Dielectric properties of anodic films on sputter-deposited Ti-Si porous columnar films

For electrolytic capacitor application of the single-phase Ti alloys containing supersaturated silicon, which form anodic oxide films with superior dielectric properties, porous Ti-7 at% Si columnar films, as well as Ti columnar films, have been prepared by oblique angle magnetron sputtering on to aluminum substrate with a concave cell structure to enhance the surface area and hence capacitance. The deposited films of both Ti and Ti-7 at% Si have isolated columnar morphology with each column revealing nanogranular texture. The distances between columns are ∼500 nm, corresponding to the cell size of the textured substrate and the gaps between columns are 100-200 nm. When the porous Ti-7 at% Si film is anodized at a constant current density in ammonium pentaborate electrolyte, the growth of a uniform amorphous oxide film continues to ∼35 V, while it is limited to less than 6 V on the porous Ti film. The maximum voltage of the growth of uniform amorphous oxide films on the Ti-7 at% Si films is similar for both the flat and porous columnar films, suggesting little influence of surface roughness on the amorphous-to-crystalline transition of growing anodic oxide under the high electric field. Due to the suppression of crystallization to sufficiently high voltages, the anodic oxide films formed on the porous Ti-7 at% Si film shows markedly improved dielectric properties, in comparison with those on the porous Ti film.     

Superhydrophobic hierarchical surfaces fabricated by anodizing of oblique angle deposited Al-Nb alloy columnar films

A combined process of oblique angle magnetron sputtering and anodizing has been developed to tailor superhydrophobic surfaces with hierarchical morphology. Isolated submicron columns of single-phase Al-Nb alloys are deposited by magnetron sputtering at several oblique deposition angles on a scalloped substrate surface, with the gaps between columns increasing with an increase in the deposition angle from 70° to 110°. Then, the columnar films have been anodized in hot phosphate-glycerol electrolyte to form a nanoporous anodic oxide layer on each column. Such surfaces with submicron-/nano-porous structure have been coated with a fluoroalkyl phosphate layer to reduce the surface energy. The porous surface before coating is superhydrophilic with a contact angle for water is less than 10°, while after coating the contact angles are larger than 150°, being superhydrophobic. The beneficial effect of dual-scale porosity to enhance the water repellency is found from the comparison of the contact angles of the submicron columnar films with and without nanoporous oxide layers. The larger submicron gaps between columns are also preferable to increase the water repellency.     

Surface phenomena of HA/TiN coatings on the nanotubular-structured beta Ti-29Nb-5Zr alloy for biomaterials

Surface phenomena of HA/TiN coatings on the nanotubular-structured beta Ti-29Nb-5Zr alloy for biomaterials have been investigated by several experimental methods. The nanotubular structure was formed by anodizing the Ti-29Nb-5Zr alloy in 1 M H₃PO₄ electrolytes with 1.0 wt.% NaF at room temperature. Hydroxyapatite (HA)/titanium nitride (TiN) films were deposited on Ti-29Nb-5Zr alloy specimens using a magnetron sputtering system. The HA target was made of human tooth-ash by sintering at 1300 °C for 1 h, and the HA target had an average Ca/P ratio of 1.9. The HA/TiN depositions were performed, using the pure HA target, on Ti-29Nb-5Zr alloy following the initial deposition of a TiN buffer layer coating. Microstructures and nanotubular morphology of the coated alloy specimens were examined by FE-SEM, EDX, XRD, and XPS. The Ti-29Nb-5Zr alloy substrate had small grain size and preferred orientation along the drawing direction. The HA/TiN coating was stable with a uniform morphology at the tips of the nanotubes.     

