Growth of 5 mm GaN Single Crystals at 750 °C from an Na-Ga Melt Aoki,†
Masato Hisanori Francis J. DiSalvo§
Yamane,*,†
Masahiko
Shimada,†
Seiji
Sarayama,‡
and
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Institute for Advanced Materials Processing, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, Department 5, R & D Center, Research and Development Group, Ricoh Company, Ltd., Natori 981-1241, Japan, and Department of Chemistry, Cornell University, Ithaca, New York 14853 Received October 22, 2000
ABSTRACT: A platelet GaN single crystal having an area of 5 × 3 mm2 was obtained by heating a Na-Ga melt in a BN crucible at 750 °C under 5 MPa of N2 for 360 h. Prismatic single crystals with a size of more than 0.5 × 0.5 × 1.0 mm3 grew at a lower content of Na in the starting melt. Laser diodes using GaN-based III-V nitrides have been developed, and nitride semiconductor devices are now of considerable interest.1,2 High-quality substrates, in particular GaN bulk single-crystal substrates, are required for fabrication of high-performance devices by epitaxial growth of thin films. GaN bulk single crystals have been synthesized by vapor growth techniques or a vapor-liquid-solid mechanism above 1000 °C using NH3.3-5 The preparation of GaN single crystals by the direct reaction of a Ga melt and N2 gas has been carried out at high temperature (around 1500 °C) and high N2 pressure (about 1 GPa).6-8 The largest bulk single crystals obtained by this method are over 15 mm.8 We have reported that GaN crystals can be synthesized at 650-800 °C and below 10 MPa of N2 by using a Na-Ga melt. In this method, the starting materials of Ga, Na, and NaN3 were sealed in a stainless steel tube and heated to grow the crystals.9-13 NaN3 decomposes into Na metal and N2 gas around 300 °C. In the sealed tube, the N2 pressure decreased as GaN crystals formed. Recently, we prepared GaN single crystals from a Na-Ga melt heated to 750 °C for 200 h but at a constant N2 pressure of 5 MPa by introducing N2 gas from outside the container.14 Colorless and transparent GaN platelet single crystals with a size of 3 mm in the longest direction were obtained at rNa ) 0.60, where rNa is the mole fraction of Na/(Ga + Na) in the starting melt. High crystalline quality was demonstrated for the platelet single crystals with a size of about 1 mm2 by X-ray diffraction, Hall effect measurements, and cathodoluminescence spectroscopy. In the growth experiment, Ga was completely consumed in 200 h to form GaN single crystals. The present study aimed to demonstrate that much larger crystals could be obtained by continuation of crystal growth over 200 h in the Na-Ga melt. Within the limitation of the container volume, we used the Ga source more effectively by designing the inside shape of the crucible. We found that a cone-shaped cavity * To whom correspondence should be addressed. Tel./Fax: 81-22217-5160. E-mail:
[email protected]. † Tohoku University. E-mail: M.A.,
[email protected]; M.S.,
[email protected]. ‡ Ricoh Company, Ltd. E-mail:
[email protected]. § Cornell University. E-mail:
[email protected].
Figure 1. Schematic view of the BN crucibles (a) with a cylinder-shaped cavity used in the previous study14 and (b) with a cone-shaped cavity used in the present study.
controlled the formation of GaN crystals at the surface of the melt and enabled us to continue crystal growth in the melt over 360 h. As a result, we obtained platelet single crystals with a longest dimension of 5 mm. We also obtained pyramidal and prismatic GaN single crystals with a size of about 1 mm at rNa ) 0.54. The present paper describes the formation of GaN single crystals in Na-Ga melts charged in the cone-shaped BN crucible. Figure 1 shows the BN crucible with a cylindershaped cavity, which we used in the previous study (Figure 1a), and the newly designed crucible used in the present study (Figure 1b). The latter has a cone-shaped interior volume. Ga metal (99.9999% purity) and Na metal (99% purity) were weighed and charged in the BN crucible. This loaded crucible was set in a stainless steel container. These manipulations were carried out
10.1021/cg005521c CCC: $20.00 © 2001 American Chemical Society Published on Web 02/03/2001
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Figure 2. Photographs (a-c) and an optical micrograph (d) of the sample prepared at rNa ) 0.60 in the BN crucible with the cone-shaped cavity: (a) taken just after opening the container; (b) taken after removal of Na; (c) taken after removal of Na and Na-Ga intermetallic compound.
