High quality transparent conductive ZnO/Ag/ZnO multilayer films deposited at room temperature

We have fabricated, by simultaneous DC and RF magnetron sputtering, multilayer transparent electrodes having much lower electrical resistance than the widely used transparent conductive oxide electrodes. The multilayer structure consists of three layers (ZnO/Ag/ZnO). Ag films with different film thickness were used as metallic layers. Optimum thicknesses of Ag and ZnO films were determined for high optical transmittance and good electrical conductivity. Several analytical tools such as spectrophotometer, atomic force microscopy, scanning electron microscopy and four-point probe were used to explore the possible changes in electrical and optical properties. A high quality transparent electrode, having resistance as low as 3 Ω/sq and high optical transmittance of 90% was obtained at room temperature and could be reproduced by controlling the preparation process parameters. The electrical and optical properties of ZnO/Ag/ZnO multilayers were determined mainly by the Ag film properties. The performance of the multilayers as transparent conducting materials was also compared using a figure of merit.     

ZnO:Sn/Ag/ZnO:Sn复合薄膜电极的防氧化性能研究

随着分辨率的提高,传统金属电极在电阻率和抗氧化性能方面已经不适合作为需要高温热处理的场致发射显示器件中的薄膜电极。本文采用5.77%(原子比)Sn掺杂的ZnO:Sn作为Ag层的保护层,利用磁控溅射法制备ZnO:Sn/Ag/ZnO:Sn复合薄膜及其电极,并采用X射线衍射、光学显微镜、扫描电子显微镜和电性能测试系统研究复合薄膜及其电极在经过不同温度退火后的晶体结构、表面形貌和电学性能的变化。ZnO:Sn膜层致密,25 nm厚的ZnO:Sn足以保护Ag层在530℃的高温中不被明显氧化,电极电阻率低达2.0×10-8Ω.m左右。     

Effect of substrate temperature and annealing treatment on the electrical and optical properties of silver-based multilayer coating electrodes

Multilayer coatings consisting of thin silver layers sandwiched between layers of transparent conducting metal oxides are investigated from the view point of low-resistance electrodes for use in flat panel displays, solar cells, etc. ZnO/Ag/ZnO multilayer films were prepared on glass substrates by simultaneous RF magnetron sputtering of ZnO and dc magnetron sputtering of Ag. Optimization of the deposition conditions of both ZnO layers and metallic layers were performed for better electrical and optical properties. The structural, electrical and optical properties of the films (deposited at room temperature, different substrate temperature and annealed at different conditions) were characterized with various techniques. We could not produce high-quality transparent conductive electrodes simply by annealing at various temperatures. However, improved electrical properties and a considerable shift in the transmittance curves was observed after heat treatment. The experimental results show that the electrical resistivity of as-grown films can be decreased to 10^− 5 Ω cm level with post-annealing at 400 °C for 2 h in vacuum atmosphere. After heat treatment, the sheet resistance was reduced as much as 20% which was due to the increased grain size of Ag film. The samples heat treated at 200-400 °C under vacuum or nitrogen atmosphere showed the best electrical properties. The key to the superior electrical and optical properties of the multilayer is the optimization of growth conditions of the silver layer by careful control of the oxide properties and the use of appropriate annealing temperature and atmosphere.     

Real time spectroscopic ellipsometry of Ag/ZnO and Al/ZnO interfaces for back-reflectors in thin film Si:H photovoltaics

Real time spectroscopic ellipsometry (RTSE) has been applied to analyze the optical characteristics of Ag/ZnO and Al/ZnO interfaces used in back-reflector (BR) structures for thin film silicon photovoltaics. The structures explored here are relevant to the substrate/BR/Si:H(n-i-p) solar cell configuration and consist of opaque Ag or Al films having controllable thicknesses of microscopic surface roughness, followed by a ZnO layer up to ~ 3000 Å thick. The thicknesses of the final surface roughness layers on both Ag and Al have been varied by adjusting magnetron sputtering conditions in order to study the effects of metal film roughness on interface formation and interface optical properties. The primary interface loss mechanisms in reflection are found to be dissipation via absorption through localized plasmon modes for Ag/ZnO and through intraband and interband transitions intrinsic to metallic Al for Al/ZnO.     

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.     

