Thermal and Optical Energy Gaps in PbTe

The optical absorption edge (indirect energy gap) for PbTe was determined in the temperature range of 80°–520°K. It was found that dEg/dT had a value of 4.1×10−4 eV/°K in the temperature range from 80°–350°K, while above 400°K, dEg/dT was approximately zero. The low-temperature linear portion extrapolated to 0.19 eV at 0°K while the high-temperature portion extrapolated to 0.36 eV. This behavior is explained by a two-valence band model, one active at low temperatures, the other in the high temperature range. A thermal energy gap of approximately 0.36eV was obtained from the temperature variation of resistivity and Hall coefficient in the intrinsic range, 500°–900°K. This value agrees with the high temperature optical data extrapolated to 0°K, thus correlating the optical and thermal energy gaps.