Structure of II-VI lattice mismatched epilayers used for blue-green lasers for underwater communication

Critical thickness (h(c)) is 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 h(c) are larger than the calculated values. However the discrepancy is much smaller than that found in InGaAs/GaAs and GeSi/Si layers. Moreover as compared to InGaAs/GaAs and GeSi/Si layers, the experimental data show a much smaller scatter and can be fitted with one curve. Strain relaxation in layers with thickness h > h(c) is also calculated. Strain relaxation in ZnSe layers grown on (100) GaAs shows good agreement with the equilibrium theory. In other cases the observed relaxation is sluggish, the residual strain is larger than its calculated value. Thick highly mismatched layers behave-differently. The residual strain agrees with theory and dislocations are distributed periodically. A model to interpret these observations is suggested. Implications of this study on the stability of II-VI strained layers are discussed.