Studies on evaporated titanium oxide thin films for oxygen gas detection

Present work deals with the preparation of TiO₂ thin films of different thicknesses by PVD technique using an electron beam and to characterize the films for oxygen gas detection. The films were characterized using optical transmission measurements, XRD and atomic force microscopy. From the spectral data, the extinction, absorption coefficient and refractive index of the films are evaluated and reported. The optical band gap energy varies between 3 and 3.68 eV. The XRD pattern confirms the formation of polycrystalline anatase structure of Titanium with preferred orientation of (110) plane. The AFM images indicate the presence of coarse and fine grains with uniform as well as smooth surfaces over the entire range of the analyzed surface. Response characteristics of TiO₂ thin films for oxygen gas detection are studied, which indicate a low response time of 120 seconds and high sensitivity of 16 at the operating temperature 450 °C. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Low temperature fabrication of vanadium oxide films for uncooled bolometric detectors

Vanadium oxide films with temperature coefficient of resistant of −2.6% K⁻¹ have been fabricated on Si₃N₄-film-coated Si substrates by ion beam sputtering in a controlled Ar/O₂ atmosphere, at a relatively low growth temperature of 200 °C. The as-deposited films show no semiconductor-to-metal phase transitions even heated up to 150 °C. X-ray diffractometry shows that the main compound of the VO_x film is a metastable phase of vanadium dioxide (VO₂(B)) and the VO₂(B) film can be transformed into VO₂ film by post-growth annealing at 450 °C in flowing Ar atmosphere.     

Hydrogen diffusion in titanium at room temperature

The diffusion of hydrogen in technically pure VT1-00 titanium is studied at room temperature under conditions of hydrogen penetration from a glow discharge plasma. Hydrogen penetration occurs under conditions of hydride phase growth. The diffusion coefficients of hydrogen in titanium at room temperature are estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–47, March, 1982.     

Low operating temperature of oxygen gas sensor based on undoped and Cr-doped ZnO films

Undoped and doped ZnO with 1 at.% (atomic percentage) chromium (Cr) was synthesized by RF reactive co-sputtering for oxygen gas sensing applications. The prepared films showed a highly c-oriented phase with a dominant (0 0 2) peak at a Bragg angle of around 34.2°. The operating temperature of the prepared ZnO sensor was around 350 °C and shifted to around 250 °C for the doped ZnO sensor which is lower than that of previously reported work. The sensitivity of the sensor toward oxygen gas was enhanced by doping ZnO with 1 at.% Cr. Good stability and repeatability of the sensor were demonstrated when tested under different concentration of oxygen atmosphere.     

Dynamic measurement of oxygen diffusion in indium-tin oxide

The diffusion of oxygen through an impervious ITO layer has been determined by measuring admittances in the range 5×10^?5 -5 s^?1. The results can be fit by a model which assumes simultaneous oxygen diffusion and generation. The diffusion coefficient does not depend noticeably on the oxygen pressure and has an activation energy comparable to that found for In₂O₃.     

Direct diffusion through interpenetrating networks: oxygen in titanium

How impurity atoms move through a crystal is a fundamental and recurrent question in materials. The previous models of oxygen diffusion in titanium relied on interstitial lattice sites that were recently found to be unstable--leaving no consistent picture of the diffusion pathways. Using first-principles quantum-mechanical methods, we find three oxygen interstitial sites in titanium, and quantify the multiple interpenetrating networks for oxygen diffusion. Surprisingly, all transitions contribute to diffusion.     

Optical constants of thin oxide films on titanium

Thin oxide films on titanium formed by heating were studied by the ellipsometric method. To obtain the complex refractive index and film thicknesses, the ellipsometric measurements were performed by means of the immersing method: each sample was measured first in air and then in a liquid of known refractive index (in our case CCl₄). The optical constants and the oxide film thickness were computed by means of a computer from two pairs of ellipsometric values. To state the optical constants of clean titanium surface the graphic-computational method was proposed and applied. The measurements were carried out at two wavelengths on oxide films grown in air and dry oxygen by thermal oxidation at temperatures from 150 to 700 °C. It has been shown that when increasing the film thickness the refractive index of the film decreases, whereas the absorption coefficient is independent on the film thickness. Optical constants of oxide films growing in dry oxygen are smaller than those growing in air.     

Low temperature quantum diffusion of muonium in KCl

T ₁ spin relaxation of muonium in KCl has been studied at low temperatures (20 mK to 2 K), where the motion is believed to be band-like, i.e. the mean free path is longer than the lattice constant. The Celio model, based on the assumption of stochastic hopping of muonium, accurately describes the field dependence ofT ₁ at higher temperatures but fails below 4 K. The measuredT ₁ spin relaxation rates vary weakly with temperature below 2 K even thoughk _BT at the lowest temperature is well below the estimated muonium bandwidth obtained from the data at higher temperatures. This is taken as evidence that muonium is not completely thermalized on the time-scale of the muon lifetime due to the weak interaction with phonons at low temperatures.     

