Investigation on the structure of TiO2 films sputtered on alloy substrates

Titanium dioxide (TiO₂) films have been successfully deposited on metal alloy substrates by radio-frequency magnetron reactive sputtering in an Ar+O₂ gas mixture. The effects of gas total pressure on the structure and phase transition of TiO₂ films were studied by X-ray diffraction and Raman spectra. It is suggested that the film structure changes from rutile to anatase while work gas total pressure changes from 0.2 to 2 Pa. The structure of TiO₂ films is not affected by the film thickness.     

Low temperature stabilized rutile phase TiO2 films grown by sputtering

The deposition of rutile phase TiO₂ films on unheated substrates by radio frequency magnetron sputtering is elaborated. The effect of total pressure and O₂/Ar flow ratio on the growth of rutile film on different substrates has been studied thoroughly. The development of crystalline phase along with film deposition rate, surface morphology, optical transmission and band gap were also investigated for various growth conditions. It was found that the rutile phase crystallinity increased with decrease in total pressure and increase in O₂ flow. In addition, the grown rutile films have interesting optical characteristics such as high transmittance (~ 85%) and high refractive index (~ 2.7) with a band gap about 3.2 eV.     

The selective growth of rutile thin films using radio-frequency magnetron sputtering

Using unbalanced radio-frequency (RF) magnetron sputtering crystalline rutile films were synthesised on glass substrates at (combined Ar and O₂) pressures of 0.4 Pa or less, at RF powers of 500 and 600 W with substrate to magnetron distances of 40 mm or longer. Anatase films were deposited at the greater pressure of 1.2 Pa (substrate to magnetron distance of 40 mm) or shorter substrate to magnetron distance of 20 mm (at 0.4 Pa). A mixture of anatase and rutile was formed at 0.5 Pa with all other conditions being as for those required for rutile or the power was reduced along with the substrate to magnetron distance (500 W and 20 mm). The crystallite sizes of rutile obtained were 1 - 3 nm. It is proposed that the greater the energy imparted to the substrate surface by the impinging positive species the greater the activation energy to crystalline phase formation that can be overcome. Hence the formation of rutile over anatase is favoured at greater power, longer magnetron to substrate distances and decreased pressure. Moreover, not only is it possible to control the phase of TiO₂ formed it appears to be possible to control the degree of oxygen non-stoichiometry in the rutile films formed. Smaller O₂ partial pressures, shorter substrate to magnetron distances and greater RF power are believed to produce an environment of reduced reaction of sputtered Ti species with O₂ and to result in the formation of non-stoichiometric rutile structures resulting in increased band gap energies and decreased refractive indices.     

Crystallinity and photocatalytic activity of TiO2 films deposited by reactive sputtering with radio frequency substrate bias

TiO₂ films with thicknesses of 400-460 nm were deposited on the unheated non-alkali glass by radio frequency (rf) reactive magnetron sputtering using a Ti metal target. Depositions were carried out using a 3-in. 1000 G magnetron cathode with various rf substrate bias voltages (V_sb, dc component of self bias) of 10-80 V under total gas pressure of 1.0 or 3.0 Pa. The oxygen flow ratio [O₂/(O₂ + Ar)] and rf sputtering power were kept constant at 60% and 200 W, respectively. Photocatalytic activity on photoinduced oxidative decomposition of acetaldehyde (CH₃CHO) of the TiO₂ films showed a clear tendency to decrease with the increase in the V_sb during the deposition. Most of the films consisted of the mixture of anatase and rutile polycrystalline portions. It was confirmed that the rutile phase content increased and anatase phase content decreased markedly with increasing V_sb, where the crystallinity of anatase phase was much higher than that of rutile phase.     

