Ordering induced direct and indirect transitions in semiconductor alloys

Abstract

Atomic ordering from random alloys results in a peculiar evolution of electronic structures. Utilizing this property, we show theoretically that the directness of a band gap can be manipulated by forming ordered alloys in specific atomic configurations. For example, we show that Ga4 N As3 can have an indirect band gap if it forms the luzonite structure, despite that the constituents GaN and GaAs in the zinc-blende (ZB) structure, and their random alloys, all have direct band gaps. On the other hand, Al2 NP can have a direct band gap if it forms the CuPt structure, although the ZB constituents AlN and AlP, and their random alloys, all have indirect band gaps. These direct and indirect transitions are explained in terms of the ordering-induced evolution of the electronic states at high symmetry k points, namely, the emergence of the symmetry-enforced segregating states that are preferentially localized on one of the two sublattices in the ternary alloy, and the level repulsion between different ZB states that are folded into the same symmetry states in the ordered ternary alloy. © 2006 The American Physical Society.