Bandgap nature of chalcopyrite ZnXP2 (X = Si, Ge, Sn).

The bandgap nature and near-edge band structure of the chalcopyrite ZnXP 2 (X = Si, Ge, Sn) are investigated using the first-principles approaches. For these materials, whereas the valence band maximums are well defined, there are several competing near-extrema at the conduction band-edge. Moreover, the energetically near-degenerate conduction states are usually derived from different electron orbitals, which results in distinct optical properties and thus different bandgap types. A pseudo-direct bandgap, which is related to a weak optical absorption onset, can also be found beside the well-known direct and indirect bandgaps. In this work, we first investigate the fundamental properties of the band-edge electronic states using the density functional theory with the generalized gradient approximation. The origins of bandgap anomaly of the ZnXP 2 systems are then analyzed. Finally, we perform the many-body electronic structure calculations for more reliable band orderings. The quasiparticle calculations indicate that whereas ZnSiP 2 and ZnSnP 2 have a pseudo-direct and a direct bandgap, respectively. However, ZnGeP 2 has a bandgap with mixed pseudo-direct and indirect characters. [ABSTRACT FROM AUTHOR]