Amorphous Titanium Oxide Films Research Articles

Impact of amorphous titanium oxide film on the device stability of Al/TiO2/Al resistive memory

We have investigated the role of amorphous titanium oxide film in the reliable bipolar resistive switching of Al/TiO2/Al resistive random access memory devices. As TiO2 deposition temperature decreased, a more stable endurance characteristic was obtained. We proposed that the degradation of the bipolar resistive switching property of Al/TiO2/Al devices is closely related to the imperfect migration of oxygen ions between the top insulating interface layer and the oxygen-deficient titanium oxide during the set and reset operations. In addition, the dependence of the TiO2 film thickness on the switching property was also studied. As the thickness of the film increased, a reduction in the resistance of the high resistance state rapidly appeared. We attribute the improved endurance performance of thin and low-temperature grown TiO2 devices to the amorphous state with a low film density.

Nanoporous titania films produced by pulsed interference lithography

We describe a simple, inexpensive technique for producing deep nanopores on the surface of titania films using laser exposure in a four-beam interference configuration. In addition to producing nanopores, laser pulses convert amorphous titania films to a polycrystalline state. The effect of laser exposure on the TiO2 surface can be used to improve its biophotocatalytic properties, optimise solar cells, etc.

Determination of photocatalytic activity in amorphous and crystalline titanium oxide films prepared using plasma-enhanced chemical vapor deposition

Hydro-oxygenated amorphous titanium oxide (a-TiO x :OH) films were prepared by plasma-enhanced chemical vapor deposition (PECVD) using precursors of titanium tetraisopropoxide (TTIP) and oxygen. The influences of chemical states and crystal quality on the photocatalytic activity were systematically investigated in the as-deposited and post-annealed films. The degree of the photocatalytic activity was deeply correlated with the porosity related to the hydroxyl (OH) groups in the as-deposited amorphous film. The crystallized anatase structures was observed from the 200 °C-deposited a-TiO x :OH film after a post-annealing treatment at 400 °C. The photocatalytic activity related to the film with anatase structure was markedly superior to that of an amorphous film with porous structures. The larger the crystal size of the anatase structure, the higher the photocatalytic activity obtained. At elevated annealed temperatures, the inferior anatase structure due to the crystalline transformation led to a low photocatalytic activity. It was concluded that the photocatalytic activity of an amorphous TiO x film prepared using PECVD was determined by the porosity originating from the functional OH groups in the film, whereas the crystalline quality of anatase phase in the annealed poly-TiO x film was crucial to the photocatalytic activity.

Direct observation of microscopic change induced by oxygen vacancy drift in amorphous TiO2 thin films

To clarify the resistive switching and failure mechanisms in Al/amorphous TiO2/Al devices we investigate the microscopic change in amorphous titanium oxide films and interface layers after the set process according to film deposition temperatures. For low temperature (<150 °C) samples, the thickness of top interface layer decreased after the set process due to the dissociation of a top interface layer by uniform migration of oxygen vacancies. Meanwhile, for high temperature samples, crystalline TiO phases emerged in the failed state, meaning the formation of conducting paths from the local clustering of oxygen vacancies in nonhomogeneous titanium oxide film.

Optimized‐geometry ARROW waveguides using TiO2 as anti‐resonant layer

AbstractThe simulation, fabrication and characterization of ARROW waveguides using dielectric films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and Sputtering techniques, are presented in this work. Amorphous titanium oxide (TiO2) films were used as first cladding layer and silicon oxynitride (SiOxNy) films, as core layer. Furthermore, homemade routines based in two computational methods were used, for numerical simulations: Transfer Matrix Method (TMM) for the determination of the optimum thickness values of the Fabry‐Perot layers, and the Finite Difference Method (FDM) for 2D design and determination of the maximum width that allows single‐mode operation. The utilization of thermally grown silicon oxide as second anti‐resonant layer, along with improvements in the Reactive Ion Etching conditions for the definition of sidewalls of the optical waveguides were responsible for diminishing optical attenuations. Optimization of the waveguide rib height was done both through FDM simulations and experimentally. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of carbon and oxygen on phase transformation of titania films during annealing

Pure and carbon-doped amorphous titania films were prepared in an argon–oxygen plasma by reactive sputtering of a titanium metal target and co-sputtering of a Ti and a graphite target, respectively. The as-deposited films were subsequently annealed in vacuum and in air. The influence of the annealing atmosphere and temperature on the composition, phase transformation and crystallinity of the films was systematically studied. The results indicate that carbon incorporated in titania and an oxygen deficient environment both facilitate the formation of oxygen vacancies in titania which favors the phase transformation to rutile during annealing at low temperatures. A mechanism is proposed to explain why carbon incorporation and oxygen defects in titania favors the formation of rutile during annealing.

