Local properties of grain boundaries in semiconducting ceramics

Zinc oxide-based varistors and barium titanate-based thermistors exhibit important variations in microstructure and chemistry in the vicinity of the grain boundaries. The resulting electrical interface barriers give rise to valuable electrical properties which are sensitive to ceramic formulation and processing conditions. Most models that describe the properties depend on the nature of the grain boundaries and relevant defect states. Traditional measurement techniques for these polycrystalline semiconductors yield only bulk or averaged properties such as resistivities of grains or grain boundaries. Greater understanding of the microstructural control of the properties at the grain to grain level has come through the utilisation of local investigative techniques. These include SEM/AFM-based procedures including Local Impedance Imaging, Scanning Surface Potential Microscopy, Remote Electron Beam Induced Current and Deep Level Transient Spectroscopy (DLTS). The capabilities and achievements of these techniques are highlighted with reference to semiconducting ceramics.