Effect of isotope substitution and doping on the Raman spectrum of galena (PbS)

The Raman spectrum of natural PbS has been shown to exhibit features related to the IR-active (Raman forbidden) LO phonons and to allowed scattering by two phonons. In order to confirm the vibrational nature of these features, and their peculiar temperature dependence, we compare here the spectra obtained for natural samples (i.e. mineral as well as synthetic with the natural isotopic abundance) with those measured for crystals prepared from highly enriched 34S. The observed isotopic shifts confirm the vibrational nature of the structures mentioned above. We have also measured the spectra of synthetic samples of PbS annealed at several different pressures of sulphur. Because of their non-stoichiometric sulphur content they have carrier concentrations (either electrons or holes) in the 1×1017–3×1018 carriers/cm3 range (as determined from their IR reflection spectra). The measured spectra, as well as their temperature dependence, do not depend on the bulk carrier concentration, a fact that must result from surface pinning of the Fermi level within the fundamental energy gap. ally discuss the apparent paradox of why the IR spectra reveal coupled LO-phonon–plasmon modes whereas the Raman spectra exhibit the bare LO-phonons.