Light-extraction enhancement in GaN-based light-emitting diodes using grade-refractive-index amorphous titanium oxide films with porous structures

Amorphous titanium oxide (a-TiOx:OH) films prepared by plasma-enhanced chemical-vapor deposition at 200 and 25 °C are in turn deposited onto the GaN-based light-emitting diode (LED) to enhance the associated light extraction efficiency. The refractive index, porosity, and photocatalytic effect of the deposited films are correlated strongly with the deposition temperatures. The efficiency is enhanced by a factor of ∼1.31 over that of the uncoated LEDs and exhibited an excellent photocatalytic property after an external UV light irradiation. The increase in the light extraction is related to the reduction in the Fresnel transmission loss and the enhancement of the light scattering into the escape cone by using the graded-refractive-index a-TiOx:OH film with porous structures.

Biocompatibility and bio-corrosion resistance of amorphous oxide thin films

ABSTRACTThe corrosion resistance of biocompatible materials in body fluids is one of the essential factors in the determination of the lifetime of medical implants. Therefore, it is of great relevance to understand the interface processes that occur when a surface is exposed to body fluids. To this end, amorphous titanium and niobium oxide films were deposited on medical grade stainless steel using a magnetron sputtering system. The biocompatibility of the films was evaluated by adhesion and viability/proliferation assays using human cells, showing non-toxic response. The electrochemical response of the films was evaluated by poteontiodynamic polarization and electrochemical impedance spectroscopy (EIS) as a function of time, up to 500 hrs, using three different simulated body fluids; the NaCl solution and Hartman (Ringer's + Lactate) and Gey's (Ringer's + Phosphates + Glucose) solution. The results indicated that the chemical composition of the solution was very important since different electrochemical behavior was observed for each case. For example, NbOxshowed a better resistance than the TiOxfilms in the Hartman's solution but it failed when Gey's solution was used. Meanwhile TiOxshowed a well passivated response for both NaCl and Gey's solution.

Formation of ultraviolet laser-annealed mesoporous anatase films by a sol–gel process

We report a novel technique for generation of nanocrystalline mesoporous anatase-based titania films by laser annealing through a CeO 2 modified sol–gel system. Amorphous titania films deposited on Pt(Si) substrates were subjected to a KrF excimer laser beam (248 nm, 13 ns) for crystallization. Anatase as a dominant phase was achieved at a laser fluence of 30–40 mJ cm −2 while higher fluence favored rutile phase. Raman spectroscopy , field emission gun scanning electron microscopy and atomic force microscopy were used to characterize the crystalline structure and morphology of the CeO 2 -doped titania films.

Amorphous to anatase transformation in atomic layer deposited titania thin films induced by hydrothermal treatment at 120 °C

Hydrothermal treatment has been applied successfully to convert amorphous titania films to crystalline anatase at 120 °C, a temperature compatible with most polymeric substrates. The amorphous films were deposited at 80 °C using atomic layer deposition (ALD). The crystallinity of the films was monitored by x-ray absorption near edge structure (XANES), and the film composition was determined by x-ray photoelectron spectroscopy (XPS). The effect of precursor chemistry and substrate material was investigated. It was found that titania films produced from Ti isopropoxide are easier to crystallize than those from Ti tetrachloride as the Ti precursor. The amorphous to crystalline transformation can be achieved more readily with films deposited on Si than polycarbonate substrates. The effect of a “seed” layer on the amorphous to crystalline transformation was also studied. Preformed anatase crystallites between the Si substrate and the amorphous film were shown to accelerate the crystallization process. The possible mechanisms responsible for the phase transformation are discussed.

Tio2 Anti-Resonant Layer Arrow Waveguides

The simulation, fabrication and characterization of ARROW waveguides using dielectric films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and Sputtering techniques, are presented in this work. Amorphous titanium oxide (TiO2) films were used as first cladding layer and silicon oxynitride (SiOxNy) films, as core and second cladding layers. Furthermore, homemade routines based in two computational methods were developed, for numerical simulations: Transfer Matrix Method (TMM) for the determination of the optimum thickness values of the Fabry-Perot layers, and the Finite Difference Method (FDM) for 2D design and determination of the maximum width that allows single-mode operation. The utilization of titanium oxide as first anti-resonant layer resulted in low optical attenuations, since high interlayer refractive index differences can be obtained.

