Materials Research Bulletin 44 (2009) 594–599

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Materials Research Bulletin

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Growth of colorless transparent GaN single crystals on prismatic GaN seedsusing a Ga melt and Na vapor

Takahiro Yamada a,*, Hisanori Yamane a, Yongzhao Yao b, Masaaki Yokoyama b,c, Takashi Sekiguchi b

a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japanb Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japanc Horiba Ltd., 1-7-8 Higashi-Kanda, Chiyoda-ku, Tokyo 101-0031, Japan

A R T I C L E I N F O

Article history:

Received 4 February 2008

Received in revised form 12 June 2008

Accepted 10 July 2008

Available online 29 July 2008

Keywords:

A. Nitrides

A. Semiconductors

B. Crystal growth

B. Luminescence

A B S T R A C T

Growth of GaN on seeds of GaN prismatic single crystals was carried out at 900 8C and N2 pressure ðPN2Þ of

0.8–7.0 MPa for 72 h by the Na flux method using premixed Na–Ga melts or Ga melt and Na vapor. Black

GaN crystals, having some pits and striations on the facets, grew on the seeds when the premixed Na–Ga

melts were used. A full-width at half maximum (FWHM) of the X-ray rocking curve measured for the

ð101̄0Þ (m plane) of the grown crystals was over 360 arcsec. Colorless and transparent GaN crystals with

smooth facets were grown on the m plane of the seed crystals by using a Ga melt and Na vapor. The

FWHM measured for the m plane of the colorless crystals was 112–204 arcsec. Cathodoluminescence (CL)

spectra from the m plane of the crystals were measured at room temperature. Besides a near-band-edge

(NBE) emission at 361–363 nm, the specimens grown with Ga melt and Na vapor at higher PN2had a

broad deep emission peak at 617 nm, while the specimens grown at lower PN2had a shallow-level

emission peak at 380 nm and a broad deep emission peak at 550 nm.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The use of GaN bulk single crystal substrates for thehomoepitaxial growth of GaN-based semiconductor thin films isexpected to overcome the difficulties faced in the heteroepitaxialgrowth of the films, which result from dislocations and defectscaused by the large differences in the thermal expansion andlattice mismatch between GaN and the materials of the foreignsubstances. Various methods for the growth of GaN bulk singlecrystals which can be used as substrates have been attempted [1].The Na flux method enables the growth of GaN single crystals witha Na–Ga melt heated at 600–850 8C and a N2 pressure ðPN2

Þ of lessthan 10 MPa, while the crystal growth from a Ga melt without Narequires conditions of around 1500 8C and PN2

of 1 GPa [2].Aoki et al. reported single crystal growth of GaN on platelet GaN

seeds by the Na flux method under a temperature–pressurecondition (850 8C and PN2

of 2 MPa) which was between theconditions of GaN formation without seed crystals and GaNdecomposition in the melt [3]. However, the crystals grown onthe seed crystals were black and had many hillocks and rough

* Corresponding author. Tel.: +81 22 217 5813; fax: +81 22 217 5813.

E-mail address: yamataka@tagen.tohoku.ac.jp (T. Yamada).

0025-5408/$ – see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2008.07.007

pyramidal faces. A full-width at half maximum (FWHM) of the X-ray rocking curve measured for 0 0 0 2 diffraction was 486 arcsec[4]. Recently, Morishita et al. have reported an increase of nitrogensolubility in the Na–Ga melt by heating it at higher temperatureand/or by the addition of Li and Ca [5]. They succeeded infabricating thick GaN layers by liquid-phase epitaxy with the melton GaN substrates prepared by metalorganic chemical vapordeposition [6]. An increase of the GaN dissolution mass in an Na–Ga melt by using Li3N additives has also been reported by Aokiet al. [7] Growth of a colorless transparent GaN crystal wasperformed on a prismatic GaN single crystal seed by dissolutionand recrystallization of GaN in an Na–Ga melt with the addition ofa small amount of Li3N [8].

