Photoreflectance measurements on ZnSe/ZnS0.25Se0.75 SQW

A series of ZnSe/ZnS0.25Se0.75 single quantum wells (SQW) of various well thicknesses are grown on (001) GaAs with low pressure metalorganic vapour phase epitaxy (MOVPE). Photoreflectance measurements (PR) show pronounced structures which are assigned to excitonic transitions between quantum well states. The structural properties of the samples are determined with high resolution X-ray diffraction and transmission electron microscopy. The model of Mathieu et al. is used to calculate excitonic states in a QW and a fitting procedure is developed which allows an estimate of the band offset. The structure exhibits a type TT alignment which is also supported by photoluminescence (PL) measurements and which is mainly the result of a strain induced shift of the ZnS0.25Se0.75 conduction band.