Self-contained multi-sample ellipsometry reveals voltage-composition co-regulation of defects in Nb–Ta anodic gradient oxides

By developing a self-contained multi-sample SE fitting method, we reveal voltage-composition correlated defect regulation in Nb–Ta anodic oxides formed across a compositional gradient (11.3–81.5 at.% Nb) under varying anodizing voltages (3–10 V). The oxide formation factor К derived from this approach aligns more closely with electrochemical data than traditional sequential fitting. Analysis of the extinction coefficient k reveals voltage-dependent Urbach tail absorption ( E u ) and defect density trends: low E u values (∼53.8 meV) and defect densities (∼3.5 × 10 19 cm −3 ) dominate at mid-voltages (5–8 V), defining a “voltage stabilization window”- a new discovery uniquely enabled by our self-contained fitting method. Conversely, high-voltage (9–10 V) and low-voltage (3–4 V) regions exhibit elevated E u (>150 meV) and defect densities (>2 × 10 20 cm −3 ). A dual-layer model demonstrates voltage-controlled optical heterogeneity, with significant surface/bottom bandgap differences (Δ E g_ max = 0.99eV) at voltage extremes, while homogeneous E g distributions characterize the mid-voltage window. Differences in oxidation kinetics between Ta–O and Nb–O systems drive voltage-dependent defect dynamics. The higher ionic mobility of Nb ions and its octahedral distortion in amorphous Nb 2 O 5 amplify defects generation at voltage extremes, while mid-voltage stabilization enables homogeneous oxide growth. These findings establish a voltage- and composition-coordinated regulation paradigm for engineering defects in complex alloy oxides, with the identified stabilization window enabling optimal memristor performance (e.g., 61 at.% Nb at 8 V). This method offers a scalable strategy for industrial oxide production, replacing tedious electrical screening with rapid optical characterization.