TiO2 Buffer Layer Research Articles

Vanadium Dioxide Thin Films Deposited on TiO<sub>2</sub> Buffer Layer for Smart Thermochromic Glazing of Windows

Vanadium dioxide (VO2) films were prepared on quartz glass and TiO2-coated quartz glass substrates by reactive RF-magnetron sputtering. The VO2 thin film with film thickness of 50 nm deposited on quartz glass substrates showed two kinds of regions with different color visible to the naked eye, i.e., the earth yellow region and the cyan region. Both XRD and Raman spectroscopy showed that the different color regions of the films had the different crystallinity quality and phase composition. Whereas, the VO2 thin films with film thickness of 50 nm fabricated on TiO2-coated quartz glass (with TiO2 film thickness of 50 nm) had uniform color and exhibited a larger change in transmittance at near infrared region than the VO2 thin films deposited on quartz glass did. A TiO2 buffer layer improved the crystallinity and uniformity of the VO2 film. Such very thin VO2 films with a TiO2 buffer layer have high potential for practical application in smart thermal glazing of windows.

Enhancing the performance of poly(3-methylthiophene)-based electrochromic devices via insertion of a TiO2 buffer layer

Anatase-titanium dioxide (TiO2) nanoparticles were spin-coated on indium-tin oxide (ITO) glass substrates to fabricate TiO2 films through annealing at 450°C/30min. 3-Methylthiophene (3MT) was electrochemically deposited onto mesoporous TiO2 films to form poly(3-methylthiophene) (P3MT/TiO2) composite electrochromic electrodes. The introduction of TiO2 mesoporous films enhanced the adhesion of P3MT polymers to the TiO2/ITO substrate, thereby increasing the long-term stability of the corresponding electrochromic devices. Experimental results reveal that P3MT/TiO2 composite electrodes retained up to 84% of the maximum optical contrast (ΔT%), relative to 56% remaining ΔT% of pure ITO electrodes. This illustrates the enhanced long-term stability achieved through the introduction of a TiO2 buffer layer in electrochromic devices. These devices demonstrated excellent response time characteristics and ΔT% value of 4s and ca. of 70%, respectively. This work has shown that conductive polymer/TiO2 composite electrodes are well suited to electrochromic devices for the promotion of performance and stability.

Effects of the Nano-Tubular Anodic TiO2 Buffer Layer on Bioactive Hydroxyapatite Coating

We studied the effect of nano-tubular anodic TiO2 buffer layers on hydroxyapatite (HA) coating. The pulsed laser deposition (PLD) method was used to deposit HA on a well arranged nano-tubular anodic TiO2 (NT-ATO) buffer layer prepared by an electrochemical anodization technique. The surface morphology and chemical composition of HA coatings were characterized by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and contact angle measurement. We found that crystalline HA coatings show well arranged porous morphologies with a favorable surface wettability. We also found that an anodic nano-tubular TiO2 buffer layer with a relatively short tube length shows a better coating morphology. The deposition process of HA on the nanotubular TiO2 buffer layer was also proposed.

The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells

The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing oxide films of TiO2 and Nb2O5approximately 10 nm in thickness deposited onto FTO substrates. The experimental results demonstrated that the thin oxide layer serving to separate the electron collecting electrode and the photoactive film of a blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) was necessary to promote the formation of continuous uniform PCBM film to block holes in P3HT from being recombined with electrons in collecting electrode. A use of TiO2 buffer layer leads to power conversion efficiency as high as 2.8%. As for Nb2O5, in spite the fact that its conduction band is higher than the LUMO level of PCBM polymer acting as electron transport material, a power conversion of 2.7%, which was only slightly different from the 2.8% achieved for the cell employing TiO2. These experimental results suggest a tunneling mechanism for the electrons to transport from the PCBM to collecting electrode over the oxide film, instead of a diffusion through the oxide film arising from either energy or concentration difference of the photogenerated electrons.

Ti-Doped VO2 Films Grown on Glass Substrates by Excimer-Laser-Assisted Metal Organic Deposition Process

V1-xTixO2 films were prepared on glass substrates by excimer-laser-assisted metal organic deposition (ELAMOD) and thermal MOD processes. The VO2 phase was successfully formed in the range of x = 0–0.35 at 300 °C in air using the ELAMOD process. This is a big advantage in terms of fabrication process for glass or organic substrates. It was also confirmed that dense films were obtained by preparing TiO2 buffer layers on glass substrates. Furthermore, the detailed effect of Ti content in VO2 films on metal–insulator (MI) transition properties was examined. As Ti content increased, the broadening of MI transition and the decreasing of the hysteresis width occurred. At x = 0.14, the hysteresis disappeared. The temperature coefficient of electrical resistance (TCR) value of the film (x = 0.14) remained at approximately -3%/K from 300 to 340 K and was more than -4%/K from 340 to 350 K.

