Charge Carrier Transport Mechanism in Ta O , TaON, and Ta N Studied by Applying Polaron Hopping and Bandlike Models.

TaON and Ta N are considered promising materials for photocatalytic and photoelectrochemical water splitting. In contrast, their counterpart Ta O does not exhibit good photocatalytic performance. This may be explained with the different charge carrier transport mechanisms in these materials, which are not well understood yet. Herein, we investigate the charge transport properties in Ta O , TaON, and Ta N by polaron hopping and bandlike models. First, the polaron binding energies were calculated to evaluate whether the small polaron occurs in these materials. Then we performed calculations to localize the excess carriers as small polarons using a hybrid density functional. We find that the small polaron hopping is the charge transfer mechanism in Ta O whereas our calculations indicate that this mechanism may not occur in TaON and Ta N . We also investigated the bandlike model mechanism by calculating the charge carrier mobility of these materials using the effective mass approximation, but the calculated mobility is not consistent with experimental results. This study is a first step towards understanding charge transport in oxynitrides and nitrides and furthermore establishes a simple rule to determine whether a small polaron occurs in a material.