ZnO transparent thin films for gas sensor applications

Zinc oxide (ZnO) transparent thin films were deposited onto silicon and Corning glass substrates by dc magnetron sputtering using metallic and ceramic targets. Surface investigations carried out by Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) have shown a strong influence of deposition technique parameters on film surface topography. Film roughness (RMS), grain shape and dimensions are correlated with the deposition technique parameters as well as with the target material. XRD measurements have proven that the dc sputtered films are polycrystalline with the (002) as preferential crystallographic orientation. AFM analysis of thin films sputtered from a ceramic target has shown a completely different surface behavior compared with that of the films grown from a metallic target. This work demonstrates that the target material and the growth conditions determine the film surface characteristics. The gas sensing characteristics of these films are strongly influenced by surface morphology. Thus correlating the optical and electrical film properties with surface parameters (i.e. RMS and Grain Radius) can lead to an enhancement of the material's potential for gas sensing applications.     

High performance multilayered nano-crystalline silicon/silicon-oxide light-emitting diodes on glass substrates

A low-temperature hydrogenation-assisted sequential deposition and crystallization technique is reported for the preparation of nano-scale silicon quantum dots suitable for light-emitting applications. Radio-frequency plasma-enhanced deposition was used to realize multiple layers of nano-crystalline silicon while reactive ion etching was employed to create nano-scale features. The physical characteristics of the films prepared using different plasma conditions were investigated using scanning electron microscopy, transmission electron microscopy, room temperature photoluminescence and infrared spectroscopy. The formation of multilayered structures improved the photon-emission properties as observed by photoluminescence and a thin layer of silicon oxy-nitride was then used for electrical isolation between adjacent silicon layers. The preparation of light-emitting diodes directly on glass substrates has been demonstrated and the electroluminescence spectrum has been measured.     

The effects of deposition variables on spray-deposited ZnO thin film prepared from Zn(C2H3O2)2

Undoped Zn0 thin films have been prepared by a modified spray technique under various deposition conditions and the effects of the different deposition variables on the electrical and optical properties of the prepared films have been studied in detail. Substrate temperature has been found to be the most important parameter for the deposition process. To obtain a low resistive highly transparent Zn0 film with appreciable Hall mobility and carrier concentration, a post-deposition heat treatment in vacuum is always necessary and is a precondition for undoped films. From the study of the optical transmission spectra in the visible and near-UV regions it was observed that the films are highly transparent (98%) in the visible range. The fundamental absorption edge of the films lies in the UV region. Values of refractive indices of these films were also determined.     

反应溅射SiO_xN_y栅介质薄膜的成分及C-V特性研究

在前期工作基础上,研究了不同反应气体组合条件下SiQxNy栅介质薄膜的反应溅射制备.反应气体的不同组合首先导致薄膜氮氧比以及沉积速率的不同,并进一步导致薄膜电学性能的差异.对薄膜电容-电压特性的分析表明,沉积速率降低会使薄膜体缺陷密度减小,而氮含量的减少则有助于提高薄膜界面质量.最终结果显示,采用合适的反应气体组合(QN2=1.0sccm,Qo2=1.0sccm)制得的SiOxNy薄膜具有较低的体缺陷密度和较好的界面质量,适合于MOS栅介质领域的应用.     

Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition

A novel deposition technique has been used to grow ZnO films. Good quality films were obtained on glass substrates by single source chemical vapor deposition (SSCVD), for gas sensing applications. The properties of ZnO films were investigated at different deposition temperatures 300, 350 and 400 °C. X-ray diffraction results show that all deposited films were polycrystalline. The morphological, structural, optical and electrical properties of the films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), cathodoluminescence (CL) and Hall effect techniques. The morphology of the deposited films evolves from columnar grains, to parallel plates as the substrate temperature increases. A significant increase in the relative intensities of the green and red emission with increasing deposition temperature has been observed. Electrical properties, relevant for gas sensing behavior have been investigated as well. In the particular case of CO an operating temperature of 300 °C seems to yield the best sensitivity.     

Chemical bath deposition of indium sulphide thin films: preparation and characterization

Indium sulphide (In₂S₃) thin films have been successfully deposited on different substrates under varying deposition conditions using chemical bath deposition technique. The deposition mechanism of In₂S₃ thin films from thioacetamide deposition bath has been proposed. Films have been characterized with respect to their crystalline structure, composition, optical and electrical properties by means of X-ray diffraction, TEM, EDAX, optical absorption, TRMC (time resolved microwave conductivity) and RBS. Films on glass substrates were amorphous and on FTO (flourine doped tin oxide coated) glass substrates were polycrystalline ( phase). The optical band gap of In₂S₃ thin film was estimated to be 2.75 eV. The as-deposited films were photoactive as evidenced by TRMC studies. The presence of oxygen in the film was detected by RBS analysis.     

