Spin-polarized photoemission is used to study Si${}_{1-x}$Ge${}_x$ alloys epitaxially grown on a Si substrate. Spin-oriented electrons are generated in the conduction band upon excitation with circularly polarized light. We prove that in these structures it is possible to lower the vacuum level of the system below the conduction band minimum at the $\Gamma$ point of the Brillouin zone and show that electron spin polarization values $P=40\pm 3\%$ can be achieved when promoting electrons with direct transitions at $\Gamma$. Such values can be greatly increased, up to $P=72\pm 3\%$, in compressively strained thin epitaxial films, thus obtaining performances very close to those of III-V semiconductor heterostructures.

• Received 10 June 2013

DOI:https://doi.org/10.1103/PhysRevB.88.115209