Resonant Raman scattering in germanium

We have studied the spectral dependence of the first order Raman scattering cross section of Ge at room and liquid nitrogen temperatures in the energy region containing the E₁ and E₁ + Δ₁ optical gaps. This region was covered by a fine mesh of points obtained from the discrete lines of three gas lasers and a cw continuously tunable dye laser. Only one resonant peak was observed, as opposed to the two peaks that characterise the absorption and reflection spectra in this region. The shape of this resonance peak can be explained as due to the changes in the electronic polarizability produced by phonon-induced wave function mixing of the spin-orbit split Λ valence band doublet. The observed temperature shift in the resonant energy is much smaller than the one predicted from the known shifts of the optical gaps with temperature. Furthermore the resonant peak at room temperature appears shifted to higher energies when compared with the theoretical peak calculated from the room temperature optical constants. The resonant Raman peak appears to shift with increasing temperature by the full thermal expansion effect plus only a fraction of the electron-phonon interaction shift seen in the optical constants.