Removal of spin degeneracy in p-SiGe quantum wells demonstrated by spin photocurrents

Spin photocurrents requiring a system lacking inversion symmetry, become possible in SiGe based quantum well (QW) structures due to their built-in asymmetry. We report on circular and linear photogalvanic effects induced by infrared radiation in (001)- and (113)-oriented p-Si/Si_1-xGe_x QW structures and analyze the observations in view of the possible symmetry of these structures. The circular photogalvanic effect arises due to optical spin orientation of free carriers in QW's with spin-split subbands. It results in a directed motion of free carriers in the plane of the QW. We discuss possible microscopic mechanisms that could remove the spin degeneracy of the electronic subband states.