Ti Metal Films Research Articles

Dimensionally stable anodes for the oxygen evolution reaction: Ruthenium dioxide on a nickel metal substrate

Dimensionally stable anodes (DSA) for the oxygen evolution reaction (OER) gradually passivate under operating conditions due to the formation of insulating TiO2 at the interface of the Ti-metal substrate and the catalytically active metal oxide layer (typically a mixture of ruthenium and iridium oxides). The incorporation of a catalytically active, yet stable, interfacial buffer layer or substrate is a potential means of promoting the overall performance. Here, we prepared DSA-like RuO2 on Ni films by the thermal decomposition method and contrast their OER activity with an analogous RuO2 on Ti-metal film. The fabrication process resulted in the presence of NiO and Ni oxides/(oxy)hydroxides at the surface of the rutile RuO2 layer. Along with RuO2, the former species were active in the water splitting reaction under the testing conditions. Ni–RuO2 films showed lower OER overpotentials relative to Ti–RuO2 suggesting a synergistic effect between Ni- and Ru-oxides and the Ni layer between Ti and RuO2 to improve overall performance. Further studies should optimize the NiO–RuO2 loading levels, understand the NiO–RuO2 synergy toward catalytic performance and determine the stability under accelerated testing conditions.

Resistive Switching Behavior of Titanium Oxynitride Fabricated Using a Thermal Nitridation Process

A titanium oxynitride (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> ) resistive random-access memory (RRAM) with write-once-read-many-times (WORM) characteristics is proposed. The TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> resistive switching (RS) layer with a thickness of 62 nm was fabricated using a thermal nitridation process of an evaporated Ti metal film. The residual Ti layer with a thickness of 8 nm is used as the bottom contact. The Al/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> /Ti/n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si RRAM shows a large memory window of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> at a low read voltage of 0.2 V. Stable high-resistance state (HRS) and low-resistance state (LRS) are observed in the endurance test for 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> read cycles. High read-disturb immunity can be found for over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s. The energy band diagram, carrier conduction and the RS mechanisms of the RRAM are investigated.

(Invited) Precursors for the Thermal Atomic Layer Deposition of Cobalt Metal Films and Alloys Thereof

Herein, we will describe recent progress in our laboratory on the development of chemical precursors for the thermal atomic layer deposition (ALD) of cobalt metal, titanium metal, cobalt-titanium alloy, and other metal and metal alloy films. The nanoscale dimensions of current and future microelectronics devices now require alternative, ultrathin barrier materials for copper and other interconnect metals as well as new ultrathin liner materials. Sputtered cobalt-titanium alloys have been demonstrated to function as barrier materials and liners at dimensions as small as 1 nm. Hence, these materials may replace current barrier and liner materials such as TiN, TaN, and W metal. However, the narrow and deep nanoscale features require growth of cobalt-titanium alloy films by ALD to meet the strict conformality and thickness requirements. Moreover, thermal ALD is preferred over plasma-based processes, since recombination of atomic hydrogen on feature walls can afford poor conformal coverage. Thermal ALD processes for Co and Ti metal films are poorly developed, because the negative electrochemical potentials of the ions in precursors (Co2+ ↔ Co, E° = -0.280 V, Ti2+ ↔ Ti, E° = -1.628 V) require strong reducing co-reactants, which are not available. We will report new thermal ALD processes for cobalt metal films that employ various ligands, in combination with hydrazine-based co-reactants. These processes afford high purity cobalt metal films. We will also describe our efforts to develop new precursors for the thermal ALD of titanium metal films. Initial depositions of cobalt-titanium metal films will also be described.

Electroplating titanium film on 316L stainless steel in LiCl–KCl–Tix+ (2 &lt; x&lt;3) molten salts

To improve the anti-corrosion properties of 316L stainless steel, titanium (Ti) metal films were electroplated on it using LiCl–KCl molten salts as the electrolyte, and low-valence Tix+ (2 < x<3) as the solute. The solute was produced by the reaction of Ti4+ and Ti0 via mixing K2TiF6, and Ti metal (sponge Ti) in the melts. Anti-corrosion test has shown significant enhancement in the stability of the Ti-coated 316L due to the presence of Ti film. However, the anti-corrosion properties did not enhance with an increase in the thickness of the Ti layers owing to a possible defect in the thicker Ti layers.

