Optical absorption and the band structure of PbTe

The optical characteristics of PbTe in the region of fundamental absorption, viz. the absorption spectrum α(h/2πω) and dispersion of the refractive index n(h/2πω) are determined by parallel measurements at T=313 K of the optical transmittance and reflection of an epitaxial film on a BaE2 substrate. At 77 K the hole concentration is approximately 1016 cm-3. The calculation of α(h/2πω) and n(h/2πω) is performed by interference analysis with a computer. The width of the direct forbidden band at point L is Esub(gL)=0.321 eV. The longitudinal effective mass of the electrons and holes, msub(l)=0.195msub(0), is close to the value derived from the experiments at low temperatures. This indicates the validity of the Cohen model for the energy spectrum. Beside the main band exstrema at point L, there are two auxiliary valence band extrema directed toward point Σ (Esub(gΣ) approximately 0.335 eV) and toward point Λ (Esub(gΛ) approximately 0.41 eV). The maximum of the valence band at point L changes its energy position with respect to the remaining extrema on variation of the temperature. The existence of an additional extremum of the conductivity band near point W is assumed which is separated from the minimum at point L by an energy approximately 0.195 eV. Steps and resonance peaks are observed against the background of the absorption spectrum. They can be ascribed to phonon - assisted transitions between levels near the band extrema