Photovoltaic effect of TiO2 thick films with an ultrathin BiFeO3 as buffer layer

The photovoltaic (PV) effect of a bilayer anatase TiO₂/BiFeO₃ (BFO) film has been studied. The 20-nm ultrathin BFO layers were deposited on the fluorine-doped tin oxide (FTO) glass substrates by the chemical solution deposition method. An anatase TiO₂ layer is deposited subsequently on the BFO surface via a screen-printing technique. It is found that the FTO/TiO₂/Au cell exhibits negligible PV effect under solar exposure, while the one after introducing an ultrathin BFO film between TiO₂ and FTO leads to a considerable PV effect with an open-circuit voltage of ?0.58 V and a photocurrent density of 18.27 µA/cm². The FTO/BiVO₄ (BVO)/TiO₂/Au cell was constructed to investigate the underlying mechanism for the observed effect. A negligible PV effect of the FTO/BVO/TiO₂/Au cell indicates that the PV effect of the FTO/BFO/TiO₂/Au cell arises mainly from a built-in electric field in the BFO film induced by the self-polarization. Our work opens up a new path to utilize TiO₂ and may influence the future design of solar cells.     

Influence of Au thickness on the optical and electrical properties of transparent and conducting TiO2/Au/TiO2 multilayer films

Au-intermediate TiO₂/Au/TiO₂ (TAT) multilayer films were deposited by RF magnetron sputtering onto glass substrates. Changes in the optical and electrical properties of the films were investigated with respect to the thickness of the Au interlayer.The observed optical and electrical properties were dependent on the thickness of the Au interlayer. The resistivity decreased to 3.3 × 10⁻⁴ Ω cm for TiO₂ films with a 20 nm-thick Au interlayer and the optical transmittance was also influenced by the Au interlayer. Although optical transmittance deteriorated as Au thickness increased, TiO₂ films with a 5 nm-thick Au interlayer showed a relatively high optical transmittance of 80% at a wavelength of 550 nm. In addition, since a TAT film with a 5 nm-thick Au interlayer showed a relatively high work function value, it is an alternative candidate for use as a transparent anode in OLEDs and flat panel displays.     

Specific contact resistance of IGZO thin film transistors with metallic and transparent conductive oxides electrodes and XPS study of the contact/semiconductor interfaces

In this work, the specific contact resistance (ρ_c) between amorphous indium-gallium-zinc-oxide (IGZO) semiconductor and different contact electrodes was obtained from thin film transistors (TFTs). Ti/Au (10/100 nm), aluminum doped zinc oxide (AZO, 100 nm) and indium tin oxide (ITO, 100 nm) were used as source/drain electrodes to fabricate IGZO TFTs. Chemical states of the contacts/semiconductor interfaces were examined by depth profile X-ray photoelectron spectroscopy (XPS) analysis to explain the origin of the differences on specific contact resistance. The lowest ρ_c achieved using Ti/Au was related to the formation of a TiO_x interlayer due to oxygen atoms diffusing out from the semiconductor under layer, increasing the carrier concentration of IGZO at the interface and lowering the ρ_c. On the contrary, no interfacial reactions were observed between IGZO and AZO or ITO source/drain. However, IGZO resistivity increased with ITO contacts likely due to oxygen vacancies filling during ITO deposition. This fact seems to be the origin of the high contact resistance between IGZO and ITO, compared to IGZO-AZO and IGZO-Ti/Au interfaces.     

Deposition of Au/TiO2 film by pulsed laser

Au nanoparticles, which were photoreduced by a Nd:YAG laser in HAuCl₄ solution containing TiO₂ colloid and accompanied by the TiO₂ particles, were deposited on the substrate surface. The film consisting of Au/TiO₂ particles was characterized by the absorption spectra, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The adhesion between the film and substrate was evaluated by using adhesive tape test. It was found that the presence of TiO₂ dramatically enhanced the adhesion strength between the film and the substrate, as well as the deposition rate of film. The mechanism for the deposition of Au/TiO₂ film was also discussed.     

