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.

Sprache

Englisch