Mechanism of formation and growth of sunflower-shaped imperfections in anodic oxide films on niobium

Anodizing of niobium has been investigated to develop niobium solid electrolytic capacitors. Chemically polished niobium specimens were anodized in a diluted phosphoric acid solution, initially galvanostatically at i_a = 4 A m⁻² up to E_a = 100 V, and then potentiostatically at E_a = 100 V for t_pa = 43.2 ks. During the galvanostatic anodizing, the anode potential increased almost linearly with time, while, during potentiostatic anodizing, the anodic current decreased up to t_pa = 3.6 ks, and then increased slowly before decreasing again after t_pa = 30.0 ks. Images of FE-SEM and in situ AFM showed that nuclei of imperfections were formed at the ridge of cell structures before t_pa = 3.6 ks. After formation, the imperfection nuclei grew, showing cracking and rolling-up of the anodic oxide film, and crystalline oxide was formed at the center of imperfections after t_pa = 3.6 ks. The growth of imperfections caused increases in the anodic current between t_pa = 3.6 and 30.0 ks. Long-term anodizing caused a coalescence of the imperfections, leading to decreases in the anodic current after t_pa = 30.0 ks. As the imperfections grew, the dielectric dispersion of the anodic oxide films became serious, showing a bias voltage dependence of the parallel equivalent capacitance, C_p, and a dielectric dissipation factor, tan δ. The mechanism of formation and growth of the imperfections, and the correlation between the structure and dielectric properties of anodic oxide films is discussed.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Influence of silicon species on the electric properties of anodic niobia

The influence of incorporation of silicon species on the electric properties of anodic niobia, formed in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, has been examined using sputter-deposited Nb-Si alloys containing 5 and 17 at.% silicon. The potential dependence of the capacitance of anodic niobia, originating from its n-type semiconducting properties, becomes less significant by incorporation of silicon species. In addition, the leakage current decreases with increasing silicon content in the alloy. The thermal stability of the anodic niobia is also enhanced by silicon species; the capacitance and leakage current, which increase significantly for niobium, are little influenced by annealing up to 523 K. The silicon species are incorporated in the inner 72% of the film thickness, as a consequence of immobility of the species in growing anodic niobia. The immobility of silicon species is associated with a strong Si⁴⁺O bond, which may also contribute to the reduction of leakage current.     

Growth of anodic oxide films on oxygen-containing niobium

The present study is directed at understanding of the influence of oxygen in the metal on anodic film growth on niobium, using sputter-deposited niobium containing from about 0-52 at.% oxygen, with anodizing carried out at high efficiency in phosphoric acid electrolyte. The findings reveal amorphous anodic niobia films, with no significant effect of oxygen on the field strength, transport numbers, mobility of impurity species and capacitance. However, since niobium is partially oxidized due to presence of oxygen in the substrate, less charge is required to form the films, hence reducing the time to reach a particular film thickness and anodizing voltage. Further, the relative thickness of film material formed at the metal/film interface is increased by the incorporation of oxygen species into the films from the substrate, with an associated altered depth of incorporation of phosphorus species into the films.     

A study of the chemical breakdown of the anodic oxide formed on (100) oriented silicon in tetramethylammonium hydroxide (TMAH) solutions

An investigation was carried out into the chemical breakdown of the anodic oxides grown on p- and n-doped (100) orientated silicon in tetramethylammonium hydroxide (TMAH) solutions. The anodic oxides were grown by sweeping the potential anodic of the passivation potential (PP) and into the passive region. The effect of various parameters on the chemical breakdown of the anodic oxide was studied. In particular, an extensive study on the influence of temperature, concentration and carrier type on the breakdown of the anodic oxide was carried out, with the effect of sweep rate studied to a lesser extent. The oxide breakdown time (OBT) was found to decrease with increasing temperature for all concentrations investigated. The OBT was found to increase with increasing concentration at lower temperatures with the concentration not having a major effect on the breakdown at higher temperatures. The OBT was consistently found to be greater for those anodic oxides grown on p-type (100) Si with the values for the two carrier types converging at higher temperatures. The OBT also decreased when grown at higher sweep rates. It is shown that a number of factors are involved in the explanation of the effect of temperature and concentration on the breakdown time of the anodic oxides.     

