Self-contained multi-sample ellipsometry reveals voltage-composition co-regulation of defects in Nb–Ta anodic gradient oxides
Year: 2025
Type: article
Abstract: 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.%) 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 (Eu) and defect density trends: low Eu values (~53.8 meV) and defect densities (~3.5×1019 cm-3) dominate at mid-voltages (5-8 V), while high-voltage (9-10 V) and low-voltage (3-4 V) regions exhibit elevated Eu (>150 meV) and defect densities (>2×1020 cm-3). A dual-layer model demonstrates voltage-controlled optical heterogeneity, with significant surface/bottom bandgap differences (DEg_max=0.99eV) at voltage extremes, while homogeneous Eg distributions are present at mid-voltages. 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 Nb2O5 amplify defect generation at voltage extremes, while mid-voltage stabilization enables homogeneous oxide growth with minimal defects. These findings establish a voltage- and composition-coordinated regulation paradigm for engineering defects in complex alloy oxides.
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,640
Awards: G6180380761, G7188639927
OpenAlex ID: W4413951024