Spin-resolved study of direct band-gap recombination in bulk Ge

Germanium and silicon-germanium alloys have found entry into Si technology thanks to their compatibility with Si processing and their ability to tailor electronic properties by strain and band-gap engineering. Noticeably, the quasi-direct band structure of Ge allows for optical spin orientation bridging the fields of spintronics and Si photonics where Ge is increasingly applied. Despite recent progress in the investigation of spin dynamics in Ge, very little is known about spin flip scattering by dopants and the role played by impurities in determining spin properties in different temperature regimes. Here we present a combined experimental and theoretical investigation of the spin and energy relaxation of electrons in Ge. By taking advantage of polarization-resolved photoluminescence measurements and Monte Carlo simulations of carrier dynamics we unveil a unique and very rich spin physics and highlight the importance of the multi-valley conduction band of this material. We demonstrate full control over the angular momentum of the direct gap emission with a complete reversal between right- and left-handed circular polarization. In the low temperature regime, the observed state of light polarization is ascribed to the so-far overlooked carrier cooling process that stems from the Coulomb scattering originated within the X-valleys. In the high temperature range, we disclose the role of backscattering from the L-valley in defining the depolarization of direct gap luminescence. This detailed investigation of spin properties of Ge is expected to open new opportunities connected to photonics and spintronics on a CMOS-compatible platform