STRUCTURAL AND OPTICAL-PROPERTIES OF CUINS2 BULK CRYSTALS

The optical properties of CuInS2 bulk crystals were studied by photoreflectance (PR) and photoluminescence (PL) spectroscopy in comparison with the material structure, verified by energy dispersive x-ray (EDX) microprobe analysis and x-ray diffraction. The samples have been produced by the gradient freeze technique under different sulfur pressures. It was shown by EDX that growth-induced cracking along the ingot is strongly reduced with higher sulfur pressures. The PR spectra at 80 K exhibit two well-resolved transitions. From a comparison with the reflectivity spectra at 80 K, these transitions are explained by the presence of two Wannier excitons. The fit of the spectra yields a broadening parameter of the lower-energy transition which increases approximately linearly with increasing Cu/In ratio. Thus, it is deduced that PR is sensitive to small inhomogeneities in the molecularity of the compound. In agreement with the EDX data photoluminescence indicates the material to be Cu rich. However, a continuous change of the significant PL peaks as a function of the Cu/In ratio could not be observed. According to this, PL is not well suitable to determine small inhomogeneities of the molecularity, in contrast to PR.