Low temperature properties of magnetic films

The effect of surface oxides on the low temperature magnetic properties of permalloy films is reviewed. Emphasis is placed on the work of Hagedorn and Mitchell and coworkers. It is shown that the unidirectional anisotropy in permalloy (typically exhibited at temperatures below 40 °K) can be correlated with the presence of -Fe₂O₃ as detected in electron diffraction studies. A possible mechanism for the phenomena is discussed which hinges on the depression of the Morin transition in -Fe₂O₃ as a consequence of a change in thed spacing of the -Fe₂O₃.The author would like to thank Professors L. D. Roberts and C. S. Smith of the University North Carolina for their helpful discussions during the course of this work.     

The structure and properties of titanium oxide nanoheterogeneous films doped with molybdenum oxide

Thin films of titanium oxide nanoparticles doped with molybdenum oxide were synthesized by vacuum cocondensation of titanium atom and molybdenum oxide molecule flows onto a substrate cooled with liquid nitrogen. Atomic-force, scanning tunnel, and scanning electron microscopy were used to study the structure and composition of the films. It was shown that doping caused the formation of a large number of point defects in the titanium oxide lattice and an increase in its specific conductivity. Point defects were three-dimensional F centers and surface hydrate complexes. Tests of films as photoanodes in a photoelectrochemical cell showed that doping of titanium oxide with molybdenum oxide offered promise for increasing the effectiveness of sources for the decentralized production of hydrogen by electrochemical methods.     

Thickness dependent optical properties of titanium oxide thin films

TiO₂ thin films of different thickness were prepared by the Electron Beam Evaporation (EBE) method on crystal silicon. A variable angle spectroscopic ellipsometer (VASE) was used to determine the optical constants and thickness of the investigated films in the spectral range from 300 to 800 nm at incident angles of 60°, 70°, and 75°, respectively. The whole spectra have been fitted by Forouhi–Bloomer (FB) model, whose best-fit parameters reveal that both electron lifetime and band gap of TiO₂ thin film have positive correlation to the film thickness. The refractive indices of TiO₂ thin film increase monotonically with an increase in film thickness in the investigated spectral range. The refractive index spectra of TiO₂ thin films have maxima at around 320 nm and the maxima exhibit a marginally blue-shift from 327.9 to 310.0 nm with an increase in film thickness. The evolution of structural disorder in the TiO₂ thin film growth can be used to explain these phenomena.     

Low temperature magnetoresistance of Al-doped ZnO films

The magnetoresistances of aluminum-doped zinc oxide thin films with thickness of 463.63, 203.03, and 66.85 nm were measured at low temperatures from 2.5 to 30 K. It is found that the samples exhibit negative magnetoresistance at all measuring temperatures. However, neither the three-dimensional nor the two-dimensional weak-localization theories can reproduce the behavior of the magnetoresistance. We find that the magnetoresistance of the three films can be well described by a semiempirical expression that takes into account the third order s-d exchange Hamiltonians describing a negative part and a two-band model for the positive contribution. This strongly suggests that the negative magnetoresistance in ZnO:Al film originates from the scattering of conduction electrons due to localized magnetic moments.     

Aluminium diffusion in titanium nitride films. Efficiency of TiN barrier layers

Two kinds of reactively evaporated titanium nitride films with columnar (B ₀ films) and fine-grained (B ₊ films) film structures, respectively, have been examined as diffusion barriers for preventing aluminium diffusion. The aluminium diffusion profiles have been investigated by 2 MeV ⁴He⁺ Rutherford backscattering spectrometry (RBS) at temperatures up to 550° C. The diffusivity from 300° C to 550° C is: D[m²s^–1]=3×10^–18 exp[–30/(RT)] in B ₀ layers and D[m²s^–1]=1.4×10^–16 exp[–48/(RT)] in B ₊ TiN layers. The activation-energy values determined indicate a grain boundary diffusion mechanism. The difference between the diffusion values is determined implicitly by the microstructure of the layers. Thus, the porous B ₀ layers contain a considerable amount of oxygen absorbed in the intercolumnar voids and distributed throughout the film thickness. As found by AES depth profiling, this oxygen supply allows the formation of Al₂O₃ during annealing the latter preventing the subsequent diffusion of the aluminium atoms.     

Composite titanium oxide nanocolumnar thin films for low-emissivity coating

The TiO_X thin films were grown on the glass substrate for the low-e film applications by direct-current magnetron sputtering methods. The composite nature of dielectric and metallic phases including Ti₂O₃, TiO₂ and amorphous TiOx were identified from the comparative study of x-ray diffraction and Raman spectroscopy. The coalescent nanocolumnar structure revealed from scanning electron microscopy is believed to be responsible for the absence of the interference in the transmission. Most interestingly, the narrow transmittance bands from the films were observed in the visible region, especially in the violet-blue-green color portion, which could be ideal for the applications of narrow band-pass filters or red/infrared reflectors.     

