Recently, growth of bulk GaN crystals has been intensively researched because GaN wafers sliced from bulk crystals will solve all fundamental growth problems arising from heteroepitaxy. However, the extremely high price of state-of-the-art GaN wafers does not meet the cost requirement of LEDs for solid-state lighting. Among several bulk growth methods, the ammonothermal growth, which is the solvothermal growth using supercritical ammonia, has a high potential of realizing low-cost, high-quality GaN substrates due to its excellent scalability. In the early stage of our research, we demonstrated single-phase synthesis of wurtzite GaN powders with basic mineralizers and retrograde solubility of GaN in supercritical ammonobasic solutions. Through these experiments, we have achieved uniform growth of GaN films on an over-1-inch oval shaped seed crystal through fluid transport of Ga nutrient. However, it turned out that Ga nutrient was transformed to GaN in the nutrient crucible resulting in abrupt drop of the growth rate in about a day.
In this presentation, we report on consistent growth of GaN for extended time in the ammonothermal method with polycrystalline GaN nutrient and higher mineralizer concentration. GaN was grown in a cylindrical high-pressure autoclave having its internal diameter of 40 mm. Free-standing c-plane GaN grown by HVPE was used as seed crystals. The estimated defect density of the seeds was at the order of 108 cm-2. The reactor was divided into an upper region and a lower region with a baffle to set a temperature difference in the dissolving region and the crystallization region. Since GaN has retrograde solubility in supercritical ammonobasic solution, the nutrient was placed in the colder region (upper region) and the seed crystals were placed in the hotter region (lower region). The temperature of the external heaters was maintained at about 507˚C and 700˚C for upper and lower region, respectively. Due to thick wall of the autoclave, the temperature difference between upper and lower region is estimated to be less than 50˚C. The resulting ammonia pressure was about 180-190 MPa.
Single crystalline growth of GaN on both sides of seeds was confirmed with XRD and TEM. GaN also grew on the sidewalls (i.e. m-plane and a-plane) of the seeds. Unlike the Ga nutrient, which causes growth saturation problem, GaN nutrient with higher concentration of a mineralizer allowed continuous growth over extended time. Continuous growth was confirmed up to 50 days with growth rates of 0.8, 3.6, and 6 µm/day along +c, -c and m direction, respectively. Growth on Ga-face (+c) has stronger tendency to form (10-11) facet at the edge of the seeds compared to N-face (-c). The FWHM of X-ray rocking curve obtained from N-polar side was 843 arcsec for 002 reflections and 489 arcsec for 201 reflections. Since the film on Ga-face was not thick enough, X-ray rocking curve was affected by a signal from the seed, thus did not represent the structural quality of the grown layer.