Fabrication and optical property of vertically-aligned ZnO/Si double nanostructures

We fabricated the vertically-aligned zinc oxide (ZnO)/silicon (Si) double nanostructures by simple processes using the metal-assisted chemical etching and a subsequent hydrothermal synthesis, and their optical property was investigated. For efficient antireflection characteristics, Si nanostructures were optimized by changing the size of the dewetted silver (Ag) at different etching times. The thermally dewetted Ag nanoparticles or semi-island films as metal catalysts were controlled by the Ag film thickness and dewetting temperature. To form the ZnO/Si double nanostructures, ZnO nanorods were synthesized on the chemically etched Si nanostructures using a thin sputtered ZnO seed layer. The grown ZnO nanorod arrays (NRAs) exhibited good crystallinity and further reduced the surface reflection due to their antireflective property. The ZnO/Si double nanostructures showed the increased peak intensity of X-ray diffraction as well as the significantly reduced solar weight reflectance of 6.05% compared to 11.71% in the ZnO NRAs on the flat Si substrate. Also, the enhanced antireflection property of ZnO/Si double nanostructures was theoretically analyzed by performing the rigorous coupled wave analysis simulation.     

Tunable Spectrum Selectivity for Multiphoton Absorption with Enhanced Visible Light Trapping in ZnO Nanorods

Observation of visible light trapping in zinc oxide (ZnO) nanorods (NRs) correlated to the optical and photoelectrochemical properties is reported. In this study, ZnO NR diameter and c‐axis length respond primarily at two different regions, UV and visible light, respectively. ZnO NR diameter exhibits UV absorption where large ZnO NR diameter area increases light absorption ability leading to high efficient electron–hole pair separation. On the other hand, ZnO NR c‐axis length has a dominant effect in visible light resulting from a multiphoton absorption mechanism due to light reflection and trapping behavior in the free space between adjacent ZnO NRs. Furthermore, oxygen vacancies and defects in ZnO NRs are associated with the broad visible emission band of different energy levels also highlighting the possibility of the multiphoton absorption mechanism. It is demonstrated that the minimum average of ZnO NR c‐axis length must satisfy the linear regression model of Z _p,min = 6.31d to initiate the multiphoton absorption mechanism under visible light. This work indicates the broadening of absorption spectrum from UV to visible light region by incorporating a controllable diameter and c‐axis length on vertically aligned ZnO NRs, which is important in optimizing the design and functionality of electronic devices based on light absorption mechanism.     

Investigation of thin solid ZnO films prepared by sputtering

Zinc oxide (ZnO) thin films have attracted great attention in recent years due to their unique piezoelectric and piezooptic properties, making them suitable for various microelectronics and optoelectronics applications, such as surface acoustic wave devices, optical fibers, solar cells etc. ZnO is a semiconductor with a band gap of 3.3 eV and a large exciton binding energy of 60 meV. Undoped ZnO exhibits intrinsic n-type conductivity and it enables achieving high electron concentration. However, it may be doped to obtain low resistivity p-type thin films. Among group V of the periodic table, nitrogen is used as a popular p-type dopant due to its small atomic size. However, it is difficult to achieve p-type conduction in ZnO films due to the low solubility of nitrogen and its high intensity in self compensating process upon doping.Sputtering techniques enable us to form dense and homogeneous films due to the relatively high energy of the sputtered atoms. Thus we can grow high quality ZnO films with c-axis orientation, low growth temperature, high deposition rate, large area deposition, and availability in various growths ambient. In this work, the zinc oxide films were prepared using various DC sputtering methods in an atmosphere of pure argon and an atmosphere of mixed argon with nitrogen. Optical and electrical properties of the films were investigated.     