Modification of erbium photoluminescence excitation spectra for the emission wavelength 1.54 μm in mesoscopic structures

Photoluminescence excitation (PLE) spectra for the emission wavelength 1.54 μm were studied for erbium-doped xerogels embedded in artificial opals and porous anodic alumina films. Opals were chosen with photonic stop-band in green spectral range, where excitation of 1.54 μm occurs most efficiently. In comparison to the structure erbium-doped titania xerogel/porous anodic alumina/silicon the photoluminescence excitation spectra for 1.54 μm emission wavelength significantly changes for the same xerogels embedded in artificial opals. Enhancement of erbium-related 1.54 μm emission was observed from the structure Fe₂O₃ xerogel/porous anodic alumina fabricated on silicon, having some incompletely anodized aluminium, under excitation with either the lasing source at 532 nm or xenon lamp. Evident difference in PLE spectra for erbium doped TiO₂ and Fe₂O₃ xerogels in porous anodic alumina is observed.     

Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition

The antimony doped tin oxide (SnO₂:Sb) (ATO) thin films were prepared by oblique angle electron beam evaporation technique. X-ray diffraction, field emission scanning electron microscopy, UV-vis-NIR spectrophotometer and four-point probe resistor were employed to characterize the structure, morphology, optical and electrical properties. The results show that oblique angle deposition ATO thin films with tilted columns structure are anisotropic. The in-plane birefringence of optical anisotropy is up to 0.035 at α = 70°, which means that it is suitable as wave plate and polarizer. The electrical anisotropy of sheet resistance shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane and it can be changed from 900 Ω/□ to 3500 Ω/□ for deposition angle from 40° to 85°, which means that the sheet resistance can be effectively tuned by changing the deposition angle. Additionally, the sandwich structure of SiO₂ buffer layer plus normal ATO films and oblique angle deposition ATO films can reduce the resistance, which can balance the optical and electrical anisotropy. It is suggested that oblique angle deposition ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display.     

Fabrication of self-ordered nanohole arrays on Si by localized anodization and subsequent chemical etching

Nanohole arrays with a 60 nm hole periodicity were fabricated on a Si substrate by the anodization of an aluminum film sputtered on a Si substrate in sulfuric acid and subsequent chemical etching. The transfer of the nanoporous pattern of anodic alumina into the Si substrate was achieved by the selective removal of silicon oxide, which was produced by the anodic oxidation of the underlying Si substrate through the anodic porous alumina used as a mask.     

Magnetic properties of Fe0.95Pd0.05 nanowire arrays

Fe_0.95Pd_0.05 nanowires were fabricated by the electrodeposition in porous anodic aluminum oxide templates and post-annealed at 300–700 °C. Transmission electron microscopy observations demonstrated the isolated nanowires to have polycrystalline structure. Magnetic measurements, however, showed improvement of both coercivity and squareness with the addition of 5 at% Pd in the Fe nanowires as well as proper annealing temperatures of about 500 °C.     

Electrodeposited ruthenium oxide thin films for supercapacitor: Effect of surface treatments

Amorphous and porous ruthenium oxide thin films have been deposited from aqueous Ru(III)Cl₃ solution on stainless steel substrates using electrodeposition method. Cyclic voltammetry study of a film showed a maximum specific capacitance of 650 F g⁻¹ in 0.5 M H₂SO₄ electrolyte. The surface treatments such as air annealing, anodization and ultrasonic weltering affected surface morphology. The supercapacitance of ruthenium oxide electrode is found to be dependent on the surface morphology.     

Multiple UV-blue luminescence emissions in electrochemical anodic etched n-type silicon wafer

This very paper is focusing on the preparation of porous nanostructures in n-type silicon (1 1 1) wafer by chemical etching technique in alkaline aqueous solutions of 5 M NaOH, 5 M K₂CO₃ and 5 M K₃PO₄, and particularly, on its ultraviolet-blue photoluminescence emission. The anodic chemical etched silicon wafer has been characterized by means of optical microscopy, scanning electron microscopy, fluorescence spectroscopy, atomic force microscopy and Fourier transform infrared spectroscopy. This very surface morphology characterization has been clearly shown - the effect of anodic-chemical-etching procedure processed in K₂CO₃ or K₃PO₄ was much vigorous than that processed in NaOH. The FTIR spectra indicate that the silicon oxide was formed on the surface of electrochemical etched n-Si (1 1 1) wafers, yet not on that of the pure chemical etched ones anyhow. And an intense ultraviolet-blue photoluminescence emission is observed, which then differs well from the silicon specimen etched in alkaline solution with no anodic potential applied. The proper photoluminescence mechanism is discussed, and hence there may be a belief that the intense ultraviolet-blue photoluminescence emission would be attributed to the silicon oxide coating formed on silicon wafer in anodic-chemical-etching process.     