Apatite formation from simulated body fluid on various phases of TiO2 thin films prepared by filtered cathodic vacuum arc deposition

Hydroxyl (OH⁻)-free TiO₂ thin films with amorphous and crystalline phases were deposited onto (100) silicon substrates using filtered cathodic vacuum arc deposition in order to investigate the in vitro apatite formation in simulated body fluid (SBF). The surface morphology, composition and structure of the TiO₂ thin films were characterized. The X-ray photoelectron spectroscopy results confirmed the presence of calcium and phosphorus on all TiO₂ thin film surfaces after immersion in SBF at 37 °C. Fourier transform infra red results showed the presence of carbonated apatite on the surface of these films. Amorphous structured TiO₂ thin film showed poor ability to form apatite on its surface in SBF. Apatite formation was more pronounced on the surfaces of the anatase films in comparison to those of rutile. The carbonated apatite deposition rate increased significantly when the TiO₂ film was illuminated with UV light prior to immersing in the SBF. In particular, the UV-treated anatase and rutile films showed increased rates of carbonated apatite formation on their surfaces in comparison to samples not treated with radiation. The increase in hydrophilicity due to UV treatment appears beneficial for the apatite growth on these surfaces.     

Phase transformation in semi-transparent TiO2 films irradiated with CO2 laser

Anatase (TiO₂) thin films were obtained by immersion of glass plates into a titanium sol-gel precursor followed by calcination at 450 °C for 3 h. The Raman results for the CO₂ laser irradiated TiO₂ films show that laser radiation is able to promote favorable changes of anatase phase in anatase/rutile mixtures. Nevertheless, the transformation process level depends on laser characteristics and scan speed of the radiation treatment.     

The dry etching property of TiO2 thin films using metal-insulator-metal capacitor in inductively coupled plasma system

In this work, we investigated the etching characteristics of TiO₂ thin films and the selectivity of TiO₂ to SiO₂ in a BCl₃/Ar inductively coupled plasma (ICP) system. The maximum etch rate of 84.68 nm/min was obtained for TiO₂ thin films at a gas mixture ratio of BCl₃/Ar (25:75%). In addition, etch rates were measured as a function of etching parameters, such as the RF power, DC-bias voltage and process pressure. Using the X-ray photoelectron spectroscopy analysis the accumulation of chemical reaction on the etched surface was investigated. Based on these data, the ion-assisted physical sputtering was proposed as the main etch mechanism for the BCl₃-containing plasmas.     

TiO2 nanorod films grown on Si wafers by a nanodot-assisted hydrothermal growth

We report the controlled hydrothermal growth of rutile TiO₂ nanorods on Si wafers by using an anatase TiO₂ nanodot film as an assisted growth layer. The anatase nanodot film was prepared on the wafer by phase-separation-induced self-assembly and subsequent heat-treatment at 500 °C. The nanodots on the wafer were then subjected to hydrothermal treatment to induce the growth of rutile TiO₂ nanorod films. The size and dispersion density of the resulting TiO₂ nanorods could be varied by adjusting the Ti ion concentration in the growth solution. The TiO₂ nanorods were of the rutile phase and grew in the [001] direction. The growth mechanism reveals that the growth of the rutile nanorods was wholly dependent on the existence of rutile TiO₂ seeds, which could be formed by the dissolution-reprecipitation of the anatase nanodots during hydrothermal treatment or under the high-temperature conditions of the subsequent heat-treatment of the as-prepared nanodots. In controlling the rutile nanorod growth, the anatase nanodots show more efficiency than a dense anatase film. Preliminary evaluations of the rutile nanorod films have demonstrated that the wettability changed from highly hydrophobic to superhydrophilic and that the photocatalytic activity was enhanced with increasing nanorod dispersion density.     

Characterization of N-doped TiO2 films prepared by reactive sputtering using air/Ar mixtures

Nitrogen-doped titanium dioxide (TiO_2 − xN_x) thin films desirable for visible light photocatalysts were prepared by reactive sputtering using air/Ar mixtures. Using air as the reactive gas allows the process to conduct at high base pressures (low vacuum), which reduces substantially the processing time. The obtained films transformed from mixed phases to anatase phase as the air/Ar flow ratio increased. Substitutional doping of nitrogen verified by X-ray photoelectron spectroscopy accounts for the red-shift of absorption edge in the absorption spectra. Anatase TiO_2 − xN_x films could incorporate up to about 7.5 at.% substitutional nitrogen and a maximum of 23 at.% nitrogen was determined in the films with mixed phases. The optical band gaps of the TiO_2 − xN_x films calculated from Tauc plots varied from 3.05 to 3.11 eV and those of the mixed phase ranged from 2.77 to 3.00 eV, which are all lower than that for pure anatase TiO₂ and fall into the visible light regime.     

