Submillimeter spectroscopy of TO-phonon mode softening in PbTe

Reflectivity spectra of lead telluride epitaxial layers on BaF2 and NaCl substrates have been investigated in the wave-number range of 5–100 cm−1. Due to multiple reflections and interference effects in the sandwich samples, weak structures in the reflectivity spectra are accentuated. This leads to a more precise determination of the lattice parameters, especially the TO-phonon mode frequency as compared to measurements using bulk samples. By measuring the back-surface (NaCl or BaF2) reflectivity, an additional advantage caused by a kind of index matching is achieved. The temperature dependence (T = 4.2–300 K) and the carrier concentration dependence of the PbTe TO-phonon mode frequency was determined.