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 ${E}_{0}$ (${\ensuremath{\Gamma}}_{15}^{v}$\ensuremath{\rightarrow}${\ensuremath{\Gamma}}_{1}^{c}$) increases sublinearly with pressure whereas its dependence on density \ensuremath{\rho} is linear (${\mathrm{dE}}_{0}$/d ln\ensuremath{\rho}=8.5). The energy ${E}_{i}$ of the indirect edge (${\ensuremath{\Gamma}}_{15}^{v}$\ensuremath{\rightarrow}${X}_{1}^{c}$) decreases with pressure at a rate of -1.35\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ 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.