Addressing spin-optoelectronic properties of Ge by polarization and time-resolved PL investigations

The polarization of the photoluminescence (PL) from the indirect gap of bulk Ge is studied after optical orientation by absorption of circularly polarized light through the direct gap. The spin relaxation time ts is then estimated by combining the polarization degree and the decay time of the indirect gap emission. Ge is a promising material for spin-optoelectronics due to its feasible integration on Si, its noticeable spin properties and the possibility of achieving room temperature lasing action. In order to merge the spin degree of freedom and the photonics functionalities, the knowledge of properties such as the electron spin lifetime is of practical and fundamental importance. In this work we address this issue reporting the first evidence that low temperature PL from the indirect gap of bulk Ge is circularly polarized. This is the smoking gun proof that spin-polarized electrons preserve their orientation during the long dwell time in the L valley. We experimentally verified the selection rules for phonon mediated transitions showing an excellent agreement with the theory. Finally, to estimate ts we applied time-resolved PL, yielding in addition accurate data for the carrier lifetime. Hundreds of ns long ts is found providing the longest estimation reported at T<30K. Our results demonstrate the prominent and viable role of Ge for merging photonics and spintronics on the same materials platform.