Achieving Mass Transfer and Saturation Homogeneity in the Acidic Ammonothermal Process for Uniform GaN Growth

Abstract The ammonothermal method is a promising route for growing large-diameter and high-quality gallium nitride (GaN) bulk single crystals, but the extreme high-temperature, high-pressure environment inside the autoclave renders direct observation of growth processes extremely difficult. Conventional horizontal baffle structures tend to induce flow-field disturbances that limit mass-transport efficiency and growth-rate uniformity. Here, a two-dimensional axisymmetric multiphysics-coupled model is established for an acidic mineralizer system, in which a vertical baffle is introduced and a bottom-heated boundary is applied to systematically investigate the coupled laminar-flow, heat-transfer, and dilute-species transport mechanisms. The radial position of the baffle is found to strongly govern the distribution of circulation resistance. At a baffle radius of 1.1 rT (reference tube radius) and a bottom temperature of 798.15 K, the central upward and outer return channels are optimally matched, local vortices are suppressed, and the directed precursor transport is markedly enhanced, yielding an average seed-surface growth rate of 1.93 μm/h, a surface nonuniformity of 0.0002, and a relative supersaturation of 0.36 within a suitable metastable regime. These numerical findings provide theoretical guidance for the efficient and uniform growth of large bulk GaN crystals. The uncertainty in the thermochemical properties in the mode necessitates experimental validation.