Repair of thin thermally grown silicon dioxide by anodic oxidation

Anodic oxidation in 0.1 M HCl followed by a post-rapid thermal annealing process has been used to repair defects existing in thin thermally grown oxide layers (3 and 6 nm) on a p-type silicon substrate. The improved quality of the insulator layer is particularly useful for applications that require large gate areas in Metal-Insulator-Semiconductor (MIS) or Electrolyte-Insulator-Semiconductor (EIS) devices such as Light-Addressable Potentiometric Sensors (LAPS) and Scanning Photo-induced Impedance Microscopy (SPIM). Different methods have been used to characterize the oxide. High-frequency capacitance-voltage curves and ac impedance spectra showed that there was no significant change of the oxide thickness after repair, and the number of the interface states of the oxide was increased for both types of samples. Ramped voltage stress (RVS) measurements of Metal-Oxide-Semiconductor (MOS) structures with gate electrodes 2 mm in diameter showed leakage currents of 0.75 nA cm⁻² for the repaired and annealed 3 nm thick oxide and 1.31 nA cm⁻² for the repaired and annealed 6 nm thick oxide at accumulation voltage. XPS measurements confirmed that there was no change of the oxide thickness and no contamination with other ionic species after repair. AFM results showed a good agreement with the other characterization methods.     

Surfactant-assisted growth of anodic nanoporous niobium oxide with a grained surface

Nanoporous niobium oxide film with a maximum thickness of 520 nm was prepared by anodizing niobium in a mixture of 1 wt% HF, 1 M H₃PO₄, and a small amount of Sodium Dodecyl Sulfate (SDS) surfactant. The porosity of the anodic niobium oxide prepared without SDS is irregular with the surface of the oxide suggesting a grained surface pattern rather than an ordered porous structure. A proper amount of SDS addition can prepare a pore arrangement with stripe patterns. The pore depth and surface pattern were strongly affected by the concentration of SDS and bath temperature. We found that the addition of SDS surfactant facilitated improvement in the chemical resistance of niobium oxide, leading to the formation of pores with a longer length compared to those prepared without a SDS surfactant. This can be in part ascribed to the protection of the surface by the physical adsorption of SDS on the surface due to a charge-charge interaction and be in part attributed to the formation of NbO bonding on the outermost oxide layer by SDS. When anodization was carried out for 4 h, the surface dissolution of niobium oxide was observed, which means that the maximum tolerance time against chemical dissolution was less than 4 h.     

Solid-state reactions of silicon carbide and chemical vapor deposited niobium

Niobium films were deposited on silicon carbide by chemical vapor deposition using niobium chloride and hydrogen at a temperature range of 900–1300°C. The solid-state reactions between the deposited niobium and silicon carbide matrix were studied by examining the obtained films using X-ray diffraction and energy dispersion spectroscopy. The results indicated that niobium silicides could be formed at the beginning, which blocked further reactions between carbon and niobium to form niobium carbides. When the deposition temperature was increased, silicon would diffuse outward, which allowed the formation of niobium carbides. The reaction process and mechanism are discussed based on the thermodynamics and kinetics.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

Characterization of anodic silicon oxide films grown in room temperature ionic liquids

The electroformation of silicon oxide was performed in two room temperature ionic liquids (RTIL), 1-butyl-3-methyl-imidazolium bis(trifluoromethane sulfonyl) imide (BMITFSI) and N-n-butyl-N-methylpiperidinium bis(trifluoromethane sulfonyl) imide (BMPTFSI). This phenomenon was studied by electrochemical techniques and it was observed that the oxide growth follows a high-field mechanism. X-ray Photoelectron Spectroscopy experiments have shown that a non-stoichiometric oxide film was formed, related to the low water content present in both RTILs (<30 ppm). The roughness values obtained by using AFM technique of the silicon surface after etching with HF was 1.5 nm (RMS). The electrochemical impedance spectroscopy at low frequencies range was interpreted as a resistance in parallel with a CPE element, the capacitance obtained was associated with the dielectric nature of the oxide formed and the resistance was interpreted considering the chemical dissolution of the oxide by the presence of the TFSI anion. The CPE element was associated with the surface roughness and the very thin oxide film obtained.     