Yamada et al. reported that a Ga melt heated in Na vapor at720–800 8C and PN2

of 5 MPa absorbed Na from the vapor and wastransformed into an Na–Ga melt [9,10]. Colorless transparentprismatic GaN single crystals grew on the wall of a boron nitride(BN) crucible from this melt. They have also attempted seededgrowth of GaN by heating a Ga melt with Na vapor at 780 8C andPN2

of 5 MPa. Brownish transparent GaN crystals grew on prismaticGaN seed crystals, but they could not obtain colorless transparentcrystals [11]. The growth planes of the crystals tilted by a fewdegrees from the ð101̄0Þ (m plane) of the seed crystals. The FWHMof the 101̄0 X-ray rocking curve of the crystals was 360 arcsec. In

Fig. 1. Schematic drawing of the apparatus used for GaN seeded growth by the Na

flux method. Seed crystals are set into small holes on the inside bottom wall of the

upper BN crucible. Na used as a source of Na vapor is loaded into the lower crucible.

T. Yamada et al. / Materials Research Bulletin 44 (2009) 594–599 595

this seeded growth, Na vapor pressure was reduced in the processof heating to prevent spontaneous nucleation. However, it wasdifficult to reproduce this process.

In a previous study in which a premixed melt was employed [3],the condition of seeded growth was achieved at a highertemperature or lower PN2

. In the present study, we made a newapparatus which can heat a sample at 900 8C in order to growcolorless transparent GaN on the prismatic seed crystals. Wecarried out seeded growth of GaN with an Na–Ga premixed melt orwith a Ga melt and Na vapor and compared the results. The growncrystals were characterized by optical and electron scanningmicroscopy, X-ray diffraction and cathodoluminescence (CL)spectroscopy.

2. Experimental

Fig. 1 shows a schematic illustration of the apparatus used forthe present study. Two crucibles are set and heated up to 900 8Cwith two heaters which are located beside and under the crucibles,respectively. The reaction temperature described in the presentstudy was measured with two chromel–alumel thermocouples setbeside the upper crucible and under the lower crucible. Thepositions of the two thermocouples were kept at the sametemperature. The employed crucibles were made of sintered boronnitride (BN, Showa Denko, 99.5%) with an inner volume size ofØ16 mm � 13 mm. A cap and a cover made of nickel were used toprevent the escape of Na vapor from the crucible zone. N2 and Arwere introduced into the container so as to set partial N2 pressureðPN2Þ at 0.8–7.0 MPa and a total pressure of 7.0 MPa at 900 8C.

Seeds of prismatic GaN single crystals (approximately 1 mmlong and 0.3 mm wide) were prepared at 800 8C and PN2

of 5.0 MPaby the conventional Na flux method with a Na–Ga premixed melt.Two seed crystals were inset into holes drilled on the inside bottomwall of the BN crucible. In an Ar-filled glove box, 2 g of Ga melt(99.99995%) heated at about 60 8C was poured onto the seeds.

For the experiment with the Na–Ga premixed melt, 0.44 or1.55 g of Na (99.95%) was added to the upper crucible; Na molefractions in the melt (rNa = Na/(Na + Ga)) were 0.4 and 0.7,respectively. For the experiment starting with the Ga melt andNa vapor, 5 g of Na metal was separately loaded into the lower BNcrucible. After heating for 72 h, the sample was cooled to roomtemperature by shutting off the furnace power.

The GaN crystals grown in the crucible were separated from thesolidified Na–Ga melt by first adding ethanol to react with Na andthen by using nitrohydrochloric acid solution to dissolve theremaining Na–Ga intermetallic compounds. The yield of GaN wascalculated based on the masses of the crystals grown on the seedcrystals and the crystals grown by spontaneous nucleation againstthe mass of the Ga source. The amount of crystals grown on theseed crystals was estimated based on the volume increase and thedensity of GaN. The mole fraction of Na in the solidified Na–Ga melt(rNa = Na/(Na + Ga)) was calculated based on the masses of thesample measured before and after the removal of Na and Ga andwith the total mass of the grown GaN crystals.

Table 1N2 pressure, Na fraction in the melt at 0 and 72 h, apparent seeded growth rate, yield of G

rocking curve

Sample no. PN2(MPa) rNa (0 h) rNa (72 h)

1 1.6 0.4 0.38

2 1.6 0.7 0.74

3 1.6 0 0.42

4 0.8 0 0.39

5 5.0 0 0.40

6 7.0 0 0.45

The diffraction peak of 101̄0 from the m plane was measured inthe v-scan mode using a high-resolution four-circle diffractometer(Philips X’Pert-MRD) and Cu Ka1 radiation obtained from a Ge(2 2 0) four-crystal monochromator. The crystal shape andmorphology were observed with an optical microscope and ascanning electron microscope (SEM) (Hitachi S3500N). The CLspectra were measured at room temperature by using a CL system(Horiba) attached to an SEM (Hitachi S4200) [12].