ZnO/Cu/ZnO multilayer films: Structure optimization and investigation on photoelectric properties

A series of ZnO/Cu/ZnO multilayer films has been fabricated from zinc and copper metallic targets by simultaneous RF and DC magnetron sputtering. Numerical simulation of the optical properties of the multilayer films has been carried out in order to guide the experimental work. The influences of the ZnO and Cu layer thicknesses, and of O₂/Ar ratio on the photoelectric and structural properties of the films were investigated. The optical and electrical properties of the multilayers were studied by optical spectrometry and four point probe measurements, respectively. The structural properties were investigated using X-ray diffraction. The performance of the multilayers as transparent conducting coatings was compared using a figure of merit. In experiments, the thickness of the ZnO layers was varied between 4 and 70 nm and those of Cu were between 8 and 37 nm. The O₂/Ar ratios range from 1:5 to 2:1. Low sheet resistance and high transmittance were obtained when the film was prepared using an O₂/Ar ratio of 1:4 and a thickness of ZnO (60 nm)/Cu (15 nm)/ZnO (60 nm).     

Optimization of ZnO/Ag/ZnO multilayer electrodes obtained by Ion Beam Sputtering for optoelectronic devices

We investigated the electrical and optical properties of ZnO/Ag/ZnO multi-layer electrodes obtained by ion beam sputtering for flexible optoelectronic devices. This multi-layer structure has the advantage of adjusting the layer thickness to favor antireflection and the surface plasmon resonance of the metallic layer. Inserting a thin (Ag) metallic layer between two (ZnO) oxide layers decreases the sheet resistance while widening the optical transmittance window in the visible. We found that the optimal electrode is made up of a 10 nm thin Ag layer between two 35 nm and 20 nm thick ZnO layers, which resulted in a low sheet resistance (R_sq = 6 Ω/square), a high transmittance (T ≥ 80% in the visible) and the highest figure of merit of 1.65 × 10^-2 square/Ω.     

Microstructure and optical properties of Ag-doped ZnO nanostructures prepared by a wet oxidation doping process

Silver-doped zinc oxide (Ag:ZnO) nanostructures were prepared by a facile and efficient wet oxidation method. This method included two steps: metallic Zn thin films mixed with Ag atoms were prepared by magnetron sputtering as the precursors, and then the precursors were oxidized in an O(2) atmosphere with water vapour present to form Ag:ZnO nanostructures. By controlling the oxidation conditions, pure ZnO and Ag:ZnO nanobelts/nanowires with a thickness of ～ 20 nm and length of up to several tens of microns were synthesized. Scanning electron microscopy, transmission electron microscopy, cathodoluminescence and low temperature photoluminescence (PL) measurements were adopted to characterize the microstructure and optical properties of the prepared samples. The results indicated that Ag doping during magnetron sputtering was a feasible method to tune the optical properties of ZnO nanostructures. For the Ag:ZnO nanostructures, the intensity of ultraviolet emission was increased up to three times compared with the pure ones. The detailed PL intensity variation with the increasing temperature is also discussed based on the ionization energy of acceptor in ZnO induced by Ag dopants.     

Effect of ZnO buffer layer on AZO film properties and photovoltaic applications

Aluminum-doped ZnO (AZO) transparent conducting films were deposited on glass substrates with and without intrinsic ZnO (i-ZnO) buffer layers by a home made and low cost radio-frequency (RF) magnetron sputtering system at room temperature in pure argon ambient and under a low vacuum level. The films were examined and characterized for electrical, optical, and structural properties for the application of CIGS solar cells. The influence of sputter power, deposition pressure, film thickness and residual pressure on electrical and optical properties of layered films of AZO, i-ZnO and AZO/i-ZnO was investigated. The optimization of coating process parameters (RF power, sputtering pressure, thickness) was carried out. The effects of i-ZnO buffer layer on AZO films were investigated. By inserting thin i-ZnO layers with a thickness not greater than 125 nm under the AZO layers, both the carrier concentration and Hall mobility were increased. The resistivity of these layered films was lower than that of single layered AZO films. The related mechanisms and plasma physics were discussed. Copper indium gallium selenide (CIGS) thin film solar cells were fabricated by incorporating bi-layer ZnO films on CdS/CIGS/Mo/glass substrates. Efficiencies of the order of 7–8% were achieved for the manufactured CIGS solar cells (4–5 cm² in size) without antireflective films. The results demonstrated that RF sputtered layered AZO/i-ZnO films are suitable for application in low cost CIGS solar cells as transparent conductive electrodes.     

