single crystal growth of gallium nitride by slow-cooling

Joint 20th AIRAPT – 43th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005

Single crystal growth of Gallium Nitride by slow-cooling of its congruent melt under high pressure

W. Utsumi*, H. Saitoh, H. Kaneko, K.Kiriyama and K. Aoki

Synchrotron Radiation Research Center, Japan Atomic Energy Research Institute

Mikazuki, Hyogo 679-5148, Japan, [email protected]

We were successful in synthesizing single crystals of GaN (Gallium Nitride) by slow cooling of its congruent melt under high pressure. It was confiremed by in situ x-ray diffraction that applying high pressures above 6.0 GPa completely prevented the decomposition and allowed the congruent melt of GaN at 2220°C. Using a cubic-anvil-type large volume high-pressure apparatus and GaN powder as a starting material, single crystal growth was performed by decreasing temperature from 2400°C at 6.5 GPa. The x-ray rocking curve of the recovered sample showed very narrow line-width smaller than 30 arcsec, suggesting its low dislocation density.

Introduction GaN (Gallium Nitride) is a very important material in optoelectronic devices for blue light-

emitting diodes and lasers (Nakamura S.,1994). These devices are usually fabricated by epitaxial growth on sapphire (Al2O3) substrates since large-size GaN single crystals are unavailable. However, there is a large mismatch in the lattice constants of sapphire and GaN. This mismatch causes high-density dislocations in the deposited layer and is a major obstacle for improving device quality. Hence, large-size single crystals of GaN, which are suitable as substrates, are desired. Generally, bulk nitride crystals are difficult to grow since they usually decompose or evaporate before melting at ambient pressure. Large GaN single crystals from its stoichiometric melt cannot be synthesized by standard methods such as Czochralski or Bridgman growth. Many attempts have been made such as hydride vapor phase epitaxy (Motoki K.,2001), Na flux method (Yamane H.,1998), ammonothermal method (Yoshikawa A,2004), and laser heating with a diamond anvil cell (Hasegawa M. and Yagi T.,2000), but all still have problems in growing large single crystals suitable for substrates of optoelectronic devices.

High pressure is effective in suppressing the decomposition of GaN into Ga and N2 at high temperatures. A Polish group has conducted extensive studies using their high-pressure N2 gas apparatus and has successfully obtained thin plates of GaN single crystals (Porowski S,1996). However, the maximum pressure (2 GPa) in their gas apparatus was not high enough to prevent decomposition perfectly. Thus, their method for growing GaN crystals was not by cooling a GaN melt, but was based on the solution of GaN in Ga melt under the pressure/temperature conditions. If applying higher pressures completely hinders the decomposition and allows the stoichiometric melt of GaN, then this will lead to a new synthetic technique for preparing high-quality bulk single crystals with a larger size. In this paper, we report our new approach using a large volume multi-anvil high-pressure apparatus that can generate much higher pressures and temperatures. The first in situ observation of congruent melting in GaN, which occurred above 6.0 GPa and 2220°C, and the single crystal growth of GaN by slow-cooling of its melt, are described (Utsumi W., 2003).

Experimental Method Experiments were performed using SMAP2, a multi-anvil high-pressure apparatus for the

synchrotron radiation, installed on beamline BL14B1 (bending magnet source) at the SPring-8 in Harima Science Garden City, Japan (Utsumi W.,2002). SMAP2 is a 180-ton hydraulic


press combined with a DIA type guide-block, where a cubic-shape pressure medium is compressed by tungsten carbide anvils with a top area of 6mm*6mm. High-pressure cell assembly in the present study was a modification of our previous experiment (Okada T., 2002). High temperature was achieved by a cylindrical graphite furnace embedded in a pressure medium made of pyrophillite. A LaCrO3 sleeve surrounded the graphite furnace for thermal insulation, which allowed stable high temperatures above 2000°C to be generated. Temperature was measured by a PtRh6%-PtRh30% thermocouple that was inserted in the high-pressure cell. The temperature was estimated above 1800°C by extrapolating the input power versus temperature relationship because the thermocouple did not survive. Since this relationship lies on a straight line and can be accurately reproduced, the temperature uncertainty was estimated to be less than ±15°C.

