Electrochemistry of Binary Valve Metal Combination Library: Niobium-Tantalum Thin Films

The microstructure of the niobium-tantalum composite library was studied by scanning electron microscopy, and the results are shown in Figure 1. Cross-sectional observations of selected compositions (not shown here) confirmed the presence of columnar structures throughout the niobium-tantalum composition distribution, which is typical for thin films formed by tantalum vapor deposition techniques. The images of the alloy surfaces are presented as 500 nm × 500 nm squares, with the various compositions starting from a pure tantalum surface and ending with a pure niobium film.

Figure 1 SEM images of niobium-tantalum combinatorial library with different concentrations and pure tantalum and niobium as reference

Figure2 GIXRD patterns measured in different concentrations of niobium-tantalum library

Figure3 Cyclic voltammograms recorded during potentiodynamic oxidative growth on niobium-tantalum alloys with different concentrations

Figure 4 Electrochemical impedance spectra of anodic oxides grown at 3 V potentiostatically on niobium-tantalum alloys with different concentrations. Impedance Modulus (a) and Phase (b) vs Frequency

Figure 5 (a) Reverse capacitance and (b) anodizing resistance measured after each 1 volt increase in anodizing potential for various compositions of niobium-tantalum thin film alloys

Figure 5 presents the inverse capacitance (a) and resistance (b) of anodic oxide grown on the surface of a niobium-tantalum binary thin film library as a function of anodization potential. The inverse capacitance value has a good linear relationship with the potential, while the resistance value has a more dispersed distribution. This is mainly due to studies with very low currents measured at low frequencies, often with low signal-to-noise ratios. Since the anodic oxidation potential is directly related to the oxide thickness through the oxide formation factor, (a) and (b) will allow the direct calculation of the dielectric constant and resistivity of the anodic oxide, respectively.

Figure 6. Mixed niobium-tantalum anodic oxide capacitors depend on the niobium content on the niobium-tantalum composition distribution. The figure shows three different anodizing potentials (1V, 5V and 10V)

Scan the surface of the niobium-tantalum composition distribution. The resolution of the curve describing the relationship between inverse squared capacitance and applied bias voltage is 5 at.% for different concentrations of parent metal. This relationship is determined by the well-known Mott-Schottky equation:

Figure 7 Mott-Schottky plot of potentiostatically grown anodic oxides at 3 V

The slopes of the linear fits of the Mott-Schottky plots and their intercepts (not shown here) can be used directly to calculate the flat band potential and donor concentration of the anodic oxide in the niobium-tantalum library.

Figure8 Flat band potential and donor concentration of anodic oxides grown potentiostatically at 3 V in niobium-tantalum library as a function of concentration

Figure 9 XPS depth profiling spectra of anodic oxides grown on niobium-tantalum combinatorial libraries of different compositions at a maximum applied potential of 10 V

The XPS spectra recorded on the library surface (before deposition) were quantitatively evaluated by integrating the XPS peaks and the results are summarized in Table 1. Quantitative evaluation showed that the ratio between the oxide species of tantalum and niobium in the mixed anodic oxide was changing compared to the ratio between the metal precursors in the entire library.

Table 1 The metal concentration ratio of parent metal alloy (Me) and anodic oxide (Ox) and their relative metal concentration (Me; Ox)