Variation of the energy gap with composition in ${\mathit{A}}^{\mathrm{I}}$${\mathit{B}}^{\mathrm{III}}$${\mathit{C}}_2^{\mathrm{VI}}$ chalcopyrite-structure alloys

The nonlinear variation of the fundamental energy gap ${\mathit{E}}_{\mathit{g}}$ as a function of composition x observed in ${\mathit{A}}^{\mathrm{I}}$ ${\mathit{B}}^{\mathrm{III}}$ ${\mathit{C}}_2^{\mathrm{VI}}$ chalcopyrite-structure alloys is discussed. The downward optical-bowing parameter scrc observed in these alloys is analyzed in terms of current models given in the literature. A relationship between scrc and the difference in electronegativities ΔX of the atoms mixed to form the alloy is obtained. Also, an expression to predict the ${\mathit{E}}_{\mathit{g}\mathrm{-}}$vs-x curve for these alloys is proposed and good agreement with experimental data is found.