Influence of silicon on the growth of barrier-type anodic films on titanium

Amorphous anodic titania, stabilised by incorporation of silicon species, is shown to grow to high voltages on sputter-deposited, single-phase Ti-Si alloys during anodizing at a constant current density in ammonium pentaborate electrolyte. The films comprise two main layers, with silicon species confined to the inner layers. An amorphous-to-crystalline transition occurs at ∼60 V on the Ti-6 at.% Si alloy, while the transition is suppressed to voltages above 140 V on alloys with 12 and 26 at.% silicon. The crystalline oxide, nucleated at a depth of ∼40% of the film thickness, is associated with the presence of a precursor of crystalline oxide in the pre-existing air-formed oxide. The modified structure of the air-formed oxide due to increased incorporation of silicon species suppresses the amorphous-to-crystalline transition until the onset of dielectric breakdown. The transport numbers of cations and anions during growth of the anodic oxides are independent of the concentration of silicon species in the inner layer, despite the marked change in the field strength.     

Porous niobium oxide films prepared by anodization in HF/H3PO4

Ordered porous niobium oxide with the diameter of less than 10 nm and the aspect ratio of more than 20 is prepared by anodization of niobium foils at 2.5 V in the mixture of 1 wt% HF and 1 M H₃PO₄ for 1 h. In this study, the effects of the mixed electrolytes, anodic potential and anodization time on the preparation of porous niobium oxide are described based on the current-time transients during anodization and morphological observations. It is founded that a single HF electrolyte leads to the formation of pores as well as the fast dissolution of formed pores at the surface. The dissolution of the formed oxide is significantly retarded by the addition of appropriate amount of H₃PO₄.     

Highly selective modification of silicon oxide structures fabricated by an AFM anodic oxidation

Anodic oxidation via atomic force microscopy is a promising method for creating submicron-sized silicon dioxide patterns on a local surface. The area patterned by AFM anodic oxidation (AAO) has different chemical properties from the non-patterned area, and thus site-selective modification of patterned surfaces is quite possible. In this study, we combined the AAO with self-assembly method and/or wet chemical etching method for the fabrication of positive and/or negative structures. These locally modified surfaces could be used to the site-selective arrangement and integration of various materials based on a pre-described pattern.      

Influence of substrate microstructure on the growth of anodic oxide layers

The effects of permanent mold cast microstructure on the growth of anodic oxide layers on three different aluminum substrates (i.e. Al99.8, AlSi10, and AlSi10Cu3, wt.%) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LSCM). The anodic oxidation was performed galvanostatically in 2.25 M H₂SO₄, at 0 °C. The oxide layers developed a microscale topography mainly determined by the morphology of aluminum grains and cells. A low amount of insoluble impurities, uniformly distributed, would contribute to the growth of oxide layers with minimum defects and uniform thickness on the pure aluminum substrate whereas for the binary and ternary systems, a fine cell structure and a modified morphology of Si particles would be favorable. The Al-Fe and Al-Fe-Si particles were occluded in the oxide layers next to Si particles, blocking locally the oxide growth whereas Al₂Cu particles were preferentially oxidized. In addition, the presence of Si particles in the layer influenced pore morphology by development of deflected pores around the particles.     

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

The photoelectrochemical polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, was successfully realized on anodic film grown to 50 V on magnetron sputtered niobium. Photocurrent Spectroscopy was employed to study the optical properties of Nb/Nb₂O₅/PEDOT/electrolyte interface in a large range of potential, and to get an estimate of the band gap and flat band potential of both the oxide and the polymer. Scanning Electron Microscopy was used to study the morphology of PEDOT. Both the optical and morphological features of the photoelectrochemically grown polymer were compared with those showed by PEDOT electropolymerized on gold conducting substrate.     