Effects of a TiO2 Buffer Layer on Solution-Deposited VO2 Films: Enhanced Oxidization Durability

In this article, thermochromic VO2 films were deposited on fused quartz and rutile TiO2-buffered fused quartz substrates via a solution-phase process. The incorporation of a TiO2 buffer layer endures an enhanced oxidization durability of VO2 films under an environment with high oxygen partial pressures. Oxidization in furnace during a cooling stage and rapid thermal oxidization (RTO) treatments were employed to investigate the evolution of microstructures and compositions of the films in the gradual oxidization processes. Oxidization treatments transformed VO2 into V2O5 for films grown on fused quartz substrates, whereas the oxidation process was significantly hindered for films prepared on a TiO2 buffer layer, especially around the VO2/TiO2 interface. The phenomenon is first reported in this article and is important for practical applications.

Anomalous Hall effect suppression in anatase Co:TiO2 by the insertion of an interfacial TiO2 buffer layer

We present the effect of introducing a TiO2 buffer layer at the SrTiO3/Co:TiO2 interface on the magnetic and structural properties of anatase Co:TiO2 (1.4 at. % Co). Inserting the buffer layer leads to suppression of the room-temperature anomalous Hall effect, accompanied by a reduced density of Co clusters, and a different depth distribution of such clusters. Co clusters in Co:TiO2 with a buffer layer are mostly formed at the surface, such that they are situated outside the current path and cannot contribute to the transverse anomalous Hall resistivity. These results indicate extrinsic origins of magnetism in anatase Co:TiO2.

Epitaxial Growth and Ferroelectricity of BaTiO3 on SrRuO3/TiO2 Buffered GaN

Epitaxial growth of (111)-oriented BaTiO3 thin films on GaN/Al2O3 (0001) substrates has been realized by intervening epitaxial (111) SrRuO3/(100)TiO2 buffer layers. Rutile TiO2 was deposited by Laser molecular beam epitaxial firstly and the deposition process of TiO2 layer was in-situ monitored by reflective high energy electron diffractions. After that SrRuO3 conductive films and BaTiO3 ferroelectric films were fabricated via pulsed laser deposition. The perovskite BaTiO3 films were epitaxially grown on wurtzite GaN as revealed by X-ray diffraction. Electrical measurements demonstrate that the BaTiO3 films possess a saturation ferroelectric hysteresis loop, butterfly-shape C-V curve and low leakage current density. These results show that BaTiO3 films with favorable electrical performance could be epitaxially grown on GaN using SrRuO3/TiO2 bufferlayer.

Microstructural investigation of strontium titanate films grown by interval pulsed laser deposition

Strontium titanate (SrTiO3) films were deposited on (001) magnesium oxide (MgO) substrates by pulsed laser interval deposition (PLID). The growth modes of the films were monitored by in-situ reflection high-energy electron diffraction (RHEED). The microstructures were investigated by transmission electron microscopy (TEM). It has been shown that PLID can enhance the two dimensional (2D) growth of this system, which has the large lattice misfit of 7.9%. The addition of a mono TiO2 buffer layer to the MgO substrate surface further improved the 2D growth mode. TEM results revealed that the films grown in 2D mode with large lattice mismatch were dominated by antiphase boundaries (APBs) and holes. All the holes were connected by antiphase boundaries. The holes were rectangular in shape, with facets along the [100] and [010] directions, and they ran though the whole film thickness. Threading and misfit dislocations have also been identified as growth defects in the films grown with and without a TiO2 buffer layer. However, the density of defects varies with growth mode.

Study of phase change memory cell with inserting buffer layer

AbstractTiO2 and HfO2 with different electrical conductivity were used as buffer layer for phase change memory (PCRAM) cell, where Si‐Sb‐Te alloy was used as storage medium in the PCRAM cell. The electrical properties of the cells with and without buffer layer were compared through resistance‐voltage measurements. It is found that the PCRAM cell with both TiO2 and HfO2 buffer layer has relative low operation voltage. Furthermore, the inserting TiO2 buffer layer can also improve the device stability. It is considered that buffer layer with suitable electrical conductivity is needed for the PCRAM cell, especially in future high density storage. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation of polycrystalline VO2 films on glass and TiO2/glass substrates by means of excimer laser assisted metal organic deposition