Starting material was a high-purity (99.99%) and fine- grain (less than 0.1 mm in diameter) powder of GaN supplied by Kojundo Chemical Lab. Co., Lit. It was compacted in advance into a disk shape (1.0mm in diameter and 0.4mm in height) and placed in a capsule made of poly-crystal hexagonal BN.

The pressure was initially increased at room temperature and then the temperature was increased under a constant applied load. In situ powder x-ray diffraction profiles of the sample were obtained for each pressure/temperature condition by the energy dispersive method using the white synchrotron radiation beam. Incident x-ray collimated to 0.1 mm*0.3 mm2 irradiated the sample and the diffracted x-ray was measured by a Ge solid-state detector mounted on a goniometer (2-theta angle was fixed to 6.0°). The exposure time for one diffraction profile was 30 seconds. Pressure values in this study were based on the pressure calibration curve at room temperature, which was obtained by a different experimental run in which NaCl pressure standard was compressed. Pressure drops (less than 0.5 GPa) might occur above 2000°C. The recovered single crystals at ambient conditions were analyzed by scanning electron microscope, x-ray diffractometer and photoluminescence.

Result and Discussion (1) In situ observation

Figure 1 shows the variations of the x-ray diffraction profiles as the temperature increased at 2.0 GPa. The starting GaN powder at ambient conditions showed a diffraction profile of a wurtzite structure with large line-widths, which indicate a small grain size of sample (a). (Extra peaks from the BN capsule were also observed.) The diffraction profile at 2.0 GPa and 27°C were nearly identical to the ambient conditions except for the slightly higher-energy shift due to the compression (b). When temperature was increased to 800°C, the line-width of each diffraction peak became smaller, suggesting that the sample grain size became larger (c). A profile series (d) was obtained while continuously increasing the temperature at a constant rate of 10°C /min and the data was collected every 30 seconds. The sample maintained its wurtzite structure and drastic changes did not occur until 1600°C. Once the temperature reached 1600°C, the peak intensity became smaller. At 1645°C, all

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Fig. 1 A series of x-ray diffraction profiles of GaN as the temperature increased at 2.0 GPa


sharp peaks from GaN completely disappeared and a continuous broad diffraction profile was observed, which indicated that GaN completely decomposed and a Ga melt formed. This decomposition temperature 1650°C at 2.0 GPa is consistent with the previous study using a gas high-pressure apparatus (Karpinski J.,1984). The recovered sample at ambient conditions showed a similar broad diffraction profile (e). Comparing the recovered profile with the reagent Ga confirmed that the recovered product was a Ga melt.

On the other hand, GaN melted congruently at pressures above 6.0 GPa. Figure 2 shows the results at 6.0 GPa. Similar to the results at 2.0 GPa, the line-width of the diffraction peaks became smaller as the temperature increased (a,b). When temperature was greater than 1800°C, the intensity of each diffraction peak greatly varied. Some peaks were very strong, while others did not appear (c), which is common in the energy dispersive x-ray diffraction when the temperature approaches the melting point and the sample grain size becomes large. Despite of the large variations in the intensities, all observed diffraction peaks can be indexed as the wurtzite structure of GaN (and hexagonal BN). At 2215°C, all the sharp peaks from the GaN crystal vanished and a broad diffraction profile appeared. The shape of this profile is significantly different from that of Ga melt, which suggests that the GaN congruently melted at this temperature. After melting was confirmed, turning off the supplied power to the furnace rapidly decreased the temperature and then pressure was released. The diffraction of the recovered sample was taken under ambient conditions and showed that it was a poly-crystal of wurtzite structure (d). This indicates that unlike the case at 2.0 GPa, the melt formed at 6.0 GPa reversibly back-transformed into the original crystal structure when the temperature was decreased.

