Some optical properties of the Al_xGa_1-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 m c 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 AlxGa 1-xAs. We observe no bowing of band gaps in alloys is inadequate for the Γ15V-Γ1C gap in AlxGa 1-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.