Temperature Dependence of the Optical Absorption Edge ofp-Type SnTe

The optical-absorption coefficient and index of refraction of SnTe were measured at 300, 77, and 12°K over the photon energy range between 0.14 and 0.64 eV. The specimen was a single-crystal SnTe film with a hole concentration of 3.6 × ${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The threshold of absorption by bound carriers at 12°K occurs at a photon energy less than 0.38 eV. This suggests that the smallest direct gap is considerably less than the gap of 0.30 eV which Esaki and Stiles have deduced from tunneling measurements. In contrast to the temperature dependence of the tunneling gap, the temperature dependence of the absorption edge suggests that the temperature coefficients of the direct gaps that lie in the photon energy range investigated are positive. However, these coefficients decrease rapidly with decreasing photon energy, suggesting that direct gaps between states sufficiently close to the band edge will have negative temperature coefficients. The electric-susceptibility effective mass was also determined. Its variation from $0.066m_0$ at 300°K to $0.057m_0$ at 12°K is consistent with the observation of direct gaps having positive temperature coefficients. We propose new features for the band structure of SnTe which account for these results, as well as previously reported results for the PbTe-SnTe alloy system.