Effect of electrolyte temperature on the formation of self-organized anodic niobium oxide microcones in hot phosphate-glycerol electrolyte

Nanoporous niobium oxide films with microcone-type surface morphology were formed by anodizing at 10 V in glycerol electrolyte containing 0.6 mol dm⁻³ K₂HPO₄ and 0.2 mol dm⁻³ K₃PO₄ in a temperature range of 428-453 K. The microcones appeared after prolonged anodizing, but the required time was largely reduced by increasing electrolyte temperature. The anodic oxide was initially amorphous at all temperatures, but crystalline oxide nucleated during anodizing. The anodic oxide microcones, which were crystalline, appeared on surface as a consequence of preferential chemical dissolution of initially formed amorphous oxide. The chemical dissolution of an initially formed amorphous layer was accelerated by increasing the electrolyte temperature, with negligible influence of the temperature on the morphology of microcones up to 448 K.     

Deposition of aluminide and silicide based protective coatings on niobium

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl₃ in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi₂ in the inner layer and a ternary compound of Nb-Si-Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi₂ exhibits much stronger covalent character as compared to NbAl₃. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226 mJ/m² and 3545 mJ/m², respectively, indicating stronger Nb-Si bonding across the interface.     

Surface structures and osteoblast response of hydrothermally produced CaTiO3 thin film on Ti-13Nb-13Zr alloy

This study investigated the surface characteristics and in vitro biocompatibility of a titanium (Ti) oxide layer incorporating calcium ions (Ca) obtained by hydrothermal treatment with or without post heat-treatment in the Ti-13Nb-13Zr alloy. The surface characteristics were evaluated by scanning electron microscopy, thin-film X-ray diffractometry, X-ray photoelectron spectroscopy, atomic force microscopy and contact angle measurements. In vitro biocompatibility of the Ca-containing surfaces was assessed in comparison with untreated surfaces using a pre-osteoblast cell line. Hydrothermal treatment produced a crystalline CaTiO₃ layer. Post heat-treatment at 400 °C for 2 h in air significantly decreased water contact angles in the CaTiO₃ layer (p < 0.001). The Ca-incorporated alloy surfaces displayed markedly increased cell viability and ALP activity compared with untreated surfaces (p < 0.001), and also an upregulated expression of various integrin genes (α1, α2, α5, αv, β1 and β3) at an early incubation time-point. Post heat-treatment further increased attachment and ALP activity in cells grown on Ca-incorporated Ti-13Nb-13Zr alloy surfaces. The results indicate that the Ca-incorporated oxide layer produced by hydrothermal treatment and a simple post heat-treatment may be effective in improving bone healing in Ti-13Nb-13Zr alloy implants by enhancing the viability and differentiation of osteoblastic cells.     

Effects of magnesium film thickness and annealing temperature on formation of Mg2Si films on silicon (1 1 1) substrate deposited by magnetron sputtering

Magnesium films of various thicknesses were first deposited on silicon (1 1 1) substrates by magnetron sputtering method and then annealed in annealing furnace filled with argon gas. The effects of the magnesium film thickness and the annealing temperature on the formation of Mg₂Si films were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Mg₂Si thin films thus obtained were found to be polycrystalline and the Mg₂Si (2 2 0) orientation is preferred regardless of the magnesium film thickness and annealing temperature. XRD results indicate that high quality magnesium silicide films are produced if the magnesium/silicon samples are annealed at 400 °C for 5 h. Otherwise, the synthesized films annealed at annealing temperatures lower than 350 °C or higher than 450 °C contain magnesium crystallites or magnesium oxide. SEM images have revealed that microstructure grains in the polycrystalline films are about 1-5 μm in dimensions, and the texture of the Mg₂Si films becomes denser and more homogeneous as the thickness of the magnesium film increases.     