纳米Ag夹层ZnO薄膜的光电性能

用射频磁控溅射ZnO陶瓷靶、直流磁控溅射Ag靶的方法在室温下制备了Ag纳米夹层结构ZnO薄膜．用X射线衍射仪、紫外一可见分光光度计、四探针电阻测量仪和原子力显微镜对薄膜样品的结构、光学透过率、面电阻和表面形貌进行表征．结果表明,ZnO衬底有利于Ag夹层形成连续膜．随着Ag层厚度的增加,Ag夹层ZnO薄膜呈现多晶结构,Ag（111）衍射峰强度增强,面电阻先迅速下降后缓慢下降．随着ZnO膜厚度的增加,Ag夹层ZnO薄膜的透射峰红移．制得样品的最佳可见光透过率高达92．3％,面电阻小于4．2Ω／□．     

层数变化对ZnO/In_2O_3多层膜微观结构及光电性能的影响

用直流磁控溅射和热氧化法在玻璃衬底上制备ZnO/In2O3透明导电多层膜,当总厚度一定时,调节溅射沉积的层数与相应各层膜的厚度,研究该多层膜微观结构、光学性能和电学性能的变化.XRD和SEM分析表明:随着溅射沉积层数的增加,In2O3衍射峰的强度不断地减弱,ZnO衍射峰出现了不同的晶面择优取向;多层膜表面的ZnO晶粒粒径变小,光洁度增加.四探针法方块电阻测试表明:低温热氧化时,ZnO/In2O3多层膜的方块电阻随层数的增加而上升;高温氧化时,ZnO/In2O3多层膜的方块电阻随层数的增加而下降.可见光光谱分析表明:随着溅射沉积层数的增加,ZnO/In2O3多薄膜在可见光区的平均透过率增大,透过率的峰值向短波方向偏移.     

Microstructural and optical properties of nanocrystalline ZnO deposited onto vertically aligned carbon nanotubes by physical vapor deposition

Nanocrystalline ZnO films with thicknesses of 5 nm, 10 nm, 20 nm, and 50 nm were deposited via magnetron sputtering onto the surface of vertically aligned multi-walled carbon nanotubes (MWCNTs). The ZnO/CNTs heterostructures were characterized by scanning electron microscopy, high resolution transmission electron microscopy, and X-ray diffraction studies. No structural degradation of the CNTs was observed and photoluminescence (PL) measurements of the nanostructured ZnO layers show that the optical properties of these films are typical of ZnO deposited at low temperatures. The results indicate that magnetron sputtering is a viable technique for growing heterostructures and depositing functional layers onto CNTs.     

ZnO/CdTe heterojunctions prepared by r.f. sputtering

ZnO/CdTe heterojunctions were prepared by r.f. sputtering of ZnO films onto p-type CdTe single crystals. The ZnO deposition was carried out using a ZnO target and an Ar-H₂ mixture as the sputtering gas. The ZnO films obtained under these conditions show a very good optical transmission (about 90%) between 0.4 and 0.8 μm and a low resistivity. The electrical properties of the ZnO/CdTe heterojunctions were studied by means of current-voltage (I-V) and capacitance-voltage (C-V) measurements carried out at different temperatures. The dark I-V characteristics show that a multistep tunnelling mechanism controls the transport of charge carriers across the junction. The presence of interface states was shown to play an important role in the electrical properties of the junction. Tentative measurements of the solar energy conversion efficiency gave a value of about 3.5% without any attempt at optimization of the cell parameters.     

Investigation of conductive and transparent Al-doped ZnO/Ag/Al-doped ZnO multilayer coatings by electron beam evaporation

Multilayer coatings consisting of thin silver layer sandwiched between layers of Al-doped ZnO (AZO) were prepared by electron beam evaporation. The optical and electrical performances of AZO/Ag/AZO multilayers were investigated. Optimization of the multilayer coatings resulted with low sheet resistance of 7.7 Ω/sq and transmittance of 85%. The influence of thickness of each layer on the optic and electrical performance was analyzed. The sheet resistance of the multilayer was reduced to 5.34 Ω/sq. and the average transmittance was improved to 90% by the thermal treatment. The coatings had satisfactory properties of low resistance, high transmittance and thermal stability.     

Structure-composition-property dependence in reactive magnetron sputtered ZnO thin films

Thin films of zinc oxide (ZnO) were prepared by dc reactive magnetron sputtering on glass substrates at various oxygen partial pressures in the range 1×10⁻⁴–6×10⁻³ mbar and substrate temperatures in the range 548–723 K. The variation of cathode potential of zinc target on the oxygen partial pressure was explained in terms of target poisoning effects. The stoichiometry of the films has improved with the increase in the oxygen partial pressure. The films were polycrystalline with wurtzite structure. The films formed at higher substrate temperatures were (0 0 2) oriented. The temperature dependence of Hall mobility of the films formed at various substrate temperatures indicated that the grain boundary scattering of charge carriers was predominant electrical conduction mechanism in these films. The optical band gap of the films increased with the increase of substrate temperature. The ZnO films formed under optimized oxygen partial pressure of 1×10⁻³ mbar and substrate temperature of 663 K exhibited low electrical resistivity of 6.9×10⁻² Ω cm, high visible optical transmittance of 83%, optical band gap of 3.28 eV and a figure of merit of 78 Ω⁻¹ cm⁻¹.     

