Origins of bandgap bowing in compound-semiconductor common-cation ternary alloys.

We present an investigation into the existence and origins of bandgap bowing in compound-semiconductor common-cation ternary alloys. As examples, we consider CdSe(x)Te(1-x) and ZnSe(1-x)Te(x) alloys. A calculation, based on the sp(3)s(*) tight-binding method including spin-orbit coupling within the framework of the virtual crystal approximation, is employed to determine the bandgap energy, local density of states and atomic charge states versus composition and valence-band offset. The results show that (i) in the valence band, the top states are mainly contributed by Te atoms. The degree of ionicity of all atoms is found to vary linearly with mole fraction x. (ii) There is a strong competition between the anions (Se and Te) in trapping/losing charges and this competition is the main reason for the bandgap bowing character. (iii) There is a reasonable agreement between the calculated results and the available photoluminescence data. (iv) The bowing parameter is found to increase with increasing valence-band offset and increasing lattice mismatch.