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.     

无电金属沉积法硅纳米线阵列的制备研究

采用无电金属沉积法在硅衬底上制备出了大面积规整的硅纳米线阵列,并对其形貌控制的影响因素和形成机理进行了研究。用扫描电子显微镜(SEM)、X射线衍射仪(XRD)对硅纳米线阵列和相应银枝晶的形貌和结构进行了表征。结果表明,硅纳米线阵列的形貌受水热体系中溶液配比、温度和时间的影响,在温度为50℃、HF和AgNO3浓度分别为4.6和0.02mol/L的条件下,容易得到大面积排列规整的硅纳米线阵列,并且硅纳米线的长度为30～50μm,直径为200nm左右。无电金属沉积法为硅纳米线及其阵列的制备提供了一种设备简单、条件温和的制备方法。     

Surface modification of carbon post arrays by atomic layer deposition of ZnO film

The applicability of atomic layer deposition (ALD) process to the carbon microelectromechanical system technology was studied for a surface modification method of the carbon post electrodes. A conformal coating of the ALD-ZnO film was successfully demonstrated on the carbon post arrays which were fabricated by the traditional photolithography and subsequent two-step pyrolysis. A significant Zn diffusion into the underlying carbon posts was observed during the ALD process. The addition of a sputter-deposited ZnO interfacial layer efficiently blocked the Zn diffusion without altering the microstructure and surface morphology of the ALD-ZnO film.     

Template synthesis of highly ordered hydroxyapatite nanowire arrays

Highly ordered hydroxyapatite (HA) nanowire arrays were synthesized using porous anodic aluminum oxide (AAO) template from sol-gel solution containing P₂O₅ and Ca(NO₃)₂. Theresults of transmission electron microscopy (TEM) and scanning electron microscopy(SEM) revealed that the obtained HA nanowires had a uniform length and diameter andformed highly ordered arrays, which were determined by the pore diameter and thethickness of the applied AAO template. The results of X-ray diffraction (XRD) and X-rayphotoelectron spectroscopy (XPS) demonstrated that the major component was HA. Theselected-area electron diffraction (SAED) results indicated that HA was a polycrystallinestructure. This novel method of preparing highly ordered HA nanowires with a large areamight be important for many applications in biomaterials.     

Solid-state dye-sensitized solar cells based on ordered ZnO nanowire arrays

A solid-state dye-sensitized solar cell (DSC) is fabricated by using arrays of 11-12 μm long, vertically oriented ZnO nanowires as the anode and CuSCN as the solid hole-transport material. The fabricated DSC yields a remarkably higher photocurrent density (J(SC) = 8 mA cm(-2)) compared to previously reported data for solid-state DSCs based on either one-dimensional nanostructures (J(SC) = 0.34 mA cm(-2)) or nanoporous nanocrystalline structures (J(SC) = 4.5 mA cm(-2)) of ZnO. A power conversion efficiency of 1.7% under an irradiation of AM 1.5 G simulated sunlight is reported.     

Wafer-scale assembly of highly ordered semiconductor nanowire arrays by contact printing

Controlled and uniform assembly of "bottom-up" nanowire (NW) materials with high scalability presents one of the significant bottleneck challenges facing the integration of nanowires for electronic applications. Here, we demonstrate wafer-scale assembly of highly ordered, dense, and regular arrays of NWs with high uniformity and reproducibility through a simple contact printing process. The assembled NW pitch is shown to be readily modulated through the surface chemical treatment of the receiver substrate, with the highest density approaching approximately 8 NW/mum, approximately 95% directional alignment, and wafer-scale uniformity. Such fine control in the assembly is attained by applying a lubricant during the contact printing process which significantly minimizes the NW-NW mechanical interactions, therefore enabling well-controlled transfer of nanowires through surface chemical binding interactions. Furthermore, we demonstrate that our printing approach enables large-scale integration of NW arrays for various device structures on both rigid silicon and flexible plastic substrates, with a controlled semiconductor channel width ranging from a single NW ( approximately 10 nm) up to approximately 250 microm, consisting of a parallel array of over 1250 NWs and delivering over 1 mA of ON current.     

