Numerical Analysis of Gas Flow Instabilities in Simplified Vertical HVPE GaN Reactors

This paper investigates the gas flow and the mass transport in simplified axial-symmetric vertical HVPE reactors for the growth of GaN bulk crystals through numerical simulations. We evaluate the relative significance of different flow and transport phenomena in dependence on the direction of gravity. The performed simulations show that buoyancy effects due to density differences between neighboring gas lines are the main factor causing the deformation of laminar flow patterns and the formation of recirculation cells within the growth zone. Baroclinic instabilities have been identified as the source for these phenomena. In contrast, typical vertical temperature gradients show only a minor impact on the stability of the gas flow within the growth zone in the vicinity of the growing crystal. Based on these results, major differences of the species transport in vertical HVPE reactors, where the flow is parallel or anti-parallel to the direction of gravity, referred to as down-flow and up-flow, respectively, are summarized. The performed analysis of the interplay and relative significance of different flow effects in the HVPE environment allows a general recommendation for reactor design and scaling with respect to stable gas flow conditions within the growth zone.