Three-Dimensional Visualization of Conductive Filaments in TiO2 via Conductive Atomic Force Microscopy Tomography.

Direct three-dimensional visualization of conductive filaments (CFs) in valence change memory is essential for resolving whether their spatial evolution is governed by stochastic defect generation or by electrode geometry. Here, we apply conductive atomic force microscopy tomography to a TiO2 (10 nm)/Pt structure, extending this technique to TiO2-based CFs previously inaccessible to tomographic characterization. The tomogram reveals a reverse-cone morphology whose terminus preferentially connects to a Pt bottom electrode (BE) protrusion, evidencing dual field enhancement at both interfaces. Current gradient analysis reveals a nonmonotonic conductivity profile, with values of 0.1355, 0.0682, and 0.1035 nA/nm at the top, middle, and bottom, consistent with lateral electrostatic confinement by the BE protrusion. Competing exponential field dependence of vacancy generation and linear field dependence of drift accounts for the near-tip constriction. These results establish the BE morphology as an active confinement mechanism and identify nanoscale BE roughness as a key parameter influencing switching variability in resistive random access memory.