Mesoscopic models for the mechanically modulated electrical conductivity of piezoelectric semiconductors

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The piezoelectric polarisation induced by internal or applied mechanical stresses in oxide ceramic semiconductors is known to modify the potential barriers at grain boundaries and therefore the electrical conductivity across these boundaries. Besides this modification in the microscopic scale, the effective electrical conductivity of polycrystalline bulk material depends also on the microstructural disorder associated with stochastic variations in grain size, grain boundary properties, crystallographic orientation and the related stress field distribution. The effect of microstructural disorder is manifested in the current flow properties, in particular, in the current filamentation phenomenon consisting in the concentration of current density along a few low resistivity paths within the material. Thus, the effective conductivity of polycrystalline materials depends not only on the piezoelectric grain boundary modification but also on the mesoscopic scale current distribution within the material for different applied voltages and mechanical stress conditions. Both, microscopic and mesoscopic effects are closely connected and should be investigated as coupled phenomena within a common research platform.The project is dedicated to the numerical modeling and simulation of the mechanically modulated electrical conductivity of ZnO. Microscopic-scale charge transport models for bicrystals and multigrain arrangements taking into account drift-diffusion, thermionic emission at grain boundaries as well as the direct and inverse piezoelectric effects will be developed. These models will be incorporated into 3D mechanically informed mesoscopic current flow simulations for polycrystalline ZnO using accurate mechanical stress distributions and taking into account stochastic microstructural variations. The microscopic-scale approach will provide insight into the range of validity of mesoscopic current simulations based on equivalent network and finite element models. The proposed models will be employed in the simulation based electromechanical characterization of ZnO varistors. Furthermore, textured ZnO films with intentionally oriented polarity will be investigated. All proposed investigations will be performed in close cooperation with our project partners.