Pulsed laser deposition of Pb(Zr0.52Ti0.48)O3 thin film on cobalt ferrite nano-seed layered Pt(111)/Si substrate: effect of oxygen pressure

The effect of oxygen pressure during pulsed laser deposition of Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films on CoFe₂O₄ nano-seed layered Pt(111)/Si substrate was investigated. The PZT film deposited at oxygen pressure lower than 25 mTorr is identified as both perovskite and pyrochlore phases and the films deposited at high oxygen pressure (50-100 mTorr) show the single-phase perovskite PZT that has a perfect (111)-orientation. In addition, the film deposited at P_O2 of 50 mTorr has a uniform surface morphology, whereas the film deposited at P_O2 of 100 mTorr has a non-uniform surface morphology and more incompacted columnar cross-section microstructure. The polarization of film deposited at 100 mTorr is higher than that deposited at 50 mTorr, but shift of the hysteresis loop along the electrical field axis in the film deposited at P_O2 of 100 mTorr is larger than that of the film deposited at P_O2 of 50 mTorr.     

Characterization of TiO2/Au/TiO2 films deposited by magnetron sputtering on polycarbonate substrates

Transparent and conducting TiO₂/Au/TiO₂ (TAuT) films were deposited by reactive magnetron sputtering on polycarbonate substrates to investigate the effect of the Au interlayer on the optical, electrical, and structural properties of the films. In TAuT films, the Au interlayer thickness was kept at 5 nm. Although total thickness was maintained at 100 nm, the stack structure was varied as 50/5/45, 70/5/25, and 90/5/5 nm.In XRD pattern, the intermediate Au films were crystallized, while all TAuT films did not show any diffraction peaks for TiO₂ films with regardless of stack structure. The optical and electrical properties were dependent on the stack structure of the films. The lowest sheet resistance of 23 Ω/□ and highest optical transmittance of 76% at 550 nm were obtained from TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm films. The work function was dependent on the film stack. The highest work function (4.8 eV) was observed with the TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm film stack. The TAuT film stack of TiO₂ 90 nm/Au 5 nm/TiO₂ 5 nm films is an optimized stack that may be an alternative candidate for transparent electrodes in flat panel displays.     

Bipolar switching properties of amorphous TiO2 thin film grown on TiN/Si substrate

Pt/TiO₂/TiN device with the amorphous TiO₂ film grown at room temperature under an oxygen partial pressure of 1.0 mTorr showed reliable bipolar switching behavior. During the electroforming process, a large number of oxygen vacancies formed in the TiO₂ film and accumulated at the Pt/TiO₂ interface. The barrier height of the Schottky contact of the Pt/TiO₂ interface was reduced owing to the presence of these oxygen vacancies, resulting in the low-resistance state (LRS). Moreover, oxygen ions diffused into the TiN electrode during the electroforming and set processes. On the other hand, the oxygen ions in the TiN electrode diffused out and reacted with oxygen vacancies in the TiO₂ film during the reset process, and the device changed from the LRS to the high-resistance state (HRS). Conduction in the LRS and HRS can be attributed to Ohmic conduction and the trap controlled space charged limited mechanism, respectively.     

退火气压对Bi3.25La0.75Ti3O12铁电薄膜的微观结构和铁电性能的影响

采用sol-gel法在Pt/TiO₂/SiO₂/p-Si(100)衬底上制备了Bi_3.25La_0.75Ti₃O₁₂(BLT)铁电薄膜,研究了在750 ℃时不同退火气压(p_{O2:10^-4—3 atm)对薄膜微观结构和电学性能的影响.XRD和拉曼光谱结果表明在10^-4和3 atm氧气压下退火 关键词： 3.25}La_0.75Ti₃O₁₂')" href="#">Bi_3.25La_0.75Ti₃O₁₂ 铁电性能 sol-gel法 正交化度     

Photosensitization of nanocrystalline TiO2 film electrode with cadmium sulphoselenide