Effect of sputtering pressure and post-annealing on hydrophilicity of TiO2 thin films deposited by reactive magnetron sputtering

A series of TiO₂ thin films was deposited onto glass substrates without intentional heating or biasing by magnetron sputtering of a titanium target using Ar/O₂ reactive mixtures over a broad range of total sputtering pressures from 0.12 Pa to 2.24 Pa. Each of the film types was deposited by the threshold poisoned mode at a specific given oxygen flow rate monitored in-situ by optical emission spectroscopy. Both the sputtering pressure and thermal annealing are the key factors for the TiO₂ films to yield fast-response superhydrophilicity with a water contact angle of 5°. The mechanism of superhydrophilicity for the TiO₂ films deposited by high-pressure sputtering will be discussed based on empirical studies of X-ray diffractometry, high-resolution scanning microscopy and atomic force spectroscopy.     

Photocatalytic activities of TiO2 thin films prepared on Galvanized Iron substrate by plasma-enhanced atomic layer deposition

Titanium dioxide (TiO₂) thin films were prepared on Galvanized Iron (GI) substrate by plasma-enhanced atomic layer deposition (PE-ALD) using tetrakis-dimethylamido titanium and O₂ plasma to investigate the photocatalytic activities. The PE-ALD TiO₂ thin films exhibited relatively high growth rate and the crystal structures of TiO₂ thin films depended on the growth temperatures. TiO₂ thin films deposited at 200 °C have amorphous phase, whereas those with anatase phase and bandgap energy about 3.2 eV were deposited at growth temperature of 250 °C and 300 °C. From contact angles measurement of water droplet, TiO₂ thin films with anatase phase and Activ™ glass exhibited superhydrophilic surfaces after UV light exposure. And from photo-induced degradation test of organic solution, anatase TiO₂ thin films and Activ™ glass decomposed organic solution under UV illumination. The anatase TiO₂ thin film on GI substrate showed higher photocatalytic efficiency than Activ™ glass after 5 h UV light exposure. Thus, we suggest that the anatase phase in TiO₂ thin film contributes to both superhydrophilicity and photocatalytic decomposition of 4-chlorophenol solution and anatase TiO₂ thin films are suitable for self-cleaning applications.     

Ion sputter etching of ZnO:Ga thin film surfaces

In this article the modification of surface morphology of ZnO:Ga (GZO) thin films by ion sputter etching is presented. GZO thin films were prepared at room temperature on Corning glass substrates by both normal and oblique angle RF diode sputtering from ZnO:2%Ga ceramic target in Ar gas. Ion sputter etching was performed by RF re-sputtering of GZO thin films on substrates. During RF sputter etching, Ar pressure of 1.3 Pa and RF power of 250 W were kept constant, only the time of sputter etching was changed. Ion sputter etching had remarkable influence on surface morphology of GZO thin films: increase of roughness R_q and the “homogenization” of film surfaces, i.e. skewness (R_sk) and spikiness (R_ku) parameters (R_sk ≈ 0/R_ku ≈ 3).Surface root-mean-square roughness (R_q) increased from 15.3 nm (after sputter deposition) to 29.1 nm (after ion sputter etching). For obliquely thin films increased from 16.5 nm (after sputter deposition) to 38.2 nm. Changes of these parameters R_q, R_sk, R_ku influenced optical properties of GZO films, increased Haze parameter up to values 7.7% and width of optical band gap 3.44 eV, respectively.     

