Thermal Diffusion in Metal‐Organic Chemical Vapor Deposition

Abstract

The rate of chemical vapor deposition (CVD) can be significantly affected by thermal diffusion. Accurate predictions of growth rate and growth rate uniformity require accurate estimations of thermal diffusion factors. A complete first approximation of thermal diffusion factors, however, can be a complex process. We present a simple expression for estimating thermal diffusion factors for heavy molecules present in dilute concentrations in a light carrier gas, conditions that are usually valid for metal‐organic chemical vapor deposition (MOCVD). This simple expression yields approximations within a few percent of those calculated from a complete first approximation. The error for the methyl and ethyl organometallics of aluminum, gallium, and indium in hydrogen or helium is less than 1% at concentrations below 0.1% and less than 4% for concentrations below 0.5%. For hydrides of phosphorus, arsenic, or silicon in hydrogen or helium, the error is less than 3% at concentrations below 1%. Thermal diffusion factors for dilute concentrations of organometallics and hydrides in helium are greater than in hydrogen. Thermal diffusion factors are dependent on temperature, much more so for hydrogen carrier gas than for helium. Simple models are used to demonstrate the influence of thermal diffusion on MOCVD growth rate under both mass‐transfer controlled and kinetically controlled growth and on the composition of species incorporated under equilibrium‐controlled growth conditions.