Enhanced orbital mixing in the valence band of strained germanium

We give a theoretical explanation of the very high electron-spin polarization measured in compressively strained Ge layers through spin-polarized photoemission experiments, based on the enhancement of the band orbital mixing between light-hole (LH) and split-off (SO) bands along the growth direction of Ge thin films. We deduce an increased valence-band mixing, with respect to the bulk case, from the measured spin-polarization values, which exceed the bulk limit when optical transitions away from the Gamma point of the tetragonally distorted Brillouin zone are taken into account. Furthermore, we show that the degree of LH-SO band orbital mixing is directly proportional to the degree of compressive strain in the Ge layer.