Charge transport properties of bulkTa3N5from first principles

Abstract

Tantalum nitride is considered a promising material for photoelectrochemical water splitting, however, its charge transport properties remain poorly understood. We investigated polaronic and band transport in ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ using first-principles calculations. We first studied the formation of small polarons using density-functional theory (DFT) including $\mathrm{DFT}+U$ and hybrid functionals. We found that electron small polarons may occur but hole polarons are not energetically favorable. The estimated polaronic mobility for electrons is at least three orders of magnitude smaller than that measured in ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ films, suggesting that the main transport mechanism for both electrons and holes is bandlike. Since band transport is strongly affected by the carrier effective masses, and ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ is known to have large electron and hole effective masses, we also investigated whether substitutional impurities or strain may help lower the effective masses. We found a significant reduction in both electron and hole effective masses (up to 17% for electrons and 39% for holes) under applied strain, which may lead to a substantial improvement (up to 30% for electrons and 15% for holes) in the carrier mobilities.