in an Ar gas filled glovebox. The container was heated from room temperature to 750 °C over 1 h with an electric furnace under an argon atmosphere. Then, N2 gas (>99.9999% purity) was introduced up to 5 MPa and kept at this pressure using a pressure regulator. The apparatus used has been described before.14 The temperature gradient at the position of the crucible in the furnace was about 10 °C/cm. After it was heated at 750 °C for 360-450 h, the sample was cooled by shutting off the furnace power. Some Na metal evaporated and deposited on the cooler part of the container. Na metal, remaining in the crucible and covering GaN crystals, was removed by reaction with methanol and ethanol. The crystals were identified as wurtzite-type GaN by single-crystal X-ray diffractometry (hexagonal, P63mc, a ) 3.189(1) Å and c ) 5.185(1) Å). Photographs of the sample synthesized at rNa ) 0.60 (Ga ) 18 mmol, Na ) 27 mmol) for 360 h in the BN crucible with the cone-shaped cavity are shown in Figure 2. Figure 2a was taken in air just after opening the stainless steel container. The surface of the Na metal turned to white immediately by reaction of water and oxygen in air. Part of the GaN platelet single crystals were seen at the surface. Since some Na-Ga intermetallic compound remained as metallic luster needle crystals which crystallized during cooling (Figure 2b), the crystal growth of GaN could have been continued for a longer time. Most of the GaN crystals grown from the BN wall were hexagonal platelets, but some
small prismatic single crystals were included in the matrix (Figure 2c,d). These crystals were colorless and transparent. The optical micrograph of the largest platelet crystal obtained under these conditions is shown in Figure 3a. The crystal has an area of about 5 × 3 mm2 by 0.030.10 mm thick. As indicated with an arrow in Figure 2a, most of the crystal was in the Na metal. Since the crystal was detached during Na removal, we could not confirm whether the crystal nucleated on the BN wall or in the melt near the melt surface. In the second run under the same conditions, we obtained 3 × 2.5 × 0.10 mm3 single crystals (Figure 3b). Owing to spontaneous nucleation, platelet crystals bumped together and blocked the growth of each other. Thus, we did not obtain 5 mm crystals for every run. One side of the platelet single crystals has a mirror surface, but many step edges are observed on the other side. Many crystals contain some lustrous thin platelike Na-Ga inclusions in the thicker parts (Figure 3). When the inclusions were exposed to air, they reacted with water vapor in air and produced sodium hydroxide and small gallium metal droplets at the fracture surface of the crystal. Figure 4a is a scanning electron micrograph of the prismatic and pyramidal GaN crystals which grew from the BN wall at rNa ) 0.54 (Ga, 18 mmol; Na, 21 mmol) and a growth time of 450 h. The crucible wall was detached in the micrograph. Black small grains of GaN
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Figure 3. Optical micrographs of GaN platelet single crystals prepared at rNa ) 0.60 in the BN crucible of (a) the first run and (b) the second run.
Figure 4. (a) Scanning electron micrograph and (b) optical micrograph of GaN prismatic single crystals prepared at rNa ) 0.54.
crystals deposited densely on the BN wall, and some of the grains grew toward the melt. The larger crystals were colorless and transparent. In the crucible, no NaGa intermetallic compound remained. Some platelet crystals with a maximum size of 2 mm precipitated from the BN wall only near the melt-gas interface. As shown in Figure 4b, the prismatic and pyramidal single crystals have mirror surfaces. The size of these crystals (more than 0.5 × 0.5 × 1.0 mm3) is also larger than the size of prismatic and pyramidal crystals prepared in the previous study. Some inclusions or defects were observed in the larger crystals. The platelet crystal with a longest dimension of 5 mm (Figure 3a) has the largest (0001) surface area among the GaN single crystals prepared so far using a NaGa melt at 750 °C. In previous studies, the platelet single crystals having a size of 3 mm in the longest direction were obtained with the same amount of starting materials at rNa ) 0.60. One-third of the melt surface in the previously used cylindrical crucible was covered with a layer that consisted of platelet crystals of size 0.2-0.5 mm. No Na-Ga intermetallic compound remained in the melt, and all the Ga reacted with nitrogen to form GaN within 200 h. This is likely due to the consumption of most of the Ga to form a GaN platelet polycrystalline layer near the melt surface. We speculate that the slope of the cone-shaped cavity surface in the crucible may limit the formation of a GaN layer near the melt surface. This might be related to