Effect of ethylene glycol monomethyl ether ratio in mixed solvent on surface morphology,wettability and photocatalytic properties of ZnO thin films

Zinc oxide (ZnO) thin films with different ethylene glycol monomethyl ether (EGME) ratio were prepared on Si substrates using a two-step process. The results show that they possess a polycrystalline hexagonal wurtzite crystal structure. The topography of the ZnO thin films evolves from nanoparticles to hexagonal nanorods with the decrease of EGME content. The photoluminescence spectra consist of a near-band-edge emission and two visible emissions. The optical band gap energy decreases first and then increases with the increase of EGME ratio in mixed solvent, the broadening of the optical band gap can be explained by Moss–Burstein effect. The wetting behavior of all the samples can switch from hydrophobicity to hydrophilicity through UV illumination. The degradation efficiency of the thin films increases with decreasing EGME content, photocatalytic reaction mechanism of the ZnO thin films is discussed in detail.     

Broadband optical absorption enhancement in Au-coated ZnO nanotips

The enhancement of broadband optical absorption in zinc oxide (ZnO) nanotip (NT) arrays coated with evaporated gold (Au) on fluorine-doped SnO2 (FTO)/glass by a simple hydrothermal growth and subsequent Au evaporation is reported. As the core of ZnO NT arrays is gradually coated with evaporated Au, the reflectance is slightly increased at lambda > approximately 800 nm while the transmittance is decreased at lambda approximately 400-1800 nm. For both FTO/glass and ZnO NT arrays on FTO/glass, the coating of Au improves the light absorption due to the antireflective geometry compared to the flat Au films and the absorptance is also enhanced by increasing the nominal thickness of Au with evaporation time. For the ZnO NT arrays with an Au evaporated for 600 s at 0.5 A/s, a high absorptance of >72% is achieved over the wavelength range of 250-2000 nm, indicating a significant increase due to the enhanced antireflection property as well as the increased surface area compared to the Au-coated FTO/glass without ZnO NT arrays.     

Progress in the growth and characterization of nonpolar ZnO films

Zinc oxide (ZnO) is an important material for its potential applicability to short-wavelength optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs). Nonpolar ZnO materials have been developed in recent years to avoid the strong internal electric fields in active regions of optoelectronic devices and improve luminescence efficiency. The growth and physical properties of nonpolar ZnO films, which are essential for fabricating optoelectronic devices and improving device performance, still remains not well understood. In this review, the technologies for preparation of nonpolar ZnO epitaxial films are summarized, and recent developments are described. Then the main characteristics of nonpolar ZnO films are discussed with the deviations from those of polar ZnO films, including morphology, structural defects, anisotropic strain, optical, and electrical properties. The anisotropic electron transport and strains correlated strongly with the anisotropic surface morphologies of nonpolar ZnO films. Fabricating nonpolar ZnO films with high quality should be further developed to decrease the structural defect densities for substantial improvement of device performance, and intensive studies on their characteristics are especially important for device applications.     

Effect of Al concentrations on the electrodeposition and properties of transparent Al-doped ZnO thin films

Al-doped zinc oxide (AZO) thin films are prepared on polycrystalline fluorine-doped tin oxide-coated conducting glass substrates from nitrates baths by the electrodeposition process at 70 °C. The electrochemical, morphological, structural and optical properties of the AZO thin films were investigated in terms of different Al concentration in the starting solution. It was found that the carrier density of AZO thin films varied between ?3.11 and ?5.56 × 10²⁰ cm^?3 when the Al concentration was between 0 and 5 at.%. Atomic force microscopy images reveal that the concentration of Al has a very significant influence on the surface morphology and roughness of thin AZO. X-ray diffraction spectra demonstrate preferential (002) crystallographic orientation having c-axis perpendicular to the surface of the substrate and average crystallites size of the films was about 33–54 nm. With increasing Al doping, AZO films have a strong improved crystalline quality. As compared to pure ZnO, Al-doped ZnO exhibited lower crystallinity and there is a shift in the (002) diffraction peak to higher angles. Due to the doping of Al of any concentration, the films were found to be showing >80 % transparency. As Al concentration increased the optical band gap was also found to be increase from 3.22 to 3.47 eV. The room-temperature photoluminescence spectra indicated that the introduction of Al can improve the intensity of ultraviolet (UV) emission, thus suggesting its greater prospects in UV optoelectronic devices. A detailed comparison and apprehension of electrochemical, optical and structural properties of ZnO and ZnO:Al thin films is done for the determination of optimum concentration of Al doping.     

