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We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 cm-2. The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the "+" and "-" c directions, resulting in a Na concentration of 4.2×1014 cm-3 in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.

We developed a new liquid phase epitaxy (LPE) growth method for growing large GaN single crystals. In this method, the homo-epitaxial growth of the GaN crystal proceeds by nitrogen dissolution against a Na-Ga solution film coating the GaN thin-film substrate. A growth rate of 4 µm/h was achieved though nitrogen pressure was relatively low at 9.5 atm. Furthermore, the GaN film grown by this method showed a flat surface.

Growth of large and transparent GaN single crystals was carried out by applying the liquid phase epitaxy (LPE) method in a Ca-Na mixed flux system. We have previously reported LPE growth of GaN in a Na flux system, and that GaN crystals grown by LPE have extreme low dislocations and show excellent photoluminescence characteristics. In this study, use of a Ca-Na mixed flux system enabled us to grow transparent GaN crystals under low nitrogen pressure and to further improve the photoluminescence (PL) characteristic. The dislocation density of this crystal is very low (2 ×105 cm-2 in highest point).

We found the yield of GaN grown in a Li-Na flux system to be much higher than that grown in Na flux. Nitrogen dissolution seemed to increase with Li in solution, which promotes the growth of GaN crystals. In this paper, we report that the yield of GaN and habit modifications greatly depend on the composition of the Li-Na flux system and that GaN can be grown in a pure Li flux at about 50 atm.

We grew a 12×5×0.8 mm GaN single crystal using the liquid phase epitaxy (LPE) method with Na flux. This GaN single crystal was grown on a 3 µm-thick GaN thin film synthesized on sapphire using the metal organic chemical vapor deposition (MOCVD) method. It grew in a polyhedral form larger than the MOCVD-GaN substrate. Results indicate that a MOCVD-GaN thin film on a sapphire substrate functions as a seed crystal in Na flux. The use of mixed nitrogen gas containing 40% ammonia instead of pure N2 gas also enabled the growth of a 10 µm-thick GaN homo-epitaxial film on an MOCVD-GaN film under 5 atm, the lowest reported pressure for growing GaN in Na flux. In this paper, we describe the liquid phase epitaxy (LPE) technique for growing bulk GaN single crystals, as well as the results of photoluminescence (PL) measurements. We also compare the PL intensity of the bulk GaN obtained in this study and the MOCVD-GaN. PL measurements revealed that the peak intensity of GaN single crystal grown by LPE indicates 40 times larger than MOCVD-GaN film. Also, dislocation density of bulk GaN crystals could be drastically reduced by the LPE growth technique.