Pressure dependence of direct and indirect optical absorption in GaAs

Abstract

The dependence on hydrostatic pressure of the direct and indirect optical-absorption edges of GaAs has been measured for pressures up to the first structural transition near 17 GPa (T=300 K). The energy of the lowest direct gap $E_0$ (${\Gamma }_{15}^v$→${\Gamma }_1^c$) increases sublinearly with pressure whereas its dependence on density ρ is linear (${\mathrm{dE}}_0$/d lnρ=8.5). The energy $E_i$ of the indirect edge (${\Gamma }_{15}^v$→$X_1^c$) decreases with pressure at a rate of -1.35×${10}^{\mathrm{-}2}$ eV/GPa in the range 4.2 to 17.5 GPa. The strength of the direct absorption grows linearly with increasing gap $E_0$ due to excitonic coupling. A strong enhancement of the indirect absorption with pressure is attributed, in part, to an increase of the transition-matrix element for virtual direct transitions involved in the indirect absorption process.