Temperature-dependent direct transition energy in Ge0.99Sn0.01 film grown on Si measured by photoreflectance spectroscopy

Optical properties of a p-Ge0.99Sn0.01 film grown on an n-Si substrate have been investigated as a function of temperature and excitation laser intensity using photoreflectance (PR) spectroscopy. The Ge0.99Sn0.01 film was grown by ultra-high vacuum chemical vapor deposition method. Room temperature PR spectrum shows a dominant signal assigned to a direct transition from the conduction Γ valley to valence band at around 0.73 eV. The transition to spin-orbit split-off band is also observed at around 1.0 eV. In addition, Franz-Keldysh oscillations (FKOs) due to the internal electric field are observed above the direct bandgap transition energy. The direct transition energy obtained by an analysis of FKO extremum was 0.728 eV at room temperature. The internal electric fields are reduced as the laser excitation intensity increases due to the photovoltage effect. The temperature dependence of direct transition energy was also investigated.