An excimer laser assisted metal organic deposition process for the preparation of vanadium dioxide (VO2) films is described. When a precursor film grown on a silica glass substrate by preheating at 300°C was irradiated with a KrF laser at room temperature in air, the film was crystallized into polycrystalline VO2 phases. Generally, the preparation of VO2 films requires strict atmosphere control, but in this process, the polycrystalline VO2 films were successfully prepared without it. Moreover, it was revealed that crystallization into the VO2 phases was promoted by a TiO2 buffer layer placed on the glass substrate. This means that the photochemical reaction induced by the TiO2 absorbing the laser participates in the crystallization. The electrical resistivity of the film grown on the TiO2/glass substrate was lowered compared to that of the film grown on the glass substrate due to improvement of the film uniformity. The crystallinity of the obtained film was dependent on the irradiation time. With the improved crystallinity of the film, the resistivity drop through the metal-insulator transition became large.

Using a low temperature crystallization process to prepare anatase TiO2 buffer layers for air-stable inverted polymer solar cells

In this study, we fabricated inverted polymer solar cells featuring titanium dioxide (TiO2) as the electron collection layer and vanadium (V) oxide (V2O5) as the hole collection layer. TiO2 films (anatase phase) were prepared by combining electrochemical deposition with high-pressure crystallization. The low temperature process used to obtain the TiO2 films minimized interdiffusion of Ti and In species between the TiO2 and ITO films and maintained the conductivity of the indium tin oxide substrate. The inverted device reached a power conversion efficiency of 3.22% and exhibited much better stability under ambient conditions relative to that of the corresponding conventional device.

Tunable dual-mode filter using varactor and variable ring resonator on integrated BST/TiO2/Si substrate

A RF tunable dual-mode filter employing a BST (BaxSr1−xTiO3)-based ring resonator and a capacitor is proposed. BST thin film was deposited on TiO2/Si substrate and a TiO2 buffer layer grown by ALD (atomic layer deposition). The device with a BST varactor capacitance of 0.5 pF and 40% of tunability applied voltage of 40 V, and a tunable ring resonator frequency of 1.53–2.02 GHz with a bias of up to 40 V, has been realised. The fabricated bandpass filter has a centre frequency of 1.53 GHz and shows a tunability of approximately 20% with an applied voltage of 40 V. It also has a bandwidth of 74 MHz and shows a tunability of approximately 27% with an applied voltage of 40 V.

Improvement of Photovoltaic Efficiency of Dye-Sensitized Solar Cell by Introducing Highly Transparent Nanoporous TiO<SUB>2</SUB> Buffer Layer

13 nm-sized highly-dispersible TiO2 nanoparticle was synthesized by solvothermal reaction of titanium isopropoxide in a basic condition with tetrabutylammonium hydroxide (TBAH). The prepared TiO2 nanoparticle was applied to fabrication of the transparent nanoporous TiO2 layer with 1.2 microm-thickness. By introducing this buffer layer between FTO and main TiO2 layer in the dye-sensitized solar cell (DSSC), the photovoltaic conversion efficiency was improved from 5.92% to 7.13%. Due to the excellent antireflective role of nanoporous TiO2 buffer layer, the transmittance of FTO glass was increased by 9.2%, and this seemed to be one of the major factors in enhancing photovoltaic conversion efficiency. Moreover, the presence of nanoporous TiO2 buffer layer induces excellent adhesion between FTO and main TiO2 layer, as well as it suppresses the back reaction by blocking direct contact between I3- and FTO electrode.

Effects of SiO2 and TiO2 on resistance stabilities of flexible indium-tin-oxide films prepared by ion assisted deposition

Inorganic buffer layers such as SiO2 or TiO2 and transparent conductive indium-tin-oxide (ITO) films were prepared on polyethylene terephthalate (PET) substrates by ion assisted deposition (IAD) at room temperature, and the effects of SiO2 and TiO2 on the bending resistance performance of flexible ITO films were investigated. The results show that ITO films with SiO2 or TiO2 buffer layer have better resistance stabilities compared to ones without the buffer layer when the ITO films are inwards bent at a bending radius more than 1.2 cm and when the ITO films are outwards bent at a bending radius from 0.8 cm to 1.2 cm. ITO films with SiO2 buffer layer have better resistance stabilities compared to ones with TiO2 buffer layer after the ITO films are bent several hundreds of cycles at the same bending radius, for the adhesion of SiO2 is stronger than that of TiO2. The compressive stress resulted from inward bending leads to the formation of more defects in the ITO films compared with the tensile stress arising from outward bending. SiO2 and TiO2 buffer layers can effectively improve the crystallinity of ITO films in (400), (440) directions.

