Optical absorption in epitaxial PbTe films on BaF2

Results are presented for below-band-gap (λ∼5−12 μm) optical-absorption measurements at 300 and 77 K on epitaxial p-type PbTe films grown on BaF2. These results indicate a significant reduction in the absorption previously reported on PbTe epitaxial films on NaCl substrates. A few samples show an additional absorption suggestive of impurity effects. He4 ion-backscatter spectra of these films show a surface contamination of chlorine. The results of room-temperature optical-absorption measurements on less contaminated samples agree reasonably well with bulk sample behavior. In low-carrier-concentration films (p < 1017 cm−3) impurity effects appear to eliminate any temperature dependence of the absorption. The presence of the surface contamination does not affect the dc transport or refractive-index properties.