, we weighed Ga (99.99995%,0.75 g) and Na (99.95%, 1.0 g), loaded them separately intosintered boron nitride crucibles (16mm inner diameter and12mm depth), and set them into a stainless steel container.Details of the apparatus used were described in a previousreport [17]. The crucible containing the Na was placed on astainless steel stage and the crucible of Ga was placedabove it. The container was heated to 800 1C with anelectric furnace and the N2 pressure was maintained at5MPa during the heating and growth. The reactiontemperature was measured with a chromel–alumel thermo-couple that touched the outer bottom wall of the lower Nacrucible. The equilibrium vapor pressure of Na is 45 kPa at800 1C [18,19]. Some of the Na loaded in the lower cruciblestill remained after the growth experiment. This means thatthe Na vapor was always present during the crystal growth.

After heating for 8–200 h, the sample was cooled toroom temperature by shutting off the furnace power. TheGaN crystals grown in the crucible were separated from asolidified Na–Ga melt by adding ethanol first to react withNa and then by using a nitrohydrochloric acid solution todissolve the remaining Na–Ga intermetallic compounds.The yield of GaN based on the initial Ga mass wascalculated by the weighing the GaN crystals obtained in thecrucible. The mole fraction of Na (rNa ¼ Na/(Na+Ga)) inthe solidified Na–Ga melt was calculated with the massesof the sample measured before and after removal of Naand Ga and with the mass of GaN.

The resulting GaN crystals were powdered and char-acterized by X-ray diffraction (XRD). CuKa radiation wasused on a diffractometer with a pyrolytic graphitemonochromator (Rigaku RINT2000). The crystals wereobserved with an optical microscope and a scanningelectron microscope (SEM, Hitachi S3500N).

3. Results and discussion

The yields of GaN and the Na mole fractions ofsolidified Na–Ga melts are plotted in Fig. 1 as a functionof the heating time. The Ga melt immediately started toabsorb Na from the vapor and the Na fraction rNa

increased from 0.17 at 8 h to 0.38 at 75 h. After heatingfor 100 h, rNa almost remained constant at 0.39–0.45. Theyield of GaN was less than 2% at 50 h. The yield increasedlinearly from 7.5% to 57% with increasing heating timefrom 75 to 200 h. The X-ray powder diffraction analysisshowed that GaN crystals obtained in the present studywere wurtzite-type.

The cross-sectional view of the sample prepared byheating for 50 h is schematically illustrated in Fig. 2(a). Avery small amount of thin aggregates of GaN grains with asize of micrometers crystallized on the Na–Ga droplet

surface that contacted the gas phase or the crucible wall.Some parts of the aggregates were black or brown possiblyindicating contamination by impurities or some nitrogendeficiency, but others were colorless and transparent.Fig. 3(a) and (b) shows the SEM micrographs of GaN

crystals obtained in the melt by heating for 75 h. Prismaticcrystals of GaN with an approximate size of 40 mm wideand 70 mm long grew from a 3–5 mm thick GaN micro-crystalline layer precipitated on the bottom of the cruciblewall and at the melt-gas interface near the bottom. Asshown in Figs. 2(b) and 3(b), larger crystals having a widthof 100–150 mm and a length of around 200–300 mm grew atthe edge of the GaN formation area. The yield of GaN at75 h was 7.5% and the crystals obtained were colorless andtransparent.The colorless transparent GaN prismatic crystals kept

growing at 100 h as illustrated in Fig. 2(c). The dimensionof the crystals obtained at 150 h increased to 200–400 mm inlength. However, the width did not change so much (seeFig. 3(c)). A few crystals grew to a larger size ofapproximately 600 mm–1mm long and 100–400 mm wide.The GaN crystals observed at the margin of the crystalformation area were hopper crystals in which the outerprism planes 1 0 1 0

� �of the crystals were smooth, but the

inside of the crystals was hollow (Fig. 3(d)).Fig. 4 shows the optical micrographs of the crystals

obtained by heating for 200 h. The maximum length of thecolorless and transparent prismatic crystal was 1.5mm(Fig. 4(a)). The hopper crystals that grew at the marginalarea had a size of over 3mm long and 1.2mm wide (Fig.4(b)). The top surface of the prismatic crystals wasuncovered with the Na–Ga melt as illustrated in Fig.2(d). Some of the hopper crystals contained inclusions ofNa or a Na–Ga melt.

