Indium-doped ZnO thin films deposited by the sol–gel technique

Conducting and transparent indium-doped ZnO thin films were deposited on sodocalcic glass substrates by the sol–gel technique. Zinc acetate and indium chloride were used as precursor materials. The electrical resistivity, structure, morphology and optical transmittance of the films were analyzed as a function of the film thickness and the post-deposition annealing treatments in vacuum, oxygen or argon. The obtained films exhibited a (002) preferential growth in all the cases. Surface morphology studies showed that an increase in the films' thickness causes an increase in the grain size. Films with 0.18 μm thickness, prepared under optimal deposition conditions followed by an annealing treatment in vacuum showed electrical resistivity of 1.3 × 10^− 2 Ωcm and optical transmittance higher than 85%. These results make ZnO:In thin films an attractive material for transparent electrodes in thin film solar cells.     

Thermal stability of carrier centers in various types of transparent conductive ZnO and Ga-doped ZnO films

We investigated the thermal stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films when they were annealed in a high vacuum with stepwise increasing temperature. The ZnO samples included V_O-rich and Zn-rich ZnO films; the primary donors were respectively oxygen vacancies (V_O) or Zn atoms highly unsaturated with oxygen atoms. V_O-rich ZnO was the most unstable against annealing; resistivity initially within the 10⁻³ Ω cm range diverged higher than 10 Ω cm when a critical temperature was exceeded. The critical temperature between 350 and 450 °C depended on the film thickness, which indicated that V_O's were diminished through recombination with migrating interstitial oxygen atoms. In contrast, Zn-rich ZnO films remained highly conductive up to 550 °C. They became more and more transparent and their crystallinity improved at higher annealing temperatures, which was the consequence of metallic-like Zn atoms being removed through desorption from the surface or being accommodated into the crystalline lattice. Comparatively, GZO films were more robust against annealing with their resistivities remaining unchanged up to 350 °C.     

Thermally stable, highly conductive, and transparent Ga-doped ZnO thin films

Highly conductive and transparent films of Ga-doped ZnO (GZO) have been prepared by pulsed laser deposition using a ZnO target with Ga₂O₃ dopant of 3 wt.% in content added. Films with resistivity as low as 3.3 × 10^− 4 Ω cm and transmittance above 80% at the wavelength between 400 and 800 nm can be produced on glass substrate at room temperature. It is shown that a stable resistivity for use in oxidation ambient at high temperature can be attained for the films. The electrical and optical properties, as well as the thermal stability of resistivity, of GZO films were comparable to those of undoped ZnO films.     

Preparation of uniform titania nanocoating on ZnS-based phosphors by a sol–gel process

A sol–gel process has been developed to coat micron-sized ZnS:Cu,Au,Al phosphors with a smooth and uniform layer of amorphous titania having nanosize thickness. The titania nanocoating is based on the hydrolysis and condensation of titanium tetrabutoxide Ti(OBu)₄. Acetylacetone was used to decrease the reactivity of Ti(OBu)₄. The experimental variables such as water concentration, the amount of ZnS particles, and reaction time were investigated. The thickness of the titania nanocoating was homogeneous and can easily be controlled from 20 to 54 nm by adjusting the experimental variables. The as-prepared titania nanocoating was amorphous phase and could be crystallized to anatase phase upon heating at 500 °C in Ar atmosphere.     

Advanced light trapping materials: Double layer ZnO:B based transparent conductive oxide

We present double layer structures consisting of ZnO:B/ZnO:B (BZO) and In₂O₃:Mo (IMO)/BZO films. The structure offers the unique opportunity of separating the conductivity of transparent conductive oxides from their light scattering behavior and allows their optimization for use in thin film solar cells. The layers serve as carrier transport and light trapping layers, respectively. BZO films were prepared by mid-frequency magnetic sputtering from a ZnO:B₂O₃ ceramic target. In order to enhance the conductivity of the BZO films, hydrogen was introduced into the sputtering atmosphere. Introducing hydrogen increased the mobility of the BZO-based double layer films to near 30 cm²/V•s. Efficient scattering was achieved by etching the film in dilute hydrochloric acid. IMO films were also tested as the transport layer. An unconventional surface morphology was obtained by etching the IMO/BZO double layer film. Using this cascading multilayer structure IMO/BZO film as the front contact in a-Si:H solar cell, 20.4% and 7.4% enhancements in short circuit current density were obtained compared to smooth IMO films and textured single layer BZO films.     

