X-ray diffraction study of ciialcopyrite ZnSnP2 epitaxial layers

We apply the technique of x-ray diffraction to the determination of the crystallographic structure and the quantitative measurement of the order parameter of ZnSnPa epitaxial layers. In bulk, ZnSnPa it is possible to obtain highly ordered distribution of Zn and Sn atoms in the cation sublattice, but epitaxial growth often produces partially ordered layers. The ordered and disordered phases correspond to the chalcopyrite and sphalerite structures and their respective band gaps are 1.66 and 1.24 eV. Since ZnSnP2 is almost lattice-matched to GaAs, it is interesting candidate for optoelectronic applications. Samples used in this work were grown by gas source molecular beam epitaxy on GaAs substrates. Slight variations in growth conditions could be induced to produce partially ordered and disordered structures. Chalcopyrite ordering is determined by the observation of several characteristic reflections identifying the lower symmetry of this structure. For example, reflections from (101), (217) and (611) planes, strictly forbidden for sphalerite, were measured. The quantitative determination of the order parameter could be made by comparing intensities of a carefully chosen set of measured and calculated reflections. We show that while kinematic approximation can be used to model weak superstructure reflections, in the calculation of the strong, low-angle, fundamental reflections used for intensity normalization it is necessary to take into account extinction effects. Order parameters varying from 0 to 30% were obtained.