Reflectivity and Wavelength-Modulation-Derivative Reflectivity ofPb1−xSnxTe

Reflectivity and wavelength-modulation-derivative-reflectivity spectroscopy have been performed on the alloy series ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ throughout the compositional range $0\le x\le 1$. PbTe and SnTe reflectivities were studied from 0.5 to 21 eV and the ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ alloy reflectivities from 0.5 to 13 eV. Wavelength-modulation-derivative data were obtained from 1.6 to 4.4 eV for all samples studied. Optical constants and characteristic energy-loss functions were obtained through a Kramers-Kronig computation. Line-shape analysis of derivative spectra in conjunction with recent band calculations indicate that saddle-point singularities give rise to the $E_2$ and $E_3$ reflectivity peaks. Derivative spectra show additional structure in these spectral regions consistent with theoretical predictions. Observed reflectivity of PbTe and the derived optical constants $n$ and $k$ are compared with the recent calculation of Buss and Parada. Reflectivity peak positions as a function of alloy composition are discussed. Characteristic energy-loss spectra of PbTe and SnTe as found from optical data are in very good agreement with electron energy-loss experiments.