Segregation and properties at curved vs straight (0001) inversion boundaries in piezotronic ZnO bicrystals

TEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain-boundary structure, segregation, and elec-trical transport perpendicular to the interface. To this end, varistor-like ZnO bicrystals with piezotronic characteristics were chosen with (0001)║(0001) tail-to-tail orienta-tion with respect to the c-axis. In order to contrast different local grain-boundary structures with different coherency and segregation of bismuth, but identical macro-scopic polarization state, two complementary processing techniques were applied. A diffusion-bonded bicrystal with an intermediate thin film containing Zn–Bi–Co–O provided a straight interface as reference. In contrast, a ZnO bicrystal prepared by epitaxial solid-state transformation was manufactured by bonding two ZnO single crystals with a 100 μm thick polycrystalline ZnO varistor material with a typical dopant composition including bismuth and cobalt. This structure was annealed to the point that a bicrystal was formed with the varistor concentration at the boundary, which was strongly curved due to the polycrystalline microstructure still providing a shadow image at the interface. The results highlight a distinct correlation between local interfacial morphology, degree of segregation of bismuth, and degree of non-linearity of the electrical transport across the interface