Sr-doping effects on conductivity,charge transport,and ferroelectricity of Ba_(0.7)La_(0.3)TiO_3 epitaxial thin films

Sr-doped Ba_(0.7)La_(0.3)TiO_3(BSLTO) thin films are deposited by pulsed laser deposition, and their microstructure, conductivity, carrier transport mechanism, and ferroelectricity are systematically investigated. The x-ray diffraction measurements demonstrate that Sr-doping reduces the lattice constant of BSLTO thin films, resulting in the enhanced phonon energy in the films as evidenced by the Raman measurements. Resistivity-temperature and Hall effect measurements demonstrate that Sr can gradually reduce electrical resistivity while the electron concentration remains almost unchanged at high temperatures. For the films with semiconducting behavior, the charge transport model transforms from variable range hopping to small polaron hopping as the measurement temperature increases. The metalic conductive behaviors in the films with Sr = 0.30, 0.40 conform to thermal phonon scattering mode. The difference in charge transport behavior dependent on the A-site cation doping, is clarified. It is revealed that the increasing of phonon energy by Sr doping is responsible for lower activation energy of small polaron hopping, higher carrier mobility, and lower electrical resistivity. Interestingly, the piezoelectric force microscopy(PFM) results demonstrate that all the BSLTO films can exhibit ferroelectricity, especially for the room temperature metallic conduction film with Sr = 0.40. These results imply that Sr-doping could be a potential way to explore ferroelectric metal materials for other perovskite oxides.