Pure UV photoluminescence from ZnO thin film by thermal retardation and using an amorphous SiO2 buffer layer

ZnO/SiO₂ thin films were fabricated on Si substrates by E-beam evaporation with thermal retardation. The as-prepared films were annealed for 2 h every 100 °C in the temperature range 400-800 °C under ambient air. The structural and optical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL). The XRD analysis indicated that all ZnO thin films had a highly preferred orientation with the c-axis perpendicular to the substrate. From AFM images (AFM scan size is 1 μm×1 μm), the RMS roughnesses of the films were 3.82, 5.18, 3.65, 3.40 and 13.2 nm, respectively. PL measurements indicated that UV luminescence at only 374 nm was observed for all samples. The optical quality of the ZnO film was increased by thermal retardation and by using an amorphous SiO₂ buffer layer.     

Comparative study of ZnO layers prepared by PLD from different targets at various oxygen pressure levels

Two series of polycrystalline zinc oxide (ZnO) layers, from Zn or ZnO targets, were grown on silicon (1 1 1) substrates by pulsed laser deposition (PLD) at ambient oxygen pressure levels, stepwise increased from 1 to 35 Pa. For ablation of targets, a pulsed Nd:YAG laser was used. The structural and morphological properties of the layers were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). The SEM images of ZnO layers in SE mode show a uniform granular structure and modified surface morphology, depending on oxygen pressure. The mean grain size in height and lateral directions decreases with an increase of oxygen pressure from 1 to 5 Pa, while a subsequent rise of oxygen pressure from 5 to 35 Pa will cause an increase in the grain size. The AFM measurement revealed that the surface structures of zinc oxide layers grown from different targets were similar, and the layers formed at an ambient oxygen pressure of 5 Pa exhibited the smallest values of calculated roughness and granularity. SIMS depth profiling analyses confirmed that the ZnO composition was homogenous across the layer, up to the abrupt change of chemical composition at the interface between the ZnO layer and the Si substrate.      

Effects of sapphire substrate annealing on ZnO epitaxial films grown by MOCVD

The annealing effects of sapphire substrate on the quality of epitaxial ZnO films grown by metalorganic chemical vapor deposition (MOCVD) were studied. The atomic steps formed on (0 0 0 1) sapphire (α-Al₂O₃) substrate surface by annealing at high temperature was analyzed by atomic force microscopy (AFM). The annealing effects of sapphire substrate on the ZnO films were examined by X-ray diffraction (XRD), AFM and photoluminescence (PL) measurements. Experimental results indicate that the film quality is strongly affected by annealing treatment of the sapphire substrate surface. The optimum annealing temperature of sapphire substrates is given.     

Effects of sapphire substrates surface treatment on the ZnO thin films grown by magnetron sputtering

The surface treatment effects of sapphire substrate on the ZnO thin films grown by magnetron sputtering were studied. The sapphire substrates properties have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD) and photoluminescence (PL) measurements. Results show that the intensity of (0 0 2) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching was strongest, FWHM of (0 0 2) diffraction peak is the narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property.     

p-type ZnO thin films grown by RF plasma beam assisted Pulsed Laser Deposition

The high exciton binding energy and band gap energy of ZnO thin films open the prospect of fabricating semiconductor lasers in the ultraviolet spectral range. A prerequisite for laser diode fabrication is highly p-doped ZnO which was not reproducibly obtained up to now. Without intentional doping ZnO exhibits n-type conduction. ZnO thin films have been obtained by radio-frequency assisted pulsed laser deposition. A metallic Zn target was used for ablation in an oxygen and nitrogen RF discharge. The electrical and morphological properties of the films grown on Si were studied by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), optical absorption and Hall Effect measurements for different ratios between the nitrogen and oxygen content. The AFM images of the as-grown ZnO films reveal high quality surfaces with low values for the surface roughness and a sharp distribution of grains sizes as an effect of the RF discharge. The XRD patterns for all samples exhibit only (002) and (004) peaks indicating that the c-axis is always oriented normal to the substrate surface. The films present p-type conductivity with different carrier concentration and mobility depending on the nitrogen/oxygen ratio.     

