Effect of Bond Ionicity on the Bandgap Bowing in Compound Semiconductor Alloys

The existence and the origins of the bowing character in the bandgap of compound semiconductor alloys are investigated using the Sp(3)S* tight-binding method which includes the spin-orbit coupling effects. As examples, we consider several ternary alloys among the composition of the ZnyCd1-ySexTe1-x family. The results show that the common-cation ternary alloys (either y = 0 or 1) exhibit the bowing behavior as a consequence of the ionicity competition between the anions induced by the valence-band offset (VBO). On the other hand, the common-anion ternary alloys (either x = 0 or 1) do not possess the bowing behavior and their bandgaps vary almost linearly with composition (y). Such a linear behavior is claimed to be due to the lack of ionicity competition between the cations which is, in turn, consistent with the vanishing VBO. The favorable agreement between our theoretical results and the available experimental data corroborates our claim.