Control of electrical resistance of TiO2 films by short-pulse laser irradiation

Titanium dioxide (TiO₂) films were irradiated with a femtosecond laser beam to alter their electrical resistances. The TiO₂ film was produced by aerosol beam deposition. The wavelength, pulse duration, and repetition rate of the femtosecond laser scanned across the sample surface were 800 nm, 100 fs, and 1 kHz, respectively. By attenuating the laser fluence on the TiO₂ film, a range was found in which the electrical resistance of the TiO₂ film was varied even though the morphology of the film surface was not changed.     

Formation of TiO2 film with lower electrical resistance by aerosol beam and fiber laser irradiation

Titanium dioxide (TiO₂) is a functional ceramic with unique photoconductive and photocatalytic properties. In our previous study, a TiO₂ film was formed by aerosol beam irradiation. The films were darkened by femtosecond laser irradiation in air. Then electrical resistance of the darkened area on the film decreased. The heating process is also a useful method to vary the TiO₂ film property. Local heating can be performed by using a continuous wave (CW) fiber laser. In this study, the film was irradiated with a commercial CW fiber laser in vacuum. Laser irradiated area on the film was also darkened after CW fiber laser irradiation. The electrical resistance of the darkened area on the films was decreased as laser fluence was increased. Electrical resistance of the darkened area after CW fiber laser irradiation in vacuum was much smaller than that after femtosecond laser irradiation.     

Phase transformation at the surface of TiO<Subscript>2</Subscript> single crystal irradiated by femtosecond laser pulse

Phase transformation of a titanium oxide crystal irradiated by a femtosecond laser from rutile to anatase was studied by Raman spectroscopy. In the case of the high temperature phase of TiO₂ single crystal rutile, irradiated by the 120 fs, 800 nm, 250 kHz femtosecond laser with an average power of 300 mW for a short time, the intensity of Raman active mode E_g (446 cm^-1) of TiO₂ would decrease, while that of A_1g (611 cm^-1) increased, which indicated the color-center-defect-cluster was formed. After the longer irradiation time (less than 600 s), four new Raman active modes would occur, so a part of rutile in the irradiated region was transformed into anatase phase. As the irradiation time increased, the component of anatase increased to a constant, while that of rutile decreased. By this means, we can selectively induce anatase on the rutile surface through controlling the femtosecond laser exposure region. We suggest that this technique can be applied to fabricate micro patterns of anatase. PACS 52.38.Dx; 36.20.Ng; 36.40.Ei; 42.62.-b; 42.65.Dr     

Raman study of phase transformation of TiO2 rutile single crystal irradiated by infrared femtosecond laser

Titanium dioxide (TiO₂) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO₂ was observed. Micro-Raman spectra show that the intensity of E_g Raman vibrating mode of rutile phase increases and that of A_1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO₂ single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a constant with increasing of fs pulse laser irradiation time. The study indicates the more stable rutile phase is transformed into anatase phase by the high pressure produced by fs pulse laser irradiation.     

Microfabrication of gold dots in SiO2/TiO2 glass films by two-photon absorption

Photogeneration of Au nanoparticles in SiO₂/TiO₂ glass films was carried out by two-photon absorption with a femtosecond pulse laser. Exquisite microdot-arrays of Au with micrometer spatial resolution were achieved by scanning of the focused laser beam. These structures were constructed in SiO₂/TiO₂ glass films by a sol–gel method. The sol–gel method demonstrated that Au dots microarray are fabricated at any position by two-photon absorption in the glass. The results show the utility of a two-photon absorption technique in the fabrication of complicated patterns with metal particles.     

Preparation of transparent conductive TiO2:Nb thin films by pulsed laser deposition

This study investigated the optical and electrical properties of Nb-doped TiO₂ thin films prepared by pulsed laser deposition (PLD). The PLD conditions were optimized to fabricate Nb-doped TiO₂ thin films with an improved electrical conductivity and crystalline structure. XRD analyses revealed that the deposition at room temperature in 0.92 Pa O₂ was suitable to produce anatase-type TiO₂. A Nb-doped TiO₂ thin film attained a resistivity as low as 6.7 × 10⁻⁴ Ω cm after annealing at 350 °C in vacuum (<10⁻⁵ Pa), thereby maintaining the transmittance as high as 60% in the UV-vis region.     

