Band-gap engineering via local environment in complex oxides

We examine band-structure engineering in extremely tetragonal ferroelectric perovskites (ABO₃) to make these materials suitable for photovoltaic applications. Using first-principles calculations, we study how B-site ordering, lattice strain, cation identity, and oxygen octahedral cage tilts affect the energies and the compositions of the valence and conduction bands. We find that extreme tetragonality makes the band gap highly sensitive to the B-cation ordering, with a layered B-site arrangement exhibiting a small band gap. It also leads to a strong sensitivity of the band gap to the oxygen octahedral tilting. These effects only occur for cations with filled d states located near the valence-band maximum or empty d states at the conduction-band minimum; this criterion is explained by crystal field theory. In addition to a smaller band gap, the layered B-site ordering has a strong impact on the carrier mobility. We find that the excited electron effective mass is similar to that found in Si and other classic semiconductors. Moreover, the hole effective mass is strongly anisotropic, indicating a two-dimensional hole gas in the layered B-cation ordering.