薄膜厚度对TGZO透明导电薄膜光电性能的影响

利用直流磁控溅射法,在室温水冷玻璃衬底上成功制备出了可见光透过率高、电阻率低的钛镓共掺杂氧化锌(TG-ZO)透明导电薄膜。X射线衍射和扫描电子显微镜研究结果表明,TGZO薄膜为六角纤锌矿结构的多晶薄膜,且具有c轴择优取向。研究了厚度对TGZO透明导电薄膜电学和光学性能的影响,结果表明厚度对薄膜的光电性能有重要影响。当薄膜厚度为628 nm时,薄膜具有最小电阻率2.01×10-4Ω.cm。所制备薄膜在波长为400~760 nm的可见光中平均透过率都超过了91%,TGZO薄膜可以用作薄膜太阳能电池和液晶显示器的透明电极。     

Deposition and characterization of PEDOT/ZnO layers onto PET substrates

ZnO thin films of different thicknesses were deposited by pulsed direct-current magnetron sputtering onto poly(ethylene terephthalate) (PET) substrates and afterwards poly 3, 4-ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) was spin-coated onto the ZnO film. Spectroscopic ellipsometry in the Vis–fUV energy range (1.5–6.5 eV), X-ray diffraction and atomic force microscopy were used to reveal the properties of the deposited films. The size of crystallites increased from 5.1 to 7.4 nm, whereas the crystallinity of the ZnO films has been improved. The influence of different ZnO thickness on the optical properties of the PEDOT:PSS layer was studied as well. As the thickness of ZnO films increased, the surface roughness increased but the energy gap decreased after a critical thickness. Concerning the consequences to the PEDOT:PSS optical properties, no major changes occurred in the transition energies.     

High quality ZnO layers with adjustable refractive indices for integrated optics applications

Thin ( 1 μm) crystalline ZnO films with a good optical quality and good (0002) texture are grown under two considerably different process parameter sets using a r.f. planar magnetron sputtering unit. The optical parameters of the two corresponding ZnO layers are distinctly different: high refractive index ( 2.0 at λ = 632.8 nm) ZnO films resembling the single crystal form, and ZnO films with considerably lower (typical difference 0.05) refractive indices. The refractive index of the latter ZnO layers is adjustable ( 1.93–1.96 at λ = 632.8 nm) through the process deposition parameters. It is shown that the difference in refractive index between the two ZnO types most probably results from a difference in package density of the crystal columns. The optical waveguide losses of both ZnO types are typically 1–3 dB/cm at λ = 632.8 nm, however the low refractive index ZnO layers need a post-deposition anneal step to obtain these values. The two ZnO types are used to fabricate optical channel-and slab waveguides with small refractive index differences.     

Deposition of ZnO thin film on polytetrafluoroethylene substrate by the magnetron sputtering method

Due to the excellent optical and electrical properties, ZnO thin films deposited on flexible substrates can be used as technologically promising electron devices. Polytetrafluoroethylene (Teflon) had many advanced properties, such as high dielectric strength over various frequencies, low dissipation factor, and high surface electrical resistivity, which had made Teflon a competitive polymer choice in a variety of microelectronic applications. In our work, ZnO film was firstly deposited on Teflon substrate by the magnetron sputtering method. X-ray diffraction data revealed that the ZnO grains were highly c-axis-oriented and nanostructured with the size of 10–30 nm, which was in accordance with the experimental result of Scanning Electron Microscopy.     

溅射功率对ZnO：Si透明导电薄膜性能的影响

采用直流磁控溅射法在室温玻璃基片上制备出了掺硅氧化锌（ZnO：Si）透明导电薄膜,研究了溅射功率对ZnO：Si薄膜结构、形貌、光学及电学性能的影响,实验结果表明,溅射功率对ZnO：Si薄膜的生长速率、结晶质量及电学性能有很大影响,而对其光学性能影响不大。实验制备的ZnO：LSi薄膜为六方纤锌矿结构的多晶薄膜,且具有垂直于基片方向的c轴择优取向。当溅射功率从45W增加到105W时,薄膜的晶化程度提高、晶粒尺寸增大,薄膜的电阻率减小;当溅射功率为105W时,薄膜的电阻率达到最小值3．83~104n·cm,其可见光透过率为94．41％。实验制备的ZnO：Si薄膜可以用作薄膜太阳能电池和液晶显示器的透明电极。