Complex dielectric functions of anodic bi-layer tantalum oxide

The optical properties and structure of anodic oxides are dependent on the anodization conditions. For tantalum oxide formed in dilute phosphoric acid, the anodic oxide forms as two chemically distinct layers, referred to as a bi-layer, where the inner layer is pure tantalum oxide and the outer layer contains incorporated phosphate. The complex dielectric functions of the individual inner and outer layers are determined using spectroscopic ellipsometry. The dielectric functions of the bi-layers are compared to mono-layer oxides formed in sodium sulfate, and the effects of hydrothermal sealing are explored. For bi-layer oxides formed to 70 V in phosphoric acid, the inner layer band gap is 4.37 ± 0.02 eV and the outer layer band gap is 3.86 ± 0.06 eV. Thin anodic oxides (∼6-15 nm) are best described by a mono-layer oxide model and exhibit higher optical absorption with a band gap of 3.98 ± 0.08 eV. This study shows that spectroscopic ellipsometry is a valuable tool in assessing processing-property relationships of multi-layer anodic films.     

Micro-EIS of anodic thin oxide films on titanium for capacitor applications

The present work focuses on the investigation of the effect of the different crystallographic orientation of titanium grains on the formation of anodic oxide films and consequently their dielectric and semiconductive properties. By using a microcapillary cell the formation process and the electrochemical impedance spectroscopy (EIS) can be performed at high lateral resolution on variously orientated single grains of polycrystalline titanium. The oxide films were potentiodynamically formed by cyclovoltammetry. EIS measurements immediately followed by the oxide formation were used for a detailed investigation of the film properties, in particular, the relative permittivity ?_r and the donor concentration N_D. In contrast to the most publications it was found that under the chosen conditions the crystallographic orientation of titanium substrate has no significant influence on the oxide thickness d, the relative permittivity ?_r or on the donor concentration N_D of the oxide films. The relative permittivity ?_r is approximately 50. The donor concentration depends on the film thickness and amounts to approximately 3 × 10¹⁸ cm⁻³ in minimum.     

Preparation and characterization of niobium oxide for the catalytic aldol condensation of acetone

Niobium oxide (Nb₂O₅) has been prepared from a precursor solution of NbCl₅ in ethanol. The effect of pH, water content for hydrolysis and calcination temperature on microstructural, spectroscopic and acidic properties has been studied using N₂ adsorption, X-ray diffraction (XRD), FTIR spectroscopy, temperature programmed desorption (TPD) of ammonia and DRIFTS of adsorbed ammonia. The activity for the aldol condensation of acetone was studied as well.     

Morphological and structural studies of nanoporous nickel oxide films fabricated by anodic electrochemical deposition techniques

Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate, nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved Ni²⁺ to Ni³⁺, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small pores is obtained by CV deposition in a potential range of 0.7-1.1 V. A galvanostatically deposited film is structurally denser near the surface of the substrate, and becomes less dense further away from the surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration, potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited films resemble closely to cubic NiO when the annealing temperature exceeds 200 °C.     

A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys

Transmission electron microscopy (TEM) has been employed to examine anodic oxide film formation on 99.8 wt.% aluminium, Al-10 wt.%Si and Al-10 wt.%Si-3 wt.%Cu alloys under conditions relevant to hard anodizing. In particular, anodic oxidation of silicon particles proceeded at a significantly reduced rate compared with that of the adjacent aluminium matrix. This gave rise to alumina film encroachment beneath the particles with development of tortuous porosity and, eventually, occlusion of partially anodized particle in the anodic film. Additional effects included the presence of gas-filled cavities above the silicon particles, associated with oxygen generation above the anodizing particle. The presence of such particles and the corresponding gas-filled voids across the anodic film thickness and at the alloy/film interface is considered responsible for the continuous voltage rise during anodizing of the Al-10 wt.%Si alloy, effectively blocking electrolyte access to the pore base and providing local region of high resistance at the alloy/film interface. A direct consequence of the voltage rise was a thickening of the barrier layer at the base of the porous anodic film. For the ternary alloy, with the additional presence of copper and the CuAl₂ particles, the latter appear to have undergone complete oxidation, with copper detected in local film regions.