Tuning the Phase and Microstructural Properties of TiO<Subscript>2</Subscript> Films Through Pulsed Laser Deposition and Exploring Their Role as Buffer Layers for Conductive Films

Titanium oxide (TiO₂) is a semiconducting oxide of increasing interest due to its chemical and thermal stability and broad applicability. In this study, thin films of TiO₂ were deposited by pulsed laser deposition on sapphire and silicon substrates under various growth conditions, and characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption spectroscopy and Hall-effect measurements. XRD patterns revealed that a sapphire substrate is more suitable for the formation of the rutile phase in TiO₂, while a silicon substrate yields a pure anatase phase, even at high-temperature growth. AFM images showed that the rutile TiO₂ films grown at 805°C on a sapphire substrate have a smoother surface than anatase films grown at 620°C. Optical absorption spectra confirmed the band gap energy of 3.08 eV for the rutile phase and 3.29 eV for the anatase phase. All the deposited films exhibited the usual high resistivity of TiO₂; however, when employed as a buffer layer, anatase TiO₂ deposited on sapphire significantly improves the conductivity of indium gallium zinc oxide thin films. The study illustrates how to control the formation of TiO₂ phases and reveals another interesting application for TiO₂ as a buffer layer for transparent conducting oxides.     

Effect of titanium dioxide (TiO2) seed layer on the phase composition of 0.7Pb (Mg1/3Nb2/3)O3-0.3PbTiO3 film prepared by pulsed laser deposition

Lead-magnesium niobate-lead titanate (PMN-PT) thin films with and without the TiO₂ seed layer were deposited on Pt/Ti/SiO₂/Si substrates through pulsed laser deposition. The study aimed to characterize the effect of the TiO₂ seed layer on the phase composition and properties of PMN-PT film. Without the TiO₂ seed layer, the pure perovskite phase could be obtained in the thinner PMN-PT film while with the TiO₂ seed layer, the pure perovskite phase was formed in the thicker PMN-PT film. The ferroelectric properties of PMN-PT films with the TiO₂ seed layer were exhibited. As a result, the maximum amount of remnant polarization reached the amount of 32 μC/cm² for the PMN-PT thin film with the TiO₂ seed layer.     

Reactive pulsed laser deposited N–C codoped TiO2 thin films

TiO₂ is a large bandgap chemically stable oxide useful for several applications that involve photo-activated processes, including photocatalysis, photovoltaics, photoelectrolysis, etc. However, the large band gap renders this material not a very efficient absorber of the solar spectrum. Various schemes of cation and anion doping have been utilized that reduce this deficiency to a certain extent. In this paper we present the results of N–C codoping of TiO₂ thin films deposited by a reactive pulsed laser deposition technique. These films were compared for their optical and structural properties with undoped, N doped and C doped TiO₂ films prepared by the same technique. While all samples contained polycrystalline anatase phase, varying N₂ and CH₄ partial pressures resulted in change in TiO₂ lattice parameters due to codoping. X-ray diffraction high-resolution scans show the evidence of C incorporation into TiO₂ lattice by 2θ shift in (101) reflections due to large ionic radius of C. N doping was confirmed by XPS analyses. Direct relationship between oxygen vacancies and doping concentration was established by the deconvolution of XPS peaks. Considerable bandgap reduction occurred that was measured by using UV–vis diffuse reflectance spectroscopy. Results show that reactive pulsed laser deposition is indeed a useful method for the synthesis of codoped TiO₂ thin films as bandgap reduction of ~1.00 eV via N–C codoping was successfully achieved.     

Effects of annealing technique and TiO2 seed layer on the phase composition of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 film

The lead magnesium niobate–lead titanate (PMN–PT) thin films with and without the TiO₂ seed layer were prepared by a pulsed laser deposition (PLD) deposited on Pt/Ti/SiO₂/Si substrates. The films were treated by two-step annealing and normal annealing with rapid thermal annealing (RTA). The effects of two-step annealing and the TiO₂ seed layer on the phase composition of PMN–PT films were studied. The results show that the PMN–PT film with TiO₂ seed layer can gain a pure perovskite phase with a high (1 0 0) preferential orientation after the two-step annealing technique.     