Evaporated nanostructured Y2O3:Eu thin films

Europium-doped yttrium oxide (Y2O3:Eu) is a well-known luminescent material that in recent years has been studied in thin-film form. However, to date there has not been a great effort put into altering the nanostructure of these films. A thin-film deposition technique called glancing angle deposition allows for a high degree of control over the nanostructure of the thin film, resulting in thin films with nanostructure geometries ranging from chevron and post to helix. Glancing-angle deposition was used to make europium-doped yttrium oxide thin films with slanted-post nanostructures. Portions of the films were annealed in air at 850 degrees C for 10 hours following deposition. Scanning electron microscopy was used to characterize the nanostructures of the films, while UV laser excitation was used to characterize the photoluminescence properties of the films. The annealed samples exhibited increased photoluminescent responses compared to unannealed samples; however, the porous nanoscale geometry of the films was unaffected. In order to optimize the photoluminescence properties of the films, both the partial pressure of oxygen during film deposition and the level of europium doping in the source material used were varied. Films fabricated from the source material with a greater amount of europium doping had larger photoluminescent responses, while the optimal partial pressure of oxygen during electron-beam evaporation was found to be less than 1.0 x 10(-4) torr.     

纳米银-聚合物薄膜的制备及力电性能测试

聚合物基底上纳米银颗粒薄膜的制备工艺相对简单,成本较低,且该薄膜具有成为高敏感性压阻应力/应变传感材料的潜力。本文采用银镜制备法在聚酰亚胺(PI)和聚乙烯(PE)上合成了纳米银颗粒薄膜,系统研究了该薄膜制备工艺、结构特性、材料性能之间的关系。实验考察了材料“浸泡”时间及聚合物材料前处理等因素对材料表面吸附纳米颗粒含量的影响,研究了 “浸泡”时间对纳米银颗粒粒径大小、颗粒含量及分布的影响,并探讨了不同聚合物基体的颗粒特性对薄膜二维导电渗滤,压阻特性及拉伸性能的影响。研究表明,增加“浸泡”时间能够增加纳米银颗粒粒径大小,提高银颗粒的含量及分布均匀性；在相同的制备条件下,PI基底较PE基底对纳米银颗粒具有更加优异的吸附效果；在PI 和PE基底上的纳米银颗粒薄膜均表现出显著的压阻性能,且电阻对应变的敏感性随应变的增大及银颗粒含量的减少而显著提高。     

Fabrication, structural and electrical characterization of lanthanum tungstate films by pulsed laser deposition

Films of lanthanum tungstate, 3 μm in thickness, were fabricated by means of pulsed laser deposition on a Pd foil. The films were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and their electrical conductivity was measured at temperatures between 400 and 800 °C in different gas atmospheres. The films' structure and electrical characteristics are close to what is reported in the literature for corresponding polycrystalline material. The films exhibit fairly high proton conductivity at elevated temperatures, which make them interesting for components in hydrogen-related technologies. Changes in microstructure and the crystallographic orientation observed at higher temperatures were accompanied by changes in the conductivity characteristics.     

Electrical and optical studies of Ga-doped ZnO thin films

Nanostructure Ga-doped zinc oxide (GZO) thin films with highly (0 0 2) preferred orientation were fabricated on glass substrates, using radio frequency magnetron sputtering with an GZO ceramic target (The Ga₂O₃ contents was about 3 wt%) and different deposition conditions. The structural features, surface morphology and electrical and optical properties of the GZO thin films were studied, in terms of the deposition parameters. A Grey-based Taguchi method was used to determine the optimal deposition parameters for GZO thin films by considering multiple performance characteristics. The response graph and table for each level of the deposition parameters forms the Grey relational grade and the optimal levels of the deposition parameters were chosen. The experimental results show that the process pressure and the thickness make the most significant contribution to the overall performance. In the confirmation runs, Grey relational analysis showed that the improvement in deposition rate is 14.2 %, the improvement in electrical resistivity 38.1 % and the improvement in optical transmittance is 1.2 %. Annealing in a vacuum further improved the crystalline quality and optoelectronic performances of the GZO thin films.     

