Effect of pressure on the optical absorption in GaP and ${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$P (x=0.36 and 0.5)

The pressure dependence of the fundamental gaps of GaP and of the two alloys ${\mathrm{Ga}}_{\mathrm{x}}$ ${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$P (x=0.36 and 0.5) has been determined from optical-absorption measurements up to 13 GPa (T=300 K). The pressure coefficients of the lowest direct band gap (${\Gamma }_{8v}$-${\Gamma }_{6c}$) in the alloys are nearly identical to that of GaP. For the alloys we find a crossover from direct to indirect (${\Gamma }_{8v}$-$X_{6c}$) fundamental gap at 5.0 GPa (x=0.36) and 2.7 GPa (x=0.5). Phase-transition pressures for the alloys are 15.0 GPa (for X=0.36) and 17.0 GPa (for x=0.5). A quantitative study of the strength of the indirect absorption reveals a strong enhancement with pressure or increasing direct energy gap relative to the effect estimated for Γ→X transitions within the parabolic-band approximation of the Hartman model. The excitonic character of the virtual direct transition involved in the indirect absorption process does not account for the magnitude of this effect. We suggest that an additional enhancement arises from the nonparabolic nature of the conduction-band dispersion for k vectors outside the immediate vicinity of the zone center.