Preparation of bioactive titania films on titanium metal via anodic oxidation

Objectives To research the crystal structure and surface morphology of anodic films on titanium metal in different electrolytes under various electrochemical conditions and investigate the effect of the crystal structure of the oxide films on apatite-forming ability in simulated body fluid (SBF). Methods Titanium oxide films were prepared using an anodic oxidation method on the surface of titanium metal in four different electrolytes: sulfuric acid, acetic acid, phosphoric acid and sodium sulfate solutions with different voltages for 1 min at room temperature. Results Anodic films that consisted of rutile and/or anatase phases with porous structures were formed on titanium metal after anodizing in H 2SO 4 and Na 2SO 4 electrolytes, while amorphous titania films were produced after anodizing in CH 3COOH and H 3PO 4 electrolytes. Titanium metal with the anatase and/or rutile crystal structure films showed excellent apatite-forming ability and produced a compact apatite layer covering all the surface of titanium after soaking in SBF for 7 d, but titanium metal with amorphous titania layers was not able to induce apatite formation. Significance The resultant apatite layer formed on titanium metal in SBF could enhance the bonding strength between living tissue and the implant. Anodic oxidation is believed to be an effective method for preparing bioactive titanium metal as an artificial bone substitute even under load-bearing conditions.

Local Tuning of Conductivity in Amorphous Titanium Oxide Films by Selective Electron Beam Irradiation

Amorphous TiO2 (a-TiO2) films electrodeposited on steel substrates have recently been found highly e-beam sensitive, allowing for reduction and crystallization with high lateral resolution. We here report on the strong correlation between the e-beam-induced changes in oxidation state and film structure, as observed by transmission electron microscopy and Micro-Raman spectroscopy, and the local film conductivity (σ), measured by conductive atomic force microscopy (c-AFM). c-AFM at constant applied potentials reveals a low σ of 1.9 × 10-4 Ω-1 cm-1 for a-TiO2 and a further decrease of σ by up to a factor 5.5 × 10-3 at locally reduced spots, rather than an enhancement in conductivity consistently reported in literature upon reduction of crystalline TiO2. However, σ significantly rises with the first appearance of anatase phase in micro-Raman spectra, and makes a second large jump coinciding with the formation of TiO, with σ being 2 × 10+4 times higher than in the surrounding a-TiO2. We further observe a rectifying effect due to the presence of micrometer-sized anatase/steel Schottky barrier diodes at locally crystallized spots. The absolute probe current rather than the current density is found responsible for e-beam-induced crystallization, underlining the thermal nature of crystallization at electron energies disqualifying atomic displacement. Structural changes are strictly limited to the irradiated area with anatase nanocrystals being extended nearly throughout the 60 nm thick film, not showing preferential orientation. The results demonstrate the possibility of e-beam-induced confined tuning of σ in the amorphous oxide film in both directions. This observation is novel and will allow for high-resolution patterning of electrical properties in amorphous semiconducting oxides.

Preparation of Niobium-Doped Titanium Oxide Film by Liquid Phase Deposition

Niobium-doped amorphous titanium oxide film can be deposited on a Corning glass substrate using a mixture of ammonium hexafluorotitanate, boric acid, niobium oxide and ammonium hydrogen fluoride aqueous solutions. After thermal annealing, the nanocrystallite niobium-doped titanium oxide film can be obtained. The smaller crystallite size of nanocrystallite niobium-doped titanium oxide than that of undoped-titanium oxide benefits the catalytic activity for sensing gas. A thin-film niobium-doped titanium oxide oxygen sensor with a higher sensitivity and a faster response is demonstrated.

Electron-Beam-Induced Topographical, Chemical, and Structural Patterning of Amorphous Titanium Oxide Films

