Leakage-current characteristics of vanadium- and scandium-doped barium strontium titanate ceramics over a wide range of DC electric fields

We report the leakage-current characteristics of bulk vanadium (donor)- and scandium (acceptor)-doped Ba_0.7Sr_0.3TiO₃ ceramic structures measured using gold electrical contacts. Vanadium doping reduces the ohmic leakage current that dominates the transport characteristics by up to 5 kV cm^-1. The Arrhenius activation energy is 0.18, 0.20 and 0.23 eV for 1, 2 and 4 at.% V-doped samples, respectively. Above this field, the current-voltage characteristics exhibit discontinuous current transitions associated with trap filling by electronic carriers. At higher fields, trap-controlled, space-charge-limited conduction (SCLC) is observed with an effective mobility of 4 ± 1 × 10^-7 cm² V^-1 s^-1, characteristic of an electronic transport process that involves quasi-equilibrium between conduction in the band and trapping. In contrast, the leakage current of Sc-doped samples increases with impurity concentration and exhibits a 0.60 eV activation energy. In this case, the limiting current conduction mechanism is the transport of holes over the electrostatic barrier at grain boundaries. Comparison of these results to those on similarly doped homoepitaxial SrTiO₃ thin films deposited on single-crystal and bicrystal substrates helped to identify the characteristics of transport in the bulk and across grain boundaries for this class of materials. The Sc- and V-doped thin films and ceramics did not exhibit electrical properties characteristic of shallow-level impurity doping, suggesting that V and Sc impurities form deep-level point defects and /or complexes, or are strongly compensated.