Measurements by X-ray topography of the critical thickness of ZnSe grown on GaAs

Abstract

The initial stages of relaxation in ZnSe layers grown by molecular beam epitaxy on vertical-gradient-freeze Bridgeman GaAs substrates have been studied using synchrotron-generated X-ray topography. We have detected the formation of the very first strain-relieving misfit dislocations in the ZnSe layers in a layer of thickness 100 nm while no dislocations are found in a layer of 95 nm thickness. The critical thickness for this material system is therefore estimated to be around 97 nm, which is markedly lower than the widely accepted value of 150 nm. These data are used to infer the maximum critical thickness of ZnCdSe quantum wells possible in II–VI laser diodes.