Nanocrystalline titanium dioxide (TiO₂) thin films composed of densely packed nanometer-sized grains have been successfully deposited onto an indium-doped-tin oxide (ITO) substrate. Then cadmium sulphoselenide (CdSSe) thin film was deposited onto pre-deposited TiO₂ to form a TiO₂/CdSSe film, at low temperature using a simple and inexpensive chemical method. The X-ray diffraction, selected area electron diffraction, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and water contact angle techniques were used for film characterization. Purely rutile phase of TiO₂ with super-hydrophilic and densely packed nanometer-sized spherical grains of approximate diameter 30-40 (±2) nm was observed. The increase in optical absorption was observed after CdSSe film deposition. Nest like surface morphology of CdSSe on TiO₂ surface results in air trapping in the crevices which prevents water from adhering to the film with increase in water contact angle. Photosensitization of TiO₂ with CdSSe was confirmed with light illumination intensity of 80 mW/cm².     

Enhanced optical properties of Al-doped TiO2 thin films in oxygen or nitrogen atmosphere

Al-doped TiO₂ (TiO₂:Al) films were deposited by simultaneous RF magnetron sputtering of TiO₂ and DC magnetron sputtering of Al. The advantage of this method is that the Al content could be independently controlled. By depositing in a mixed Ar-O₂ or a mixed Ar-N₂ atmosphere, the TiO₂:Al film became more stoichiometric and the nanocrystallinity was enhanced. The nonlinear refractive index of TiO₂:Al film deposited in a pure Ar, a mixed Ar-O₂ or a mixed Ar-N₂ atmosphere was measured by Moiré deflectometry, and was of the order of 10⁻⁸ cm² W⁻¹. For the TiO₂:Al film deposited in a pure Ar atmosphere, the porosity was higher corresponding to the lower transmission. However, the porosity of TiO₂:Al film decreased as the oxygen or nitrogen pressure increased. Especially, as the ratio of O₂ to Ar pressure increased to 0.22, TiO₂:Al film exhibited lower porosity, higher visible transmission, higher linear refractive index, lower stress and lower stress-optical coefficient.     

TiO2 Nanotube Array/Monolayer Graphene Film Schottky Junction Ultraviolet Light Photodetectors

Schottky junctions made from a titanium dioxide nanotube (TiO₂NT) array in contact with a monolayer graphene (MLG) film are fabricated and utilized for UV light detection. The TiO₂NT array is synthesized by the anodization and the MLG through a simple chemical vapor deposition process. Photoconductive analysis shows that the fabricated Schottky junction photodetector (PD) is sensitive to UV light illumination with good stability and reproducibility. The corresponding responsivity (R), photoconductive gain (G), and detectivity (D*) are calculated to be 15 A W^?1, 51, and 1.5 × 10¹² cm Hz^1/2 W^?1, respectively. It is observed that the fabricated PD exhibits spectral sensitivity and a simple power‐law dependence on light intensity. Moreover, the height of the Schottky junction diode is derived to be 0.59 V by using a low temperature I–V measurement. Finally, the working mechanism of the TiO₂NT array/MLG film Schottky junction PD is elucidated.     

纳米TiO_2薄膜的低温等离子体制备技术

以四氯化钛为源物质,氩气为载气,氧气为反应气体,利用低温等离子体增强化学气相沉积在硅基表面制备出了TiO2薄膜。使用场发射扫描电子显微镜、X射线衍射仪等检测分析表征TiO2薄膜的性能与性质,并探讨了工艺条件如基片材料、沉积时间和基片温度对薄膜性能的影响。结果表明:制备的薄膜表面光滑均匀,结构致密,最小晶粒尺寸约15 nm;薄膜的晶型主要依赖于沉积温度,低于300℃沉积的薄膜是无定形的,300℃之上沉积的薄膜是锐钛矿结构。     

Effects of 1064 nm laser on the structural and optical properties of nanostructured TiO2 thin film

