Oxygen-Impurity-Induced Direct-Indirect Band Gap in Perovskite SrTaO₂N

Oxynitride semiconductors are considered to be promising candidates for solar water splitting. In this work, we show that oxygen-rich SrTaO₂N has a band gap with direct-indirect character through twin valence-band maximums (VBMs), resulting in good photoelectronic responses. Compared with the direct band gap of ideal SrTaO₂N, the additional indirect VBM of the oxygen-rich solid solution was found to be due to strontium-oxygen hybridization, using orbital projections based on hybrid/GW density functional theory (DFT). This twin-VBM character was validated by strontium K-edge absorption through extended X-ray absorption fine structure (EXAFS) analysis. The twin-VBM character of the band structure can enhance the photoelectronic response and hole transport. Our findings provide a viable strategy for enhancing the solar water splitting performance of oxynitrides.