Surface and interface characterization of nanoporous alumina templates produced in oxalic acid and submitted to etching procedures

High purity aluminum (Al) sheets were used to produce ordered nanoporous alumina templates by two step anodization technique using oxalic acid as electrolyte. Electrochemical polishing of Al sheets was performed in order to study the surface roughness influence on the pore roundness of the alumina templates. The templates were submitted to progressive reduction of anodizing voltage (electrochemical etching) to thin the Al₂O₃ barrier layer on the bottom of the pores, and to chemical etching to widen the diameter of the pores. The pore diameter and porosity increase linearly with the chemical etching time, while interpore distance, circularity, pore density and pore distribution do not change. Electropolished Al substrates produce nanoporous Al₂O₃ templates with more circular pores, and the Al₂O₃ barrier layer at the Al₂O₃/Al interfaces is completely removed after the electrochemical and chemical etching processes.     

Penetrating the oxide barrier in situ and separating freestanding porous anodic alumina films in one step

A simple method for penetrating the barrier layer of an anodic aluminum oxide (AAO) film and for detaching the AAO film from residual Al foil was developed by reversing the bias voltage in situ after the anodization process is completed. With this technique, we have been able to obtain large pieces of free-standing AAO membranes with regular pore sizes of sub-10 nm. By combining Ar ion milling and wetting enhancement processes, Au nanowires were grown in the sub-10 nm pores of the AAO films. Further scaling down of the pore size and extension to the deposition of nanowires and nanotubes of materials other than Au should be possible by further optimizing this procedure.     

Fabrication of highly ordered anodic aluminium oxide templates on silicon substrates

The controlled fabrication of highly ordered anodic aluminium oxide (AAO) templates of unprecedented pore uniformity directly on Si, enabled by new advances on two fronts - direct and timed anodisation of a high-purity Al film of unprecedented thickness (50 mum) on Si, and anodising a thin but pre-textured Al film on Si, has been reported. To deposit high-quality and ultra-thick Al on a non-compliant substrate, a prerequisite for obtaining highly ordered pore arrays on Si by self-organisation while retaining a good adhesion, a specially designed process of e-beam evaporation followed by in situ annealing has been deployed. To obtain an AAO template with the same high degree of ordering and uniformity but from a thin Al film, which is not achievable by the self-organisation alone, pre-patterning of the thin Al surface by reactive ion etching using a freestanding AAO mask that was formed in a separate process was performed. The resultant AAO/Si template provides a good platform for integrated growth of nanotube, nanowire or nanodot arrays on Si. Template-assisted growth of carbon nanotubes (CNTs) directly on Si was demonstrated via a chemical vapour deposition method. By controllably removing the AAO barrier layer at the bottom of the pores and partially etching back the AAO top surface, new CNT/Si structures were obtained with potential applications in field emitters, sensors, oscillators and photodetectors.     

Effect of process parameters on growth rate and diameter of nano-porous alumina templates

Anodic aluminium oxide (AAO) template with hexagonal shaped nano-pores with high aspect ratio was fabricated by two-step anodization processes from high purity aluminium foil. It was observed that pore dimensions were affected by anodizing voltage, electrolyte temperature and the duration of anodization time. The vertical growth rate of the pores (10?C250?nm/min) was found to vary exponentially with anodizing voltage; however, it exhibits linear increment with the electrolyte temperature. The measured pore diameter (50?C130?nm) shows a linear variation with anodizing voltage. The bottom barrier oxide layer was etched out by pore widening treatment to obtain through holes.     

Ru nanostructure fabrication using an anodic aluminum oxide nanotemplate and highly conformal Ru atomic layer deposition

We fabricated metallic nanostructures directly on Si substrates through a hybrid nanoprocess combining atomic layer deposition (ALD) and a self-assembled anodic aluminum oxide (AAO) nanotemplate. ALD Ru films with Ru(DMPD)(EtCp) as a precursor and O(2) as a reactant exhibited high purity and low resistivity with negligible nucleation delay and low roughness. These good growth characteristics resulted in the excellent conformality for nanometer-scale vias and trenches. Additionally, AAO nanotemplates were fabricated directly on Si and Ti/Si substrates through a multiple anodization process. AAO nanotemplates with various hole sizes (30-100?nm) and aspect ratios (2:1-20:1) were fabricated by controlling the anodizing process parameters. The barrier layers between AAO nanotemplates and Si substrates were completely removed by reactive ion etching (RIE) using BCl(3) plasma. By combining the ALD Ru and the AAO nanotemplate, Ru nanostructures with controllable sizes and shapes were prepared on Si and Ti/Si substrates. The Ru nanowire array devices as a platform for sensor devices exhibited befitting properties of good ohmic contact and high surface/volume ratio.     

