Investigation of phase evolution within ZnO–Bi₂O₃ varistors utilizing thin film prototypes

Varistors are technologically important for their large energy handling capabilities and highly nonlinear electrical behavior when voltages above a characteristic switch field are applied. It is generally accepted that the prototypical ZnO–Bi₂O₃ varistor system forms electrostatic Schottky barriers at grain boundaries in response to residual Bi and other dopants left at grain surfaces during Bi₂O₃ segregation. While barrier heights can be modulated with formulation and defect chemistry, mechanisms by which dopant locations, defect compensation, and local phases determine varistor behavior are not completely understood. Bulk studies are challenging due to random grain boundary formation and difficulties studying individual boundaries. To circumvent these challenges in the ZnO–Bi₂O₃ varistor system, we use as-deposited and post-heat-treated thin film ZnO–Bi₂O₃ prototypes to simulate bulk varistor grain boundary phase formation and investigate resulting defect chemistry. Characterizing interactions between Bi₂O₃ films deposited on thin film and single-crystal ZnO by XRD and TEM-EDS revealed primarily Zn-out diffusion, resulting in two (Bi₂O₃)_1−x(ZnO)_x, or BZO, phases. Using these results, we present a saturated front model correlating changes in Bi₂O₃ thickness to phase evolution. We subsequently explore the influence of MnO doping leading to substantial changes in phase evolution for post-heat-treated (Mn:ZnO)–Bi₂O₃ stacks. Dopant-controlled Bi₂O₃ phase formations yield a 12 × difference, on average, between nonlinear coefficients for γ*- and β*-BZO. Graphical Abstract: [Figure not available: see fulltext.].