On the Surface Structure of the La-Doped BaTiO3-Bi2O3 Composite Ceramic Showing a Two-Step Anomalous Increase of Resistivity

Abstract

A band model for the La-doped BaTi3-Bi2O3 composite ceramic which exhibits a two-step anomalous increase in resistivity with increasing temperature is tentatively proposed. The surface barrier layer built up between the semiconducting ceramic body and a silver electrode is made up of Bi2O3 and the semiconductor's surface depletion layer, and gives rise to the first anomaly around room temperature. The current-voltage characteristics with asymmetric electrodes (an In-Sn alloy applied on one side and Ag on the other side of the specimen) demonstrate that the surface barrier layer limits the current through the ceramic body below the Curie point. The width of the surface insulating layer of Bi2O3 is considered to be 10-6–10-5 cm, taking a value of 3–5 as the optical dielectric constant of Bi2O3. Capacitance-voltage characteristics around room temperature suggest that the diffusion potential and the width of the depletion layer extending from the surface to the inside of the semiconducting BaTiO3 ceramic are estimated as about 1.3 eV and of the order of 10-4 cm, respectively.