Field electron emission from amorphous CNx:B films

CN_x:B thin films were prepared on titanium coated ceramic substrate by pulsed laser deposition technique (PLD). The microstructure of the film was examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The analyses indicate that the deposited samples are amorphous CN_x:B thin films. Field electron emission characteristics of amorphous CN_x:B thin films were measured in a vacuum chamber with a base pressure of about 3.2×10⁻⁵ Pa. The turn-on field of the film was 3.5 V/μm. The current density was 60 μA/cm² at an electric field of 9 V/μm. The experimental results indicate that this film could be a promising material applicable to cold cathodes.     

Pulsed laser deposition of titanium nitride and diamond-like carbon films on polymers

We have investigated the deposition of titanium nitride (TiN) and diamond-like carbon (DLC ) films on polymethylmethacrylate (PMMA) substrates using pulsed laser deposition (PLD) technique. The TiN and diamond-like films were deposited by laser ablation (KrF excimer laser λ = 248 nm, pulse duration τ～25 × 10^?9 s, energy density ～2?15J/cm²) of TiN and graphite targets, respectively, at room temperature. These films were characterized by transmission electron microscopy, scanning electron microscopy, x-ray diffraction, Auger electron spectroscopy, UV-visible absorption spectroscopy, and Raman spectroscopy. The TiN films were smooth and found to be polycrystalline with average grain size of 120Å. The diamond-like carbon films were amorphous with a characteristic Raman peak at 1550 cm^?1. The TiN films are highly adherent to the polymer substrates as compare to DLC films. The adhesion strength of DLC films on polymers was increased by interposing thin TiN layer (200Å) on polymers byin-situ pulsed laser deposition. The DLC films were found to be amorphous with good adhesion to TiN/PMMA substrates.     

A study of ultrasonic spray pyrolysis deposited rutile-TiO2-based metal-semiconductor-metal ultraviolet photodetector

This work uses ultrasonic spray pyrolysis deposition to grow the TiO₂ film on a Si substrate. The TiO₂ film was annealed at 800 °C for 2 h to form rutile phase. X-ray diffraction, Raman spectrum, X-ray photoelectron spectroscopy were used to characterized rutile phase TiO₂. The optical characteristics like refractive index, extinction coefficient and absorption coefficient were measured. The rutile TiO₂-based metal-semiconductor-metal ultraviolet photodetector was fabricated and investigated, including current-voltage characteristic, photoresponsivity, external quantum efficiency, response time, noise equivalent power, and detectivity.     

Growth of Nanocrystalline ZnSe:N Films by Pulsed Laser Deposition

Nanocrystalline zinc-blende-structured ZnSe:N films have been deposited on GaAs(100) substrates by pulsed laser deposition (PLD). The growth of the nanocrystalline ZnSe:N films is found to be greatly affected by the pressure of ambient N₂. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) results show that the morphologies of the as-grown films are sensitive to the ambient pressure at a fixed substrate temperature of 300 °C, and the sizes of the as-grown ZnSe:N nanocrystals increase as the ambient pressure increases from 0.1 Pa to 100 Pa. The average sizes of the as-grown nanocrystals are estimated to be about 19 nm, 29 nm, and 71 nm for 0.1 Pa, 1 Pa, and 100 Pa ambient N₂ pressure, respectively. X-ray photoelectron spectroscopy analyses show that the N-doping concentration in the as-grown film is over 10²¹ cm⁻³. Raman spectra demonstrate the broadening of the longitudinal optical (LO) phonon and transverse optical (TO) phonon modes of the ZnSe nanocrystals. Based on these analyses, the mechanism of the formation of ZnSe:N nanocrystals is discussed.     

Effect of thermal annealing and exposure to oxygen plasma on the properties of TiO2-Si structures

