ZnO‐based single crystal‐polycrystal structures for piezotronic applications

Electrostatic potential barriers at doped ZnO‐ZnO interfaces can be modified bystress‐induced polarization charges. This concept was enhanced by preparingZnO‐based single crystal‐polycrystal‐single crystal structures by diffusion bond-ing. Increasing time for epitaxial solid‐state transformation results in structureswith a decreasing thickness of residual polycrystalline material in between twowell‐oriented single crystals. Microstructural and electrical analysis quantifies theinfluence of high‐temperature treatment during epitaxial growth on the stress sen-sitivity of the prepared structures. The orientation of the single crystals is definedto maximize the interaction between stress‐induced polarization charges and thepotential barriers at doped ZnO‐ZnO interfaces. With decreasing thickness ofresidual polycrystalline material, the percentage of grain boundaries with favor-ably aligned polarization vectors is increased resulting in a higher stress sensitiv-ity. This effect is compensated by an adverse effect of the high‐temperaturetreatment on the initial potential barrier height. Hence, a maximum in stress sensi-tivity can be observed for intermediate times of epitaxial growth. The preparedstructures close the gap between the varistor piezotronics based on bulk ceramicswith random orientation of the polarization vector and the bicrystal piezotronicswith perfect orientation of the polarization vector, demonstrating the capability ofmicrostructural engineering for varistor‐based piezotronic devices.