Crystalline structure of ZnSe and ZnSSe epilayers grown on (100)GaAs by metalorganic vapour-phase epitaxy

The crystalline properties of ZnSe/(100)GaAs and ZnSSe/(100)GaAs structures are presented. ZnSe and ZnSSe were grown by MOVPE using dimethyldiselenide and diethyldisulphide along with dimethylzinc:triethylammine. Double-crystal X-ray diffraction (DC-XRD) patterns in the vicinity of the (4 0 0) reciprocal lattice points of ZnSe and ZnSSe were studied. No macroscopic misorientations of the ZnSe crystal with respect to GaAs were detected, suggesting an isotropic defect distribution along the [0 1 1] in-plane directions. The generation/propagation of defects due to the epilayer strain relaxation causes a mosaic-like structure which yields a characteristic broadening of the DC-XRD curves. The line shape of ZnSe and ZnSSe DC-XRD patterns was analysed by a statistical XRD theory under the kinematical approximation to determine the epilayer structural parameters. Simulations of ZnSe/GaAs DC-XRD curves gave a mean mosaic block radius of 140-150 nm and a random mean (microscopic) misorientation angle between the blocks which decreases from 1.3 to 0.4 mrad by increasing the ZnSe thickness from 0.3 to 1.85 um. ZnSSe layers having comparably small (tensile) lattice mismatches showed a lower crystal quality, in agreement with the higher defect density expected for ZnSSe.