Electronic Raman scattering due to spin-orbit split-off band in tensilely-strained Ge

Electronic Raman scattering is an inelastic two-photon process involving electronic transitions rather than elementary excitations like phonons. Ge exhibits ERS spectra dominated by the light-hole valence subband when excited by the 1064-nm light. Here we attempt to observe the strain-induced development of the ERS due to the spin-orbit split-off (SO) valence subband, which is invisible otherwise. To this end, a sub-ns pulse laser was used, which helped discriminate the relevant transition as a pronounced spectral peak with negative circular polarization, as opposed to the cw or fs excitations. A similar negative-helicity peak was confirmed for the ERS due to the light-hole subband as expected. We therefore infer that the otherwise inexplicable negative helicity near the direct band-edge in unstrained Ge is most likely due to the ERS involving SO. Based on these, an argument is invoked which takes account of the electron-electron scattering as the microscopic mechanism of an enhanced ERS in the strained multivalleyed Ge where the intervalley scattering of electrons in the conduction band plays a pivotal role. The possibility of a near-edge Raman gain will be discussed in light of the absorption characteristics.