Inversion of Conduction and Valence Bands inPb1−xSnxSeAlloys

Single-crystal films of ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$ alloys with rocksalt structure ($0\le x\le 0.33$) have been deposited by evaporation on cleaved (100) faces of KCl substrates. The energy gap $E_g$ at $T=300, 195, \mathrm{and} 77°$K has been determined by measuring the optical transmission between 2 and 25μm. As $x$ increases or $T$ decreases, $E_g$ decreases for sufficiently low values of $x$, but increases for $x>\mathrm{}0.2\mathrm{}$ and $T=77\mathrm{}°$K. The sign reversal in the composition and temperature dependence of $E_g$ shows that the ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$ alloys exhibit the inversion in band structure recently proposed for the ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ alloys. If $E_g$ is assigned negative values for samples in which the conduction and valence bands are inverted from those in PbSe, the data are well represented by the equation $E_g=0.13+(4.5\mathrm{}\times {10}^{-4\mathrm{}}{\mathrm{deg}}^{-1\mathrm{}}) T-0.89x$ eV. The inversion point, at which $E_g=0\mathrm{}$ because the ${L_6}^{+}$ and ${L_6}^{-}$ band extrema are degenerate, is $x=0.19\mathrm{}$ at 77°K and $x=0.30\mathrm{}$ at 300°K.