Strain effects on conductivity and charge transport in La-doped BaTiO₃ thin films

We investigate how epitaxial strain affects the conductivity and charge transport of Ba_0.6La_0.4TiO₃ (BLTO) films deposited by pulsed laser deposition. Depositing BLTO films with thicknesses varying from 14.1 to 95.9 nm gives differing epitaxial strains, with a maximum strain up to ~6% for the thinnest film of 14.1 nm. Transport measurements demonstrate that film thickness (i.e. epitaxial strain) affects the conductivity, carrier mobility, and concentration. Measurements of resistivity versus temperature demonstrate that all the BLTO films undergo a distinct semiconductor-metal phase transition, and the transition temperature T _SM also depends clearly on strain. For all the films, the charge transport follows the small-polaron hopping mechanism below T _SM and the thermal phonon scattering mechanism above T _SM. These results prove that strain in epitaxial films strongly affects the conductivity and charge transport of traditional insulating ferroelectric oxide films. (paper)