Preferred orientations in nano-gold/silica/silicon interfaces

Gold in contact with silicon substrates Si(1 0 0), Si(1 1 1), and SiO₂ is studied by thermal evaporation and annealing in N₂ using the modified sphere-plate technique. The final orientation distribution of crystalline Au films grown on Si substrate systems that incorporate a native amorphous oxide layer of silica and Au on amorphous silica (SiO₂ glass) substrates is influenced by preferred orientations and twinning. Experimental evidence suggests that the orientation of Au{1 1 1} close packed planes (multiply twinned) was found to be of low-energy as the annealing temperature was increased to 530 °C and 920 °C. Additional orientations were observed for Au{1 0 0} on Si(1 0 0) substrates and Au{1 0 0}, {1 1 0}, and {3 1 1} on SiO₂ substrates. After annealing at 920 °C the size distribution of the gold particles was determined to be within the range of 20-800 nm while the morphology of gold surface appears spherical to faceted in character. These results show similarities to recent findings for smaller nano-size 1D particles, islands and thin Au films on silicon annealed over lower temperature ranges.     

Crystallization and surface morphology evolution of erbium fluoride films on different substrates

Erbium fluoride (ErF₃) films were thermally deposited on Ge(1 1 1), Si(0 0 1) and copper mesh grid with different substrate temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the structure and morphology of the films. The structure of ErF₃ films deposited on germanium and silicon changed from amorphous to crystalline with increasing the substrate temperature, while the crystallization temperature of the films on silicon is higher than that of on germanium. The infrared optical properties of the films change greatly with the evolution of crystal structure. It is also found that the morphology of ErF₃ film on Ge(1 1 1) at 200 °C is modulated by the stress between the substrate and film. The SEM and TEM results confirmed that the ErF₃ films on copper mesh grid were crystalline even at 100 °C. Interestingly, the ErF₃ films show flower-like surface morphology when deposited on copper mesh at 200 °C. The crystallization temperature (T_c) of ErF₃ films on the three substrates has the relation which is which is induced by the wetting angle of ErF₃ films on different substrates.     

Fabrication of nanostructures on Si(1 0 0) and GaAs(1 0 0) by local anodic oxidation

Atomic force microscopes have become useful tools not only for observing surface morphology and nanostructure topography but also for fabrication of various nanostructures itself. In this paper, the application of AFM for fabrication of nanostructures by local anodic oxidation (LAO) of Si(1 0 0) and GaAs(1 0 0) surfaces is presented. A special attention is paid to finding relations between the size of oxide nanolines (height and half-width) and operational parameters as tip-sample voltage and tip writing speed. It was demonstrated that the formation of silicon oxide lines obeys the Cabrera-Mott theory, i.e. the height of the lines grow, linearly with tip-sample voltage and is inversely proportional to logarithm of tip writing speed. As for GaAs substrates, the oxide line height grows linearly with tip-sample voltage as well but LAO exhibits a certain deviation from this theory. It is shown that the selective chemical etching of Si or GaAs ultrathin films processed by LAO makes it possible to use these films as nanolithographic masks for further nanotechnologies, e.g. fabrication of metallic nanostructures by ion-beam bombardment. The ability to control LAO and tip motion can be utilized in fabrication of complex nanostructures finding their applications in nanoelectronic devices, nanophotonics and other high-tech areas.     

