Tight-binding study of ZnSe/ZnTe strained superlattices: Determination of the band offset from the optical properties

We have theoretically studied large-, short-, and very-short-period ZnSe/ZnTe strained superlattices. We use an ${\mathit{sp}}^3$ ${\mathit{s}}^{\mathrm{*}}$ tight-binding model with spin-orbit coupling in order to calculate energies of superlattice electronic states and optical transition probabilities. This enables us to estimate the valence-band offset considering many experimental studies on a large range of samples with various thicknesses and strain states. For large-period superlattices, our work evidences the important role of unconfined excited states in optical transitions and explains that low-energy structures observed in the absorption spectra are due to transitions between spatially separated conduction and valence superlattice confined states. The main absorption contribution at higher energy is explained by strong transitions occurring between valence and unconfined conduction states. These features result from the type-II nature of this superlattice. Our calculation leads to a 1.02±0.02 eV unstrained band offset, which allows a very good comparison with the studied experimental data.