Suppression of polycrystalline formation during Na–flux GaN growth on point seeds via high-temperature and high N2 pressure conditions

The Na–flux method is suitable for producing large-diameter GaN substrates using the multi-point seed (MPS) method, in which numerous tiny seed crystals are arrayed on sapphire; however, a major challenge is achieving both stable growth on MPS substrates and suppression of polycrystalline formation outside the seed region. In this study, we investigated high-temperature, high N2 pressure (HTHP) conditions, which are expected to expand the range of conditions under which only the seed crystals grow without polycrystalline formation. In short-duration growth experiments, we compared the feasibility of crystal growth on MPS substrates and the supersaturation behavior under various temperature and pressure conditions. Stable crystal growth on MPS substrates was achieved under HTHP conditions even at supersaturation levels where growth was not possible under conventional conditions. Furthermore, polycrystalline formation tended to be suppressed even under conditions, exhibiting relatively high supersaturation. Moreover, we conducted long-duration growth experiments incorporating planarization and thick-film growth using the flux-film-coated method. The polycrystalline yield was reduced to approximately 1% under HTHP conditions (900 °C/5 MPa), compared with approximately 11% under conventional conditions (870 °C/3 MPa). These results demonstrate that HTHP conditions suppress polycrystalline formation by expanding the growth region without polycrystals, thereby contributing to the development of a Na–flux GaN substrate fabrication process.