Effects of Solution Height on the Growth of GaN Crystals in the Na-Flux Liquid-Phase Method

The effects of solution height on the growth of GaN crystals in the liquid-phase epitaxial growth (LPE) of Na-Li-Ca flux were investigated by using a combination of experimental and numerical calculations. Vertically placed hydride vapor phase epitaxial (HVPE) GaN crystals were used as seeds in the experiments. Two different solution heights were employed: 13.2 mm (H-13.2 mm) and 26.8 mm (H-26.8 mm). The distribution of the nitrogen concentration and supersaturation of GaN in both solutions were simulated and calculated through COMSOL Multiphysics field simulation. The experimental results show that the LPE thickness of GaN crystals grown in both solutions shows a top-down decreasing trend; however, there is a significant decrease in the LPE thickness range of about 375-159 mu m in the H-13.2 mm solution and an insignificant decline in the thickness range of approximately 101-55 mu m in the H-26.8 mm solution. The trend in the thickness of as-grown crystals is highly consistent with the calculated trends of nitrogen concentration and GaN supersaturation in the solution. In the H-26.8 mm solution, the fluid appears to be stratified, which inhibits nitrogen transport downward from the gas-liquid interface. When crystal growth in both solutions lies in the epitaxial growth of the substable zone, the nitrogen concentration and GaN supersaturation in the H-26.8 mm solution near the bottom of the crucible are reduced compared to the H-13.2 mm solution height, resulting in a lower growth rate with the near-equilibrium growth mode. This is consistent with XRC measurements showing a higher crystalline quality in the H-26.8 mm solution. Controlling the height of the growth solution may offer a new idea to growing high-quality GaN crystals with flatter surfaces, better crystalline quality, and improved homogeneity closer to near-equilibrium growth in the Na-flux liquid-phase epitaxial method.