MBE growth of HgCdTe epilayers with reduced visible defect densities: Kinetics considerations and substrate limitations

A semi-empirical constraint to the thermodynamical model for growth of Hg_1−xCd_xTe (MCT) by molecular beam epitaxy is described. This constraint, derived by forcing the population of Hg atoms in a surface layer to be proportional to the HgTe fractional growth rate, can determine an optimal total growth rate for specific beam fluxes and substrate temperature. Utilizing improved growth conditions determined by this model has resulted in MCT layers with consistently lower visible defect density (e.g., voids). The majority of recent layers grown using the constrained conditions has achieved defect densities limited by the CdZnTe substrate. On the highest quality substrates, total defect densities have consistently been reduced to the 100–200 cm⁻² range using the improved conditions for compositions x=0.2 to x=0.6. On more typical substrates, the total defect density is 1000–1500 cm⁻². This compares with densities of 3000–5000⁺ cm⁻² for old layers grown under non-optimized conditions. The density of voids has remained about the same upon using the improved conditions, and is determined primarily by the Te precipitate content of the substrate, but microdefect (hillock) density has been reduced by almost a factor of ten.