3. Results and discussion

The main products in the upper crucible were a solidified meltconsisting of Na and a Na–Ga intermetallic compound. Afterremoving these products by using ethanol and nitrohydrochloricacid solution, seeded grown GaN crystals and GaN crystals formedby spontaneous nucleation at the surface of the melt and/or on thecrucible wall remained in the crucible. Experimental conditionsand resulting data on the seeded growth are summarized inTable 1.

3.1. Seeded growth with a Na–Ga premixed melt

Fig. 2 shows optical microscope and SEM photographs of thecrystals grown on the seeds in the premixed Na–Ga melts withrNa(0 h) = 0.4 and 0.7 at 900 8C and PN2

of 1.6 MPa for 72 h (samples1 and 2). The crystals grown on the seeds were black. (In thephotographs of the crystal shown in Fig. 2(a and c), the part of theseed crystal buried in the crucible broke away when the crystal

aN grown by spontaneous nucleation and seeded growth, FWHM of 101̄0 diffraction

Growth rate (mm/h) GaN/Ga (%) FWHM (10.0) (arcsec)

1.6 3.7 >600

2.0 33.1 >360

1.20 2.6 113

0.38 0.3 112

1.45 4.4 131

1.60 13.2 204

Fig. 2. Optical and scanning electron micrographs of GaN grown with the Na–Ga

melts of rNa = 0.4: sample 1 (a and b) and 0.7: sample 2 (c and d) on the seed crystals

at 900 8C and PN2of 1.6 MPa for 72 h.

T. Yamada et al. / Materials Research Bulletin 44 (2009) 594–599596

was removed from the crucible.) Growth bandings and manystriations were observed on the surface and in the crystals.Triangular pits were seen on the surface of the crystal grown withrNa = 0.4. Pyramidal facets and hillocks were formed on the N-polarc plane ð0001̄Þ. The FWHMs of the X-ray rocking curves measuredfor 101̄0 reflection from the m plane were over 360 arcsec, whilethose of seed crystals were approximately 25 arcsec. Seededgrowth of similar black crystals on platelet seeds of GaN with alarge FWHM has been reported in previous studies using apremixed Na–Ga melt [3,4].

We immersed a prismatic seed crystal into the Na–Ga melt withrNa = 0.7 and heated it for 1 h at the same temperature and PN2

condition (900 8C and 1.6 MPa). As shown in Fig. 3, the surface ofthe crystal was eroded. This result is consistent with the Aoki’sreport that GaN dissolved into a Na–Ga melt [7]. The striations andpits on the facets and the large FWHM of the seeded grown GaNwere probably due to the rough surface resulting from the erosionin the premixed Na–Ga melt before the introduction of a sufficientamount of nitrogen from the vapor phase into the melt to initiatecrystal growth on the seeds.

The increases in the width of the crystals were 231 and 287 mmfor the seeded growth with the premixed melts of rNa = 0.4 and 0.7,respectively. Assuming that the crystal growth rate was constantduring heating, the apparent growth rates perpendicular to the m

plane were 1.6 and 2.0 mm/h, respectively. The actual growth ratesshould be faster than these due to the initial erosion describedabove.

The Na mole fraction in the premixed Na–Ga melt (rNa(0 h) = 0.4and 0.7) was only slightly changed by heating for 72 h(rNa(72 h) = 0.38 and 0.74). However, the yields of GaN crystalsformed by spontaneous nucleation with the premixed melts ofrNa = 0.4 and 0.7 were 3.7 and 33.1%, respectively. Some amounts ofNa evaporated from the melts and condensed at the cooler insidepart of the Ni cover. A high yield and high growth rate of GaN with amelt of higher rNa has also been previously reported for the crystalgrowth by spontaneous nucleation with the premixed melt [13].

