Low-temperature optical absorption inAlxGa1−xAs grown by molecular-beam epitaxy

A detailed study of optical absorption in the ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As alloy system is undertaken using optical transmission and photoluminescence data obtained at 3 K from molecular-beam epitaxial layers. Absorption coefficient spectra are calculated for the entire alloy composition range 0≤x≤1. The first experimental data concerning AlAs in the region of the ${\Gamma }_{15V}$-${\Gamma }_{1C}$ band gap are presented. The compositional dependence of threshold values of the absorption coefficient agrees with current theoretical predictions. Detailed excitonic structure is observed for the first time in the absorption spectra of ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As epilayers with x≤0.43. From this structure, free-exciton binding energies are determined. These energies display an x dependence qualitatively similar to that observed in donor activation energies in n-type ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As.