Characterisation of the anodic layers formed on 2024 aluminium alloy, in tetraborate electrolyte containing molybdate ions

Anodic layer growth on 2024 aluminium alloy at 70 °C, under 40 V, during 60 min, in 50 g L⁻¹ di-sodium tetraborate solution containing di-sodium molybdate from 0.1 to 0.5 M (pH 10) is examined. Anodising behaviours strongly depend on additive concentration. Development of anodic films is favoured with weak molybdate additions (<0.3-0.4 M). The film thicknesses increase and the porosity of anodic layers decreases. Molybdenum (+VI), detected by X-ray photoelectron spectroscopy (XPS) analysis, is present in the anodic films and the Mo incorporation, studied by energy dispersive spectroscopy (EDS) analysis, increases with molybdate concentration. However, for high molybdate concentrations (>0.4 M), anodising behaviour becomes complex with the formation of a blue molybdenum oxide at the cathode. The growth of aluminium oxide is hindered. As the anodic layers are thinner, the Mo(+VI) incorporation significantly decreases. These two configurations implicate different corrosion performances in 5% sodium chloride solution at 35 °C. As the alkaline anodic layer formed with 0.3 M molybdate species is the thickest and the Mo incorporation is the more pronounced, its corrosion resistance is the highest. The effect of morphology and composition of anodic films on pitting corrosion is also discussed.     

Electronic properties of electrolyte/anodic alumina junction during porous anodizing

The growth of porous oxide films on aluminum (99.99% purity), formed in 4% phosphoric acid was studied as a function of the anodizing voltage (23-53 V) using a re-anodizing technique and transmission electron microscopy (TEM) study. The chemical dissolution behavior of freshly anodized and annealed at 200 °C porous alumina films was studied. The obtained results indicate that porous alumina has n-type semiconductive behavior during anodizing in 4% phosphoric acid. During anodising, up to 39 V in the barrier layer of porous films, one obtains an accumulation layer (the thickness does not exceed 1 nm) where the excess electrons have been injected into the solid producing a downward bending of the conductive and valence band towards the interface. The charge on the surface of anodic oxide is negative and decreases with growing anodizing voltage. At the anodizing voltage of about 39 V, the charge on the surface of anodic oxide equals to zero. Above 39 V, anodic alumina/electrolyte junction injects protons from the electrolyte. These immobile positive charges in the surface layer of oxide together with an ionic layer of hydroxyl ions concentrated near the interface create a field, which produces an upward bending of the bands.     

Synthesis, structure, microstructure and mechanical characteristics of MOCVD deposited zirconia films

Zirconia (ZrO₂) thin films were deposited by metal organic chemical vapor deposition (MOCVD) on (1 0 0) Si over temperature and pressure ranges from 700 to 900 °C and 100 to 2000 Pa, respectively. The oxide films were characterized by field emission microscopy and X-ray diffraction so that microstructure and ratios of monoclinic and tetragonal phases could be estimated according to the process conditions. The mechanical behaviour of the substrate-film systems was investigated using Vickers micro-indentation and Berkovitch nano-indentation tests. The characteristics of silicon are not modified by the presence of a thin film of silicon oxide (10 nm), formed in the reactor during heating. Young's modulus and the hardness of tetragonal zirconia phase, 220 and 15 GPa, respectively, are greater than values obtained for monoclinic phase, 160 and 7 GPa, respectively. The zirconia films are well adherent and the toughness of tetragonal zirconia phase is greater than that of monoclinic phase.     

Gadolinium oxide high-k gate dielectrics prepared by anodic oxidation

The growth and properties of gadolinium oxide (Gd₂O₃) films prepared by anodic oxidation were investigated. Uniform Gd₂O₃ thin film with good oxide quality was obtained. The X-ray diffraction (XRD) pattern of the Gd₂O₃ films showed that they had a poly-crystalline structure. The dielectric constants of Gd₂O₃ films oxidized at 30 and 60 V are 9.4 and 12.2, respectively. The equivalent oxide thickness (EOT) of the Gd₂O₃ stacked oxide is in the range of 5.8-9.4 nm. The MOS capacitor with Gd₂O₃ exhibits interesting electrical properties. Longer oxidation time reduced the leakage current density for 30 V anodic oxidation but increased the leakage current density for 60 V anodic oxidation. This work reveals that Gd₂O₃ could also be an alternative dielectric for Si substrate and therefore, might pave the way to fabricate CMOS devices in the future.