Existence or absence of bandgap bowing in II-VI ternary alloys: Comparison between common-anion and common-cation cases

The common-anion and common-cation II-VI ternary alloys of the family Cd(Zn)Se(Te) are theoretically investigated based on two different methods. Within the virtual-crystal approximation (VCA), both the sp³s*- tight-binding method, with the inclusion of spin-orbit coupling, and the first-principle full-potential linear augmented plane waves (FP-LAPW) technique are employed to determine the alloy constituents' charge states (ionicities) and degree of bond polarity. The results show that: (i) in the common-cation ternary alloys (i.e., CdSe_xTe_1-x and ZnSe _xTe_1-x), the anions alter a strong competition in trapping more charge. Such a competition builds up a compromised effect yielding the bowing behaviour. Whereas, (ii) in the common-anion ternary alloys (i.e., Cd_yZn_1-yTe and Cd_yZn_1-ySe), the absence of such competition would cause the complete absence of bandgap bowing behaviour. The variation of the bandgap is found to be rather close to linear. The obtained good agreement between our theoretical results and the recently available photoluminescence data does further corroborate our claims.