Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence

Spin orientation of photoexcited carriers and their energy relaxation are investigated in bulk Ge by studying spin-polarized recombination across the direct band gap. The control over parameters such as doping and lattice temperature is shown to yield a high polarization degree, namely larger than 40%, as well as a fine tuning of the angular momentum of the emitted light with a complete reversal between right- and left-handed circular polarization. By combining the measurement of the optical polarization state of band-edge luminescence and Monte Carlo simulations of carrier dynamics, we show that these very rich and complex phenomena are the result of the electron thermalization and cooling in the multivalley conduction band of Ge. The circular polarization of the direct-gap radiative recombination is indeed affected by energy relaxation of hot electrons via the X valleys and the Coulomb interaction with extrinsic carriers. Finally, thermal activation of unpolarized L valley electrons accounts for the luminescence depolarization in the high temperature regime.