Optical spin injection into Ge at room temperature

A realistic opto-spintronic device draws on the knowledge and control over the optical orientation at room temperature. We investigated circularly polarized photoluminescence of tensilely strained Ge-on-Si in an attempt to inject spins into Ge by optical means at ambient temperature. More than 10-\% polarization was observed up to 300 K. The absence of indirect-gap luminescence facilitated spectral analysis. Meanwhile, very fast decay was observed, unlike bulk Ge with little spin polarization. This indicates that intervalley scattering has diminished in Ge-on-Si due presumably to dissipative channels introduced during growth. It is concluded that spin coherence remains in a short time scale. This led us to optical injection of spins, O(1\%), in bulk Ge at room temperature, which was observed in the prompt decay.