TiO₂ thin film has been widely used as photoelectrode in dye-sensitized solar cells. It can also be used in quantum dot synthesized solar cells. Study of its effects in different spectrum of light is important for its use in solar cells. We have reported effects of 1064 nm laser on the surface morphology, structural and optical properties of nanostructured TiO₂ thin film deposited on glass substrates using sol-gel spin coating technique. Q-Switched Nd:YAG pulsed laser at various power densities is used in this study. Surface morphology of the film is investigated using X-ray diffraction (XRD) and atomic force microscopy technique. The XRD pattern of as deposited TiO₂ thin film is amorphous and after laser exposure it became TiO₂ anatase structure. Atomic force microscopy of the crystalline TiO₂ thin film shows that the grain size increases by increasing laser power density. The calculations of the band gap are carried out from UV/Visible spectroscopy measurements with JASCO spectrometer. For laser power density of 25 MW/cm² there is an increase in the transmission and it decreases at the value of 38 MW/cm² and band gap decreases with increasing laser power density. Photoluminescence spectra of the crystalline TiO₂ thin film indicate two broad peaks in the range of 415 and 463 nm, one for band gap peak (415 nm) and other for oxygen defect during film deposition process.     

SnO2 thin films grown by pulsed Nd:YAG laser deposition

SnO₂ thin films have been deposited on glass substrates by pulsed Nd:YAG laser at different oxygen pressures, and the effects of oxygen pressure on the physical properties of SnO₂ films have been investigated. The films were deposited at substrate temperature of 500°C in oxygen partial pressure between 5.0 and 125 mTorr. The thin films deposited between 5.0 to 50 mTorr showed evidence of diffraction peaks, but increasing the oxygen pressure up to 100 mTorr, three diffraction peaks (110), (101) and (211) were observed containing the SnO₂ tetragonal structure. The electrical resistivity was very sensitive to the oxygen pressure. At 100 mTorr the films showed electrical resistivity of 4×10⁻² Ω cm, free carrier density of 1.03×10¹⁹ cm⁻³, mobility of 10.26 cm² V⁻¹ s⁻¹ with average visible transmittance of ∼87%, and optical band gap of 3.6 eV.     

LiMn2O4 films grown by pulsed-laser deposition

LiMn₂O₄ films have been deposited onto silicon wafer by pulsed-laser deposition (PLD) technique in order to test their reliability as cathode materials in rechargeable lithium microbatteries. The film formation has been studied as a function of the preparation conditions, i.e., composition of the target, substrate temperature, and oxygen partial pressure in the deposition chamber. Depending on the conditions of deposition, Mn₂O₃ was present as an impurity phase. When deposited onto silicon substrate maintained at 300 °C in an oxygen pressure of 100 mTorr from the target LiMn₂O₄+15 % Li₂O, the PLD films are well-textured with crystallite size of 300 nm. It is found that such a film crystallizes in the spinel structure (Fd3m symmetry) as evidenced by x-ray diffraction and Raman scattering measurements. Surface morphologies of layers were investigated by SEM. The cells Li//LiMn₂O₄ have been tested by cyclic voltammetry and galvanostatic charge-discharge techniques in the range 3.0–4.2 V. The voltage profiles show the two expected steps for Li_xMn₂O₄ with a specific capacity as high as 120 mC/cm² μm. The chemical diffusion coefficients for the Li_xMn₂O₄ thin films appear to be in the range of 10⁻¹¹-10⁻¹² cm²/s. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

电子束蒸发TiO2薄膜的光学特性

研究了不同工艺参数条件下，电子束蒸发TiO₂薄膜的光学特性。在正交实验的基础上，利用离子束辅助沉积技术，获得了影响TiO2薄膜折射率的主要因素．得到了TiO2薄膜的折射率随氧气分压的关系。对离子氧和分子氧两种情况下TiO2薄膜的折射率进行了比较．得到了TiO2薄膜的折射率与沉积速度的关系，并给出了TiO2薄膜的红外吸收光谱。     