A study of structural transition in nanocrystalline titania thin films by X-ray diffraction Rietveld method

Structural and microstructural analyses of nanocrystalline titania thin films prepared by pulsed laser deposition have been carried out. At lower oxygen partial pressures (≤10⁻⁴ mbar), rutile films were formed, whereas at 1.2 × 10⁻³ mbar of oxygen partial pressure, the thin films contained both rutile and anatase phases. At 0.04 and 0.05 mbar of oxygen partial pressure, the film was purely anatase. Addition of oxygen has also shown a profound influence on the surface morphology of the as deposited titania films. Modified Rietveld method has been used to determine crystallite size, root mean square strain and fractional coordinates of oxygen of the anatase films. The influence of crystallite size and strain on the rutile to anatase phase transition is investigated.     

Preparation of anatase TiO2 thin films for dye-sensitized solar cells by DC reactive magnetron sputtering technique

Anatase titanium dioxide (TiO₂) thin films are prepared by DC reactive magnetron sputtering using Ti target as the source material. In this work argon and oxygen are used as sputtering and reactive gas respectively. DC power is used at 100 W per 1 h. The distance between the target and substrate is fixed at 4 cm. The glass substrate temperature value varies from room temperature to 400 °C. The crystalline structure of the films is determined by X-ray diffraction analysis. All the films deposited at temperatures lower than 300 °C were amorphous, whereas films obtained at higher temperature grew in crystalline anatase phase. Phase transition from amorphous to anatase is observed at 400 °C annealing temperature. Transmittances of the TiO₂ thin films were measured using UV-visible NIR spectrophotometer. The direct and indirect optical band gap for room temperature and substrate temperature at 400 °C is found to be 3.50, 3.41 eV and 3.50, 3.54 eV respectively. The transmittance of TiO₂ thin films is noted higher than 75%. A comparison among all the films obtained at room temperature showed a transmittance value higher for films obtained at substrate temperature of 400 °C. The morphology of the films and the identification of the surface chemical stoichiometry of the deposited film at 400 °C were studied respectively, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The surface roughness and the grain size are measured using AFM.     

Preparation of anatase TiO2 thin films by vacuum arc plasma evaporation

Anatase titanium dioxide (TiO₂) thin films with high photocatalytic activity have been prepared with deposition rates as high as 16 nm/min by a newly developed vacuum arc plasma evaporation (VAPE) method using sintered TiO₂ pellets as the source material. Highly transparent TiO₂ thin films prepared at substrate temperatures from room temperature to 400 °C exhibited photocatalytic activity, regardless whether oxygen (O₂) gas was introduced during the VAPE deposition. The highest photocatalytic activity and photo-induced hydrophilicity were obtained in anatase TiO₂ thin films prepared at 300 °C, which correlated to the best crystallinity of the films, as evidenced from X-ray diffraction. In addition, a transparent and conductive anatase TiO₂ thin film with a resistivity of 2.6 × 10^− 1 Ω cm was prepared at a substrate temperature of 400 °C without the introduction of O₂ gas.     

Chemical bonding structure of TiO2 thin films grown on n-type Si

Titanium dioxide thin films were obtained by RF magnetron sputtering system with different Ar and O atmospheres. Chemical bonding structures of the thin films were investigated using the Fourier transform infrared spectroscopy (FTIR) in the range of 400-7500 cm^− 1 for as-deposited and conventionally thermal annealed films at different temperature in air. These structural characterizations of the films were carried out by describing the low-frequency fluctuations of the FTIR spectra using the noninvasive (i.e. error controllable) procedure of the optimal linear smoothing. This approach is based on the criterion of the minimal relative error in selection of the proper smoothing window. It allows the receiving an optimal separation of a possible trend from the high-frequency fluctuations, defined as a random sequence of the relative fluctuations possessing zero trends. Thus, the noise can be read and extra information about the structures was then obtained by comparing with the experimental results. In the film annealed at 900 °C, the rutile phase was the dominant crystalline phase as revealed by infrared spectroscopy. At the annealing temperatures lower than 900 °C, both the anatase and the rutile phases were coexisting. In addition, symmetric and asymmetric Si-O-Si vibrations modes were observed at around 1000 cm^− 1 and 800 cm^− 1, respectively. These peaks suggest that a thin SiO₂ film was formed at the TiO₂/Si interface during the growth and the annealing of the TiO₂ films. It was also observed that the reactivity between TiO₂ film and Si substrate is increased with the increasing annealing temperature.     