阳极氧化法制备有序纳米多孔氧化铝膜的研究

利用电化学阳极氧化的方法,在草酸溶液中,精确控制反应条件,在高纯铝片表面有序生长了纳米多孔氧化铝膜。试验中,分别采用一次阳极氧化和二次阳极氧化方法制备氧化铝膜。利用H3PO4溶液浸泡法对氧化铝膜进行扩孔处理。通过扫描电子显微镜对样品进行表征分析。结果发现,二次阳极氧化制备的氧化铝膜的孔洞分布较一次氧化的更为规则有序,并且孔径大小均匀一致。扫描电镜观察显示,氧化铝膜的扩孔过程可以去掉阻碍层,并调节孔径大小,溶去二次氧化后黏附在氧化层表面的一些杂质,从而使氧化铝模板更为规则有序,孔径均一。这种经过二次阳极氧化和扩孔处理得到多孔阳极氧化铝模板的方法简单,成本较低,可以为后续的纳米材料合成提供高质量的合成模板。     

In-situ fabrication of AAO template without oxide barrier layer and its applications

Porous anodic aluminum oxide (AAO) film is the most widely used template in combination with electrodeposition (ED) method to fabricate one-dimensional nanostructures such as nanowires, nanotubes and nanorods. However, the existing oxide barrier layer after the anodization blocks the application of AAO template in synthesis of nanostructures via direct electrodeposition. In this paper, AAO template without oxide barrier layer was successfully fabricated by stepwise voltage decrement; influence of two types of stepwise voltage decrement on the removal of oxide barrier layer was introduced. Field Emission Scanning Electron Microscopy (FESEM) images indicated that stepwise voltage decrement could make the oxide layer thin effectively. Meanwhile, highly ordered gold nanowire arrays were fabricated by using direct electrodeposition method based on AAO template with the second anodization process with stepwise voltage decrement of 1 V/min, FESEM image showed that as-prepared gold nanowires are uniform in diameter and the diameter is in accordance with the diameter of AAO template pores. XRD pattern revealed that gold nanowires were indexed as face-centered cubic phase.     

Nanopatterned silicon emitters of a solar cell fabricated by anodic aluminum oxide masks

We investigated the nanopattern transferring process by a template of anodic aluminum oxide and the formation of a nanoporous aluminum oxide layer on a Si solar cell by the anodization process of Al thin films. The anodization process provided a template to transfer the nanopattern onto the Si surface. The small-sized nanoporous alumina template was attached to be covered on the textured surface and played the role of etching mask in the F-based dry etching process. Furthermore, we deposited an Al thin film onto the Si surface and the subsequent anodization process was performed. The alumina formulated on the deposited Al thin film did not show the array of nanoporous structure and no nanopatterns were transferred onto the surface. The large-areal alumina deposited on the Si surface showed enhanced photo-absorption in the ultraviolet spectral region of 243 nm, but increased the photo-reflectance in the visible and infrared spectral regions when compared to the Si-bare sample.     

Freestanding Alumina Membrane by Double-Layer Anodization

A novel method has been developed for fabricating full or partial freestanding anodic alumina. In our method a sacrificial metal layer is introduced between an Al film and a Si₃N₄ substrate. A freestanding alumina film at wafer scale is successfully achieved by anodizing the double metal layer, during which the alumina is spontaneously stripped off the Si₃N₄ substrate due to the anodic oxidation of the sacrificial layer. The barrier oxide of the alumina film is effectively removed either by H₃PO₄ dissolution or by CF₄ reactive ion etching. The freestanding alumina film is utilized as a contact mask to transfer its nanoporous pattern to a Si substrate. By patterning the sacrificial metal layer with contact lithography, a partial freestanding alumina film is successfully achieved on the silicon chip, producing a unique micro/nanofluidic channel. Compared with previous techniques, the method reported here is advantageous for its simplicity and flexibility     

铝箔多孔阳极氧化铝膜的低温制备及其电化学性能

以多孔阳极氧化铝(AAO)膜制备纳米材料时降低AAO膜孔径至关重要,降低电压无法达到要求,而降低氧化温度可实现这一目标。在0.4 mol/L H3PO4溶液中加入70%～80%(体积分数)1,3-丙三醇(PDO),于-10～10℃下恒压110 V阳极氧化1 h制备了多孔阳极氧化铝(AAO)膜,并在0.50 mol/L H3BO3和0.05mol/L Na2B4O7溶液中于20℃下以0.5 mA/cm2进行填孔后处理。利用SEM,EDS,XRD分析了AAO膜的表面形貌与组成,并对AAO膜填孔前后的极化曲线和交流阻抗谱进行了测试。结果表明,膜孔径随氧化温度降低而降低,80%PDO,10℃所得AAO膜的成分包括65.94%(质量分数,下同)Al,12.79%C,20.29%O和0.98%P;随氧化温度升高和PDO含量下降,AAO膜的稳定电流密度增大;随氧化温度升高,膜阻挡层厚度增大;填孔试验前AAO膜只存在一个阻挡层的相位角峰,填孔后出现两个峰,中高频段体现封闭的阻挡层特性,低频段体现的是多孔层封闭部分的特性。     