ARTICLE IN PRESS

Fig. 3. Scanning electron micrographs of GaN single crystals grew at 800 1C an

(a) and at the margin (b), for 150 h on the crucible wall (c) and at the margin

Fig. 2. Schematic illustrations of cross-sections of the products prepared

by heating for 50 h (a), 75 h (b), 100 h (c) and 200h (d) at 800 1C and

5MPa of N2 with 0.75 h of Ga and Na vapor.

T. Yamada et al. / Journal of Crystal Growth 286 (2006) 494–497496

As mentioned above, Ga melt adsorbs Na probably withN from Na vapor in N2 atmosphere at 800 1C and formsthe Na–Ga melt containing nitrogen. According to theelectrochemical study on Na melts by Tamaki and Cusack,the minimum integral molar free energy of mixing (about�1160 cal/mol) was measured for the melt with a Na molefraction (rNa) of 0.385 at 570 1C [20]. They suggested astable grouping of Na5Ga8 in the Na–Ga melt. Similarstable groupings or species including N may also forms inthe melt for the GaN growth.During the period of 8–50 h the yield of GaN was less

than 2% and microcrystalline aggregates of GaN wereproduced on the surface of the melt. In this period, Nafraction in the melt increased to approximately 0.3. Themicrocrystalline aggregates of GaN prepared at this stagemay become the nuclei for the following growth ofprismatic crystals. After this aging period, the growth ofthe prismatic single crystals started and continued from 75to 200 h, keeping rNa almost constant at 0.38–0.45. This rNa

range is close to the Na fraction of the stable grouping ofNa5Ga8 proposed by Tamaki and Cusack [20].When a premixed Na–Ga melt with rNa ¼ 0.3–0.4 was

used in the conventional Na flux method, the formation ofGaN was slow [2]. The growth rate of the prismatic singlecrystals was presumably low enough that increasing rNa

caused by the formation of GaN went into the vaporphase. If the growth rate was faster than that of Naevaporation from the melt, the morphology of the crystalschanged to thin platelets as reported for the samplesprepared by using a premixed Na–Ga melt with higherrNa [4].The prismatic crystals obtained from the samples of

75–200 h were colorless and transparent crystals grew fromthe bottom of the crucible wall or the interface between themelt and gas phase near the bottom wall. (See the GaN

d 5MPa of N2 with 0.75 g of Ga and Na vapor for 75 h on the crucible wall

(d).

ARTICLE IN PRESS

Fig. 4. Optical micrograph of GaN single crystals (a) and a hopper crystal

(b) prepared at 800 1C and 5MPa of N2 for 200 h with 0.75 g of Ga and Na

vapor.

T. Yamada et al. / Journal of Crystal Growth 286 (2006) 494–497 497

single crystals which contacted with crucible bottom wall inFig. 4(a)). This means that the nitrogen content in the melthad attained a value for the growth of the colorless andtransparent single crystals at 75 h.

Although larger crystals grew at the margins of GaNcrystal formation in the crucible, the crystals are mostlyhollow and some contained Na or a Na–Ga meltinclusions. This suggested that the supersaturation of Nor GaN at the part was locally higher than that at thecentral part due to the difference of N diffusion paths fromthe liquid–gas interface.