The effect of thermal annealing and exposure to oxygen plasma on the phase composition, structure, and microprofile of titanium-dioxide films deposited by high-frequency magnetron sputtering on silicon substrates is studied. The influence exerted by processing modes on the capacitance-voltage and conductance-voltage characteristics of Me-TiO₂-Si-Me structures and on the density of surface states at the semiconductor-insulator interface is examined. It is shown that TiO₂ films are amorphous upon their fabrication. Upon the annealing of films at 500°C in an argon atmosphere, crystallites of anatase and rutile appear in the amorphous matrix. The treatment of a titanium-dioxide film in oxygen plasma gives rise to rutile crystallites with new crystallographic planes. As a result of annealing at 750°C, the anatase phase disappears and the film becomes polycrystalline, containing only rutile crystallites. The capacitance of Me-TiO₂-Si-Me structures in the accumulation mode reaches the maximum value upon annealing at 750°C, which is due to the transformation of titanium dioxide to the rutile phase. The specific capacitance is 5.9 × 10^?2 F/cm³. The decrease in the capacitance of the structures and in the amount of fixed charge in the insulator upon exposure to oxygen plasma is due to the diffusion of oxygen atoms across the titanium-dioxide layer to give a SiO₂ film at the TiO₂-Si interface. As a result of the annealing and treatment of a titanium-dioxide film in oxygen plasma, the energy density of surface states decreases by more than an order of magnitude as compared with the unannealed samples.     

Influence of Ti film thickness and oxidation temperature on TiO2 thin film formation via thermal oxidation of sputtered Ti film

Single-phase rutile TiO₂ films with good crystallinity were obtained by thermal oxidation of sputtered Ti films on Si and quartz substrates. The influence of the Ti film thickness on oxidation was systematically investigated. A temperature of 823 K was sufficient to fully oxidize Ti films of <0.2 μm in thickness, but 923 K was required for complete oxidation of thicker films. The crystal structure, phase, composition, and optical properties of the TiO₂ films were investigated using X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray analysis (EDAX), and UV-vis-NIR spectroscopy. XRD and Raman analyses showed that the TiO₂ films are rutile phase. The bandgap of the TiO₂ films decreased with increasing thickness. A growth mechanism for TiO₂ thin films due to thermal oxidation of sputtered Ti films is proposed. Oxidation commences from the surface and proceeds inside the bulk and Ti→TiO₂ phase transformation occurs via different intermediate phases. We found that the oxidation temperature rather than the duration is the dominant factor in the growth of TiO₂ thin films.     

Influence of Oxygen Pressure on the Properties of Ni-Mn-Zn Ferrite Films on Silicon Substrate

Ni-Mn-Zn ferrite films were prepared on silicon substrate with CeO₂/YSZ (YSZ: yttria-stabilized ZrO₂) as buffer layer by pulsed laser deposition. The influence of oxygen pressure on the microstructures and magnetic properties of films were studied. It was shown that the film crystallinity, microstructure and soft magnetic properties were sensitive to the oxygen pressure. High quality film with preferred (311) orientation, uniform surface structure, high purity and excellent soft magnetic properties can be obtained at the oxygen pressure of 1 Pa. The saturation magnetization (M _s) is higher than 230 emu/cm³ with a coercivity (H _c) smaller than 30Oe, which is suitable to be used in high-frequency devices.     

Low temperature TiO2 rutile phase thin film synthesis by chemical spray pyrolysis (CSP) of titanyl acetylacetonate

Rutile phase TiO₂ thin films have been synthesized using chemical spray pyrolysis of titanyl acetylacetonate TiAcAc in ethanol at 500 °C. The first part of the paper focuses on the thermal decomposition behavior of the precursor by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) coupled with differential scanning calorimetry (DSC). The second part of the paper focuses on the evolution of TiO₂ thin films and their structural transformation with substrate temperature. XRD revealed amorphous TiO₂ thin film at low substrate temperatures (<350 °C) and on high substrate temperatures anatase (3.84 g/cm³) or rutile (4.25 g/cm³) crystalline structure was obtained. The lattice constant, grain size, microstrain and the dislocation density of the film were obtained from the peak width. FTIR spectra of both anatase and rutile TiO₂ revealed stretching vibration of the Ti–O bond for tetrahedral and octahedral surroundings of the titanium atom. Scanning electron micrograph showed the compactness of the rutile film.     