Rapid thermal annealing effects on the structural and optical properties of ZnO films deposited on Si substrates

The structural and optical properties of ZnO films deposited on Si substrate following rapid thermal annealing (RTA) have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. After RTA treatment, the XRD spectra have shown an effective relaxation of the residual compressive stress, an increase of the intensity and narrowing of the full-width at half-maximum (FWHM) of the (0 0 2) diffraction peak of the as-grown ZnO film. AFM images show roughening of the film surface due to increase of grain size after RTA. The PL spectrum reveals a significant improvement in the UV luminescence of ZnO films following RTA at 800 °C for 1 min.     

Structural characterization of supported nanocrystalline ZnO thin films prepared by dip-coating

Nanocrystalline ZnO thin films prepared by the sol-gel dip-coating technique were characterized by grazing incidence X-ray diffraction (GIXD), atomic force microscopy (AFM), X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS). The structures of several thin films subjected to (i) isochronous annealing at 350, 450 and 550 °C, and (ii) isothermal annealing at 450 °C during different time periods, were characterized. The studied thin films are composed of ZnO nanocrystals as revealed by analysing several GIXD patterns, from which their average sizes were determined. Thin film thickness and roughness were determined from quantitative analyses of AFM images and XR patterns. The analysis of XR patterns also yielded the average density of the studied films. Our GISAXS study indicates that the studied ZnO thin films contain nanopores with an ellipsoidal shape, and flattened along the direction normal to the substrate surface. The thin film annealed at the highest temperature, T = 550 °C, exhibits higher density and lower thickness and nanoporosity volume fraction, than those annealed at 350 and 450 °C. These results indicate that thermal annealing at the highest temperature (550 °C) induces a noticeable compaction effect on the structure of the studied thin films.     

用锌有机源和CO2/H2混合气源PECVD沉积ZnO薄膜

在等离子体作用下,以CO2/H2混合气为氧源,Zn(C2H5)2锌为锌源,在单晶硅上生长出高度择优取向的氧化锌薄膜。X射线衍射分析表明,薄膜为六方结构,c轴高度择优;原子力显微镜观察到晶粒是有规律地按六方排布,薄膜的表面粗糙度较小;从光致发光谱还发现在380 nm处有非常强的紫外峰。     

ZnO薄膜的分子束外延生长及性能

利用分子束外延(MBE)和氧等离子体源辅助MBE方法分别在Si(100)、GaAs(100)和蓝宝石Al2O3(0001)衬底上用Zn、ZnS或以一定Zn-O化学计量比作缓冲层,改变衬底生长温度和氧压,并在氧气氛下,进行原位退火处理,得到ZnO薄膜。依据X射线衍射(XRD)图,表明样品的结晶性能尚好,且呈c轴择优取向;实验结果表明在不同衬底上生长的ZnO薄膜,由于晶格失配度不同,其衍射峰也有区别。用原子力显微镜(AFM)观测薄膜的表面形貌,为晶粒尺寸约几十纳米的ZnO纳米晶,且ZnO晶粒呈六边形柱状垂直于衬底的表面。采用掠入射X射线反射率法测膜厚。在360nm激发下,样品的发光光谱是峰值为410,510nm的双峰谱,是与样品表面氧缺陷有关的深能级发光。     

Room-temperature deposition of crystalline patterned ZnO films by confined dewetting lithography

In this work patterned ZnO films were prepared at room-temperature by deposition of ∼5 nm size ZnO nanoparticles using confined dewetting lithography, a process which induces their assembly, by drying a drop of ZnO colloidal dispersion between a floating template and the substrate. Crystalline ZnO nanoparticles exhibit a strong visible (525 nm) light emission upon UV excitation (λ = 350 nm). The resulting films were characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM). The method described herein presents a simple and low cost method to prepare crystalline ZnO films with geometric patterns without additional annealing. Such transparent conducting films are attractive for applications like light emitting diodes (LEDs). As the process is carried out at room temperature, the patterned crystalline ZnO films can even be deposited on flexible substrates.     

Effect of ultrasonic treatment on the morphology of casein particles

In this study, the effect of ultrasonic treatment duration on the morphology of self-assembled casein particles was investigated by atomic force microscopy (AFM), low vacuum scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the case of AFM images, the particle analysis which was carried out by the SPIP program showed that the self-assembled casein particles after being ultrasonically treated for 2 min got smaller in size compared to the casein particles that have not been exposed to any ultrasonic treatment. Surprisingly, however, increasing the ultrasonic time exposure of the particles resulted in an opposite effect where larger particles or aggregates seemed to be present. We show that by comparing the results obtained by AFM, SEM and TEM, the information extracted from the AFM images and analyzed by SPIP program give more detailed insights into particle sizes and morphology at the molecular level compared to SEM and TEM images, respectively.     

