Electronic origin of defect states in Fe-doped LiNbO3 ferroelectrics

Abstract

We investigate the role of Fe in the electronic structure of ferroelectric LiNbO3 by density-functional theory calculations. We show that Fe2+ on the Li site (Fe2+ Li) features a displacement opposite to the direction of spontaneous polarization and acts as a trigger for the bulk photovoltaic (PV) effect. In contrast to Fe3+ on the Li site that forms the defect states (1e, a, and 2e) below the conduction band minimum, the reduction from Fe3+ to Fe2+ accompanied by a lattice relaxation markedly lowers only the a state (dz2) owing to a strong orbital hybridization with Nb-4d. The a state of Fe2+ L i provides the highest electron-occupied defect state in the middle of the band gap. A reduction treatment of Fe-LN is expected to increase the concentration of Fe2+ and therefore to enhance the PV effect under visible light illumination.