Selective area growth of GaN nanowires on Si(1 1 1) substrate with Ti masks by molecular beam epitaxy

We present results on the selective area growth (SAG) of Gallium Nitride (GaN) nanowires on Si substrate without any buffer layer by radio frequency plasma-assisted molecular beam epitaxy. Full selectivity was achieved with a thin Ti metal film used as a mask. The mask was fabricated with a lift-off method to avoid damage to the mask or Si substrate. The growth on the Ti film was totally suppressed at a low substrate temperature of 840 °C which is by far the lowest critical temperature for SAG. An InGaN thin layer was embedded in the GaN nanowires to realize a site controllable single photon source. Transmission electron microscopy (TEM) indicates single crystalline quality of defect-free GaN nanowires and the embedded InGaN quantum dot structure.

Conformal High-K Dielectric Coating of Suspended Single-Walled Carbon Nanotubes by Atomic Layer Deposition.

As one of the highest mobility semiconductor materials, carbon nanotubes (CNTs) have been extensively studied for use in field effect transistors (FETs). To fabricate surround-gate FETs— which offer the best switching performance—deposition of conformal, weakly-interacting dielectric layers is necessary. This is challenging due to the chemically inert surface of CNTs and a lack of nucleation sites—especially for defect-free CNTs. As a result, a technique that enables integration of uniform high-k dielectrics, while preserving the CNT’s exceptional properties is required. In this work, we show a method that enables conformal atomic layer deposition (ALD) of high-k dielectrics on defect-free CNTs. By depositing a thin Ti metal film, followed by oxidation to TiO2 under ambient conditions, a nucleation layer is formed for subsequent ALD deposition of Al2O3. The technique is easy to implement and is VLSI-compatible. We show that the ALD coatings are uniform, continuous and conformal, and Raman spectroscopy reveals that the technique does not induce defects in the CNT. The resulting bilayer TiO2/Al2O3 thin-film shows an improved dielectric constant of 21.7 and an equivalent oxide thickness of 2.7 nm. The electrical properties of back-gated and top-gated devices fabricated using this method are presented.

Anodic TiO2 nanotube arrays directly grown on quartz glass used in front‐ and back‐side irradiation configuration for photocatalytic H2 generation

In the present work, we explore the front‐ and back‐side performance of a photocatalytic platform consisting of self‐organized TiO2 nanotube layers formed by complete anodization of Ti metal films evaporated on quartz slides. The adhesion and light transmission of the tube layers on the quartz surfaces are optimized by a suitable anodization procedure. After their growth, the nanotube arrays were converted into crystalline structures and sputter‐coated with co‐catalytic Pt nanoparticles. The optically transparent quartz substrates enable the use of the Pt‐decorated tube layers for photocatalytic H2 generation under either front‐ or back‐side illumination configurations. The nanotube films on quartz are characterized in view of their physico‐chemical properties, including their light transmission features measured using different light sources. The results show that the front‐side illumination under optimized factors, i.e., the amount of loaded co‐catalyst, yields a maximized photocatalytic performance in terms of H2 generation.

Vertical Alignment TiO2 Nanotube Based on Ti Film Prepared via Anodization Technique

A highly ordered nanotube TiO2 was successfully prepared from sputtered Ti metal film using anodization process. Ethylene glycol and ammonium fluoride was introduced as the electrolyte solution. The applied of anodizing voltage was systematically controlled between 20 - 60 volts along fabrication process, respectively. The physical characteristic of the fabricated TiO2 nanotube including anodizing rate, tube diameter and tube width was investigated through the characterization system as field emission scanning electron microscope (FE-SEM). According to cross-section FE-SEM photograph, the anodizing rate and tube width significantly increases when the anodizing voltage was future increased due to higher the electric field. Moreover, the tube diameter directly depends with the anodizing voltage also. The anodizing voltage provides a significant role on the feature of TiO2 nanotube. Finally, the fabricated nanotube TiO2 is potentially promising for Photo-activated application and Nanostructure template.

Investigation of Sheet Resistance Distribution of Ti, Al, Ni, Cr and Au Metal Films on Silicon Substrates

Ti, Al, Ni, Cr and Au metal films have been deposited onto silicon (100) n−type 100 mm substrates by thermal evaporation technique. The film thickness and sheet resistance distributions have been measured. We show that the increase in the sheet resistance towards the substrate edge occurs due to both a decrease in the film thickness and an increase in the metal resistivity.