The seed crystals having a high crystal quality were obtained byspontaneous growth from a premixed Na–Ga melt with rNa = 0.7 ata higher pressure and lower temperature ðPN2

¼ 5:0 MPa; 800�CÞrather than the growth conditions of the seeded growth in apremixed Na–Ga melt ðPN2

¼ 1:6 MPa; 900�CÞ. Before the spon-taneous growth of the transparent colorless crystals for the seeds,many black and minute crystals were precipitated on a cruciblesurface [14]. It was surmised that the black coloration was derivedfrom nitrogen deficiency or Ga excess in the crystals because thenitrogen content in the Na–Ga melt must be low at the beginning of

Fig. 3. Scanning electron micrograph of a GaN seed crystal heated at 900 8C and PN2

of 1.6 MPa for 1 h in the Na–Ga melt with rNa = 0.7.

T. Yamada et al. / Materials Research Bulletin 44 (2009) 594–599 597

the crystal growth [15]. Transparent colorless crystals grew at thelater stage, accompanying increasing nitrogen content in the meltwith growth time. Seeded growth could not be realized under thegrowth condition of the seed crystals due to the heavy spontaneousgrowth of the black crystals at the early stage of the crystal growth.

3.2. Seeded growth with the Ga melt and Na vapor

A part of the Na loaded into the lower crucible still remainedafter the seed growth experiment with the Ga melt and Na vapor.This indicated that the crucibles were exposed to the Na vaporduring the crystal growth. Although the equilibrium vaporpressure of Na at 900 8C is 0.1 MPa [16], the actual vapor pressurewas probably lower because some Na condensed at a cooler insidepart of the Ni cover.

Fig. 4 shows optical microscope and SEM photographs of theseed crystals and the crystals grown on the seeds at 900 8C and PN2

of 1.6 MPa for 72 h (sample 3), the same temperature and PN2

condition of the seeded growth with a premixed Na–Ga melt(samples 1 and 2). The N–polar c plane ð0001̄Þ of one seed crystalwas set to face the melt (N-polar seed crystal, Fig. 4(a)) and that ofanother crystal was set to face the BN crucible wall (Ga-polar seedcrystal, Fig. 4(d)). The parts of the seed crystals immersed in themelt grew as shown in Fig. 4(b and e). The crystals that grew on theð0001̄Þ plane (Fig. 4(b)) and the ð101̄1Þ plane (Fig. 4(e)) werebrownish, but the m plane of the seeded grown crystals wastransparent and colorless. Small inclusions with a size less of than10 mm were observed in the grown layer on the seeds. However, asseen in the SEM photos (Fig. 4(c and f)), the surface of the m planeand other facet planes of the seeded grown crystals were as smoothas the facets of the seed crystals. The FWHM measured for the m

plane of the seeded grown crystal shown in Fig. 4(e) was113 arcsec, which was broader than that of the seed crystals

Fig. 4. Optical micrographs of seed crystals buried in the holes of the bottom wall of the

micrographs (b and e) and scanning electron micrographs (c and f) of GaN grown on t

(25 arcsec) but narrower than that of the seeded grown GaNcrystals with the premixed melt (over 360 arcsec).

The width of the Ga-polar crystal increased from 377 to546 mm, and the apparent growth rate perpendicular to the m

plane was 1.2 mm/h. The same m-plane growth rate was observedon the N-polar seed crystal. The length of the Ga-polar crystal inthe +c direction increased from 1.507 to 1.788 mm and theapparent growth rate was 3.9 mm/h. The increase in the length bythe growth for 72 h and the apparent growth rate in the �c

direction were 193 mm and the 2.7 mm/h, respectively. The growthrate in the +c direction was also higher in the previous study ofseeded growth on platelet seeds with a premixed Na–Ga melt [3].

Yamada et al. reported that crystal growth on the crucible wallwith a Ga melt and Na vapor by spontaneous nucleation started afterheating for 50 h at 800 8C and PN2

of 5.0 MPa [10]. The Ga melt wastransformed into a Na–Ga melt with rNa of 0.38 after heating for 75 hin the Na vapor. In present study, the composition of the Ga melt alsochanged to a Na–Ga melt during heating at 900 8C and the crystalgrowth probably started earlier than at 50 h. However, it still needssome period to form the Na–Ga melt before the start of crystalgrowth and to increase the Na content rNa. This is a reason why theapparent growth rate of the grown crystal with the Ga melt and Navapor (sample 3, 1.20 mm/h) is lower than that with Na–Gapremixed melt (sample 1, 1.60 mm/h), despite the crystals grew atalmost the same rNa in the final melt and PN2

(samples 1 and 3).When we immersed a prismatic seed crystal in the Ga melt and

heated it at 900 8C and PN2of 1.6 MPa for 1 h without Na vapor,

there was no change on the surface morphology of the crystal. Thisheating temperature and PN2

condition were the same as those atwhich a GaN crystal was eroded in the Na–Ga melt. According tothe report by Porowski and Grzegory [2], the solubility of nitrogenin a Ga melt was negligibly small at 900 8C. Thus, the seed crystalsheated in a Ga melt are not eroded.

crucible with �c (N-polar) (a) and +c (Ga-polar) (d) directions to the melt; optical

he seed crystals at 900 8C and PN2of 1.6 MPa for 72 h): sample 3.