Conductive Transparent TiNx/TiO2 Hybrid Films Deposited on Plastics in Air Using Atmospheric Plasma Processing

The successful deposition of conductive transparent TiN_x/TiO₂ hybrid films on both polycarbonate and silicon substrates from a titanium ethoxide precursor is demonstrated in air using atmospheric plasma processing equipped with a high‐temperature precursor delivery system. The hybrid film chemical composition, deposition rates, optical and electrical properties along with the adhesion energy to the polycarbonate substrate are investigated as a function of plasma power and plasma gas composition. The film is a hybrid of amorphous and crystalline rutile titanium oxide phases and amorphous titanium nitride that depend on the processing conditions. The visible transmittance increases from 71% to 83% with decreasing plasma power and increasing nitrogen content of the plasma gas. The film resistivity is in the range of ～8.5 × 10¹ to 2.4 × 10⁵ ohm cm. The adhesion energy to the polycarbonate substrate varies from ～1.2 to 8.5 J/m² with increasing plasma power and decreasing plasma gas nitrogen content. Finally, annealing the film or introducing hydrogen to the primary plasma gas significantly affects the composition and decreases thin‐film resistivity.     

Structural phase-dependent resistivity of intrinsic-extrinsic co-doped transparent titanium dioxide films

The authors report a method of enhancing the conductivity of TiO₂ films by controlling their structural phases. Thin films of Nb:TiO₂ (TNO) were prepared on glass and silicon substrates by RF sputtering with varying Nb content at 200 °C. It is shown that fine control over the structural phases of TiO₂ is critical for achieving low resistivity. The resistivity values of the films doped with oxygen vacancies and Nb⁺⁵ decreased from 3.8 × 10⁻¹ to 4.1 × 10⁻³ Ω cm when the weight percent of rutile in anatase-rutile phase mixture decreases from 52.8% to 32%. Furthermore, the lowest resistivity value of 2.37 × 10⁻³ Ω cm was obtained for the doped TiO₂ films having single phase anatase structure. The physical processes responsible for the diverse electrical properties are discussed and are associated with the growth conditions. Our result indicates that highly conductive doped-TiO₂ film can be obtained by controlling the anatase phase formation via the growth temperature. The obtained results can significantly contribute to the development of transparent electrodes by RF sputtering, a suitable technique for coating large area substrates.     

Dielectric properties of low loss Ba<Subscript>6−3x</Subscript>Nd<Subscript>8+2x</Subscript>Ti<Subscript>18</Subscript>O<Subscript>54</Subscript> thin films prepared by pulsed laser deposition for microwave applications

Ba_6−3xNd_8+2xTi₁₈O₅₄ with x=0.25 (BNT-0.25, or simply, BNT) dielectric thin films with a thickness of 320 nm have been prepared on Pt-coated silicon substrates by pulsed laser deposition (PLD) at the substrate temperature of 650°C in 20 Pa oxygen ambient. X-ray analysis showed that the as-deposited films are amorphous and the films remain amorphous after a postannealing at 750°C for 30 min. The dielectric constant of the BNT films has been determined to be about 80 with a low loss tan δ of about 0.006 at 1 MHz. The capacitance-voltage (C-V), capacitance-frequency, and capacitance-temperature characteristics of a BNT capacitor with Pt top electrode were measured. A low leakage-current density of 4×10⁻⁶ A/cm² at 6 V was measured, and a preliminary discussion of the leakage-current mechanism is also given. It is proposed that amorphous BNT-0.25 thin films will be a potential dielectric material for microwave applications.     

Preparation and characterization of TiO2 thin film by thermal oxidation of sputtered Ti film

Titanium dioxide (TiO₂) thin films were successfully prepared on quartz substrate by thermal oxidation of sputtered titanium film in air. The structure, composition, morphology and optical properties of oxidized TiO₂ films were characterized by Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and UV-visible spectroscopy. Meanwhile, the photocatalytic activity of the films was evaluated on the basis of the degradation of methyl orange solution under UV irradiation. Ti films after oxidation present mainly in TiO₂ form with a larger amount of adsorbed O₂, and oxidation temperature has a strong impact on the crystal structure and properties of the films. A phase transformation of anatase to rutile for oxidized TiO₂ films occurred in the temperature range of 700–800 °C. The energy band gap of oxidized TiO₂ films decreased first and then increased with annealing temperature. Furthermore, TiO₂ film oxidized at 600 °C exhibited the best photocatalytic activity due to suitable crystal phase and size. These results might contribute to the synthesis of metal oxide thin films with expectant structural morphology and properties by thermal oxidation methods.     

