Band Structure Engineering: Insights from Defects, Band Gap, and Electron Mobility, from Study of Magnesium Tantalate

Anion doping of semiconductors with nitrogen is a strategy often adopted to narrow the band gap of semiconductors and increase the range of light absorption. However, the influence of nitrogen doping on the electron mobility in the semiconductor is not fully understood and characterized. In this work, we used magnesium tantalate MgTa₂O₆ as a model system and hybrid density-functional theory calculations to characterize the mobility of electrons using the small polaron model in the presence of nitrogen-doping defects as well as oxygen-vacancy defects. We found that electron mobility is not significantly affected when MgTa₂O₆ is doped with a molar ratio N/O of ∼2%. However, in the presence of oxygen vacancies combined with nitrogen doping with the same molar ratio N/O of ∼2%, the barrier to electron hopping in the vicinity of the defects is much lower than that in pristine MgTa₂O₆ and in MgTa₂O₆ with oxygen-vacancy defects only. These results suggest that nitrogen doping combined with anion vacancy not only narrows band gap but also enhances electron mobility, a finding that may lead to new strategies toward synthesizing more efficient photocatalysts. (Figure Presented).