A comprehensive electron energy-loss spectroscopy study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO2, Nb2O5) was carried out. In this work (Part I), the plasmons and energy-loss near-edge structures (ELNES) of all relevant Nb edges (Nb-N2,3, Nb-M4,5, Nb-M2,3, Nb-M1, and Nb-L2,3) up to energy losses of about 2600 eV and the O-K edge are analyzed with respect to achieving characteristic fingerprints of Nb in different formal oxidation states (0 for metallic Nb, +2 for NbO, +4 for NbO2, and +5 for Nb2O5). Chemical shifts of the Nb-N2,3, Nb-M4,5, Nb-M2,3, and Nb-L2,3 edges are extracted from the spectra that amount to about 4 eV as the oxidation state increases from 0 for Nb to +5 for Nb2O5. Four different microscopes, including a 200 keV ZEISS Libra with monochromator, were used. The corresponding wide range of experimental parameters with respect to the primary electron energy, convergence, and collection semi-angles as well as energy resolution allows an assessment of the influence of the experimental setup on the ELNES of the different edges. Finally, the intensity of the Nb-L2,3 white-line edges is correlated with niobium 4d-state occupancy in the different reference materials.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S143192760999105X

EELS of niobium and stoichiometric niobium-oxide phases--Part I: plasmon and near-edges fine structure.

Electron energy loss spectra in conjunction with near-edge fine structures of purely stoichiometric niobium monoxide (NbO) and niobium pentoxide (Nb2O5) reference materials were recorded. The structures of the niobium oxide reference materials were checked by selected area electron diffraction to ensure a proper assignment of the fine structures. NbO and Nb2O5 show clearly different energy loss near-edge fine structures of the Nb-M4,5 and -M2,3 edges and of the O-K edge, reflecting the specific local environments of the ionized atoms. To distinguish the two oxides in a quantitative manner, the intensities under the Nb-M4,5 as well as Nb-M2,3 edges and the O-K edge were measured and their ratios calculated. k-factors were also derived from these measurements.

http://ematweb.cmi.ua.ac.be/emat/pdf/1463.pdf

EELS investigations of different niobium oxide phases.

The electrical and structural properties of nanoscale niobium pentoxide (Nb2O5) dielectric layers in niobium-based solid electrolyte capacitors were studied. The Nb2O5 layers are formed by anodic oxidation of Nb-powder compacts. Capacitance measurements show a strong bias-voltage dependence of the capacitance after anodization. Heat treatments at temperatures up to 320 °C, which are applied in the capacitor-production process, lead to an increase of the capacitance and a reduction of the bias dependence. Based on the electrical and structural properties, which are characterized by electron microscopic techniques, a model is presented which explains the behavior of the specific capacitance after the various processing steps.

Anodically formed oxide films on niobium: Microstructural and electrical properties

We report nonvolatile resistive switching in anodic niobium pentoxide thin-film memory cells. Highly dielectric Nb2O5 films were prepared at room temperature by the anodic oxidation of submicrometer-thick Nb films sputtered onto an Si wafer. After the electroforming process, Au/Nb2O5/Nb/Si sandwich memory cells demonstrate reproducible direct current and pulse mode switching between two resistance states with a resistance on-off ratio around 103. Low and high resististive states show ohmic conductivity and field-assisted Poole-Frenkel-type conductivity, respectively. Nonvolatile resistance storage was traced within 40 days to quantify retention characteristics of the Nb2O5 memristor. The low-temperature anodic oxidation of Nb was found to be feasible to fabricate high-density cross-point memory with 3-D stack structures.

Anodic $\hbox{Nb}_{2}\hbox{O}_{5}$ Nonvolatile RRAM

The anodic oxidation behavior of niobium has been investigated under nearly industrial conditions. High porosity sintered bodies as well as flat niobium sheets were anodized between 20-100 V in I wt % aqueous solution of o-phosphoric acid at 65°C, applying current densities between 0.01 and 0.067 mA/cm 2 . During formation at constant current, changes in the rate of voltage increase has been observed. Stopping oxidation at such changes in the V(t) curve, the electrical properties, surface composition and in-depth homogeneity of the oxide was analyzed. The formed oxide layers have also been investigated after a heat-treatment in vacuum and in air. Based on the measured composition profiles, the role of the solid suboxide phases [Nb(O), NbO, NbO 2 ] in the oxide growth and in the annealing could be clarified.

Anodic Oxidation of Niobium Sheets and Porous Bodies Heat-Treatment of the Nb/Nb-Oxide System

The replacement of the anode material in tantalum capacitors by a new generation of high CV niobium powders offers the possibility to get an economical alternative to tantalum for a wide range of applications. Due to the high CV potential of niobium powder there is also an alternative to low voltage aluminum electrolytic capacitors. We developed a new niobium capacitor which shows stable electrical values. By optimizing the structure of the dielectric and the cathodic layers as well as the process parameters we gained a capacitor which can be used up to 105&#x2009;&#x00B0;C . Electrical characteristics and lifetest behavior of niobium capacitors out of 100 k–150 k CV/g powder will be discussed.

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W. Lohwasser

Papers

New Niobium Capacitors with Stable Electrical Parameters