Pure and Nb-doped TiO1.5 films grown by pulsed-laser deposition for transparent p-n homojunctions

Pure and Nb-doped titanium oxide thin films were grown on sapphire substrates by pulsed-laser deposition in vacuum (10⁻⁷ mbar). The PLD growth leads to titanium oxide thin films displaying a high oxygen deficiency (TiO_1.5) compared with the stoichiometric TiO₂ compound. The structural and electrical properties (phase, crystalline orientation, nature and concentration of charge carriers, etc.) of these titanium oxide films were studied by XRD measurements and Hall effect experiments, respectively. The undoped TiO_1.5 phase displayed a p-type semiconductivity. Doping this titanium oxide phase with Nb⁵⁺ leads to an n-type behaviour as is generally observed for titanium oxide films with oxygen deficiency (TiO_x with 1.7 < x < 2). Multilayer homojunctions were obtained by the stacking of TiO_1.5 (p-type) and Nb-TiO_1.5 (n-type) thin films deposited onto sapphire substrates. Each layer is 75 nm thick and the resulting heterostructure shows a good transparency in the visible range. Finally, the I-V curves obtained for such systems exhibit a rectifying response and demonstrate that it is possible to fabricate p-n homojunctions based only on transparent conductive oxide thin films and on a single chemical compound (TiO_x).     

Influence of ambient oxygen pressure on the preferred orientation, microstructures, and dielectric properties of (Ba1-xSrx)TiO3 thin films with compositionally graded structures

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) thin films, with x decreasing from 0.25 to 0.0, were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by pulsed-laser ablation at 600 °C and under ambient oxygen pressures ranging from 50 to 400 mTorr. The influence of the ambient gas pressure on the preferred orientation, microstructures, and dielectric properties of compositionally graded BST films was investigated by X-ray diffraction, scanning electron microscopy, and dielectric frequency spectra, respectively. As the ambient oxygen pressure was increased, the preferred orientation evolved in the order: (100)+(110)(110)+(111) random orientation, and the surface roughness of the graded BST films also increased. The graded BST films deposited at high ambient oxygen pressures (300400 mTorr) exhibited a grainy structure with polycrystalline grains throughout the film thickness, whereas the graded films deposited at low ambient oxygen pressures (50200 mTorr) possessed a columnar structure. The evolution of the microstructure was ascribed to the different physical and chemical properties of the species that were incident onto the substrates at the various oxygen pressures. The dielectric properties of the graded BST films were dependent upon the ambient oxygen pressures. The graded BST films deposited at 200 mTorr exhibited the highest dielectric constant. PACS 77.55.+f; 77.22.Ch; 81.15.Fg     

On the origin of ferromagnetism in semiconducting TiO2−δ:Co oxide

In Co-doped TiO_2?δ oxide films deposited on SrTiO₃(100) substrates, a room-temperature ferromagnetism is found to occur only in a limited charge-carrier concentration interval from 2×10¹⁸? 5×10²² cm^?3. This indirectly testifies that ferromagnetism in the aforementioned n-type semiconductor is associated with the exchange interaction of magnetic ions via conduction electrons rather than with the formation of Co clusters in the material. The magnetic moment per Co atom is 0.8μ_B in the TiO cubic phase and 0.5μ_B in the anatase tetragonal phase of TiO₂.     

Influence of oxygen pressure on the ferroelectric properties of BiFeO3 thin films on LaNiO3/Si substrates via laser ablation

BiFeO₃(BFO) thin films of about 200 nm in thickness have been successfully grown on oxide bottom electrode, LaNiO₃(LNO), via pulsed laser ablation. X-ray diffraction spectrum of the as-deposited BFO film reveals a (100) preferred textured structure. The morphology of the BFO film is found to be strongly dependent on oxygen partial pressure in laser ablation. A saturated hysteresis loop with remanent polarization of 42 μC/cm² and coercive field of 100 kV/cm is obtained at the film deposition at 50 mTorr. The dielectric properties have also been obtained based on the influence of the oxygen pressure.