Fig. 5-12 High pressure and temperature apparatus installed on the JAERI beam line at SPring-8 High pressures, up to 13 GPa, and high temperatures, to 2500 degree Celsius, can be generated in a 1 mm3 sample chamber. Real time observation of a sample under high pressure-temperature conditions is possible using bright X-rays from the synchrotron radiation source.

Fig. 5-13 High pressure-temperature phase diagram of GaN determined by in situ X-ray observation Congruent melting of GaN occurs at high pressures, above 6 GPa.

Fig. 5-14 Scanning electron microscope image of the GaN single crystals obtained by slow cooling of the melt at high pressure

Gallium nitride (GaN) is a very important material for optoelectronic devices such as blue light-emitting diodes and lasers. These devices are usually fabricated by epitaxial growth on sapphire substrates. There is a large mismatch in the lattice constants of sapphire and GaN. This causes high-density dislocations in the deposited layer, which are a major obstacle for improved device quality. Hence, large single crystals of GaN suitable for substrates are urgently needed, since these crystals make it possible to reduce the dislocations greatly. However, such single crystals are difficult to grow by the standard slow cooling method because at ambient pressure GaN usually decomposes into Ga and N2 before melting. The decomposition and melting behaviors of GaN under high pressure and temperature have been observed by means of an in situ X-ray diffraction technique using the synchrotron radiation beam at SPring-8 (Fig. 5-12). These observations confirmed for the first time that high-pressure suppresses the decomposition of GaN and allows congruent melting above 6.0 GPa (Fig. 5-13). This leads to a new method for single crystal growth of GaN by slow cooling of its melt under high pressure (Fig. 5-14). High quality, single crystals of GaN have been successfully obtained by this method. Although the crystal sizes are still very small due to the size limitations of our high-pressure facility, much larger single crystals, several inches long, are expected to be synthesized by applying the established high-pressure technology now used in industrial diamond production.

Reference W. Utsumi et al., Congruent Melting of Gallium Nitride at 6 GPa and Its Application to Single Crystal Growth, Nature Mater., 2, 735 (2003).

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