Theoretical study on chemical vapor transport of ZnS–I 2 system. Part II: numerical modeling

Abstract

Two-dimensional numerical modeling on closed ampoule chemical vapor transport of ZnS–I 2 system is conducted by using SIMPLE code. The transport rate (TR) and its transition from diffusion-dominated to convection-dominated regimes in relation to total pressure ( p), average temperature ( T avg), temperature difference between source and crystal ends (Δ T), ampoule length ( L) and diameter ( d) are studied intensively. Modeling results reveal that: (1) TR varies with p, T avg, Δ T, L and d in different rules for different transport regimes. The variation of TR cannot be explained by the aspect ratio and Grashof number, as previous researchers thought. (2) In the diffusion-dominated regime, with increasing p, the main tendency of TR is to increase to a maximum and then decrease, with the curvature concave down. In the convection-dominated regime, TR increases sharply with increasing p until it reaches the upper limit, whence TR stops increasing, but tends to some constant value or even decreases, due to secondary convection and early return flow. (3) The transition of TR from diffusion-dominated to convection-dominated regimes depends on p, Δ T and d, but not on T avg and L. The transition occurs at higher p for smaller d or Δ T and is independent of the aspect ratio. (4) At the same Δ T but different T avg, lower T avg yields higher TR and vice versa. The variation rates of TR with p in both diffusion-dominated and convection-dominated regimes depend mainly on T avg. The higher the T avg, the steeper the slope of the curve, i.e., the faster change of TR with p. (5) The flow transition from pure diffusion/advection to circulatory convection starts at much lower p than the transition of TR from diffusion-dominated to convection-dominated regimes, that is, the flow may be in circulatory convection mode while the mass transport is still dominated by diffusion. Compared to the experimental literature values for all growth regimes, the modeling results agree with the experimental ones qualitatively in TR values and accurately in the transitions of TRs.