Observation of temperature-dependent heavy- and light-hole split direct bandgap and tensile strain from Ge_0.985Sn_0.015 using photoreflectance spectroscopy

Temperature- (T-) dependent photoreflectance (PR) measurements have been made for the tensile-strained, undoped Ge_0.985Sn_0.015 film grown on n-Si substrate by ultra-high vacuum chemical vapor deposition method. The PR spectra at room temperature consist of two signals at around 0.739 and 1.022 eV, which are assigned to the direct transitions from conduction Γ valley to valence and spin-orbit split-off bands, respectively. The T-dependent PR measurements show tensile-strain split direct bandgap transitions from the Γ valley to the light-hole (E_LH) and heavy-hole (E_HH) bands at energies of 0.772 and 0.803 eV at 12 K, respectively, which are not usually observable from the photoluminescence measurements for relatively high Sn content Ge_1-ySn_y samples. The PR signals for both HH and LH bands are blue shifted and their intensities decrease with increasing temperature, but both LH and HH PR signals persist through 240 K and only one HH PR signal is observed at room temperature. It has been observed that the separation energy between the E_HH and E_LH increases as T decreases, which clearly indicates an increase in tensile strain as T decreases. From the analysis of the T-dependent separation energy between the E_LH and E_HH, the T-dependent tensile strain in the Ge_0.985Sn_0.015 film was obtained, which might not be easily measured using the X-ray diffraction method.