Lattice strain relaxation of ZnS layers grown by vapour-phase epitaxy on (100)GaAs

The structural characterization of ZnS epilayers grown on (100)GaAs by H2 transport vapour-phase epitaxy is reported. High resolution X-ray diffraction and ion channelling Rutherford backscattering spectrometric measurements were used to evaluate the overall crystalline perfection of the epilayers and the distortion and reciprocal orientation of the ZnS and GaAs lattices and to measure the epilayer built-in lattice strain. High crystalline quality can be achieved at the surface of relatively thick (above ca. 1 µm) ZnS layers, although a dense distribution of extended defects was observed close to the ZnS/GaAs interface. It turns out that the ZnS unit cell is orthorombically distorted, although its deviation from a purely tetragonal distortion is very small. Systematic measurements of the ZnS lattice strain were performed on the epilayers and the results were compared with the thermal strain value obtained by temperature-dependent X-ray diffraction measurements. The data indicate that the ZnS built-in lattice strain can be entirely ascribed to a thickness dependent thermal strain contribution, the initial lattice misfit at the growth temperature being almost totally relaxed even for relatively thin epilayers. Finally, no appreciable strain was found for epilayer thicknesses above about 3 µm.