Enhanced photoconduction of free-standing ZnO nanowire films by L-lysine treatment

Flexible paper-like ZnO nanowire films are fabricated and the effect of L-lysine passivation of the nanowire surfaces on improving the UV photoresponse is studied. We prepare three types of nanowires with different defect contents, and find that the L-lysine treatment can suppress the oxygen-vacancy-related photoluminescence as well as enhance the UV photoconduction. The nanowires with fewer defects gain larger enhancement of UV photoconduction after L-lysine treatment. Reproducible UV photoresponse of the devices in humid air is obtained due to L-lysine surface passivation, ruling out the influence of water molecules in degrading the UV photocurrent.     

Enhancement of crystallinity and optical properties of bilayer TiO2/ZnO thin films prepared by atomic layer deposition

Bilayer and multilayer thin films are becoming increasingly important in the development of faster, smaller and more efficient electronic and optoelectronic devices. One of the motivations of applying bilayer or multilayer structures is to modify the optical properties of materials. Atomic layer deposition (ALD) is a variant of Chemical Vapour Deposition that can produce uniform and conformal thin films with well controlled nanostructures. In this study, we have demonstrated new findings of the use of ALD fabricated bilayer TiO2/ZnO thin films with enhanced crystallinity and optical properties. TiO2 films have been deposited at 300 degrees C for 1000 (51 nm in thickness) or 3000 (161 nm in thickness) deposition cycles onto glass and Si substrates. ZnO films are subsequently deposited on the TiO2 layers at 280 degrees C for 500 deposition cycles (55 nm). The crystallinity and optical properties of the TiO2/ZnO thin films have been analysed by X-ray diffraction, photoluminescence, UV-Vis spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy. XRD diffraction pattern confirmed the presence of ZnO with wutrtize crystal structure and TiO2 with anatase structure. It shows that the crystallinity of the TiO2 films has been improved with the deposition of ZnO. The intensity of UV luminescence has increased by almost 30% for TiO2/ZnO bilayer as compared to the single layer TiO2. The possible mechanism for the enhancement of the optical properties of bilayer TiO2/ZnO thin films will be discussed.     

Structural transition of ZnO thin films produced by RF magnetron sputtering at low temperatures

Zinc oxide (ZnO) thin films were prepared using reactive radio-frequency magnetron sputtering of a pure metallic zinc target onto glass substrates. The evolution of the surface morphology and the optical properties of the films were studied as a function of the substrate temperature, which was varied from 50 to 250 °C. The surface topography of the samples was examined using atomic force microscopy (AFM), and their optical properties were studied via transmittance measurements in the UV–Vis–NIR region. DRX and AFM analyses showed that the surface morphology undergoes a structural transition at substrate temperatures of around 150 °C. Actually, at 50 °C the formation of small grains was observed while at 250 °C the grains observed were larger and had more irregular shapes. The optical gap remained constant at ~3.3 eV for all films. In the visible region, the average optical transmittance was 80 %. From these results, one can conclude that the morphological properties of the ZnO thin films were more greatly affected by the substrate temperature, due to mis-orientation of polycrystalline grains, than were the optical properties.     

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

In this work, c-axis-oriented continuous GaN films have been synthesized by the chemical vapor deposition (CVD) method using ZnO material as the intermediate buffer layer. The GaN films with different growth temperatures exhibit high crystal quality and small surface roughness due to the same crystal structure and low lattice mismatches rate between GaN and ZnO materials. Meanwhile, the UV photodetector based on the CVD-grown GaN film exhibits a relatively high responsivity, fast rise and decay time, and good thermal stability. Our work provides a simple and promising CVD method to fabricate continuous GaN film for electronic and optoelectronic devices.