Pulsed laser deposition of semiconducting crystalline double-doped barium titanate thin films on nickel substrates

We synthesized by pulsed laser deposition (Ba,Sr,Y)TiO₃ and (Ba,Pb,Y)TiO₃ thin films on mechanically polished nickel substrates.The synthesized thin films were analyzed for: crystalline structure by X-ray diffractometry, morphology and surface topography by atomic force microscopy, optical and scanning electron microscopy, and elemental composition by energy dispersive X-ray spectroscopy and electrical properties by electrical measurements.We have shown that film properties were determined by the dopants, target composition, and deposition parameters (oxygen pressure, substrate temperature and incident laser fluence). All films exhibited a semiconducting behavior, as proved by the decrease of electrical resistance with heating temperature.     

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films

A novel technique based on the excimer laser induced crystallization and modification of TiO₂ thin films is being reported. W⁺⁶ ions loaded TiO₂ (WTO) precursor films were prepared by a modified sol–gel method and spin-coated onto microscopic glass slides. Pulsed KrF (248 nm, 13 ns) excimer laser was used to irradiate the WTO amorphous films at various laser parameters. Mesoporous and nanostructured films consisting of anatase and rutile were obtained after laser irradiation at room temperature. The effect of varying W⁺⁶ ions concentrations on structural and optical properties the WTO films was analyzed by X-ray diffraction, field-emission scanning electron microscope, UV-Vis spectrophotometer and transmission electron microscope before and after laser treatment. Films irradiated for 10 pulses at 65–75 mJ/cm² laser fluence, exhibited anatase whereas higher parameters promoted the formation of rutile. XPS results revealed WO₃ along with minor proportion of WO₂ compounds after laser irradiation. Photo-absorbance of the WTO films was increased with increase in W⁺⁶ ions concentration in the film. TEM results exhibited a crystallite size of 15 nm which was confirmed from SEM results as well.     

TiO2 nanoparticle thin film deposition by matrix assisted pulsed laser evaporation for sensing applications

The MAPLE technique has been used for the deposition of nanostructured titania (TiO₂) nanoparticles thin films to be used for gas sensors applications. An aqueous solution of TiO₂ nanoparticles, synthesised by a novel chemical route, was frozen at liquid nitrogen temperature and irradiated with a pulsed ArF excimer laser in a vacuum chamber. A uniform distribution of TiO₂ nanoparticles with an average size of about 10 nm was deposited on Si and interdigitated Al₂O₃ substrates as demonstrated by high resolution scanning electron microscopy-field emission gun inspection (SEM-FEG). Energy dispersive X-ray (EDX) analysis revealed the presence of only the titanium and oxygen signals and FTIR (Fourier transform infra-red) revealed the TiO₂ characteristic composition and bond. A comparison with a spin coated thin film obtained from the same solution of TiO₂ nanoparticles is reported. The sensing properties of the films deposited on interdigitated substrates were investigated, too.     

Photoassisted chemical solution deposition method for fabricating uniformly epitaxial VO2 films

Epitaxial VO₂ films were prepared on the TiO₂ (001) substrates by the excimer-laser-assisted metal–organic deposition (ELAMOD). The quality of the epitaxial films obtained by irradiation with a KrF laser was found to be affected by the film structure obtained after preheating at 500 or 300°C. When the films containing crystal domains, which were obtained by preheating at 500°C, were irradiated with the laser at room temperature under a base pressure of 250 Pa, epitaxial and polycrystalline VO₂ phases were simultaneously formed. In contrast, when the amorphous films containing organic components, which were obtained by preheating at 300°C, were irradiated with the laser at room temperature in air, a single phase of epitaxial VO₂ was formed. By using thermal simulations, we determined that the formation of the epitaxial phase was affected both by the temperature distribution within the film during the laser irradiation and by the laser intensity at the interface between the substrate and the film. The latter factor is considered to play a role in the nucleation of crystallization, causing the epitaxial phase to form preferentially compared to the polycrystalline phase in the amorphous matrix of the films. These results indicate that the ELAMOD process is effective for the fabrication of epitaxial VO₂ films at low temperature.     