Fig. 5. Cathodoluminescence spectra measured at room temperature for the m

plane of GaN single crystals grown with the Ga melt and Na vapor on the seed

crystals at 900 8C and PN2of 0.8 (a), 1.6 (b), 5.0 (c) and 7.0 (d) MPa for 72 h: samples

4, 3, 5 and 6, respectively, and a seed crystal (e).

T. Yamada et al. / Materials Research Bulletin 44 (2009) 594–599598

In the growth method with a Ga melt and Na vapor, the Ga melttakes a Na from vapor phase and changed into Na–Ga melt withheating time. Nitrogen is probably also dissolved into the Na–Gamelt during a Ga melt changed into Na–Ga melt. However, thegrowth rate of GaN is quite small in a Na–Ga melt with low rNa [13].When rNa reached to a value enable to start crystal growth, thecolorless and transparent crystal grows on the seed crystal in themelt. This is because the melt had already contained nitrogenenough to grow colorless transparent GaN crystal. The seededgrowth of transparent colorless crystals will continues as long asthe nitrogen is supplied enough from the gas phase. The growth oftransparent colorless crystals on seed crystals without erosion is agreat advantage of the seeded growth method with a Ga melt andNa vapor.

Not only nitrogen deficiency and excess Ga, but also impuritiesand defects have been discussed as the origin of the color of GaNcrystals grown by the Na flux method as in the studies of othercrystal growth methods. In the present study, seeded growthstarted with the Ga melt and Na vapor, the crystals grown on the m

plane were colorless and transparent, and the crystals grown onthe other facets were brownish. The reason for this was unclear,but the crystal growth rate may correlates with the coloration ofthe crystal. In addition, the non-polar structure of the m plane witha smooth growth front surface may be related to reduction of theformation of defects and uptake of impurities during the crystalgrowth.

GaN crystals grown on the m plane of the seed crystals at PN2

of 0.8, 5.0 and 7.0 MPa (samples 4, 5 and 6) were also transparentand colorless, having smooth facets as observed in the sample ofPN2

of 1.6 MPa. The FWHMs measured for the m plane were 112,131, and 204 arcsec, respectively. Apparent growth ratesperpendicular to the m plane were 0.38, 1.45, and 1.60 mm/h,respectively. The highest growth rate was observed for thecrystal grown at the highest PN2

, but it exhibited the broadest X-ray rocking curve of any of the crystals prepared with the Ga meltand Na vapor. The crystal growth with a high growth rate leadedto low crystal quality which was probably due to incorporation ofmore impurities and dislocations.

Besides the seeded growth, crystal growth of GaN by spon-taneous nucleation was observed mainly on the bottom wall of thecrucible. The yields of GaN crystals formed by spontaneousnucleation and by seeded growth against the initial Ga were 0.3–4.4 mol% at PN2

¼ 0:8� 5:0 MPa, and 13.2 mol% at 7.0 MPa. Theseresults suggest that the speed of the nitrogen uptake from the gasphase and/or nitrogen content in the melt increase at higher PN2

,which may also mean that the growth condition is apart from theequilibrium condition of GaN formation.

3.3. Cathodoluminescence

Fig. 5 shows CL spectra from the m plane of the crystals grownwith the Ga melt and Na vapor at PN2

of 0.8–7.0 MPa (samples 3–6),and from the seed part which was buried in the BN crucible walland not immersed in the melt. The near-band-edge (NBE) emissionpeak of the seed crystal was observed at 362 nm. The peak shapeand intensity from the seed part did not change so much before andafter heating.

