Residual and thermal strain of ZnS epitaxial layers grown on (100)-GaAs by vapour phase epitaxy

In this work, ZnS epitaxial layers grown by vapour phase epitaxy on [100]-oriented GaAs substrates are investigated by x-ray diffraction. The residual strain status of the as-grown samples was determined by high-resolution double-crystal x-ray diffraction measurements. Eleven diffraction curves were recorded in the vicinity of the (400), (422) and (531) Bragg reflections in different diffraction geometries and for several azimuth angles. The analysis of the experimental data was performed by using a general model, which relates the angular distances between diffraction peaks and strain tensor components in the second-order approximation. This model considers the lowest crystallographic symmetry (triclinic) for the lattice distortion of a cubic unit cell. Our results indicate that the crystallographic symmetry of the distorted ZnS unit cell is orthorhombic. In order to determine the strain contribution due to the different thermal expansion coefficients of ZnS and GaAs (thermal strain) the temperature variation of the residual strain was measured between 25°C and the growth temperature (650°C) by using a single crystal x-ray diffractometer. From our temperature-dependent measurements we determined the thermal misfit between ZnS and GaAs and the linear thermal expansion coefficient of ZnS.