Time resolved spectra of optical and electrical transient response induced by nanosecond laser pulses in amorphous AgInSbTe films

The phase transition dynamics of amorphous Ag₈In₁₄Sb₅₅Te₂₃ (AIST) thin films induced by single nanosecond laser pulses were studied by transient optical reflectivity and electrical resistance measurements with nanosecond resolution. Phase transition driven by nanosecond laser pulses can be achieved in a proper fluence range on AIST thin films. The results show that phase transition dynamics driven by nanosecond laser pulses was a multi-stage optical evolution process involving melt, solidification, recalescence, and recrystallion. However, it was found that the real-time responses of optical and electrical signals were quite different under the same irradiated condition. The recalescence process reflected by the second rising of optical reflectivity will not result in obvious changes in electrical resistance. The dependence of saturated time determined by optical and electrical evolution curve on laser pulse fluence was compared and analyzed. A two-dimensional percolation model was employed to explain the difference between electrical and optical transient responses.     

CO2 laser-induced crystallization of sol–gel-derived indium tin oxide films

Indium tin oxide (ITO) thin films prepared by the sol–gel method have been deposited by the dip-coating process on silica substrates. CO₂ laser is used for annealing treatments. The electrical resistivity of sol–gel-derived ITO thin films decreased following crystallization after exposure to CO₂ laser beam. The topological and electrical properties of the irradiated surfaces have been demonstrated to be strongly related to the coating solution and to the laser processing parameters. Optimal results have been obtained for 5 dip-coating layers film from 0.4 mol/l solution irradiated by 0.6 W/m² laser power density. In this case, homogeneous and optically transparent traces were obtained with a measured sheet resistance of 1.46×10² Ω/□.     

Enhanced luminescence of silver nanoclusters in mesoporous film

Luminescence of silver nanoparticles photodeposited on titan dioxide nanoparticles of mesoporous film is studied. Luminescence was registered under the two-photon excitation by femtosecond laser pulses of Ti:sapphire laser. It occurs that Ag/TiO₂ mesoporous films have high concentration of bright luminescence spots which reveal stability to degradation under long illumination. Various configurations of silver nanoparticles are analyzed to explain the physics of bright luminescence spots (“hot spots”). Luminescence intensity reveals “hot spots” dependence on excitation laser pulse polarization. Properties of Ag/TiO₂ system show its promising usage for single molecule spectroscopy and biological objects visualization.     

Electrical properties of Sb-doped epitaxial SnO2 thin films prepared using excimer-laser-assisted metal–organic deposition

Excimer-laser-assisted metal–organic deposition (ELAMOD) was used to prepare Sb-doped epitaxial (001) SnO₂ thin films on (001) TiO₂ substrates at room temperature. The effects of laser fluence, the number of shots with the laser, and Sb content on the electrical properties such as resistivity, carrier concentration, and carrier mobility of the films were investigated. The resistivity of the Sb-doped epitaxial (001) SnO₂ thin film prepared using an ArF laser was lower than that of the film prepared using a KrF laser. The van der Pauw method was used to measure the resistivity, carrier concentration, and carrier mobility of the Sb-doped epitaxial (001) SnO₂ thin films in order to determine the effect of Sb content on the electrical resistivity of the films. The lowest resistivity obtained for the Sb-doped epitaxial (001) SnO₂ thin films prepared using ELAMOD with the ArF laser and 2 % Sb content was 2.5 × 10^?3 Ω cm. The difference between the optimal Sb concentrations and resistivities of the films produced using either ELAMOD or conventional thermal MOD was discussed.     

Characteristics of SrBi2Ta2O9 ferroelectric films on GaAs with a TiO2 buffer layer

Mainly [115]-oriented SrBi₂Ta₂O₉ (SBT) films were prepared on GaAs(100) substrates with TiO₂ buffer layers. Both the SBT films and the TiO₂ buffer layers were deposited by pulsed laser deposition (PLD) using a KrF excimer laser. The depth profile of the constituent elements observed by Auger electron spectrometry (AES) shows no remarkable diffusion at both the interfaces between SBT and TiO₂ and between TiO₂ and the GaAs substrate. The electrical characteristics of the Pt/SBT/TiO₂/GaAs(100) structures show a ferroelectric hysteresis loop with a small remanent polarization (∼0.5 μC/cm²). Received: 1 March 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: xhliu81@hotmail.com RID="**" ID="**"Present address: Data Storage Institute, DSI Building, 5, Engineering Drive 1 (off Kent Ridge Crescent, NUS) 117608 Singapore     

