Strain tuning of electronic structure inBi4Ti3O12−LaCoO3epitaxial thin films

Abstract

We investigated the crystal and electronic structures of ferroelectric $\mathrm{B}{\mathrm{i}}_{4}\mathrm{T}{\mathrm{i}}_{3}{\mathrm{O}}_{12}$ single-crystalline thin films site-specifically substituted with $\mathrm{LaCo}{\mathrm{O}}_{3}$ (LCO). The epitaxial films were grown by pulsed laser epitaxy on $\mathrm{NdGa}{\mathrm{O}}_{3}$ and $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates to vary the degree of strain. With increasing the LCO substitution, we observed a systematic increase in the $c$-axis lattice constant of the Aurivillius phase related with the modification of pseudo-orthorhombic unit cells. These compositional and structural changes resulted in a systematic decrease in the band gap, i.e., the optical transition energy between the oxygen $2p$ and transition-metal $3d$ states, based on a spectroscopic ellipsometry study. In particular, the $\mathrm{Co}\phantom{\rule{0.16em}{0ex}}3d$ state seems to largely overlap with the Ti ${t}_{2g}$ state, decreasing the band gap. Interestingly, the applied tensile strain facilitates the band-gap narrowing, demonstrating that epitaxial strain is a useful tool to tune the electronic structure of ferroelectric transition-metal oxides.