Synthesis and characterization of (CuO)x(ZnO)1 − x composite thin films with tunable optical and electrical properties

Mixed (CuO)_x(ZnO)_1 − x composite films have been prepared on glass substrates by a sol-gel spin coating method using copper acetate hydrate and zinc acetate dihydrate as precursors. The surface morphology and crystal structure of the films were investigated by field emission scanning electron microscopy and x-ray diffraction, respectively. It was observed that the crystal structure changed from wurtzite (ZnO) to monoclinic (CuO) as the Cu content increased from 0% to 100% in the films. UV-Vis absorption and photoluminescence measurements indicated that the optical properties can be tuned continuously from pure ZnO to pure CuO as the Cu content was increased, following the expected trends for a transition from ZnO to CuO. The resistivity of the films decreased by three orders of magnitude as Cu increased from 0% to 100%. These semiconducting composite oxides with tunable optical and electrical properties have potential applications in electronics and optoelectronics.     

Dye-doped sol–gel materials for two-photon absorption induced fluorescence

Two-photon absorption (TPA) and subsequent fluorescence properties of laser dyes are retained when doped into solid state sol–gel materials. These properties were demonstrated to be applicable in true 3D displays.     

Preparation and optical properties of sol–gel derived photo-patternable organic–inorganic hybrid films for optical waveguide applications

Photo-patternable TiO₂/organically modified silane hybrid films were prepared by combining a low-temperature sol–gel technique with a spinning–coating process. A ridge waveguide pattern was fabricated by ultraviolet light irradiation through a mask placed contact with the hybrid film in direct. Optical properties and photochemical activity of the hybrid film, including refractive index, thickness, propagation mode, and propagation loss, were studied and monitored by a prism coupling technique and Fourier transform infrared spectroscopy. The change of transmittance with exposure time was also observed by ultraviolet–visible spectroscopy. These results indicate that the hybrid film is potential application for fabrication of photonic devices by ultraviolet light irradiation. The structure of ridge waveguide pattern was characterized and studied by scanning electron microscope and surface profiler. The fabrication process of the as-prepared photosensitive hybrid film as compared with traditional binary mask has a great amount of advantages of cost-effective, simple, and smooth surface over non-photosensitive material methods.     

Synthesis of c-axis preferred orientation ZnO:Al transparent conductive thin films using a novel solvent method

Transparent aluminum doped zinc oxide (ZnO:Al, AZO) conducting thin films with a high-preferential c-axis orientation were synthesized using a new sol-gel formula. The films were deposited using a spin-coating route onto borosilicate glass substrates. We used propylene glycol methyl ether (PGME) as the solvent in place of ethylene glycol monomethyl ether (EGME), which is commonly used because it is easier to deposit onto the substrates. PGME is also superior in terms of health and safety. PGME solvent does not need to settle for several days before use and can be spin-coated as soon as the raw material and solvent are mixed. The effects of this novel solvent on the structural, morphological, electrical and optical properties are discussed using XRD, SEM, a four-point probe and UV-VIS spectrophotometry. It was found that the films produced with PGME showed a high-preferential c-axis orientation and compact microstructure in comparison films produced using EGME. The electrical resistivity of AZO thin films produced with PGME solvent was lowered to 3.474 × 10^− 3Ω cm after annealing in 95 N₂/5H₂ atmosphere. In addition, the optical transmittances of AZO thin films on glass plates were higher than 90% in the visible wavelength region.     

Size-controlled synthesis of ZnO nanocrystals from diethylzinc and donor-functionalized alcohols

Nanocrystals of zinc oxide (ZnO) were prepared from diethylzinc and donor-functionalized alcohols, 2-methoxyethanol, 2-ethoxyethanol, and 2-methoxyethoxyethanol by sol-gel processing. The nanocrystals were prepared at room temperature and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Crystal sizes were controlled by the type of utilized donor-functionalized alcohol in which decreased by an increase of the chain length and type of the donor-functionalized alcohol from 16 to 27 nm. The HRTEM and SAED results revealed that the degree of crystallinity decreases with increasing donor-functionalized alcohol chain length. The particle size of the ZnO powders demonstrated high tolerance to heat treatment and grow up to approximately 65 to 80 nm upon calcination at 1000 °C.     

