Compositionally Dependent Nonlinear Optical Bandgap Behavior of Mixed Anodic Oxides in Niobium–Titanium System

Optical bandgap mapping of Nb–Ti mixed oxides anodically grown on a thin film parent metallic combinatorial library was performed via variable angle spectroscopic ellipsometry (VASE). A wide Nb–Ti compositional spread ranging from Nb-90 at.% Ti to Nb-15 at.% Ti deposited by cosputtering was used for this purpose. The Nb–Ti library was stepwise anodized at potentials up to 10 V SHE, and the anodic oxides optical properties were mapped along the Nb–Ti library with 2 at.% resolution. The surface dissimilarities along the Nb–Ti compositional gradient were minimized by tuning the deposition parameters, thus allowing a description of the mixed Nb–Ti oxides based on a single Tauc–Lorentz oscillator for data fitting. Mapping of the Nb–Ti oxides optical bandgap along the entire compositional spread showed a clear deviation from the linear model based on mixing individual Nb and Ti electronegativities proportional to their atomic fractions. This is attributed to the strong amorphization and an in-depth compositional gradient of the mixed oxides. A systematic optical bandgap decrease toward values as low as 2.0 eV was identified at approximately 50 at.% Nb. Mixing of Nb2O5 and TiO2 with both amorphous and crystalline phases is concluded, whereas the possibility of complex NbaTibOy oxide formation during anodization is unlikely.