Free-Carrier Absorption inp-Type PbTe

Abstract

Infrared absorption measurements have been made on $p$-type PbTe at wavelengths ranging from 4 to 28 μ and at temperatures ranging from 300 to 77°K. Absorption by three different mechanisms was observed. At short wavelengths, the dominant absorption is caused by transitions across the forbidden energy gap. In the long-wavelength region, commencing at a temperature-dependent short-wavelength limit, the absorption resembles intraband free-carrier absorption and is of the form $\alpha =c{\lambda }^q$, where $\alpha$ is the absorption coefficient, $\lambda$ is the wavelength, and $q=2.0\mathrm{}\pm 0.2$ at all temperatures studied. At intermediate wavelengths, additional absorption is observed. Both the long-wavelength absorption and the additional absorption are proportional to carrier density and decrease with decreasing temperature at all wavelengths observed. In contrast, $n$-type material shows no additional absorption structure.