Growth mechanism and characterization of ZnO: Al multi-layered thin films by sol-gel technique

In this study, transparent conductive Al-doped ZnO films were deposited by the sol-gel method. The growth mechanism of the film microstructure and its influences on the electrical properties were discussed. It was found that dopant and solution concentration affected the nucleation behavior considerably. The preferred growth orientation of ZnO crystallite was restrained by the film itself. The repeated dip-coating processes did not enable the crystallite size to grow obviously, but it could allow crystallite and atoms to find the suitable positions and therefore led to a better film quality. Consequently, this process led to an electrical resistivity of 7.08 × 10^− 3 Ω cm and a high transmittance of over 80% in the visible region. The best sample was obtained for an Al concentration of 1 at.% and at 600 °C for pre- and post-heat treatment.     

Optical and structural properties of ZnO for transparent electronics

During the last years there is has been an enormous research effort on the materials and processes for the production of transparent electronic devices grown on flexible polymeric substrates as well as on rigid substrates, such as Si and glass. The deposition of Transparent Conductive Oxides (TCOs) characterized by superior optical and electrical properties, in combination to desirable growth characteristics, compatible to polymeric substrates, is of considerable importance. Among all TCO materials, Zinc Oxide (ZnO) has emerged as one of the most promising materials due to its optical and electrical properties, its high chemical and mechanical stability and, due to its abundance, low cost compared with the most currently used TCO materials. In this work, we study the effect of the deposition parameters of ZnO thin films in terms of their optical, structural and nanomechanical properties by employing Spectroscopic Ellipsometry (SE) in the Vis-fUV spectral region, and X-Ray Diffraction techniques. The SE measurements allowed the determination of the optical properties of the ZnO thin films with deposition time and gas partial pressure, whereas the XRD measurements revealed that the ZnO thin films are preferentially grown parallel to (002) axis, in grains less than 10 nm. Furthermore, nanomechanical testing through nanoindentation indicates a thickness controlled fracture mechanism (pop-in events) affecting the durability of the deposited ZnO thin films.     

Microstructure and gas-sensing properties of sol-gel ZnO thin films

The paper presents the properties of zinc oxide thin films deposited on glass substrate via dip-coating technique. Zinc acetate dehydrate, ethanol and monoethanol amine were used as starting materials and N₂ gas was used as thermal annealing atmosphere for film crystallization. The effect of withdrawal speed on the crystalline structure, morphology, zinc and nitrogen chemical states, optical, electrical and gas-sensing properties of the thin films has been investigated using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical transmittance and photoreduction-ozone reoxidation data.     

Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol-gel process

High density TiO₂ nanotube film with hexagonal shape and narrow size distribution was fabricated by templating ZnO nanorod array film and sol-gel process. Well-aligned ZnO nanorod array films obtained by aqueous solution method were used as template to synthesize ZnO/TiO₂ core-shell structure through sol-gel process. Subsequently, TiO₂ nanotube array films survived by removing the ZnO nanorod cores using wet-chemical etching. Polycrystalline anatase TiO₂ nanotube films were ∼ 1.5 μm long and ∼ 100 nm in inter diameter with a wall thickness of ∼ 10 nm.     

Fabrication of highly oriented (002) ZnO film on glass by sol-gel method

In this study high quality (002) ZnO films were deposited on glass substrate by a sol-gel spin coating process. The as-coated films were post-annealed at different temperatures in air to investigate the effect of annealing temperature in particular. The chemical composition of the precursor sol and the intermediates produced in the films heating process were analyzed by thermo gravimetric analysis/differential thermal analysis (TGA/DTA). The microstructure and its optical properties of ZnO films were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), ultraviolet-visible spectroscopy (UV-Vis) and photoluminescence. TGA/DTA showed that a significant weight loss occurred at around 200-300 °C and the weight stabilized at 300 °C. An extremely sharp (002) diffracted peak in XRD patterns indicated the high preference in crystallinity of these films. FESEM micrographs revealed that the films were filled with particulates with size ranging from 10 to 25 nm as post annealing temperature increased from 400 to 500 °C and turned into porous films at 600 °C. UV-Vis has shown that the films were highly transparent under visible light and had a sharp absorption edge in the ultraviolet region at 380 nm. The measured optical band gap values of the ZnO thin films were around 3.24-3.26 eV. Photoluminescence spectra revealed a strong UV emission centered at about 390 nm corresponding to the near-band-edge emission with a weak defect-related emission at about 520 nm. The intensity of UV emission increased with the annealing temperature. This may be attributed to a higher quality ZnO film while annealed at higher temperature.     

