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Indirect-to-Direct Transition of (AlAs)n/(AlP)n Strained Short-Period Superlattices

We carried out first-principles calculation of (AlAs)n/(AlP)n superlattices (SLs) (n=1–5) using the full-potential linearized augmented plane-wave (FLAPW) method. The SLs are assumed to grow epitaxially on the (001) GaAs and GaP substrates. Indirect-to-direct transition was observed for n≧2 on the (001) GaAs substrate while all SLs grown on the (001) GaP substrate were indirect gap semiconductors. Optical properties were evaluated by the square of vertical transition matrix elements. The matrix elements of (AlAs)n/(AlP)n SLs grown on (001) GaAs exhibited oscillating behavior. The matrix elements for odd n are two orders larger than those for even.