Femtosecond Z-scan measurement of third-order optical nonlinearities in anatase TiO2 thin films

Anatase phase TiO₂ films have been grown on fused silica substrate by pulsed laser deposition technique at substrate temperature of 750 °C under the oxygen pressure of 5 Pa. From the transmission spectra, the optical band gap and linear refractive index of the TiO₂ films were determined. The third-order optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. The real and imaginary parts of third-order nonlinear susceptibility χ⁽³⁾ were determined to be −7.1 × 10⁻¹¹esu and −4.42 × 10⁻¹²esu, respectively. The figure of merit, T, defined by T=βλ/n₂, was calculated to be 0.8, which meets the requirement of all-optical switching devices. The results show that the anatase TiO₂ films have great potential applications for nonlinear optical devices.     

Damage performance of TiO2/SiO2 thin film components induced by a long-pulsed laser

In order to study the long-pulsed laser induced damage performance of optical thin films, damage experiments of TiO₂/SiO₂ films irradiated by a laser with 1 ms pulse duration and 1064 nm wavelength are performed. In the experiments, the damage threshold of the thin films is measured. The damages are observed to occur in isolated spots, which enlighten the inducement of the defects and impurities originated in the films. The threshold goes down when the laser spot size decreases. But there exists a minimum threshold, which cannot be further reduced by decreasing the laser spot size. Optical microscopy reveals a cone-shaped cavity in the film substrate. Changes of the damaged sizes in film components with laser fluence are also investigated. The results show that the damage efficiency increases with the laser fluence before the shielding effects start to act.     

Effects of γ rays irradiation on structure and property of TiO2 thin films

TiO₂ thin films were deposited on a glass substrate by the radio frequency magnetron sputtering method, and annealed for 2 h at temperatures of 550°C. Then, ⁶⁰Co γ rays with different doses were used to irradiate the resulting TiO₂ thin films. The surface features of films before and after irradiation were observed by scanning electron microscope (SEM). Simultaneously, the crystal structure and optical properties of films before and after irradiation were studied by X-ray diffraction (XRD), UV–VIS transmission spectrum and Photoluminescence (PL) spectrum, respectively. The SEM analysis shows that the film is smooth with tiny particles on the film surface, and non-crystallization trend was clear after irradiated with γ rays. The XRD results indicated that the structure of the film at the room temperature mainly exists in the form of amorphous and mixed crystal at a sputtering power of 200 W, and non-crystallinity was more obvious after irradiation. Obvious difference can be found for the transmissibility of the irradiated and pre irradiation TiO₂ films by the UV-VIS spectra. The color becomes light yellow, and the new absorption edge also appeared at about 430 nm. PL spectra and photocatalysis experiments indicate that the photocatalysis degradation rate of the TiO₂ films on methylthionine chloride solution irradiated with the maximum dose can be increased to 90%.     

Femtosecond laser direct writing of TiO2 crystalline patterns in glass

Crystalline TiO₂ was induced three dimensionally inside a glass sample by 800-nm, 250-kHz femtosecond laser irradiation. Micro-Raman spectra and X-ray diffraction analysis indicated that the laser-induced crystals were monophase TiO₂ rutile. A periodic structure consisting of TiO₂ rutile crystalline lines was inscribed in the glass sample by continuously moving the focal point of the laser beam. This technique may be useful for fabricating integrated optical devices in glasses.     

Research on temperature field characteristics of optical films under 1064 nm short-pulse laser radiation

The 3-D temperature field distribution (TFD) equation of optical films (OFs) was analyzed, based on the film temperature field distribution (FTFD) model irradiated by Gaussian short-pulse laser, and short-pulse laser-induced temperature field distribution of TiO₂/SiO₂ composite membranes on K9 glasses was obtained by means of numerical simulation. The results show that the thermal effect of optical films is very obvious under the Gaussian short-pulse laser. Temperature at the center of the spot rises faster than in other areas, and it decreases along the radius rapidly. It provides theoretical basis for progress analysis of short-pulse laser acting on optical films.