Spectroscopic ellipsometry for compositionally induced bandgap tuning of combinatorial niobium–tantalum anodic oxides

Limberger, Wolfgang; Mardare, Cezarina Cela; Cobet, Christoph; Zuo, Juan; Hassel, Achim Walter; Mardare, Andrei Ionut

doi:10.1039/c6ra13157c

Cited by 6 publications

(12 citation statements)

References 25 publications

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“…The morphology of the as-deposited metallic Nb film indicates pyramidal shaped-grains with sizes in the range of 10 to 80 nm and the film develops a columnar structure with densely packed columns. This type of microstructure is typical for sputtered Nb films and confirms previous findings [42,43].…”

Section: Resultssupporting

confidence: 91%

Downstream analytics quantification of ion release during high-voltage anodisation of niobium

Kollender

Mardare

et al. 2018

J Solid State Electrochem

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In operando quantification of field-assisted ion release during high-voltage anodisation (up to 100 V SHE) of Nb in 0.1 M sulphuric acid was performed. Electrochemical high-field oxide formation under both potential and current control was studied separately. The quantification of in situ ion release via ICP-MS revealed an increase of the oxide dissolution factor (from 337 to 422 fm V −1) when decreasing the potential scan rate from 200 to 100 mV s −1. Dissolution rates measured during galvanostatic oxide formation allowed measuring oxide dissolution factors of 719 and 837 fm V −1 for current densities of 1.0 and 0.5 mA cm −2 , respectively. As compared to the potentiodynamic case, higher dissolution rates and oxide dissolution factors were measured for galvanostatic anodisation. The overall fraction of the charge used for generation of soluble Nb species was below 0.4% for all oxide growth regimes. Cross-sectional SEM imaging proofs an oxide formation factor of 2.1 nm V −1. The surface of anodised films was extremely smooth and featureless without any cracks or voids. Based on X-ray diffraction, the films were found to be amorphous, indicating that no field crystallisation is occurring under the applied oxide growth conditions even at higher voltages.

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Section: Resultssupporting

confidence: 91%

Downstream analytics quantification of ion release during high-voltage anodisation of niobium

Kollender

Mardare

et al. 2018

J Solid State Electrochem

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“…For this reason, the optical bandgap compositional mapping of the Nb–Ti mixed anodic oxides was performed as a function of oxide thickness, and the obtained results are summarized in Figure . This finding reconfirms previously reported hints that mixed anodic oxides on valve metals may change their properties with thickness, which inadvertently may lead to bandgap modifications . In Figure , a second vertical axis was added on the right side for providing the absorbed radiation wavelength equivalent of the bandgap energy.…”

Section: Resultssupporting

confidence: 87%

“…This strong deviation of the mixed oxide composition from the parent metals can indicate a strong disorder occurring during the anodization process, which would confirm the observed bandgap decrease. Moreover, a recent similar study on anodization of Nb–Ta thin film alloys shows very similar findings and points out the possibility of an in-depth compositional change of the anodic oxides . Following this idea, the driving force for strong oxide disordering will be enhanced if stronger in-depth compositional gradients are present, as can be assumed for the Nb–Ti system due to the discussed surface enrichment of TiO 2 and its complementary depletion of Nb 2 O 5 .…”

Section: Resultssupporting

confidence: 55%

“…For all anodizations, a 1287 Solartron electrochemical interface in a three electrode configuration was used with a Hg/Hg 2 (CH 3 COO) 2 μ-reference electrode and a Pt mesh as counter electrode. , As electrolyte, a pH 6.0 CH 3 COOH/CH 3 COONa buffer solution was used. More details of the setup including detailed photos are presented elsewhere …”

Section: Methodsmentioning

confidence: 99%

“…More details of the setup including detailed photos are presented elsewhere. 14 Metallic and Oxide Thin Films Characterization. Before anodization of the thin film alloys, their microstructure was examined by a field emission Zeiss Gemini 1540 XB SEM with an acceleration voltage of 20 kV and in-lens detection.…”

Section: ■ Introductionmentioning

confidence: 99%

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

et al. 2017

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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 NbO and TiO with both amorphous and crystalline phases is concluded, whereas the possibility of complex NbTiO oxide formation during anodization is unlikely.

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Coexistence of memory and threshold resistive switching identified by combinatorial screening in niobium-tantalum system

Zrinski

Minenkov

Cancellieri

et al. 2023

Applied Surface Science

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Spectroscopic ellipsometry for compositionally induced bandgap tuning of combinatorial niobium–tantalum anodic oxides

Abstract: Spectroscopic ellipsometry maps optical properties of anodic oxides grown on Nb–Ta thin film combinatorial library. The bandgap values deviate from the linear model based on mixed matter theory due to an in-depth compositional gradient of the oxides.

Cited by 6 publications

References 25 publications

Downstream analytics quantification of ion release during high-voltage anodisation of niobium

Downstream analytics quantification of ion release during high-voltage anodisation of niobium

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

Coexistence of memory and threshold resistive switching identified by combinatorial screening in niobium-tantalum system

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