Optical orientation and electron spin dynamics in Germanium

Group IV compounds have found entry into Si technology thanks to their compatibility with Si processing and their possibility to tailor electronic properties by strain and band-gap engineering. Besides the well-established technological infrastructures associated to the microelectronic industry, the long spin coherence time and the weak hyperfine interactions in Si and Ge are expected to yield devices exploiting the full control over the spin degree of freedom. 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. In this work the electron spin polarization is studied via polarization-resolved photoluminescence (PL) excitation spectroscopy, unfolding the interplay between doping and the ultrafast electron transfer from the center of the Brillouin zone towards its edge. The investigation of the circular polarization degree of the direct gap PL provides indeed insights into the electron spin dynamics in the multi-valley conduction band. Remarkably, we were able to observe electronic Raman scattering and to address intra-valence-band excitations related to the hole excitation from the heavy hole to both light and split-off subbands. Such wave-vector-conserving transitions yielded a fast and versatile spectroscopic mapping of valence band dispersion. The detailed investigation of the spin properties of Ge reported in this work is expected to open new opportunities connected to the merging of photonics and spintronics onto the mainstream CMOS-compatible platforms.