The effect of the H2 flow rate on the structure and optical properties of TiO2 films deposited by inductively coupled plasma assisted chemical vapor deposition

The optical and photocatalytic properties of TiO₂ are closely related to crystalline structures, such as rutile and anatase. In this paper, TiO₂ films were produced by inductively coupled plasma (ICP) assisted chemical vapor deposition (CVD) without extra heating of the substrate, and the effect of H₂ addition on the structure and optical properties of the films was investigated. After increasing the partial pressure of H₂, the structure of the TiO₂ films changed from anatase to rutile, which usually appears at high temperatures (> 600 °C). The light transmittance decreased with increasing the H₂ flow rate due to the increased surface roughness. The photocatalytic activity of the anatase TiO₂ film was better than that of the rutile TiO₂ film.     

High-temperature anomalies of dielectric constant in TiO2 thin films

Anatase and rutile TiO₂ thin films were prepared by chemical vapor deposition with precursors Ti(OPrⁱ)₄ and Ti(dpm)₂(OPrⁱ)₂ (dpm = 2,2,6,6-tetramethylheptane-3,5-dione and Prⁱ = isopropyl), respectively. The dielectric properties of TiO₂ thin films have been studied in 20-1100 K temperature range in air, in controlled Ar/O₂ atmospheres, and in vacuum with silicon-based metal-insulator-semiconductor Au/TiO₂/Si capacitors. High-temperature (T_c ∼ 980 K) anomalous behavior of dielectric constant was observed in both anatase and rutile TiO₂ thin films.     

Synthesis of aluminum nitride thin films by filtered arc ion plating deposition

Thin films of aluminum nitride (AlN) were deposited on stainless steel and glass substrates by a modified deposition technique, filtered arc ion plating, at an enhanced deposition rate. X-ray diffraction spectra confirmed the exclusive presence of AlN hexagonal wurtzite phase. Under a mixed gas (Ar + N₂) pressure of 0.90 Pa and a bias voltage of − 400 V, the deposited films exhibited a fairly low surface roughness of 2.23 nm. The thin films were proved higher than 75% transparent in the visible spectral region. The bonding strength between the film and substrate was verified higher than 20 N. Thus high performance of such AlN thin films can be expected in applications.     

Effects of oxygen flow ratios and annealing temperatures on Raman and photoluminescence of titanium oxide thin films deposited by reactive magnetron sputtering

Titanium oxide (TiO_x) thin films were deposited on the Si(100) substrates by direct-current reactive magnetron sputtering at 3-15 % oxygen flow ratios (FO₂% = FO₂/(FO₂ + FAr) × 100%), and then annealed by rapid thermal annealing (RTA) at 350-750 °C for 2 min in air. The phase, bonding and luminescence behaviors of the as-deposited and annealed TiO_x thin films were analyzed by X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) spectroscopy, respectively. The as-deposited TiO_x films were amorphous from XRD and showed weak Raman intensity. In contrast, the distinct crystalline peaks of anatase and rutile phases were detected after RTA at 550-750 °C from both XRD and Raman spectra. A mixture of anatase and rutile phases was obtained by RTA at 3 FO₂% and its amount increased with annealing temperature. Only the anatase phase was detected in the 6-15 FO₂% specimens after RTA. The PL spectra of all post-annealed TiO_x films showed a broad peak in visible light region. The PL peak of TiO_x film at 3 FO₂% at 750 °C annealing can be fitted into two Gaussian peaks at ~ 486 nm (2.55 eV) and ~ 588 nm (2.11 eV) which were attributed to deep-level emissions of oxygen vacancies in the rutile and anatase phases, respectively. The peak around 550 nm was observed at 6-15 FO₂% which is attributed to electron-hole pair recombination from oxygen vacancy state in anatase phase to valence band. The variation of intensity of PL peaks is concerned with the formation of the rutile and anatase phases at different FO₂% and annealing temperatures.