Plasmon Raman scattering and photoluminescence of heavily doped n-type InP near the Gamma-X crossover

We have measured Raman scattering by coupled longitudinal-optic-phonon-plasmon modes and photoluminescence in heavily doped n-type InP under hydrostatic pressure and at low temperatures. The combination of both methods allows us to determine the pressure dependence of the carrier density and the enhancement of the effective electron mass of the Gamma conduction-band minimum due to nonparabolicity. Above 10.3 GPa a striking change in the frequency of the upper coupled mode is observed, which is attributed to the transfer of electrons from the Gamma minimum to X-related states. From the onset pressure for the Gamma to X electron transfer we determine the pressure coefficient of the indirect Gamma-X gap [-17(3) meV/GPa] and a pressure of 11.2 +/- 0.4 GPa for the Gamma-X conduction-band crossover in undoped InP.