溶胶-凝胶法制备氧化锌薄膜的压电行为

"采用溶胶-凝胶技术在单晶硅Si(111)上制备了ZnO压电薄膜,并以扫描电镜、X射线衍射仪(XRD)和原子力显微镜(AFM)进行了表征．XRD衍射实验表明ZnO薄膜随着膜厚的增大,其(002)取向逐渐增强;AFM研究了薄膜的表面形貌、粗糙度与晶粒大小的结果表明,ZnO压电薄膜的粗糙度与晶粒寸随着薄膜厚度的增大而减小．粗糙度为2.188~0.914 nm．利用PFM研究压电系数,发现随着薄膜厚度的增加,(002)生长方向增强,压电系数逐渐增大;当力参数小于薄膜的表面粗糙度时,压电系数测量不准确且在较大幅度     

The effects of porous silicon on the crystalline properties of ZnO thin films

In the present work, ZnO was deposited on porous silicon substrates by sol-gel spin coating and rf magnetron sputtering. The porous silicon (PS) substrates were formed by electrochemical anodization on p-type (1 0 0) silicon wafer, and the starting material for ZnO was Zinc acetate dehydrate. Raman spectroscopy revealed the good quality of the porous silicon substrate. XRD analysis showed that highly (0 0 2) oriented ZnO thin films were formed. SEM, AFM and optical microscope have been used to understand the effects of the substrate on crystalline properties of the samples. The results indicated that the porous silicon substrate is beneficial to improve the crystalline quality in lattice mismatch heteroepitaxy due to its sponge-like structure.     

Defect-controlled growth of ZnO nanostructures using its different zinc precursors and their application for effective photodegradation

Various zinc precursors, such as zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride, have been used to control the formation of zinc oxide (ZnO) nanostructures onto aluminum substrate by chemical means. FESEM images of the ZnO nanostructures showed the formation of different morphologies, such as flakes, nanowalls, nanopetals, and nanodisks, when the nanostructures were synthesized using zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride precursors, respectively. The TEM image of disk-like ZnO nanostructures formed using zinc chloride as a precursor revealed hexagonally shaped particles with an average diameter of 0.5 μm. Room-temperature photoluminescence (PL) spectra revealed a large quantity of surface oxygen defects in ZnO nanodisks grown from zinc chloride compared with those using other precursors. Furthermore, the ZnO nanostructures were evaluated for photocatalytic activity under ultraviolet (UV) light illumination. Nanostructures having a disk-like shape exhibited the highest photocatalytic performance (k = 0.027 min⁻¹) for all the ZnO nanostructures studied. Improved photocatalytic activity of ZnO nanodisks was attributed to their large specific surface area (4.83 m² g⁻¹), surface oxygen defects, and super-hydrophilic nature of their surface, which is particularly suitable for dye adsorption.     

Ti:sapphire femtosecond laser direct micro-cutting and profiling of graphene

Zinc oxide and titanium dioxide composite thin films were prepared on Corning 7059 glass substrates by co-sputtering. The reactive gas-surroundings used was ultrahigh purity oxygen. To analyze the structural, optical and photocatalytic properties of the ZnO?CTiO₂ samples, X-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption, Raman spectroscopy and methylene blue bleaching were carried out at room temperature. XRD patterns indicate the presence of TiO₂ (anatase and rutile phases), ZnO, ZnTiO₃, and Zn₂TiO₄ crystalline structures. AFM images allow the observation of non-homogeneous surface in the ZnO?CTiO₂ system, suggesting the separation of different crystalline phases in the composite. Raman studies exhibit different spectra in the films depending on the area analyzed, which can be interpreted as a result of the existence of well separated crystalline regions as seen in AFM images. The photocatalytic activity (PA) of TiO₂?CZnO?CZnTiO₃?CZn₂TiO₄ composite, as expected for adequate coupling semiconductors, is larger than PA of ZnO and TiO₂ oxides, used as references. A simple proposal about the probable alignment of the conduction band, the valence band, and the Fermi level is included.     

