Surface Control of AlN Single Crystal Growth on SiC Substrate

In this study, the process of surface morphology control was researched by combining the computation fluid dynamics (CFD) simulations and physical vapor transport (PVT) growth experiments. The results indicate the essentiality of preserving the surface structure of SiC substrate with macroscopic steps at the initial stage of the heterogeneous PVT growth. We identified an optimal range of growth temperature (Tg) and pressure to maintain the active Al vapor below a critical threshold. In the subsequent process, the pivotal factor for controlling the surface morphology of the AlN layer is identified as the supersaturation near the growing surface. Excessive supersaturation leads to a transition from a 2D to a 3D growth mode, resulting in a shift from a smooth to a rough surface morphology. An appropriate level of supersaturation can be achieved by carefully controlling the Tg, striking a balance between high surface quality and growth rate. Herein, we proposed a two-step PVT method for cultivating high-surface-quality AlN crystals on SiC substrates. At the first stage, the Tg is maintained below a threshold corresponding to the critical Al vapor pressure to preserve the surface structure until the SiC surface is completely covered by AlN. Then, Tg is elevated to near transition temperature (Ttran) to continue AlN single crystal growth at a proper rate for a long time, where Ttran is defined as the growth temperature at which the transition of the dominant mode from step growth to 3D growth happens. Two-inch-diameter AlN single crystals of thicknesses of nearly 1 mm with a smooth and lustrous surface have been obtained on SiC substrate by the two-step method.