Indirect-to-direct transition of (AlAs)(n)/(AlP)(n) strained short-period superlattices

We carried out first-principles calculation of (AlAs)(n)/(AlP)(n) superlatt ices (SLs) (n = 1-5) using the full-potential linearized augmented plane-wa ve (FLAPW) method. The SLs are assumed to grow epitaxially on the (001) GaA s and GaP substrates. indirect-to-direct transition was observed for n grea ter than or equal to 2 on the (001) GaAs substrate while all SLs grown on t he (001) GaP substrate were indirect gap semiconductors. Optical properties were evaluated by the square of vertical transition matrix elements. The m atrix elements of (AlAs)(n)/(AlP)(n) SLs grown on (001) GaAs exhibited osci llating behavior. The matrix elements for odd n are two orders larger than those for even.