Growth behavior of Bi and Nb Co-added-SrTiO₃ thin film exhibiting varistor characteristics by metal-organic chemical vapor deposition

Nb-doped semiconductive SrTiO₃ film and Bi and Nb co-added SrTiO₃ film were grown on (001) MgO sub-strates by metal-organic chemical vapor deposition (MOCVD). The (001)-oriented SrTiO₃ phase was grown epitaxially with cube-on-cube structure in relation to the MgO substrate. In the ease of the Nb-doped SrTiO₃ film, the film exhibited a semiconductive nature with a low electrical resistivity (0.11 Ω · cm) that is three orders of magnitude lower than that in bulk SrTiOa ceramics by adding a much higher Nb addition (1.6-3.5 mol%). Most of the added Nb is thought to substitute for the Ti⁴⁺ as valence 4⁺, as suggested by the dependency of the cell volume change. In the case of Bi and Nb co-added SrTiO₃ film, the lattice parameter was somewhat increased with the addition of Bi, but the amount of Bi was undetectable. The addition of Bi did not produce a thin film with nonlinear electrical resistivity. Introducing a SrO seed layer with ∼2 nm thickness onto (001) MgO substrate produced weak nonlinear I-V (varistor) properties. It might be thought that the grain boundary between the two kinds of in-plane oriented grains, i.e., 45° rotating and cube-on-cube grains, acts as a barrier against varistor characteristics.