the wettability and wetting angle of the melt against the BN wall. Another effect we expected for the BN crucible with the cone-shaped cavity is to reduce the surface area of the BN crucible wall which comes in contact with the melt. It was confirmed that the area over which crystal growth occurred on the wall in the present study was smaller than that in the previous study. The Ga source can be consumed effectively by crystal growth from the reduced nucleation area. The longer the growth time, the larger the crystals grew. This indicates that much larger single crystals could be obtained by this method. However, the growth rate in the fastest direction was approximately 14 µm/h for the platelet crystals (perpendicular to the c axis) and 2 µm/h for the pyramidal and prismatic crystals (parallel to the c axis). Thus, under these conditions 2 1/2 months would be needed to make 1 in. single crystals. To maintain the crystal growth, the nucleation area in the crucible must be limited, ideally producing only one seed crystal, and/or Ga could be supplied to the melt during growth without disturbing the crystal growth. It has been reported that the morphology of GaN single crystals prepared by the vapor growth methods using NH3 is changed by different growth conditions.3-5 However, a detailed knowledge of the relation between the crystal morphology and the growth conditions is barely understood. Karpinski et al. have reported that platelet GaN single crystals grow at lower supersaturation of GaN (or N) in the Ga melt and needlelike and prismatic crystals are obtained with increasing super-
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saturation,7 at least under the condition of the hightemperature (over 1500 °C) and high-N2-pressure (1-2 GPa) growth. We previously investigated the morphology of GaN single crystals prepared from the Na-Ga melt by the sealed-tube method.10 The morphology changed from prismatic to platelet with increasing rNa. This result agrees with the observations of crystals synthesized at constant N2 pressure in the present study. The solubility of nitrogen in liquid sodium is extremely low (7.1 × 10-9 mol % N at 600 °C).15 However, when Ga coexists with Na, it is possible that more nitrogen is introduced into the melt, presumably by forming Nax-(GaNy) complexes in the melt.9,13 As discussed in the previous paper,13 Na at the melt surface acts as a kind of catalyst for dissociation of the N2 molecule into N by donating electrons. At higher rNa (higher Na content at the meltgas interface), nitrogen would be introduced faster into the Na-Ga melt, forming the Nax-(GaNy) complexes in the melt. The N/Ga ratio of the complexes could affect the morphology of GaN single crystals. We are investigating the relationship between polarity and morphology of the crystal shape and surface to clarify the growth mechanism. Acknowledgment. This work was supported in part by the NEDO International Joint Research Program and by a grant from the Ministry of Education, Culture and Sports.
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References (1) Orton, J. W.; Foxon, C. T. Rep. Prog. Phys. 1998, 61, 1-75. (2) Monemar, B. J. Mater. Sci.: Mater. Electron. 1999, 10, 227254. (3) Ejder, E. J. Cryst. Growth 1974, 22, 44-46. (4) Ogino, T.; Aoki, M. Oyo Butsuri 1979, 48, 269-272. (5) Elwell, D.; Feigelson, R. S.; Simkins, M. M.; Tiller, W. A. J. Cryst. Growth 1984, 66, 45-54. (6) Madar, R.; Jacob, G.; Hallais, J.; Fruchart, R. J. Cryst. Growth 1975, 31, 197-203. (7) Karpinski, J.; Kaldis, E.; Conder, K.; Rusiecki, S.; Jilek, E. Mater. Res. Soc. Symp. Proc. 1992, 251, 291-305. (8) Krukowski, S. Cryst. Res. Technol. 1999, 34, 785-795. (9) Yamane, H.; Shimada, M.; Clarke, S. J.; DiSalvo, F. J. Chem. Mater. 1997, 9, 413-416. (10) Yamane, H.; Shimada, M.; Sekiguchi, T.; DiSalvo, F. J. J. Cryst. Growth 1998, 186, 8-12. (11) Yamane, H.; Shimada, M.; Endo, T.; DiSalvo, F. J. Jpn. J. Appl. Phys. 1998, 37, 3436-3440. (12) Yamane, H.; Kinno, D.; Shimada, M.; DiSalvo, F. J. J. Ceram. Soc. Jpn. 1999, 107, 925-929. (13) Yamane, H.; Kinno, D.; Shimada, M.; Sekiguchi, T.; DiSalvo, F. J. J. Mater. Sci. 2000, 35, 801-808. (14) Aoki, M.; Yamane, H.; Shimada, M.; Sekiguchi, T.; Hanada, T.; Yao, T.; Sarayama, S.; DiSalvo, F. J. J. Cryst. Growth 2000, 218, 7-12. (15) Hubberstey, P. In Liquid Alkali Metals; Proceedings of the International Conference Organized by the British Nuclear Energy Society; Nottingham University: London, 1973; pp 15-19.
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