Mesoscopic models for the mechanically modulated electrical conductivity of piezoelectric semiconductors

已知氧化物陶瓷半导体中由内部或施加的机械应力引起的压电极化会改变晶界处的势垒，从而改变这些边界上的电导率。除了在微观尺度上的这种修改之外，多晶块体材料的有效电导率还取决于与晶粒尺寸、晶界性质、晶体取向和相关应力场分布的随机变化相关联的微观结构无序。微观结构无序的影响表现在电流特性中，特别是表现在电流密度沿材料内的几个低电阻率路径沿着集中的电流抑制现象中。因此，多晶材料的有效导电性不仅取决于压电晶界改性，而且取决于在不同的施加电压和机械应力条件下材料内的介观尺度电流分布。微观和介观效应是紧密相连的，应该在一个共同的研究平台内作为耦合现象进行研究。本项目致力于ZnO机械调制电导率的数值建模和模拟。微观尺度的电荷输运模型的双晶体和多晶粒的安排，考虑到漂移扩散，电子发射在晶界以及直接和逆压电效应将被开发。这些模型将被纳入3D机械通知介观电流模拟多晶ZnO使用精确的机械应力分布，并考虑到随机微观结构的变化。微观尺度的方法将提供洞察的有效性范围内的等效网络和有限元模型的基础上的介观电流模拟。所提出的模型将采用基于仿真的ZnO压敏电阻器的机电特性。此外，织构ZnO薄膜与故意取向的极性将进行研究。所有拟议的调查将与我们的项目合作伙伴密切合作进行。

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.