RESIDUAL AND THERMAL STRAIN OF ZNS EPITAXIAL LAYERS GROWN ON [100]-GAAS BY VAPOR-PHASE EPITAXY

In this work, ZnS epitaxial layers grown by vapour phase epitaxy on [1 00]-oriented GaAs substrates are investigated by x-ray diffraction. Th e residual strain status of the as-grown samples was determined by hig h-resolution double-crystal x-ray diffraction measurements. Eleven dif fraction curves were recorded in the vicinity of the (400), (422) and (531) Bragg reflections in different diffraction geometries and for se veral azimuth angles. The analysis of the experimental data was perfor med by using a general model, which relates the angular distances betw een diffraction peaks and strain tenser components in the second-order approximation. This model considers the lowest crystallographic symme try (triclinic) for the lattice distortion of a cubic unit cell. Our r esults indicate that the crystallographic symmetry of the distorted Zn S unit cell is orthorhombic. In order to determine the strain contribu tion due to the different thermal expansion coefficients of ZnS and Ga As (thermal strain) the temperature variation of the residual strain w as measured between 25 degrees C and the growth temperature (650 degre es C) by using a single-crystal x-ray diffractometer. From our tempera ture-dependent measurements we determined the thermal misfit between Z nS and GaAs and the linear thermal expansion coefficient of ZnS.