     

Effects of buffer layer thickness on properties of ZnO thin films grown on porous silicon by plasma-assisted molecular beam epitaxy

ZnO thin films with different buffer layer thicknesses were grown on Si and porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The effects of PS and buffer layer thickness on the structural and optical properties of ZnO thin films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The ZnO buffer layers, the intensity of the (002) diffraction peak for the ZnO thin films and its full width at half maximum (FWHM) decreased with an increase in the thickness of the ZnO buffer layers, indicating an improvement in the crystal quality of the films. On introducing PS as a substrate, the grain sizes of the ZnO thin films became larger and their residual stress could be relaxed compared with the ZnO thin films grown on Si. The intensity ratio of the ultraviolet (UV) to visible emission peak in the PL spectra of the ZnO thin films increased with an increase in buffer layer thickness. Stronger and narrower UV emission peaks were observed for ZnO thin films grown on PS. Their structural and optical properties were enhanced by increasing the buffer layer thickness. In addition, introduction of PS as a substrate enhanced the structural and optical properties of the ZnO thin films and also suppressed Fabry-Perot interference.     

Optical and structural characteristics of ZnO thin films grown by rf magnetron sputtering

Nanostructured ZnO thin films on Pyrex glass substrates were deposited by rf magnetron sputtering at different substrate temperatures. Structural features and surface morphology were studied by X-ray diffraction and atomic force microscopy analyses. Films were found to be transparent in the visible range above 400 nm, having transparency above 90%. Sharp ultraviolet absorption edges around 370 nm were used to extract the optical band gap for samples of different particle sizes. Optical band gap energy for the films varied from 3.24 to 3.32 eV and the electronic transition was of the direct in nature. A correlation of the band gap of nanocrystalline ZnO films with particle size and strain was discussed. Photoluminescence emission in UV range, which is due to near band edge emission is more intense in comparison with the green band emission (due to defect state) was observed in all samples, indicating a good optical quality of the deposited films.     

Influence of Al concentration on structural and optical properties of Al-doped ZnO thin films

Undoped ZnO and Al-doped zinc oxide (ZnO:Al) thin films with different Al concentrations were prepared onto Si (100) substrate by pulsed filtered cathodic vacuum arc deposition system at room temperature. The influence of doping on the structural and optical properties of thin films was investigated. The preferential (002) orientation was weakened by high aluminum doping in films. Raman measurement was performed for the doping effects in the ZnO. Atomic force microscopy images revealed that the surface of undoped ZnO film grown at RT was smoother than that of the Al-doped ZnO (ZnO:Al) films. The reflectance of all films was studied as a function of wavelength using UV–Vis–NIR spectrophotometer. Average total reflectance values of about 35 % in the wavelength range of 400–800 nm were obtained. Optical band gap of the films was determined using the reflectance spectra by means of Kubelka–Munk formula. From optical properties, the band gap energy was estimated for all films.     

ZnO薄膜及其性能研究进展

ＺｎＯ薄膜是一种具有优良的压电、光电、气敏、压敏等性质的材料,在透明导体、发光元件、太阳能电池窗口材料、光波导器、单色场发射显示器材料、高频压电转换器、表面声波元件、微传感器以及低压压敏电阻器等方面具有广泛的用途．ＺｎＯ 薄膜的制备方法多样,各具优缺点;而薄膜性质的差异则取决于不同的掺杂组分,并与制备工艺紧密相关．本文综述了ＺｎＯ薄膜的制备及性质特征,并对其发展趋势及前景进行了探讨．     

衬底温度对PLD方法生长的ZnO薄膜结构和发光特性的影响

在不同的衬底温度下, 通过脉冲激光淀积的方法在Si衬底上生长出c轴高度取向的ZnO薄膜. ZnO薄膜的结构和表面形貌通过X射线衍射和原子力显微镜表征. 同时以He-Cd激光和同步辐射作为激发源来测试样品的发光特性. 实验结果表明, 在衬底温度为500℃时生长的ZnO薄膜具有非常好的晶体质量, 并且表现出很强的紫外发射. 在用同步辐射为激发源的低温(18K)光致发光谱中, 还观察到了一个位于430nm处的紫光发射, 我们认为这个紫光发射与存在于晶粒间界的界面势阱所引起的缺陷态有关, 这个势阱可能起源于Zn填隙(Zn _i)     

