Pressure-tuned resonance Raman scattering and photoluminescence studies on MBE grown bulk GaAs at the E ₀ gap

Hydrostatic pressure has been used to tune in resonance Raman scattering (RRS) in bulk GaAs. Using a diamond anvil cell, both the photoluminescence peak (PL) and the 2 LO and LO-phonon Raman scattered intensities have been monitored, to establish RRS conditions. When the E ₀ gap of GaAs matches h{combining short stroke overlay}ω _S or h{combining short stroke overlay}ω _L, the 2 LO and LO-phonon intensity, respectively, exhibit resonance Raman scattering maxima, at pressures determined by h{combining short stroke overlay}ω _L. With 647.1 nm radiation (h{combining short stroke overlay}ω _L = 1.916 eV), a sharp and narrow resonance peak at 3.75 GPa is observed for the 2 LO-phonon. At this pressure the 2 LO-phonon goes through its maximum intensity, and falls right on top of the PL peak, revealing that h{combining short stroke overlay}ω _S(2 LO) =E ₀. This is the condition for "outgoing" resonance. Experiments with other excitation energies (h{combining short stroke overlay}ω _L) show, that the 2 LO resonance peak-pressure moves to higher pressure with increasing h{combining short stroke overlay}ω _L, and the shift follows precisely the E ₀ gap. Thus, the 2 LO RRS is an excellent probe to follow the E ₀ gap, far beyond the Γ-X cross-over point. A brief discussion of the theoretical expression for resonance Raman cross section is given, and from this the possibility of a double resonance condition for the observed 2 LO resonance is suggested. The LO-phonon resonance occurs at a pressure when h{combining short stroke overlay}ω _L ≈E ₀, but the pressure-induced transparency of the GaAs masks the true resonance profile.