Hydrothermal Growth and Application of ZnO Nanowire Films with ZnO and TiO2Buffer Layers in Dye-Sensitized Solar Cells

This paper reports the effects of the seed layers prepared by spin-coating and dip-coating methods on the morphology and density of ZnO nanowire arrays, thus on the performance of ZnO nanowire-based dye-sensitized solar cells (DSSCs). The nanowire films with the thick ZnO buffer layer (~0.8–1 μm thick) can improve the open circuit voltage of the DSSCs through suppressing carrier recombination, however, and cause the decrease of dye loading absorbed on ZnO nanowires. In order to further investigate the effect of TiO2buffer layer on the performance of ZnO nanowire-based DSSCs, compared with the ZnO nanowire-based DSSCs without a compact TiO2buffer layer, the photovoltaic conversion efficiency and open circuit voltage of the ZnO DSSCs with the compact TiO2layer (~50 nm thick) were improved by 3.9–12.5 and 2.4–41.7%, respectively. This can be attributed to the introduction of the compact TiO2layer prepared by sputtering method, which effectively suppressed carrier recombination occurring across both the film–electrolyte interface and the substrate–electrolyte interface.

Effects of Non-Continuous TiO<sub>2</sub> Buffer Layers on Structural and Electrical Properties of Indium Tin Oxide Thin Films

Indium Tin Oxide (ITO) films on polyethylene terephthalate (PET) sandwiching TiO2 buffer layers with different sputtering time have been prepared by rf-magnetron sputtering. Scanning Electron Microscope images of the TiO2 buffer layers showed the non-continuous growth in a typical sputtered film growing process. (400) oriented diffraction peaks appeared in all the ITO/TiO2/PET films. The electrical properties were measured by four point probe method and van der Pauw method. The variations of the resistivity and hall mobility on the sputtering time of TiO2 layer were studied. Furthermore, an exponential decay correlation of resistivity and the grain size was fitted. But the linear relationship between the ITO grain size and the mobility indicated by the grain boundary scattering theory didn’t appear here. The reason is believed to neglect the influence of the local variations of the lattice spacing and some other important scattering mechanisms such as lattice, ionized impurity, neutral impurity scattering, etc.

Adsorption-Controlled Growth of BiFeO3 by MBE and Integration with Wide Band Gap Semiconductors

BiFeO3 thin films have been deposited on (001) SrTiO3, (101) DyScO3, (011) DyScO3, (0001) AlGaN/GaN, and (0001) 6H-SiC single crystal substrates by reactive molecular beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth over-pressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry in accordance with thermodynamic calculations. Four-circle x-ray diffraction and transmission electron microscopy reveal phase-pure, epitaxial films with rocking curve full width at half maximum values as narrow as 7.2 arc seconds (0.002 degrees). Epitaxial growth of (0001)-oriented BiFeO3 thin films on (0001) GaN, including AlGaN HEMT structures, and (0001) SiC has been realized using intervening epitaxial (111) SrTiO3 / (100) TiO2 buffer layers. The epitaxial BiFeO3 thin films have 2 in-plane orientations: [1120] BiFeO3 || [1120] GaN (SiC) plus a twin variant related by a 180 degrees in-plane rotation. This epitaxial integration of the ferroelectric with the highest known polarization, BiFeO3, with high bandgap semiconductors is an important step toward novel field-effect devices.

Fabrication of high-quality three-dimensional photonic crystal heterostructures

Three-dimensional photonic crystal (PC) heterostructures with high quality are fabricated by using a pressure controlled isothermal heating vertical deposition technique. The formed heterostructures have higher quality, such as deeper band gaps and sharper band edges, than the heterostructures reported so far. Such a significant improvement in quality is due to the introduction of a thin TiO2 buffer layer between the two constitutional PCs. It is revealed that the disorder caused by lattice mismatch is successfully removed if the buffer layer is used once. As a result, the formed heterostructures possess the main features in the band gap of constitutional PCs. The crucial role of the thin buffer layer is also verified by numerical simulations based on the finite-difference time-domain technique.

Stability of ITO Films with Oxide Buffer Layer Grown onto PES Substrates

In this paper, ITO films with the oxide buffer layers were deposited by magnetron sputtering technique onto the PES. The oxide buffer layer was formed by a radio frequency (RF) magnetron sputtering method, using SiO2 or TiO2 target. The ITO layers were deposited by a low-frequency (LF) magnetron sputtering technique onto PES/Buffer layer. The sheet resistances of ITO films with the oxide buffer layers grown onto PES substrate were investigated as a function of time in the air and the vacuum. In both cases, the increasing rates of the sheet resistance of the PES/Buffer layer/ITO films are lower than those of the PES/ITO films. In addition, the increasing rates of sheet resistances of the films with the TiO2 buffer layer are lower than those of the films with the SiO2 buffer layer.