Characteristics of Al-doped ZnO films grown by atomic layer deposition for silicon nanowire photovoltaic device

We report the structural, electrical, and optical characteristics of Al-doped ZnO (ZnO:Al) films deposited on glass by atomic layer deposition (ALD) with various Al2O3 film contents for use as transparent electrodes. Unlike films fabricated by a sputtering method, the diffraction peak position of the films deposited by ALD progressively moved to a higher angle with increasing Al2O3 film content. This indicates that Zn sites were effectively replaced by Al, due to layer-by-layer growth mechanism of ALD process which is based on alternate self-limiting surface chemical reactions. By adjusting the Al2O3 film content, a ZnO:Al film with low electrical resistivity (9.84 x 10(-4) Omega cm) was obtained at an Al2O3 film content of 3.17%, where the Al concentration, carrier mobility, optical transmittance, and bandgap energy were 2.8 wt%, 11.20 cm2 V(-1) s(-1), 94.23%, and 3.6 eV, respectively. Moreover, the estimated figure of merit value of our best sample was 8.2 m7Omega(-1). These results suggest that ZnO:Al films deposited by ALD could be useful for electronic devices in which especially require 3-dimensional conformal deposition of the transparent electrode and surface passivation.     

Array of ZnO nanoparticle-sensitized ZnO nanorods for UV photodetection

Vertically aligned ZnO nanorod (NR) arrays have been successfully synthesized on ITO-glass substrate by hydrothermal growth. Chemical bath deposition method was used to deposit ZnO nanoparticles (NPs) onto the ZnO NRs. These structures were applied in fabricating ZnO NPs sensitized ultraviolet (UV) photodetectors (PDs). Incorporation of ZnO NPs onto ZnO NRs results in distinct improvement of optical properties of ZnO NRs, i.e., significant enhancement of emission as well as effective suppression of defects emission in ZnO. Furthermore, there is a noticeable blue-shift in absorption spectra compared with that of ZnO NRs structure. I–V characteristics show that the sensitized structure improved photocurrent almost twice that of unsensitized ZnO NRs. Consequently, these findings may open new opportunities for the integration of different ZnO nanostructures for application in UV region particularly fabrication of UV PDs.     

Effect of film thickness in hybrid polymer/polymer solar cells with zinc oxide nanoparticles

We briefly report the effect of film thickness on the performance of hybrid polymer/polymer solar cells that were made using poly(3-hexylthiophene), poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and zinc oxide (ZnO) nanoparticles. The ZnO nanoparticles were introduced to improve the electron transport property of P3HT/F8BT blend films. Results showed that the open circuit voltage (V(OC)) was remarkably decreased by adding only approximately 0.5 wt% ZnO nanoparticles though the optical absorption spectra were not much changed due to the small amount of ZnO nanoparticles in the ternary blend films (approximately 1.9%). In contrast, the fill factor (FF) of devices was improved for the ternary blend devices with the ZnO nanoparticles due to the improved electron transport as evidenced by the reduced series resistance. The short circuit current density of devices was not much changed because of the enhanced charge transport. However, the addition of ZnO nanoparticles decreased the power conversion efficiency of devices owing to the larger influence of V(OC) compared to the FF improvement.     

反应磁控溅射ZnO薄膜的高温退火研究

ZnO薄膜是一种新型的II VI族直接能带化合物半导体材料 ,有可能实现短波长的探测器 ,LED和LD等光电子器件。用磁控溅射法在硅衬底上生长ZnO薄膜 ,由于薄膜与衬底之间较大的应力失配 ,以及由于较快的生长速率 ,薄膜中存在较多的Zn间隙原子和O空位 ,在薄膜中存在应力。通过高温退火 ,可以使应力得到弛豫 ,降低O空位和Zn间隙原子的浓度 ,提高薄膜的化学计量比和改善薄膜的结晶质量。本实验用XRD和AFM研究了高温退火对ZnO薄膜的晶体性能和表面的影响。对ZnO薄膜在退火处理后c轴方向的应力性质的转变作了机理上的探讨。