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Fig. 2 A series of x-ray diffraction profiles of GaN as the temperature increased at 6.0 GPa

The pressure-temperature diagram (Fig. 3) summarizes the decomposition and melting behaviours of GaN. The experiments were repeated for the different pressures in order to determine the pressure that each event occurred. At pressures less than 5.5 GPa, GaN decomposed into Ga and N2 and the decomposition temperature almost linearly increased with pressure. In contrast, at pressures higher than 6.0 GPa, congruent melting occurred at around 2220°C. The decomposition temperature rapidly increases with pressure, while the pressure dependence of melting temperature is negligible. The phase boundary between Ga+1/2N2 and GaN (L) is a hypothesis line and not confirmed experimentally. The unit cell volume of solid GaN just before melting calculated from the in situ diffraction profile

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Fig. 3 Phase diagram of GaN under high pressureand temperature determined by in situ x-raydiffracion


is 45.56Å3, almost equal to the initial value at ambient conditions (45.52Å3). The Clasius-Clapeyron’s equation, dT/dP=∆V/∆S, indicates that the volume change on the melting or solidification must be very small because the slope of melting line is almost zero.

(2) Single crystal growth

The phase diagram of GaN (Fig. 3) leads to a new synthesis method of GaN single crystal by slow-cooling of its melt under high pressure. Single crystal growth experiments were carried out at 6.5 GPa. In the same manner as the in situ experiment, powdered GaN was placed in a BN capsule and compressed by the multi-anvil apparatus. Temperature was first increased to 2300 °C, which was high enough for melting, and then it was slowly decreased to 2100 °C at a constant rate of 1 °C/min. Temperature control was made by changing the electrical power to the graphite furnace. After the temperature was further decreased to room temperature (50 °C/min), pressure was released.

The recovered BN capsule at ambient conditions was filled with many pieces of transparent GaN single crystals that had a slightly yellowish colour. A scanning electron micrograph of the obtained GaN single crystals is shown in Fig. 4. The average crystal size obtained in the present study is about 100 µm. The obtained crystal in this study does not show a clear hexagonal shape that is common by the flux method. Figure 5 is a cross section photo of the recovered BN capsule packed with GaN crystals (It was embedded in epoxy resin and polished with a lapping film). Since the BN capsule was placed in a cylindrical furnace, there was a temperature gradient in the capsule. The crystallization of GaN started at the bottom and top of the capsule, where the temperature was the lowest. Then, the crystals grew up toward the center. Owing to this temperature gradient, many crystals have the needle shape and are usually recovered as two blocks (upper and lower). The surface of the BN capsule lost its initial smoothness due to the grain growth of BN particles at high temperatures. Thus, many spontaneous nucleations of GaN took place at the surface of the BN capsule, which limited the crystal size. Further R&D studies are now in progress to synthesize much larger crystals.

Fig. 4 A scanning electron micrograph of the obtained GaN single crystals by slow cooling of its melt

It has been confirmed by x-ray diffraction and Raman spectroscopy that the obtained crystals are wurtzite structural GaN single crystals with high crystallinity (Utsumi W., 2003). Figure 6 shows the x-ray rocking curve of 100 diffraction peak of the sample. Its full width at half maximum was calculated to be 29.6 arcsec, suggesting its low dislocation density.

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Fig. 5 A cross section of the recovered BN capsule filled with GaN crystals

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Fig. 6 X-ray rocking curve of 100 peak of GaN single crystal

The present results have great potential in providing high-quality bulk single crystals of GaN by slow cooling of stoichiometric melt of GaN under high pressures. By


applying the established high-pressure diamond synthesis technology, the growth of much larger single crystals is expected. Also, this high pressure technique is useful to other nitride semiconductors. Applications to AlxGa1-xN and InN are described in elsewhere (Saitoh H., 2004, 2005).

Acknowledgements We acknowledge Drs. H.Takei and T.Taniguchi for encouragement and useful discussion.

This research is supported by Grant-in-Aid for Scientific Research (B) (17360013) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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single crystal growth of gallium nitride by slow-cooling

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