Effect of processing conditions on microstructure and electrical characteristics of Ni thin films

Ni thin films with an intermediate layer of Cr were prepared by using dc magnetron sputtering under different conditions. Effects of deposition temperature, post-deposition annealing on the microstructure and the electrical characteristics were investigated. The relationship between film microstructure and its resistivity was analyzed. It was found that the crystal grains aggregated into large ones when the deposition temperature reached or exceeded 150 °C. This could be explained that high deposition temperature conduced high activation energy, which increased surface mobility of the adatoms. Annealing treatments resulted in the densification of the films. Resistivity of the films strongly depended on grain size and crystallinity. The influence of Cr intermediate layer on the resistivity was also discussed. Compared to annealing treatment, the deposition temperature exhibited larger controlling effect on film resistivity.     

Optical and electrical characterization of r.f. sputtered ITO films developed as art protection coatings

Transparent and conductive tin-doped indium oxide (ITO) films have been prepared by r.f. plasma sputtering technique in Ar and Ar + O₂ gas mixture. The influence of the deposition conditions, film thickness, and substrate heating, as well as the post-annealing treatment on the optical and electrical properties of the ITO films has been investigated.The present study has extended the optical behaviour characterization of the ITO films in a wide UV-VIS-IR spectral region in addition to the comprehensive optical studies of this material at shorter wavelengths.The optical constants: refractive index (n), extinction (k) and absorption (α) coefficient, and the optical band gap (E_go) have been calculated for the ITO films in the spectral range between 350 and 2500 nm. A combination of several well-known theoretical models has been applied to describe precisely the complex optical behaviour of ITO films in separate spectral parts. In this approach, a good overlapping between the experimental and the simulated spectra in the whole investigated spectral region has been achieved.The deposition conditions and the optical and electrical properties of the ITO films have been optimized with respect to the requirements for their applications in art protection coatings.     

Structural and electrical studies of thermally evaporated nanostructured CdTe thin films

CdTe thin films were deposited on KCl and glass substrates using thermal evaporation technique under high vacuum conditions. CdTe bulk compound grown by vertical directional solidification (VDS) technique was used as the source material to deposit thin films. Powder X-ray diffraction technique was employed to identify the phase of the as grown bulk CdTe compound as well as its thin films. Surface morphology and the stoichiometry of the bulk compound and thin films was carried out by using scanning electron microscope (SEM) with an attachment of energy dispersive spectrometer(EDS). Microstructural features associated with the as deposited CdTe thin films were studied by using transmission electron microscope (TEM). The films deposited on to glass substrates at different temperatures have been used to study the I-V characteristics of the films. These parameters have been studied in detail in order to prepare good quality nanostructured thin films of CdTe compound. CdTe bulk compound grown by VDS method and its thin films prepared by thermal evaporation method found to have single phase with cubic structure. Size of the particles in the as deposited films vary between 5 and 40 nm In the present study efforts have been made to correlate the electrical and optical properties of the CdTe thin films with the corresponding microstructural features associated with them.     

Fabrication and characterization of CuInS2 films by chemical bath deposition in acid conditions

Non-crystalline copper indium disulphide (CuInS₂) thin films had been deposited on ITO glass by chemical bath deposition (CBD) in acid conditions. Then polycrystalline CuInS₂ films were obtained after sulfuration in sulfur atmosphere at 450 °C for 1.5 h. The films had been characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), Raman scattering measurements and energy dispersive X-ray analysis (EDX). The optical and electrical property of the thin films was also measured. The results showed that the pure, flatness, and well crystallized CuInS₂ thin films with good electrical and optical property had been obtained, meaning that the chemical bath deposition in acid conditions is suitable for the deposition of CuInS₂ thin films.     

Preparation and study of thickness dependent electrical characteristics of zinc sulfide thin films

Zinc sulfide thin films have been deposited onto glass substrates by chemical bath deposition. The various deposition parameters such as volume of sulfide ion source, pH of bath, deposition time, temperature etc are optimized. Thin films of ZnS with different thicknesses of 76–332 nm were prepared by changing the deposition time from 6–20 h at 30° C temperature. The effect of film thickness on structural and electrical properties was studied. The electrical resistivity was decreased from 1.83 × 10⁵ Ω-cm to 0.363 × 10⁵ Ω-cm as film thickness decreased from 332 nm to 76 nm. The structural and activation energy studies support this decrease in the resistivity due to improvement in crystallinity of the films which would increase the charge carrier mobility and decrease in defect levels with increase in the thickness.     

