Optical characterization and band offsets in ZnSe-ZnSxSe1−x strained-layer superlattices

Abstract

Photoluminescence and excitation experiments were carried out to study effects of the strain and carrier confinement in ZnSe-${\mathrm{ZnS}}_{\mathrm{x}}$ ${\mathrm{Se}}_{1\mathrm{-}\mathrm{x}}$ strained-layer superlattices (SLS’s) grown by molecular-beam epitaxy on GaAs substrates. For the case where the total thickness of the SLS is very small (1000 Å), the structure grows pseudomorphically to the buffer layer. The ZnSe well layers are not strained and all the blue shift in the optical spectra is attributed to the carrier confinement effects only. At the other extreme, for the case of a SLS with very large total thickness (∼4 μm), we show that it can be treated as free-standing with ZnSe layers under biaxial compression and ${\mathrm{ZnS}}_{\mathrm{x}}$ ${\mathrm{Se}}_{1\mathrm{-}\mathrm{x}}$ layers under biaxial tension. In the cases of intermediate total thicknesses, we show that SLS’s are not fully relaxed to their equilibrium states by measuring the strains directly in the ZnSe well layers. Empirical values for the band offsets are obtained from the analysis of the optical response as a function of the sample parameters. Theoretical calculations of the band offsets, based on the model solid approach, were also performed and are found to agree with the experimental observations to within 0.05 eV. They indicate that all possible ZnSe-${\mathrm{ZnS}}_{\mathrm{x}}$ ${\mathrm{Se}}_{1\mathrm{-}\mathrm{x}}$ interfaces will exhibit a very small conduction-band offset.