Control of Er3+ diffusion by co-diffused Ti4+ in LiNbO3 crystal

Er3+/Ti4+-co-doped LiNbO3 plates were prepared by annealing congruent substrates coated with stacked Er- and Ti-metal films having different thicknesses and coating sequences. The crystalline phase, OH− absorption, Er3+-doping effect on LiNbO3 refractive index and Li+-composition in the Er3+/Ti4+-co-doped layer were studied. The results show that the LiNbO3 phase dominates in the co-doped layer, the Er3+-doping affects little the LiNbO3 refractive index, slight Li2O out-diffusion occurred in the diffusion procedure, and the annealing procedure does not affect the OH− absorption band noticeably. The Er3+/Ti4+-co-diffusion characteristics were studied by secondary ion mass spectrometry. The study shows that Ti4+ diffuses much faster than Er3+. The Ti4+ co-diffusion assists and controls the Er3+-diffusion and Er3+ diffusivity increases linearly with initial Ti-metal thickness. In contrast, neither Er-metal thickness nor coating sequence of Ti- and Er-metal films influences the Ti4+ or Er3+ diffusivity. The co-diffusion features are explained. A co-diffusion model is proposed.

Zr4+/Ti4+ Codiffusion Characteristics in Lithium Niobate

Zr4+/Ti4+‐codoped LiNbO3 plates were prepared by local codiffusion of stacked ZrO2 and Ti metal films coated onto Z‐cut congruent LiNbO3 substrates in wet O2 at 1060°C. The metal and oxide films have different thicknesses and coating sequences. After diffusion, the Zr4+ doping effect on the refractive index of LiNbO3 and the Li2O out‐diffusion issue were studied by the prism coupling technique. The codiffusion characteristics of Zr4+ and Ti4+ were studied by secondary ion mass spectrometry. The results show that the Zr4+ doping has little contribution to the refractive index of the crystal. Li2O out‐diffusion is not measurable. In the Zr4+‐only diffusion case, the diffusivity of Zr4+ is four times smaller than that of Ti4+. In the Zr4+/Ti4+ codiffusion case, the Ti4+ codiffusion assists the Zr4+ diffusion. The Zr4+ diffusivity increases linearly by two more times with the increase in initial Ti film thickness from 0 to 200 nm. On the other hand, the Zr4+ affects the Ti4+ diffusion little. Neither the ZrO2 film thickness nor the coating sequence of Ti metal and ZrO2 oxide films influences the diffusivity of the two ions. All the codiffusion characteristics are explained. A Zr4+/Ti4+ codiffusion model is suggested that consists of two independent diffusion equations with a Zr4+ diffusivity dependent of Ti4+ concentration and a constant Ti4+ diffusivity. In addition, the existence of a waveguide in the Zr4+/Ti4+‐codoped layer is verified experimentally, and the optical‐damage‐resistant feature of the waveguide is verified by two‐beam hologram recording experimental results.

Preparation and characterization of TiO2 barrier layers for dye-sensitized solar cells.

A TiO2 barrier layer is critical in enhancing the performance of dye-sensitized solar cells (DSSCs). Two methods to prepare the TiO2 barrier layer on fluorine-doped tin dioxide (FTO) surface were systematically studied in order to minimize electron-hole recombination and electron backflow during photovoltaic processes of DSSCs. The film structure and materials properties were correlated with the photovoltaic characteristics and electrochemical properties. In the first approach, a porous TiO2 layer was deposited by wet chemical treatment of the sample with TiCl4 solution for time periods varying from 0 to 60 min. The N719 dye molecules were found to be able to insert into the porous barrier layers. The 20 min treatment formed a nonuniform but intact TiO2 layer of ∼100-300 nm in thickness, which gave the highest open-circuit voltage VOC, short-circuit photocurrent density JSC, and energy conversion efficiency. But thicker TiO2 barrier layers by this method caused a decrease in JSC, possibly limited by lower electrical conductance. In the second approach, a compact TiO2 barrier layer was created by sputter-coating 0-15 nm Ti metal films on FTO/glass and then oxidizing them into TiO2 with thermal treatment at 500 °C in the air for 30 min. The dye molecules were found to only attach at the outer surface of the barrier layer and slightly increased with the layer thickness. These two kinds of barrier layer showed different characteristics and may be tailored for different DSSC studies.

