Inversion of conduction levels inPbSe1−xTexcompounds

The photovoltaic effect on $p-n$ junctions, made with ternary compounds $\mathrm{Pb}{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$, has been used to measure the gap of these alloys, as a function of the temperature. The gap as a function of $x$ exhibits a curvature towards high energies. This particular behavior of the gap supports the model of the inversion of $L_6^{-}\left(L_3^{-}\right)$ level (conduction band of PbTe) and of the $L_6^{-}\left(L_2^{-}\right)$ level (conduction band of PbSe), according to the results of some band-structure calculations for PbSe and PbTe. A simple theoretical model is proposed to take into account the effect of this inversion. This model reproduces the observed variation of the gap as a function of composition.