Far-infrared reflectivity measurements have been carried out from {~}40 to 320 cm^-1 on a Pb0.88{Sn}0.12Te epitaxial film of low carrier concentration (p~10¹⁶ cm^-3) on a high-carrier-density (p~10¹⁸ cm^-3) Pb0.78{Sn}0.22$Te substrate. The reflectivity data were analyzed using a two-oscillator dielectric function including free-carrier and phonon linewidths. An upper bound for the LO-phonon frequency was determined to be 115.7 cm^-1 at room temperature and 113.5 cm^-1 at liquid-N₂ temperature. Other parameters obtained were the phonon linewidth γ=19 cm^-1 at room temperature and 12 cm^-1 at liquid-N₂ temperature, and the substrate free-carrier linewidth ν=81 cm^-1 at room temperature and {~}70 cm^-1 at liquid-N₂ temperature. An analysis of the oscillatory behavior of the reflectivity at frequencies above the highest plasmon-phonon mode permitted the determination of the spatial variation of carrier concentration in the region of the film-substrate interface.
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Tennant et al. (1976) studied this question.
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