Ion-beam pore opening of porous anodic alumina: The formation of single nanopore and nanopore arrays

Anodized aluminium oxide (AAO) fabricated by electrochemical anodization of aluminium is used as the substrate for focused ion-beam (FIB) fabrication of single nanopore and nanopore arrays. The method is based on the controlled and selective removal of AAO barrier oxide film by FIB milling. FIB process with different milling conditions is applied to open single nanopores (diameter of 60 nm) or circular nanopore arrays with a different number of opened pores on AAO substrate. This process combines a low cost AAO substrate and FIB milling, and offers an economically viable alternative for the fabrication of nanopore devices with controlled dimensions for a range of applications in nanofluidics, molecular separations and biosensing.     

Valence band and catalytic activity of Au nanoparticles in Fe2O3/SiO2/Si(100) environment

A 10-nm-thick gold film was evaporated onto a SiO₂/Si(100) substrate and was implanted by Ar⁺ ions at 40 keV and 10¹⁵ at/cm² dose creating island like Au nanoparticles. An 80-nm-thick gold film was also deposited the onto same substrate and considered as reference.Fe₂O₃ film was deposited onto the gold nanoparticles and the gold/oxide interface was modeled. The valence band and the structure were measured by means of photoelectron spectroscopy (XPS) and by transmission electron microscopy (TEM), respectively. The catalytic activity was detected by CO oxidation, which was higher after the deposition of Fe₂O₃ layer onto Au nanoparticles than that on a continuous Au film. This observation was correlated to the nanosize and the redistributed valence band density of states of gold in the Au/Fe₂O₃ interface.     

Growth of dielectric films on semiconductors and metals using a multipole plasma

A multipole plasma source (a hot electron emitter associated with magnetic confinement by permanent magnets) is very suitable for plasma deposition and anodization because it can create a high density (10¹⁰-10¹¹ cm^-3) homogeneous plasma which is free from energetic particles. The anodization kinetics of metals and semiconductors as well as technological applications of the oxide layers were investigated. Space charge effects in the oxide are shown to control the transport of negative oxygen ions and positive substrate ions during growth. Anodization through a thin CaO-stabilized ZrO₂ (CSZ) film results in strong enhancement in the anodization rates of aluminium, tantalum and silicon, probably because of an alteration in the surface chemistry between the plasma and the oxide. The applications of this process are very attractive: the room temperature plasma anodization of silicon resulting in good quality SiO₂, and the protective filter effect of the CSZ layer.A combination of a multipole source and an ultrahigh vacuum system is described and will be used to study the first steps in the interactions of a surface (mostly GaAs) with a plasma.     

Metal/nonpolar <Emphasis Type="Italic">m</Emphasis>-plane ZnO contacts with and without thin Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interlayer deposited by atomic layer deposition

Using the temperature dependent current–voltage (I–V) measurements, the electrical properties of Au/nonpolar m-plane ZnO Schottky diodes with an Al₂O₃ interlayer prepared by atomic layer deposition (ALD) was investigated. With an Al₂O₃ interlayer, it was found to have higher barrier heights and higher rectifying ratio. Modified Richardson plots produced effective Richardson constants of 30.0 and 37.6 Acm^?2K^?2 for the samples with and without Al₂O₃ interlayer, respectively, which are similar to the theoretical value of 32.0 Acm^??2K^??2 for n-ZnO. Scanning transmission electron microscope (STEM) results showed that the oxygen-contained layer on ZnO surface degraded the film quality of subsequently deposited Al₂O₃ layer. In addition, the inter-diffusion of Au and Al atoms into ZnO subsurface region also modulated the electrical properties of Au/ZnO contacts.     

