Charge Localization Near Acceptors in Transition-Metal Oxides

Controlling the conductivity in transition-metal (TM) oxides is a great challenge to make them usable in electronic and optoelectronic devices. P-type doping, in particular, is hindered by the high ionization potential of these materials; their valence bands (VB) are derived from O 2p orbitals, lying very low in energy with respect to the vacuum. Adding holes to the VB, either by introducing acceptors or by photon excitation, invariably leads to the localization of the carriers, in the form of small polarons, precluding p-type conductivity. In this presentation, we use density functional theory with the HSE06 hybrid functional to explore the impact of acceptor impurities on the electronic and optical properties of TM oxides, such as TiO₂ and SrTiO₃, paying attention to charge localization, i.e., the interaction between the acceptors and small hole polarons. We first discuss hole localization in the absence of any impurity, and then analyze the formation of small polarons and their stability near acceptor impurities through the calculation of binding energies. We address the effects of size and chemical differences between the acceptors and the host atoms on binding energies and ionization energies. Finally, we discuss possible cases where p-type conductivity could be observed.