Polaron-induced metal-to-insulator transition in vanadium oxides from density functional theory calculations

Vanadium oxides have been extensively studied as phase-change memory units in artificial synapses for neuromorphic computing due to their metal-insulator transitions (MIT) at or near room temperature. Recently, injection of charge carriers into vanadium oxides, e.g., via optically via a heterostructure, has been proposed as an alternative switching mechanism and also potentially as a means to tune the MIT temperature. In this study, we explore the formation of small polarons in the low temperature (LT) insulating phases for ${\mathrm{V}}_{3}{\mathrm{O}}_{5}, {\mathrm{VO}}_{2}$, and ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$, and the barriers to their migration using density functional theory calculations. We find that ${\mathrm{V}}_{3}{\mathrm{O}}_{5}$ exhibits very low hole and electron polaron migration barriers ($<100$ meV) compared to ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{VO}}_{2}$, leading to much higher estimated polaronic conductivity. We also link the relative migration barriers to the amount of distortion that has to travel when the polaron migrate from one site to another. Polarons in ${\mathrm{V}}_{3}{\mathrm{O}}_{5}$ also have smaller binding energies to vanadium and oxygen vacancy defects. These results suggest that the triggering of the MIT via injection of charge carriers are due to the formation of small polarons that can migrate rapidly through the crystal.