Studying the uniformity of the surface resistance of Ti, Al, Ni, Cr, and Au metal films on silicon

Nanosized Ti, Al, Ni, Cr, and Au films were deposited onto KEF-20 (100)-oriented silicon plates with a diameter of 100 mm using the method of thermal evaporation in a vacuum. The value and uniformity of the distribution of the specific electrical resistance of the metal films were estimated by measuring their thickness and surface electrical resistance via the four-probe method. The specific electrical resistance of such films was shown to be considerably higher than for bulk materials. The observed increase in the surface resistance at the edges of the film was due to both the decrease in the thickness of the film and the increase in the specific electrical resistance of the material of the film. The proved regimes were used to obtain metal layers on gallium nitride substrates.

Remote opto‐acoustic probing of single‐cell adhesion on metallic surfaces

The reflection of picosecond ultrasonic pulses from a cell-substrate interface is used to probe cell-biomaterial adhesion with a subcell resolution. We culture monocytes on top of a thin biocompatible Ti metal film, supported by a transparent sapphire substrate. Low-energy femtosecond pump laser pulses are focused at the bottom of the Ti film to a micron spot. The subsequent ultrafast thermal expansion launches a longitudinal acoustic pulse in Ti, with a broad spectrum extending up to 100 GHz. We measure the acoustic echoes reflected from the Ti-cell interface through the transient optical reflectance changes. The time-frequency analysis of the reflected acoustic pulses gives access to a map of the cell acoustic impedance Zc and to a map of the film-cell interfacial stiffness K simultaneously. Variations in Zc across the cell are attributed to rigidity and density fluctuations within the cell, whereas variations in K are related to interfacial intermolecular forces and to the nano-architecture of the transmembrane bonds.

Effects of Substrate Heating on the Photovoltaic Characteristics of Dye-Sensitized Solar Cells During Two-Step Ti Film Deposition by RF Magnetron Sputtering

Nanoporous Ti metal film electrodes for use as photoanodes in dye-sensitized solar cells (DSSCs) were deposited directly on the nanoporous TiO2 layer using the two-step RF magnetron sputtering technique. The Ti film electrode replaces the transparent conducting oxide (TCO) layer. The effect of substrate heating during the deposition of the Ti film was studied to improve the porosity and columnar array of the film pores and the resulting cell efficiency. The porous Ti layer (-41 microm) with low sheet resistance (-1.7 omega/sq) was obtained by deposition at 250 degrees C. The porous Ti layer collects electrons from the TiO2 layer and allows the diffusion of I-/I3(-) through the holes. The DSSC efficiency (eta) using porous Ti layers with highly columnar structures was measured with the highest conversion efficiency of -5.77%; the other photovoltaic properties were ff: 0.76, V(oc): 0.72 V, and J(sc): 10.6 mA/cm2.

Ti film Deposition for Fabrication of Transparent Conductive Oxide Less Electrochemical Luminescence Cell

In this work, a design of transparent conductive oxide less electrochemical luminescence (TCO-less ECL) cell has been proposed using a Ti metal layer as the secondary electrode. Ti metal films (∼500 nm in thickness) are deposited on a glass substrate without Fluorine-doped SnO2 (FTO) by RF magnetron sputtering. The TCO-less ECL cell is composed of a glass substrate/Ti metal/porous TiO2/Ru(II) complex/counter electrode. The Ti metal layer with high conductivity can collect electrons from the TiO2 layer and prevent electrons from back-transferring from the Ti to the electrolyte. The vacuum annealing treatment is important to ensure solid interconnections of porous TiO2 with underlying Ti metal layers. The ECL efficiency of the TCO-less cell consisting of the layer of Ti and porous TiO2 was ∼290 cd/m2, much higher than the efficiency (125 cd/m2) of general cell with FTO glass/Ru(II) complex in propylene carbonate/counter electrode configuration.

General Strategy for Fabricating Transparent TiO2 Nanotube Arrays for Dye-Sensitized Photoelectrodes: Illumination Geometry and Transport Properties

We report on the preparation of transparent oriented titania nanotube (NT) photoelectrodes and the effect of illumination direction on light harvesting, electron transport, and recombination in dye-sensitized solar cells (DSSCs) incorporating these electrodes. High solar conversion efficiency requires that the incident light enters the cell from the photoelectrode side. However, it has been synthetically challenging to prepare transparent TiO(2) NT electrodes by directly anodizing Ti metal films on transparent conducting oxide (TCO) substrates because of the difficulties of controlling the synthetic conditions. We describe a general synthetic strategy for fabricating transparent TiO(2) NT films on TCO substrates. With the aid of a conducting Nb-doped TiO(2) (NTO) layer between the Ti film and TCO substrate, the Ti film was anodized completely without degrading the TCO. The NTO layer was found to protect the TCO from degradation through a self-terminating mechanism by arresting the electric field-assisted dissolution process at the NT-NTO interface. The illumination direction and wavelength of the light incident on the DSSCs were shown to strongly influence the incident photon-to-current conversion efficiency, light-harvesting, and charge-collection properties, which, in turn, affect the photocurrent density, photovoltage, and solar energy conversion efficiency. Effects of NT film thickness on the properties and performance of DSSCs were also examined. Illuminating the cell from the photoelectrode substantially increased the conversion efficiency compared with illuminating it from the counter-electrode side.