Influence of drying conditions on the optical and structural properties of sol–gel-derived ZnO nanocrystalline films

Zinc oxide nanothin films were prepared on glass substrate by sol–gel dip-coating method using zinc acetate dihydrate, methanol, and monoethanolamine as precursor, solvent, and stabilizer, respectively. The relationship between drying conditions and the characteristics of ZnO nanocrystalline films (c-axis orientation, grain size, roughness and optical properties) was studied. The films were dried in an oven at different temperatures and by IR radiation. Then, the films were annealed at 500°C in a furnace. The chemical composition, transmission spectra, structure, and morphology of the samples were studied using infrared (IR) and UV–visible spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM), respectively. The XRD results show that the drying conditions affect the orientation of crystallization along the (0 0 2) plane. AFM images show that the thicknesses of the films decrease from 128 to 93 nm by changing the drying conditions. The photoluminescence (PL) of ZnO nanothin films shows the UV emission at near band edge and broad green radiation at about 465 nm wavelength.     

The influence of electronic transport across interface junction between Si substrate and the root of ZnO micro-prism on field emission performance

ZnO micro-prisms are prepared on the p-type and n-type Si substrates, separately. The $I$--$V$ curves analysed by AFM show that the interface junctions between the ZnO micro-prisms and the p-type substrate and between the ZnO micro-prisms and the n-type Si substrate exhibit p--n junction behaviour and ohmic contact behaviour, respectively. The formation of the p--n heterojunction and ohmic contact is ascribed to the intrinsic n-type conduction of ZnO material. Better field emission performance (lower onset voltage and larger emission current) is observed from an individual ZnO micro-prism grown on the n-type Si substrate. It is suggested that the n-Si/n-ZnO interfacial ohmic contact benefits the electron emission; while the p-Si/n-ZnO interface heterojunction deteriorates the electron emission.     

Influence of mesoporous substrate morphology on the structural, optical and electrical properties of RF sputtered ZnO layer deposited over porous silicon nanostructure

Morphological, optical and transport characteristics of the RF sputtered zinc oxide (ZnO) thin films over the mesoporous silicon (PS) substrates have been studied. Effect of substrate porosity on the grain growth and transport properties of ZnO has been analyzed. Physical and optical properties of ZnO-PS structures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. Our experimental results indicate that on changing porosity of the PS substrates, regularity of the spatial distribution of the ZnO nanocrystallites can be controlled. While the morphology and grain size of ZnO depended strongly on the morphology and pore size of the PS substrates, the rectifying factors of the metal semiconductor junction were found to be different by a factor of 3. The deposition of semiconducting oxides on such mesoporous substrates/templates offers the possibility to control their properties and amplify their sensing response.     

不同衬底温度下PLD法制备的氧化锌薄膜的特性

利用GCR-170型脉冲激光器Nd:YAG的三次谐波(355nm),以蓝宝石Al2O3(0001)为衬底,在不同温度下采用脉冲激光沉积法制备了ZnO薄膜.通过原子力显微镜、Raman谱、光致发光谱、红外透射谱、霍尔效应和表面粗糙度分析仪对制备的ZnO薄膜进行了测试.分析了在不同衬底温度下薄膜的表面形貌、光学特性,同时进行了薄膜结构和厚度的测试.研究表明:衬底温度对ZnO薄膜的表面形貌、光学特性、结构特性都是重要的工艺参量,尤其在500℃时沉积的ZnO薄膜致密均匀,并表现出较强的紫外发射峰.     

用于Raman光谱与微纳米结构同步检测的Raman-AFM系统研究

研究和发展了一种将微区拉曼(Raman)光谱检测与原子力显微镜(AFM)微纳米扫描成像相结合的新型Raman-AFM技术。设计了Raman光谱与AFM扫描成像的原位检测探头;研制出相应的Raman-AFM系统;利用该系统,对ZnO纳米颗粒和TiO2纳米薄膜开展了微区Raman光谱与微纳米结构的检测实验。研究表明,所获得的Raman光谱检测结果与理论值良好吻合,同时,AFM扫描检测得到的图像很好地表征了样品的微纳米结构,从而实现了微区Raman光谱与AFM图像的原位及同步检测,验证了这一技术的可行性,为Raman光谱技术与微纳米技术领域的实际应用提供了技术基础。