Conformity and structure of titanium oxide films grown by atomic layer deposition on silicon substrates

Conformity and phase structure of atomic layer deposited TiO₂ thin films grown on silicon substrates were studied. The films were grown using TiCl₄ and Ti(OC₂H₅)₄ as titanium precursors in the temperature range from 125 to 500 °C. In all cases perfect conformal growth was achieved on patterned substrates with elliptical holes of 7.5 μm depth and aspect ratio of about 1:40. Conformal growth was achieved with process parameters similar to those optimized for the growth on planar wafers. The dominant crystalline phase in the as-grown films was anatase, with some contribution from rutile at relatively higher temperatures. Annealing in the oxygen ambient resulted in (re)crystallization whereas the effect of annealing depended markedly on the precursors used in the deposition process. Compared to films grown from TiCl₄, the films grown from Ti(OC₂H₅)₄ were transformed into rutile in somewhat greater extent, whereas in terms of step coverage the films grown from Ti(OC₂H₅)₄ remained somewhat inferior compared to the films grown from TiCl₄.     

Electrochemical fabrication of ordered Ag2S nanowire arrays

We have successfully fabricated ordered crystalline Ag₂S nanowire arrays by direct current electrodeposition into the nanochannels of porous anodic aluminum oxide templates from a dimethylsulfoxide solution containing AgNO₃ and elemental S. X-ray diffraction and the selected area electron diffraction investigations demonstrate that the Ag₂S nanowires are a uniform actanthite structure. Electromicroscopy results show that the nanowires are quite ordered with diameters of about 40 nm and lengths up to 5 μm. X-ray energy dispersion analysis indicates that the atomic composition of Ag and S is very close to a 2:1 stoichiometry. Furthermore, a possible mechanism for the formation of the Ag₂S nanowires is proposed.     

Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors

The development of a robust method for integrating high-performance semiconductors on flexible plastics could enable exciting avenues in fundamental research and novel applications. One area of vital relevance is chemical and biological sensing, which if implemented on biocompatible substrates, could yield breakthroughs in implantable or wearable monitoring systems. Semiconducting nanowires (and nanotubes) are particularly sensitive chemical sensors because of their high surface-to-volume ratios. Here, we present a scalable and parallel process for transferring hundreds of pre-aligned silicon nanowires onto plastic to yield highly ordered films for low-power sensor chips. The nanowires are excellent field-effect transistors, and, as sensors, exhibit parts-per-billion sensitivity to NO2, a hazardous pollutant. We also use SiO2 surface chemistries to construct a 'nano-electronic nose' library, which can distinguish acetone and hexane vapours via distributed responses. The excellent sensing performance coupled with bendable plastic could open up opportunities in portable, wearable or even implantable sensors.     

Elaborate positioning of nanowire arrays contributed by highly adhesive superhydrophobic pillar-structured substrates

Elaborate positioning of nanowire arrays can be generated upon highly adhesive superhydrophobic pillar-structured silicon substrates. The site of each nanowire can be precisely positioned by well designed tip-structured micropillars, yielding on-demand nanowire patterns. This approach might affect existing applications and enable new opportunities in organically functional devices and bio/chemical sensors.     

Organic nanowire-templated fabrication of alumina nanotubes by atomic layer deposition

Alumina nanotubes were fabricated by a template method. Tris-(8-hydroxyquinoline) gallium (GaQ3) organic nanowires were used as a soft template for coating with alumina using an atomic layer deposition technique. The deposition was conducted at 25 degrees C by using trimethylaluminum and distilled water as the precursors of Al2O3. Amorphous alumina nanotubes were obtained after removing the GaQ3 by dissolving in toluene or by heat treatment at 350 degrees C. The amorphous nanotubes could be crystallized by heating at 900 degrees C for 1 h in vacuum.     

X-ray mirrors on flexible polymer substrates fabricated by atomic layer deposition

Atomic layer deposition (ALD) techniques were used to fabricate W/Al₂O₃ superlattices with high X-ray reflectivity on flexible Kapton® polyimide and polyethylene naphthalate (PEN) polymer substrates. Reflectivities of 78% and 74% at λ = 1.54 Å were measured for 6-bilayer W/Al₂O₃ superlattices on Kapton® polyimide and PEN, respectively. These excellent X-ray reflectivities are attributed to precise bilayer thicknesses and ultrasmooth interfaces obtained by ALD and smoothing of the initial polymer surface by an Al₂O₃ ALD layer. The conformal ALD film growth also produces correlated roughness that enhances the reflectivity. These W/Al₂O₃ superlattices on flexible polymers should be useful for ultralight and adjustable radius of curvature X-ray mirrors.     