Laser processing of TiO2 films for dye solar cells: a thermal,sintering, throughput and embodied energy investigation

We have analysed and optimised a laser process for the sintering of the TiO₂ layers in dye solar cells (DSCs). Through a thermographic characterisation of the process, we show that it is possible to scale and process large areas uniformly (16 cm²). We fabricated DSCs with nanocrystalline (nc)‐TiO₂ films sintered by using pulsed ultraviolet laser with an average output power P varying from 1 W to 7 W whilst mainting a constant power conversion efficiency η. The highest efficiency reached for a laser sintered DSC was 7%. The time required to sinter 1 m² of nc‐TiO₂ film was found to decrease hyperbolically with P, which is important for determining process takt times. We quantified the embodied energy (EE) required to sinter 1 m² of the active TiO₂ layer for a variety of different processes, and found that the EE for the laser sintering process with a system wall plug efficiency of 3.5% to be competitive with the more conventional oven and belt furnace treatments. We outline the main features required from a laser system to carry out an efficient, energetically favourable and industrially applicable automated process with competitive throughput. Copyright © 2012 John Wiley & Sons, Ltd.     

C-V characteristics of metal-titanium dioxide-silicon capacitors

Titanium dioxide capacitors were fabricated on silicon wafers using electron-beam evaporation. The TiO₂ films varied in thickness from 500 to 2000 Å. Post-deposition oxidation at 1000°C in dry O₂ was used to promote stoichiometric conversion of the films to the rutile phase. Capacitive densities of greater than 2 pf/sq. mil were obtained (dielectric constants ranged from 4 to 40). For long oxidation times, significant silicon dioxide grows under the TiO₂ as a result of oxygen diffusing through the TiO₂ film. Titanium was also shown to diffuse into the silicon during the oxidation cycle resulting in an n-type diffusion. Surface state densities ranging from 10¹¹ to 5 × 10¹¹ cm^?2 eV^?1 at midgap were obtained for good devices. Longer oxidation times result in lower capacitance, leakage current and surface state density.     

Comparative study between CBD and SILAR methods for deposited TiO2, CdS,and TiO2/CdS core-shell structure

TiO₂, CdS, and TiO₂/CdS core–shell structures were deposited on fluorine-doped tin oxide (FTO)-coated glass substrate using chemical methods. TiO₂ thin films were prepared by chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). SILAR was also utilized to deposit CdS film on TiO₂ thin film. The structural, surface morphology, and optical characteristics of FTO/TiO₂, FTO/CdS, and FTO/TiO₂/CdS core- shell structures were evaluated. The FTO/TiO₂ films produced by both methods conformed to anatase and rutile phase structures. Corresponding XRD pattern of the FTO/TiO₂/CdS sample exhibited one peak corresponding to hexagonal (101) for CdS. Scanning electron micrographs showed nanorod structures for the TiO₂ thin films deposited by CBD, contrary to the nanograin structure formed by SILAR. Optical results showed highly extended absorption edge to the visible region for the FTO/TiO₂/CdS structure deposited by the two methods. The TiO₂ thin films deposited by CBD exhibited higher absorption in the visible region than nanograined TiO₂ thin films deposited by SILAR because of the high surface area of the TiO₂ nanorod. Photoelectrochemical (PEC) properties of FTO/TiO₂, and FTO/TiO₂/CdS system were also examined. PEC behavior of FTO/TiO₂/CdS was compared with that of FTO/TiO₂ deposited by CBD and SILAR. The TiO₂ nanorod thin films deposited by CBD showed evidently enhanced PEC performance compared with nanograined TiO₂ thin films deposited by SILAR.     

Synthesis of thin p-type rutile films

The structure and electrical and optical properties of heterostructures formed on the surface of single-crystal silicon wafers as a result of the heat treatment and pulsed photon treatment of Ti films in oxygen, air, and nitrogen are investigated. It is shown that a TiO₂/Ti₅Si₃/p-Si heterostructure is formed upon heat treatment in air, whereas a TiO₂/TiSi₂/p-Si heterostructure is formed upon photon treatment. It is established that rutile films with pronounced n-type conductivity are formed as a result of the heat treatment of Ni-doped Ti films in oxygen. Rutile films with p-type conductivity are formed upon the thermal annealing of Ti films in air with subsequent photon treatment in nitrogen.