Preparation and properties of transparent conductive indium-tin-oxide/polyimide substrates via a sol-gel process

High temperature, flexible and colorless indium-tin-oxide (ITO) coated plastic substrates have been prepared from a series of thermally stable, high glass transition temperature (T_g) and colorless copolyimide films. The copolyimides were synthesized from 3,3′-diaminodiphenylsulfone, 9,9′-bis(4-aminophenyl) fluorene and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride monomers. Their T_gs were around 285-365 °C. The conductive ITO was synthesized by a sol-gel method, and then deposited onto the copolyimide films by a spin coating process. After thermal treatment at 300 °C under a nitrogen/hydrogen mixture gas for 24 h, the resistivity of the ITO film was 10⁻³ Ω cm, and its transmittance was 75% at the visible light region. Scanning electron microscopy and X-ray diffraction were used to observe the surface and morphology of the ITO films. UV-visible spectroscopy and the four-probe method were used to study their optical and electrical properties. The high performance ITO/plastic substrates can be used in the next generation flexible flat panel displays and solar cell.     

Atomic layer deposition grown composite dielectric oxides and ZnO for transparent electronic applications

In this paper, we report on transparent transistor obtained using laminar structure of two high-k dielectric oxides (hafnium dioxide, HfO₂ and aluminum oxide, Al₂O₃) and zinc oxide (ZnO) layer grown at low temperature (60 °C-100 °C) using Atomic Layer Deposition (ALD) technology. Our research was focused on the optimization of technological parameters for composite layers Al₂O₃/HfO₂/Al₂O₃ for thin film transistor structures with ZnO as a channel and a gate layer. We elaborate on the ALD growth of these oxides, finding that the 100 nm thick layers of HfO₂ and Al₂O₃ exhibit fine surface flatness and required amorphous microstructure. Growth parameters are optimized for the monolayer growth mode and maximum smoothness required for gating.     

The effect of dopants on the morphology, microstructure and electrical properties of transparent zinc oxide films prepared by the sol-gel method

The influence of doping on the morphology, physical and electrical properties of zinc oxide produced by the sol-gel method was examined. Undoped zinc oxide was observed to form relatively porous films. Addition of an Al dopant influenced the sheet resistance, but did not result in a change in morphology, examined by atomic force microscopy when compared to undoped films. In the case of electrical measurements, undoped ZnO films were extremely resistive. A minimum dopant concentration of 2 at.%. Al was required to produce materials which were more conductive, as observed by sheet resistance measurements, which were shown to vary with annealing temperature. The versatile nature of sol-gel processing was demonstrated by selective ink-jet deposition of sol-gel droplets which were annealed to form oxide materials.     

Optimization of ZnO:Al/Ag/ZnO:Al structures for ultra-thin high-performance transparent conductive electrodes

Al-doped ZnO (AZO)/Ag/AZO multilayer coatings (50-70 nm thick) were grown at room temperature on glass substrates with different silver layer thickness, from 3 to 19 nm, by using radio frequency magnetron sputtering. Thermal stability of the compositional, optical and electrical properties of the AZO/Ag/AZO structures were investigated up to 400 °C and as a function of Ag film thickness. An AZO film as thin as 20 nm is an excellent barrier to Ag diffusion. The inclusion of 9.5 nm thin silver layer within the transparent conductive oxide (TCO) material leads to a maximum enhancement of the electro-optical characteristics. The excellent measured properties of low resistance, high transmittance in the visible spectral range and thermal stability allow these ultra-thin AZO/Ag/AZO structures to compete with the 1 μm thick TCO layer currently used in thin film solar cells.