Critical thickness and strain relaxation in lattice mismatched II–VI semiconductor layers

Abstract

Critical thickness hc has been calculated for capped and uncapped lattice mismatched II–VI semiconductor epilayers. Both the old equilibrium theory and the improved theory have been used. The calculated values are compared with the experimental data on epilayers of several II–VI semiconductors and alloys. The observed values of hc are larger than the calculated values, a result similar to that observed with GeSi and InGaAs strained layers. The discrepancy is attributed to the difficulty in nucleating the dislocations. Strain relaxation in layers with thickness h>hc is also calculated. Observed strain relaxation in ZnSe layers grown on (100) GaAs shows good agreement with the equilibrium theory. In other cases, the observed relaxation is sluggish and the residual strain is larger than the calculated value. Many authors have observed that strain near the surface of the II–VI epilayers is small and increases as the depth increases. We describe an improved model to explain this observation. The agreement between the prediction of our model and the observed strain distribution is excellent. A new model based on continuum elasticity theory is described to explain strain oscillations during the initial stages of growth of highly mismatched layers. In highly mismatched layers, the dislocations are distributed uniformly. A model to interpret this observation is suggested.