Near-band-gap reflectance anisotropy in ordered Ga0.5In0.5P

We present a theory that models the reflectance difference spectrum of bulk, spontaneously ordered Ga0.5In0.5P. Near the band gap E-0 this spectrum exhibits a sharp, negative feature at E-0 and a broad positive feature that peaks near E-0+Delta(S). The zero crossing between these two peaks occurs near E-0+Delta(C). For the sample studied in this paper, the spin-orbit splitting Delta(s) and the crystal-field splitting Delta(C) are 120 and 25 meV, respectively. Two previous calculations, which assume constant transition-matrix elements, were able to produce a negative peak at E-0, but not the positive feature. In this paper, the reflectance difference spectrum near the band gap is calculated using an 8-band k.p model and an explicit treatment of the momentum or k dependence of the transition-matrix elements. The new calculation produces both the negative peak at E-0 and the positive feature that peaks near E0+Delta(S). The positive feature is attributed to the strong k dependence of the matrix element anisotropy. A strong coupling, enhanced by ordering, between three valence bands is essential. A problem associated with the analytical expression for the dielectric function epsilon used in previous calculations is discussed.