Pressure dependence of direct and indirect optical absorption in GaAs

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 E₀ (Γ₁₅ᵛ{→}Γ₁ᶜ) increases sublinearly with pressure whereas its dependence on density {ρ} is linear (dE₀/d ln{ρ}=8.5). The energy Eᵢ of the indirect edge (Γ₁₅ᵛ{→}X₁ᶜ) decreases with pressure at a rate of -1.35×{}10^-2 eV/GPa in the range 4.2 to 17.5 GPa. The strength of the direct absorption grows linearly with increasing gap E₀ 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.