Bowing character in wurtzite Zno-based ternary alloys

The existence and origins of the bandgap bowing character in the wurtzite ZnO-based ternary alloys are theoretically investigated. The tight-binding method with minimal sp ³ basis set is used to calculate the band structures, density of electronic states, and constituent charge states of both commoncation ZnO _1-xS _x and common-anion Zn _1-xMg _xO ternary alloys. The experimental data show that a well-pronounced bowing effect is induced in the common-cation case, whereas there is no apparent bowing effect in the common- anion case. The present investigation indicates that the bowing effect in the former case may result from competition between anions trapping charge made available by cations. The disappearance of this competition, in the case of common-anion alloys, eliminates the possibility of bowing, thus leading to the observed linear behavior. Furthermore, the electronegativity and its discrepancy between the two anion types are expected to play a dominant role in the magnitude of the bowing parameter. The excellent fit between our theoretical results and the available photoluminescence data support this model.