4. Summary

GaN single crystals were grown by heating a Ga melt at800 1C and 5MPa of N2 in Na vapor. We observed thegrowth of prismatic single crystals for more than 125 h inthe Na–Ga melt with a constant Na molar fraction of0.38–0.45 after an aging time of about 50 h. The initial Gamelt absorbed Na from the vapor phase (and probably

adsorbed N as well) during the aging time. This allows theformation of colorless and transparent crystals right fromthe beginning of the growth of prismatic crystals. Thesample prepared by heating for 200 h contained prismaticcrystals of 1.5mm in length on the bottom of the cruciblewall and hopper crystals over 3mm at the margins of thecrystal growth area, suggesting concentration gradients ofthe nutrients in the melt.

Acknowledgments

The authors would like to thank Prof. F.J. Disalvo(Cornell Univ.) for reading the manuscript of this paper.This work was supported in part by Special CoordinationFunds from the Ministry of Education, Culture, Sports,Science and Technology.

References

[1] H. Yamane, M. Shimada, S.J. Clarke, F.J. DiSalvo, Chem. Mater. 9

(1997) 413.

[2] H. Yamane, D. Kinno, M. Shimada, T. Sekiguchi, F.J. DiSalvo,

J. Mater. Sci. 35 (2000) 801.

[3] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, H. Iwata, F.J.

DiSalvo, J. Crystal Growth 266 (2004) 461.

[4] H. Yamane, M. Aoki, T. Yamada, M. Shimada, H. Goto, H.

Makino, T. Yao, S. Sarayama, H. Iwata, F.J. DiSalvo, Jpn. J. Appl.

Phys. 44 (2005) 3157.

[5] F. Kawamura, T. Iwahashi, K. Omae, M. Morishita, M. Yoshimura,

Y. Mori, T. Sasaki, Jpn. J. Appl. Phys. 42 (2003) L4.

[6] M. Morishita, F. Kawamura, M. Kawahara, M. Yoshimura,

Y. Mori, T. Sasaki, J. Crystal Growth 270 (2004) 402.

[7] M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher,

M. Stutzmann, Jpn. J. Appl. Phys. 38 (1999) L217.

[8] K. Motoki, T. Okahisa, N. Matsumoto, M. Matsushima, H. Kimura,

H. Kasai, K. Takemoto, K. Uematsu, T. Hirano, M. Nakayama,

S. Nakahata, M. Ueno, D. Hara, Y. Kumagai, A. Koukitu, H. Seki,

Jpn. J. Appl. Phys. 40 (2001) L140.

[9] S. Porowski, I. Grzegory, J. Crystal Growth 178 (1997) 174.

[10] S. Porowski, MRS Internet J. Nitride Semicond. Res. 4S1 (1999)

G1.3.

[11] T. Inoue, Y. Seki, O. Oda, S. Kurai, Y. Yamada, T. Taguchi, Phys.

Status Solidi B 223 (2001) 15.

[12] C.M. Balkas-, Z. Sitar, L. Bergman, I.K. Shmagin, J.F. Muth, R.

Kolbas, R.J. Nemanich, R.F. Davis, J. Crystal Growth 208 (2000)

100.

[13] G. Kamler, J. Zachara, S. Podsiad"o, L. Adamowicz, W. Gebicki,

J. Crystal Growth 212 (2000) 39.

[14] D.R. Ketchum, J.W. Kolis, J. Crystal Growth 222 (2001) 431.

[15] A.P. Purdy, R.J. Jouet, C.F. George, Cryst. Growth Des. 2 (2002)

141.

[16] W. Utsumi, H. Saitoh, H. Kaneko, T. Watanuki, K. Aoki,

O. Shimomura, Nat. Mater. 2 (2003) 735.

[17] T. Yamada, H. Yamane, H. Iwata, S. Sarayama, J. Crystal Growth

281 (2005) 242.

[18] R.W. Ditchburn, J.C. Gilmour, Rev. Mod. Phys. 13 (1941) 310.

[19] W.T. Hicks, J. Chem. Phys. 38 (1963) 1873.

[20] S. Tamaki, N.E. Cusack, J. Phys. F: Met. Phys. 9 (1979) 403.