Point defects at ferroelectric domain walls in LiNbO3

Conductive domain walls (DWs) hold promise for nanoscale, energy-efficient multi-level diodes and neuromorphic circuitry. Oxygen and lithium vacancies are intrinsic point defects that can induce electrons and holes, respectively. Controlling the formation and position of vacancies can in principle give switchable local n-type or p-type conductivity at DWs and enable new concepts for DW-based circuitry. This requires a material where mobile point defects of both positive and negative charge can accumulate at DWs. LiNbO₃ is here chosen as our model system due to the possibility of having mobile Li and O vacancies.

We use density functional theory (DFT) to study the effects of these intrinsic point defects on neutral Y-type DWs in LiNbO₃. Following an initial discussion of the defect formation energies for point defects in bulk, we show that some of these defects display significantly lower formation energies at DWs. This implies that there is a driving force for accumulation of point defects at DWs and that the local charge carrier concentration and conductivity can be controlled through point defect populations. Migration barriers for both point defects in bulk and Y-DWs over points defects are determined to assess their relative mobility and whether the point defects will pin migrating DWs. Finally, the possibility of reversible p- and n-type conductivity in neutral DWs in LiNbO3 and other similar materials is discussed.