Optical properties of porous anodic aluminum oxide thin films on quartz substrates

Porous anodic aluminum oxide (AAO) thin films on quartz substrates were fabricated via evaporation of a 100-nm thick Al, followed by anodization with different durations and pore widening and Al removal by chemical etching. The transmittance and reflectance of AAO films on quartz substrates were measured by optical spectrophotometry. The microstructure and morphology were examined by scanning electron microscopy. The pore diameter of AAO films after pore widening and Al removal is 60 ± 4 nm and the interpore distance is 88 ± 5 nm. It is found that the reflectance decreases and the transmittance increases with the increase of the anodization time and pore widening. Compared to a bare substrate, the transmittance of AAO films after pore widening and Al removal is about 3.0% higher, while the reflectance is about 3.0% lower over a wide wavelength range. Additionally, after pore widening and Al removal, when AAO films are prepared on both sides of the quartz substrate, the highest transmittance is about 99.0% in the wavelength range 570-680 nm. The optical constants and thickness of AAO films after pore widening and Al removal were retrieved from normal incidence transmittance data. Results show that the refractive index is lower than 1.25 in the visible optical region and that the porosity is about 0.70.     

Preparation and characterization of Nb2O5-Al2O3 composite oxide formed by cathodic electroplating and anodizing

Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb₂O₅-Al₂O₃ composite oxide was approximately 27% higher than that of Al₂O₃ without a Nb₂O₅ layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb₂O₅-Al₂O₃ composite oxide film is a suitable material for thin film capacitors.     

The growth and electrical transport properties of self-organized metal/oxide nanostructures formed by anodizing Ta-Al thin-film bilayers

Anodizing of Ta-Al metal bilayers (Al on Ta) sputter-deposited onto SiO₂ substrates was performed in oxalic acid electrolytes at anode potentials of 53 to 21.5 V in order to form nanoporous alumina layers and sequentially oxidize the tantalum underlayers through the alumina pores. The films formed consist of arrays of tantalum oxide nanohillocks percolating through the residual tantalum layer down to the substrate, so that a self-organized network of tantalum nanowires forms between the substrate and the alumina film. The average width (25–<10 nm), length (70–35 nm), and population density (10⁹–10¹¹ cm^-2) of the nanowires are systematically defined by the initial tantalum thickness (8–22 nm) and the anodizing conditions. The mesh-like, nano-sized morphologies of the tantalum underlayers result in a remarkably wide range of potential-dependent, controlled electrical sheet resistances (10²–10⁷ _Ω/sq). The periodical, tunable, metal/insulator film structure, allowing an increased transition to hopping or tunneling conduction at elevated temperature, leads to negative temperature coefficients of resistance, ranging 300 to 5 ppm/K. Oscillations of the potential-dependent dc conductance registered in the films at room temperature are attributed to the quantum-size effects in the metal/oxide nanostructures. The films are of technological importance for fabrication of thin-film, planar, adjustable resistors with significantly improved performances.     

Nickel Oxide Interface for Xerographic Selenium Photoreceptors on Flexible Nickel Foils

The paper reports on a method to produce a nickel oxide interface for xerographic selenium alloy photoreceptors on flexible nickel foils. The process consists of an etching treatment in phosphoric acid catalyzed by small amounts of Pd or Pt on the surface and a subsequent electrochemical treatment in a Na₂Cr₂0₇ solution. The catalytic effect of Pd and Pt on the nickel dissolution in phospheric acid is investigated and explained on the basis of electrochemical principles. The etched nickel surface provides excellent adhesion of the photoconducting layer to the nickel substrate. In the subsequent electrochemical anodization treatment, a several hundred A thick nickel oxide layer is formed which reduces electron injection from the interface.     

Self-assembly of ultrathin composite TiO2/polymer films

A wet layer-by-layer self-assembly of composite TiO₂/polymer films on Si and Al₂O₃/Al, substrates has been studied by AFM, STM, and ellipsometry techniques. The quality of the first adsorbed TiO₂ layer has been found to be the governing factor in multilayer film growth. The first layer consists of single particles and particle agglomerates 30–120 nm wide The surface coverage in the layer is determined by the chemical composition of the substrate surface and water pH in post-adsorption rinsing procedure. Well-packed TiO₂/polymer film completely covering the surface has been prepared in five adsorption cycle on Al₂O₃/Al substrate. The film remained crack-free after heat treatment at 300°C. I–V curves measurement reveals high resistivity (R∼10¹⁰ O in the voltage range from −2 to +3 V) of TiO₂/polymer films prepared in ten adsorption cycles.     

Ellipsometric analysis of porous anodized aluminum oxide films

We performed an ellipsometric study of porous anodized aluminum oxide (AAO) films on Si substrates. Regular cylindrical porous AAO films with flat bottom structure were formed by chemical etching and anodization. The data showed typical interference oscillations as a result of the transparent characteristics of the film throughout the visible spectral range. We applied a combined effective medium approximation model with anisotropic model to obtain optical properties of the films, which can be used as basic information applicable for more complex structures.