Manipulation of polycrystalline TiO2 carrier concentration via electrically active native defects

Abstract

There is good reason to believe that the properties of semiconducting metal oxides for catalytic applications can be improved when designed according to the principles of microelectronic devices. Nevertheless, defect engineering of polycrystalline TiO2 is complicated by native charged point defects and grain boundaries that alter bulk carrier concentration in a manner that depends on synthesis and post-treatment protocols. These influences are difficult to decouple with standard electrical characterization methods, which typically induce rectifying Schottky barriers to wide-bandgap semiconductors like TiO2. Here, TiO2 donor carrier concentration (Nd) is investigated as a function of film thickness and annealing time using a rigorous Schottky diode-based electrical characterization approach. Nd scales inversely with film thickness due to a reduction in the concentration of electrically active grain boundaries in the bulk. Annealing of polycrystalline TiO2 at 550 °C induces film densification that reduces the void volume surface area available for charge trapping. Strategies for defect engineering polycrystalline metal oxides must focus on tuning electrical activity of grain boundaries and intergranular voids for successful control and manipulation of Nd.