Electrolytically deposited amorphous TiO2 films on steel are remarkably sensitive to electron beam (e-beam) irradiation at moderate energies at 20 keV, resulting in controlled local oxide reduction and crystallization, opening the possibility for local topographical, chemical, and structural modifications within a biocompatible, amorphous, and semiconducting matrix. The sensitivity is shown to vary significantly with the annealing temperature of as-deposited films. Well-defined irradiation conditions in terms of probe current IP (5 microA) and beam size were achieved with an electron probe microanalyzer. As shown by atomic force and optical microscopy, micro-Raman spectroscopy, wavelength-dispersive X-ray (WDX), and Auger analyses, e-beam exposure below 1 Acm-2 immediately leads to electron-stimulated oxygen desorption, resulting in a well-defined volume loss primarily limited to the irradiated zone under the electron probe and in a blue color shift in this zone because of the presence of Ti2O3. Irradiation at 5 Acm(-2) (IP = 5 microA) results in local crystallization into anatase phase within 1 s of exposure and in reduction to TiO after an extended exposure of 60 s. Further reduction to the metallic state could be observed after 60 s of exposure at approximately 160 Acm(-2). The local reduction could be qualitatively sensed with WDX analysis and Auger line scans. An estimation of the film temperature in the beam center indicates that crystallization occurs at less than 150 degrees C, well below the atmospheric crystallization temperature of the present films. The high e-beam sensitivity in combination with the well-defined volume loss from oxygen desorption allows for precise electron lithographic topographical patterning of the present oxides. Irradiation effects leading to the observed reduction and crystallization phenomena under moderate electron energies are discussed.

Local electron beam induced reduction and crystallization of amorphous titania films

We report local electron beam induced reduction and crystallization of electrolytic amorphous TiO2 films on steel at 20keV, varying beam currents from 10nAto24μA. In situ substrate temperature measurements and beam heating calculations demonstrate the formation of anatase (Raman) below 270°C, clearly beneath the crystallization temperature of nonirradiated films (≈440°C). The evolution of the sample current was correlated to oxide reduction via electron stimulated desorption. Atomic force microscopy revealed well defined cavities with Gaussian shape. The e-beam sensitivity of amorphous TiO2 allows for patterning of macroscopic films, inducing local changes in oxidation state and phase.

Photoconductive Characteristics of Hydro-Oxygenated Amorphous Titanium Oxide Films Prepared by Remote Plasma-Enhanced Chemical Vapor Deposition

The photoconductive characteristics of amorphous titanium oxide films (a-TiOx:OH) prepared by remote plasma-enhanced (RPE) chemical vapor deposition, in inert Ar atmosphere with/without a small amount of oxygen, have been studied. In inert Ar atmosphere without oxygen, the residual decay of photocurrent (long decay time) is observed after removing UV irradiation. On the other hand, in inert Ar atmosphere with a small amount of oxygen, the photocurrent under UV irradiation is influenced strongly by an environmental gas with oxygen gas. The maximum of photocurrent (photosensitivity) is nearly inversely proportional to oxygen partial pressure, suggesting that the present films are useful for sensors of environmental oxygen gases. The decay of photocurrent, after removing UV irradiation, follows a stretched exponential function. The effective decay time τ is nearly inversely proportional to oxygen partial pressure. The results will be discussed in terms of the surface reaction of oxygen gas on the present films.

Apatite formation on CO 2 laser irradiated titanium oxide films

Biomimetic apatite-forming ability on CO 2-laser-irradiated titanium oxide films was examined. Sol–gel-derived amorphous titanium oxide films were crystallized after irradiation of a continuous wave CO 2 laser at the power of 5 W. The irradiation induced crystallization of anatase. The apatite-forming ability on the irradiated titanium oxide film was investigated by soaking in a solution with ion concentration 1.5 times those of simulated body fluid (1.5SBF). X-ray diffractometry showed that apatite formed on the laser-irradiated titanium oxide film within 5 days in 1.5SBF, while no apatite formed on the non-irradiated (amorphous) film. The apatite forming ability can be enhanced and given at a desired position by using the CO 2 laser-irradiation.

On the preferential formation of anatase in amorphous titanium oxide film

We substantiate preferential formation of anatase in a titanium oxide film deposited on a quartz-glass substrate by pulsed laser deposition technique. The key factors to obtain anatase films are reducing the repetition rate of laser pulse down to about 1 Hz and annealing the films at a lower temperature.

Photocatalytic characteristics of hydro-oxygenated amorphous titanium oxide films prepared using remote plasma enhanced chemical vapor deposition

The photocatalytic activity and photoconductive characteristics of hydro-oxygenated amorphous titanium oxide (a-TiO x :OH) films prepared using remote plasma enhanced chemical vapor deposition (RPE-CVD) were studied. The a-TiO x :OH films have OH groups, which when exposed to UV irradiation, show high photoconductivity. The photoconductivity is drastically affected by oxygen gas. The results suggest that the recombination states present in the film are non-activated by the OH endings in the dangling bond like the hydrogen endings in hydrogenated amorphous silicon. Oxygen-sensitive photoconductivity is useful for environmental sensor applications.