Some optical properties of the AlxGa1−xAs alloys system

Abstract

Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0<x<0.9. The results indicate that the fundamental absorption edge is domnnated by electron-hole interaction even in an an alloy. In the direct-gap region the bound-exciton ground-state peak is clearly resolved. In the indirect-gap region the relaxation of crystal momentum conservation in optical transitions for an alloy apparently has significant effects on the Γ15V–Γ1C absorption edge, and the discrete peak in absorption spectra disappears. From the variation of the absorption coefficient αth at the Γ15V–Γ1C edge with x, we can deduce the corresponding variation of the excitonic binding energy Eex, excitonic effective mass m*, and electron mass mc for the Γ1C minimum. Extrapolated values for these quantities in AlAs are Eex=5 meV, m*=0.060 m0, and mc=0.11 m0 for a value αth = 2.3×104 cm−1. Further, there is a shift of the Γ15V-Γ1C absorption edge towards lower energies only in the indirect-gap region. Thus the usually employed quadratic expression for the bowing of band gaps in alloys is inadequate for the Γ15V-Γ1C gap in AlxGa1−xAs. We observe no bowing of band gaps in alloys is inadequate for the Γ15V-Γ1C gap in AlxGa1−xAs. We observe no bowing at all (<0.01 eV) in the direct-gap region, in sharp disagreement with earlier experiments employing other methods. Our PL data for the indirect-gap region indicate no significant bowing in the Γ15VC gap either (<0.01 eV). The PL results for the direct-gap region establish significant deviations in near-ban-gap luminescence peak positions from the band gap established in absorption, in agreement with detailed balance predictions.