Physicochemical Characterization of Thermally Aged Anodic Films on Magnetron-Sputtered Niobium

Abstract

The influence of thermal aging, at intermediate temperature ͑1 h at 250°C͒ and in different environments, on the electronic and solid-state properties of stabilized 160 nm thick amorphous anodic niobia, grown on magnetron-sputtered niobium metal, has been studied. A detailed physicochemical characterization of the a-Nb 2 O 5 /0.5 M H 2 SO 4 electrolyte junction has been carried out by means of photocurrent and electrochemical impedance spectroscopy as well as by differential admittance ͑DA͒ measurements. A change in the optical bandgap ͑3.45 eV͒ of niobia film has been observed after aging ͑3.30 eV͒ at 250°C in air for 1 h. A cathodic shift ͑0.15-0.2 V͒ in the flatband potential of the junction has been observed. The frequency dependence of DA data agrees with expectations of the theory of amorphous semiconductor Schottky barrier. The fitting of both components of DA allowed to get information on the distribution of the electronic density of states as a function of energy and distance from the metal oxide interface. The DA measurements evidenced for vacuum-treated niobia film an insulating to semiconductor transition. These findings can help to explain the large changes in the measured values of capacitance, after aging, and the larger leakage current observed in niobia electrolytic capacitors.

Light induced electropolymerization of poly(3,4-ethylenedioxythiophene) on niobium oxide

“…(3), a non-direct band gap value of 3.42 eV (see Fig. 6b) can be estimated, which is very close to that previously estimated in aqueous solution for films grown on Niobium in the same conditions [10]. The location of characteristic energy levels of the junction (flat band potential, E fb , conduction and valence band edges) was carried out by assuming the zero photocurrent potential, E*, derived from the photocurrent vs potential plot at 320 nm (Fig.…”

“…If we assume that the organic electrolyte is equivalent to an aqueous solution at the niobium oxide isoelectric point (pH pzc ), this value compares well with the E fb reported in Ref. [10] estimated in strongly acidic electrolyte, according to the following equation:…”

“…The deposited niobium had a body-centred cubic structure with a 110 preferred orientation. These films were anodized at a constant current density of 50 A m −2 in 0.1 mol dm −3 ammonium pentaborate at 293 K, the potential being kept constant for 1 h at the final cell voltage of 50 V [9,10]. The polymerization was performed in 0.1 M LiClO 4 propylene carbonate (PC, 99.8 Sigma-Aldrich) with 0.04 M 3,4-ethylenedioxythiophene (EDOT) concentration under irradiation ( = 310 nm).…”

“…From such estimate, after location of the conduction band mobility edge of oxide around 0.37 eV above E fb [10], the energy level of the valence band edge of anodic Nb 2 O 5 was located at around 2.55 eV below the reference electrode, i.e. at about 0.75 eV below the oxidation potential of propylene carbonate (E ox = 1.8 V vs Ag/AgCl, according to Ref.…”

Assessment on the use of the amorphous semiconductor theory for the analysis of oxide films

“…Anodizing is a low-temperature, low-cost fabrication process which allows growing oxides of tuneable composition and properties on the surface of valve metals and valve metals'alloys [1][2][3][4].…”