Depth dependent properties of ITO thin films grown by pulsed DC sputtering

A systematically prepared set of ITO layers for solar cell applications has been analyzed by spectroscopic variable angle ellipsometry in order to trace the dependence of free carriers’ distribution along the film depth as a function of film thickness as well as its change upon annealing. Samples were deposited on silicon substrates with various thicknesses in steps of approximately 10–20 nm. This set was duplicated and these samples were annealed, so that for each thickness an as-deposited and an annealed sample is available. Conventionally measured electrical conductivity and morphological properties (AFM measurements) of the films have been compared with the optical constants’ inhomogeneity, i.e. material properties along the film thickness modelled by variable-angle spectroscopic ellipsometry. The obtained results show that the optical as well as electrical properties of thin ITO films prepared by pulsed DC sputtering are depth dependent. For the deposition conditions used a well-determined reproducible non-uniform distribution of free carriers within the film thickness was determined. In particular it has been found that the majority of free carriers in as-deposited ultra-thin ITO films is concentrated at sample half-depth, while their distribution becomes asymmetric for the thicker films, with a maximum located at approximately 40 nm depth. The distribution of free carriers in annealed samples is qualitatively different from that of as-deposited layers.     

A greener electrodeposition recipe for ZnO films in terawatt photovoltaics

Electrodeposition of ZnO can be performed in an aqueous solution using a greener recipe, where the solution can be reused for multiple deposition runs. The solution in this greener recipe has only one function, i.e. to provide electrical conductivity for the deposition reactions. A Zn sheet serves as the anode, which dissolves during the deposition as the Zn source. O₂ is bubbled into the solution and reduced to OH⁻ ions as the O source. This recipe minimizes concentration changes in the solution as deposition proceeds, making the solution reusable. An initial Zn^2 + concentration of a few mM in the solution is required, not to serve as a Zn source but to facilitate the deposition and prevent precipitation of ZnO in the solution. Multiple deposition runs for ZnO films in the same solution have been demonstrated. X-ray diffraction, optical transmittance and absorption spectra reveal that all the ZnO films have similar structural and optical properties. They all display high transmittance of ~ 80% and low absorbance of ~ 10%.     

Nanostructural palladium films for sensor applications

The composite films composed of palladium (Pd) nanocrystals and Pd-fullerenes nanocrystals obtained by physical vapor deposition (PVD) method were studied. These types of films can be applied as active material for sensor application.The films were prepared by PVD technique. They were deposited on different substrates and contained Pd in various concentrations. The structure and composition of the Pd films were studied by X-ray diffraction, scanning probe microanalysis, transmission and scanning electron microscopy. It was found that these films were composed of Pd nanocrystals placed in Pd-C₆₀ matrix.Changes of some electric properties of the film upon an influence of a toxic liquid have been characterized by DC electrical measurements. They were performed for different volatile organic compounds (VOCs). At 22 °C the electrical resistance was studied for VOC such as benzene and toluene.     

Sputter deposition of tungsten trioxide for gas sensing applications

Tungsten trioxide thin films were grown by reactively sputtering a circular WO3 target in different Ar–O2 atmospheres. After deposition, data on the structural, optical and electrical properties were obtained by using transmission electron microscopy (TEM) to examine the structure and the morphology of the films, UV-VIS spectrophotometry to determine optical absorption edge characteristics, and Hall effect measurements to determine the change carrier mobility and the film resistivity. In addition, the film resistance variations in controlled atmospheres were examined and the gas sensing properties of films grown under different conditions were compared. The aim of the study was to examine the effect of O2 concentration in the sputtering atmosphere on structural, optical, electrical and sensing properties. © 1998 Kluwer Academic Publishers     

Indium doped silver oxide thin films prepared by reactive electron beam evaporation technique: electrical properties

The indium doped silver oxide thin films have been prepared at 275 °C on soda lime glass and silicon substrates by reactive electron beam evaporation technique; the deposition rate has been varied (by varying the electron beam current) in the range 0.94–16.88 nm/s keeping the oxygen flow rate constant. These films are polycrystalline. The electrical resistivity for these films decreases with increasing deposition rate. The AIO films prepared with a deposition rate of 5.7 nm/s show near p-type conductivity. The work function has been measured on these films by contact potential method using Kelvin Probe. The surface morphology of the films has been evaluated using atomic force microscopy (AFM). The roles of indium doping and oxygen vacancies in the electrical properties of these films have been analyzed; the ionized impurity scattering is the dominant mechanism controlling the electrical conduction in these films.