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

Abstract

The circular polarization of direct gap emission of Ge is studied in optically excited tensile-strained Ge-on-Si heterostructures as a function of doping and temperature. Owing to the spin-dependent optical selection rules, the radiative recombinations involving strain-split light $(c\mathrm{\ensuremath{\Gamma}}\text{-LH})$ and heavy hole $(c\mathrm{\ensuremath{\Gamma}}\text{-HH})$ bands are unambiguously resolved. The fundamental $c\mathrm{\ensuremath{\Gamma}}\text{-LH}$ transition is found to have a low temperature circular polarization degree of about 85%, despite an off-resonance excitation of more than 300 meV. By photoluminescence (PL) measurements and tight-binding calculations we show that this exceptionally high value is due to the characteristic energy dependence of the optically induced electron spin population. Finally, our observation of a direct gap doublet clarifies that the light hole contribution, previously considered to be negligible, can dominate the room temperature PL even at low tensile strain values of $\ensuremath{\approx}0.2%$.