Spin-dependent direct gap emission in tensile-strained Ge-on-Si heterostructures

Ge films can be epitaxially grown on Si substrates, introducing biaxial tensile strain. This opens up applications of Ge in photonic [1], as strain contributes to enhance direct-gap emission, eventually yielding lasing action at room temperature. Noticeably, the optical access to the direct gap transitions in Ge provides the possibility of spin-injection by optical orientation. As a result, spectroscopic access to the very rich spin-physic of this semiconductor can be achieved. In this work we thus study the effect of tensile strain on the spin-properties of Ge by measuring, upon optical orientation of the carrier spin, the polarization of the radiative recombination across the direct-gap. Polarization-resolved photoluminescence (PL) technique provides an ideal tool to observe emission due to optical transitions involving spin-polarized conduction band electrons with strain-splitted valence band light (cΓ-LH) and heavy holes (cΓ-HH). Figure 1 shows that the two cΓ-LH and cΓ-HH spectral features are counter-circularly polarized and separated by about 19 meV. Surprisingly, for the fundamental cΓ-LH transition and for an offresonance excitation of more than 300 meV, we measure a low temperature polarization degree as high as 85%. This is the highest value reported so far for Ge-based structures. We will demonstrate that these findings can be explained in terms of the ultrafast-dynamics of hot conduction band electrons and of the straininduced change in the density of states of the valence band due to splitting and anticrossing between the LH and HH subbands. Finally, our results provide a step forward in the investigation of the dynamics of non-equilibrium spin populations in group IV materials, and confirm Ge as a promising candidate for the development of nextgeneration CMOS-compatible devices featuring spintronics and photonics functionalities.