Spin-dependent direct gap emission in tensile-strained Ge films on Si substrates

Epitaxial growth of Ge films on Si introduces an in-plane biaxial tensile strain, opening up Ge applications in photonic, as recently demonstrated by the room temperature lasing action in Ge-on-Si heterostructure. The optical access to the direct gap transitions remarkably provides the possibility of injecting spin-polarized carriers via absorption of circular-polarized light. Nevertheless the effects of tensile-strain on the spin properties of Ge-on-Si have still to be addressed. In this work we applied polarization-resolved photoluminescence (PL) to spectrally resolve the radiative recombination involving strain-split light and heavy hole bands. We found that at low temperature, the fundamental transition has a record-high circular polarization degree of 85%, despite an off-resonance excitation of 300 meV. We show that this very high value is due to the characteristic energy dependence of the optically induced electron spin population and their concomitant dynamics. Finally, our observation of a direct gap PL doublet clarifies that the light hole contribution to the optical spectrum can dominate the room temperature PL. Our results provide a step forward the investigation of the dynamics of non-equilibrium spin populations and of the evaluation of optical gain in group IV materials. Above all, our findings confirm Ge as a prominent candidate for the development of next-generation CMOS-compatible devices featuring spintronics and photonics functionalities.