Low temperature diffusion in thin-film couples of polycristalline silver and gold

Limited diffusion spaces which may be easily traversed by atoms diffusing along grain boundaries generally cause thin-film diffusion to differ from common bulk diffusion. These peculiarities were studied in Ag−Au thin-film couples by means of electrical resistance measurements. Diffusion coefficients were found to decrease with annealing time mainly as a consequence of recrystallisation and recovery in the films during the diffusion anneal. It is shown that the rate of homogenisation is fairly independent of the film thickness thus giving evidence that diffusion into the crystallites occurs out of the grain boundary network rather than directly through the couple interface. Effective diffusion coefficients determined between 150 and 250° C ranged from 10⁻¹⁴ to 10⁻¹⁶ cm² s⁻¹ revealing an activation energy of 25 kcal mol⁻¹.     

Local modification of thin oxide films on a titanium surface

It is shown that a controllable structural rearrangement of a thin titanium oxide film, accompanied by a change in the bandgap width, can be accomplished under the probe tip of a scanning tunneling microscope (STM). The mechanism of the rearrangement is considered to be the migration of cations and oxygen vacancies in the electric field of the STM.     

Excimer laser ablation of thin titanium oxide films on glass

Thin titanium dioxide films are deposited on glass substrates by magnetron sputter deposition. They are irradiated in air, by means of a KrF excimer laser. The ablation rate is measured as a function of the laser fluence per pulse, F, and of the number of pulses, N. Above a fluence threshold, the films are partially ablated. The ablated thickness does not vary linearly with N. This is the signature of a negative feedback between the film thickness and the ablation rate. The origin of this negative feedback is shown to be due to either thermal or electronic effects, or both. At high F, the film detachs from the substrate.     

Crystallization and thermochromism of annealed heterostructures containing titanium and tungsten oxide films

Crystalline phases in heterostructures containing titanium and tungsten oxide films are studied after step annealing in vacuum at temperatures between 500 and 750°C. The films are deposited on a silica glass substrate by dc reactive magnetron sputtering. It is found that crystalline phases in single layers and bilayer structures form in a different way. In the latter, crystallization is influenced by the order of layer arrangement on the substrate. Thermochromism in structures annealed in vacuum is due to the oxygen-deficient phase WO_{3 ? x} belonging to the hexagonal syngony. This phase intensely grows as the temperature rises from 650 to 750°C.     

Low temperature ion beam irradiation of gallium films

Ga-films in the amorphous, the -Ga, and the -Ga phase have been irradiated at low temperatures with Ar-ions and He-ions at energies 275 keV and 200 keV, respectively. The -phase can be transformed into the amorphous state by Ar-ion irradiation but not by He-ion irradiation. Rather small Ar-fluences <10¹⁴ ions/cm² are sufficient. The -phase is stable with respect to irradiation with Ar- and He-ions at small fluences. The as-quench condensed amorphous phase is stable with respect to Ar-ion irradiation, however, it transforms into the -phase at He-ion irradiation. The different short range order of the - and -phase is thought to be responsible for the different behavior at ion irradiation.This work is dedicated to Professor Dr. S. Methfessel on the occasion of his 60th birthday     

The influence of oxygen vacancies on the chemical bonding in titanium oxide

A self-consistent LAPW band structure calculation for TiO_0.75 has been performed, assuming long-range order of vacancies on the oxygen sublattice. The calculation is based on a hypothetical model structure which can be described as Ti ₃ ^[4] Ti^[6]O_3o, where _o denotes an oxygen vacancy. In the model structure two types of titanium atoms occur: Ti^[4] atoms which have two vacancies as neighbours and are quadratically surrounded by four oxygen atoms, and Ti^[6] atoms which are octahedrally surrounded by six oxygen atoms. The calculated density of states (DOS) and the local partial densities of states are compared with the respective values for stoichiometric TiO containing no vacancies. A characteristic difference is the appearance of two sharp peaks in the DOS curve below the Fermi energy which are caused by the vacancies. These vacancy states exhibit a considerable amount of charge in the vacancy muffin-tin sphere and are found to be derived from Ti 3d states extending into the vacancy sphere. The introduction of vacancies also leads to a lowering of the Fermi energy indicating a stabilizing effect. The bonding situation in TiO_0.75 as compared to TiO as well as the changes in chemical bonding in the series TiC_0.75–TiN_0.75–TiO_0.75 are discussed on the basis of electron density plots. The loss of Ti^[4]–O bonds (as compared to TiO) is compensated by the formation of Ti^[4]–_o–Ti^[4] bonds across the vacancy and by an increase of the bond strength of the Ti^[4]–O bonds. On the other hand, the Ti^[4]–Ti^[4] and particularly the Ti^[4]–Ti^[6] bonds are weakened by the introduction of vacancies.Dedicated to Professor F. Kohler on the occasion of his 65^th birthday