Photoluminescence of CdS/CdSe wurtzite superlattices and evidence for type II behaviour modified by piezoelectric fields

Abstract

We discuss photoluminescence experiments on strained-layer superlattices of CdS/CdSe produced by MOCVD on (1 1 1)A GaAs substrates. CdS and CdSe grow in wurtzite form, with the c-axis normal to the epitaxial plane. The system is the first reported semiconductor superlattice with this crystal structure and possesses novel behaviour because of the existence of extremely large piezoelectric fields (∼10 8V m −1) within the strained layers; these fields drastically modify the energy band structure. The photoluminescence from each specimen is dominated by an intense band in the red or near-infrared region. The dependence of the band position on the layer thicknesses (down to 15 Å) can be accounted for if the superlattice is of type II. The piezoelectric fields can be partly neutralised by increases in excitation power, thus shifting the photoluminescence to significantly higher energy (in some cases by up to 0.3 eV).