The NBE emission peaks of the crystals grown at PN2of 1.6 and

0.8 MPa (samples 3 and 4) were also observed at 363 and 361 nm,respectively. The peak heights were about 50 times higher thanthat of the seed. In addition, a shallow-level related broad emissionpeak and a deep-level related one were observed at approximately390 and 550 nm, respectively. A broad emission peak centered ataround 720 nm was the second-ordered emission of the NBEemission (Fig. 5(a) and (b)). A deep-level related broad emission

peak centered at 617 nm was observed in the CL spectra of thecrystals grown at PN2

of 5.0 and 7.0 MPa (samples 5 and 6). Theintensity and shape of the NBE emission peaks observed at 361 nmfor both samples were similar to those of the seed crystal.

These results indicate that the shallow-level defects orimpurities were introduced into the crystals grown at lower PN2

and deep level defects or impurities (617 nm) increased in thecrystals grown at higher PN2

. The solubility of nitrogen in the Na–Ga melt is probably lower at lower PN2

, which may cause thenonstoichiometry of the GaN crystals. The shallow-level relatedemission might be attributed to nitrogen defects or excesses of Gacaused by nonstoichiometry. The increase of the crystal growthrate at higher PN2

and rNa of 0.4–0.45 may introduce the impuritiesor defects related to the emission of the deep level at around617 nm.

Fig. 6 shows the CL spectra from the m plane of the crystalsgrown with the premixed Na–Ga melt at PN2

of 1.6 MPa (samples 1and 2). The peak shape and intensity of the CL spectrum fromsample 1 (rNa = 0.4) were similar to those of samples 3 and 4 grownwith Ga melt and Na vapor at PN2

of 1.6 and 0.8 MPa. The CL is asurface sensitive probe. The fact that CL spectra of the growncrystals with different optical appearances (samples 1, 3 and 4) aresimilar is explained that the crystals near the surface were grownat similar rNa and probably with similar nitrogen contents in a Na–Ga melts after 72 h.

The NBE emission peak intensity of sample 2 ðrNa ¼ 0:7; PN2¼

1:6 MPaÞ was approximately at the same order of magnitude tothat of the seed crystal, while the peak of sample 2 was broader.The deep-level related peak at 617 nm was not observed in thespectrum from this sample. The rNa as well as PN2

affects the crystalgrowth rate and probably the nitrogen solubility in the Na–Gamelt. The seed crystal, which showed no deep-level relatedemission, was prepared in the Na–Ga melt with rNa of 0.7 at PN2

of5 MPa. The deep-level related emission may be the feature of thecrystals grown in the Na–Ga melt with low rNa of ca. 0.4 at high PN2

.More improvement of crystal quality and detail characterization,for instance, chemical analysis of the impurities and transmission

Fig. 6. Cathodoluminescence spectra measured at room temperature for the GaN

crystals grown with the Na–Ga melts of rNa = 0.4 (a) and 0.7 (b) at 900 8C and PN2of

1.6 MPa for 72 h: samples 1 and 2, respectively.

T. Yamada et al. / Materials Research Bulletin 44 (2009) 594–599 599

electron microscopic observation of the defects, are necessary toascertain the origin of the shallow-level and deep-level relatedemission.

4. Conclusions

GaN crystals grown on prismatic GaN crystals with thepremixed Na–Ga melts at 900 8C were black with pits andstriations at the surface, exhibiting broad X-ray rocking curves.Colorless transparent GaN crystals, with a smooth surface andrelatively narrow FWHM of 113–204 arcsec measured for the 101̄0reflection, grew on the m plane of the prismatic seed crystals when

the Ga melt heated in Na vapor was used. These results indicatethat the Na flux method starting with a Ga melt and Na vapor issuperior to the conventional method with a premixed Na–Ga meltfor the seeded growth of GaN.

The CL peak of the NBE emission from the crystals grown withGa melt and Na vapor was observed at 361–363 nm. A shallow-level related broad emission peak was observed at approximately390 nm for the crystals grown at PN2

of 0.8 and 1.6 MPa. Deep levelrelated broad emission peaks were observed at 550 nm for thecrystals grown at PN2

of 0.8 and 1.6 MPa, and at 617 nm for thecrystals grown at 5.0 and 7.0 MPa.

Acknowledgments

We thank Hirokazu Iwata of Ricoh Company Ltd. for his helpwith measurement of the X-ray rocking curves. This work wassupported in part by Special Coordination Funds from the Ministryof Education, Culture, Sports, Science and Technology.

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