Layer-resolved study of the Mg to MgH 2 transformation in Mg–Ti films with short-range chemical order

Positron depth-profiling was applied to monitor the effects of hydrogenation on Mg 1− y Ti y thin films. S– W diagrams and VEPFIT analysis of the depth-profiles demonstrated the homogeneity of most metal and metal hydride films. In contrast, Mg 0.90Ti 0.10H x films consisted of a double layer, with a thin unloaded Mg 0.90Ti 0.10 or Mg–Ti–Pd alloy layer on top of a hydrogenated bottom layer. The metal-to-metal-hydride transformation of Mg domains in the nanoscale phase-segregated Mg–Ti films was monitored exclusively, enabled by the large difference in positron affinity for Mg and Ti. The changes in the Doppler broadening parameters revealed that the metal–insulator transition for fluorite MgH 2 is similar to that for rutile MgH 2. Positron lifetime spectroscopy showed the presence of di-vacancies in the metal sub-lattice of as-deposited and hydrogenated Mg–Ti metal films, which may induce the fast hydrogen sorption kinetics of the fluorite MgH 2 phase.

Scanning probe lithography for fabrication of Ti metal nanodot arrays

We report fabrication of Ti metal nanodot arrays by scanning probe microscopic indentation. A thin poly-methylmethacrylate (PMMA) layer was spin-coated on Si substrates with thickness of 70 nm. Nanometer-size pore arrays were formed by indenting the PMMA layer using a cantilever of a scanning probe microscope. Protuberances with irregular boundaries appeared during the indentation process. Control of approach and pulling-out speed during indentation was able to dispose of the protrusions. Ti metal films were deposited on the patterned PMMA layers by a radio-frequency sputtering method and subsequently lifted off to obtain metal nanodot arrays. The fabricated metal nanodot arrays have 200 nm of diameter and 500 nm of interdistance, which corresponds to a density of 4×10 8/cm 2. Scanning probe-based measurement of current–voltage ( I–V) behaviors for a single Ti metal nanodot showed asymmetric characteristics. Applying external bias is likely to induce oxidation of Ti metal, since the conductance decreased and volume change of the dots was observed. I–V behaviors of Ti metal nanodots by conventional e-beam lithography were also characterized for comparison.

Secondary ion mass spectrometry study of Ti4+diffusion properties in congruent Er:LiNbO3codoped with moderate concentration of MgO

At 1100 °C, the diffusion properties of Ti4+into congruent LiNbO3crystals codoped with 0.5 mol% Er2O3and different MgO concentrations of 0.5, 1.0, and 1.5 mol% have been studied by secondary ion mass spectrometry (SIMS). ThreeY-cut and threeZ-cut plates with different Mg doping levels were coated with a 60-nm-thick Ti film at first and then annealed at 1100 °C for 28 h in a wet O2atmosphere. SIMS was used to analyze depth profile characteristics of diffused Ti ions and the constituent elements of the substrate as well. The results show that the diffusion reservoir was exhausted and the Ti metal film was completely diffused. All measured Ti profiles follow a Gaussian function. No Mg out-diffusion accompanied the Ti in-diffusion procedure for all crystals studied. The 1/ediffusion depth is similar to 8.3/10.2, 7.4/8.7, and 6.6/8.2 ± 0.2/0.2 μm/μm for theY/Z-cut crystal with the Mg doping level of 0.5, 1.0, and 1.5 mol%, respectively, yielding a Ti4+diffusivity of 0.62/0.93, 0.49/0.67, and 0.39/0.60 ± 0.03/0.03 (μm2/h)/(μm2/h), respectively. The diffusion shows definite anisotropy and a considerable MgO doping level effect. Under the same Mg doping level, the diffusion in aZ-cut crystal is faster. The diffusivity decreases with the increase of the Mg doping level. This effect is qualitatively explained from the viewpoint of the Mg doping effect on concentration of the intrinsic defects in LiNbO3crystal.