Ordered ZnO/AZO/PAM nanowire arrays prepared by seed-layer-assisted electrochemical deposition

An Al-doped ZnO (AZO) seed layer is prepared on the back side of a porous alumina membrane (PAM) substrate by spin coating followed by annealing in a vacuum at 400 °C. Zinc oxide in ordered arrays mediated by a high aspect ratio and an ordered pore array of AZO/PAM is synthesized. The ZnO nanowire array is prepared via a 3-electrode electrochemical deposition process using ZnSO₄ and H₂O₂ solutions at a potential of − 1 V (versus saturated calomel electrode) and temperatures of 65 and 80 °C. The microstructure and chemical composition of the AZO seed layer and ZnO/AZO/PAM nanowire arrays are characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). Results indicate that the ZnO/AZO/PAM nanowire arrays were assembled in the nanochannel of the porous alumina template with diameters of 110–140 nm. The crystallinity of the ZnO nanowires depends on the AZO seed layer during the annealing process. The nucleation and growth process of ZnO/AZO/PAM nanowires are interpreted by the seed-layer-assisted growth mechanism.     

Ultralong ZnO nanowire arrays synthesized by hydrothermal method using repetitive refresh

Long ZnO nanowire arrays (> or = 10 microm) were fabricated using the hydrothermal method and the refresh process of the reactant solution. The diameter of the synthesized nanowires was controlled by varying the solution concentration of the seed layers, without reducing their length. The maximum temperature in this process was 95 degrees C and the repeated refresh process at 95 degrees C provided the driving force for the growth of ultralong nanowires by exchanging the reactants. Interestingly, the diameter of the refreshed ZnO nanowires strongly depended on the solution temperature during refresh. The exchange of the reactant solution at the same temperature as the synthesis temperature induced the synthesis of ultralong nanowires and the length of the resultant nanowires can be controlled by varying the repetition number. The illumination of the ultraviolet light induced considerably enhanced current flow in the ultralong nanowires from mid 10(-10) to 10(-7) A at 5 V.     

Site-controlled fabrication of dimension-tunable Si nanowire arrays on patterned (001)Si substrates

In this study, we fabricated well-ordered arrays of site-controlled, vertically-aligned Si nanowires on the desired areas of pre-patterned (001)Si substrates by employing the nanosphere lithographic technique in combination with the Au-assisted selective etching process. The results of transmission electron microscopy and selected-area electron diffraction analysis show that the Si nanowires that fabricated on the patterned (001)Si substrates have a single-crystalline nature and form along the [001] direction. The length of the Si nanowires was found to increase linearly with the Au-assisted etching time. Scanning electron microscopy images clearly revealed that by adjusting the sizes of the nanosphere template and the etching temperature and time, the diameter and length of the patterned Si nanowires could be effectively tuned and accurately controlled. Furthermore, the diameters of the Si nanowires produced at various temperatures and time were found to be relatively uniform over the entire length. The combined approach presented here provides the capability to fabricate a variety of size-, length-tunable 1D Si-based nanostructures on various patterned Si-based substrates.     

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.     

Growing oriented AlN films on sapphire substrates by plasma-enhanced atomic layer deposition

Technical Physics Letters - The possibility of growing oriented AlN films on Al2O3 substrates at temperatures below 300°C by plasma-enhanced atomic layer deposition was examined. The samples...     

Nitrogen-doped ZnO thin films grown on glass substrates by atomic layer deposition using NH3 as a doping source

Nitrogen-doped ZnO (ZnO:N) films were successfully grown on glass substrates by atomic layer deposition (ALD). NH3 was used as a doping source, and the substrate temperature was relatively low (90 approximately 210 degrees C). The main focus of the study was to report on the effect of the temperature on the electrical properties (e.g., carrier concentration, mobility, etc.) of the grown ZnO:N films. At all temperatures, the carrier was found to be n-type, and its electron concentration did not show much variation within the values between 3 x 10(16) and 6 x 10(16) cm-3; the mobility increased with the temperature (1 cm2/Vs at 110 degrees C, 5 cm2/Vs at 190 degrees C); and the resistivity decreased with the temperature (203 omegacm at 110 degrees C, 21 omegacm at 190 degrees C). The electrical properties are discussed in relation with the nitrogen concentration, crystallinity, crystal orientation, grain size, and surface morphology. The nitrogen concentration in the ZnO:N films was constant at all temperatures (approximately 2.5 atomic percent); the crystallinity and crystal orientation improved with the temperature; and the mean grain size increased with the temperature (13.2 nm at 110 degrees C, 35.3 nm at 190 degrees C). The results for the ZnO:N films were also compared with the results for the undoped ZnO films.     

TiO2纳米线阵列生长机理的研究

采用两步氧化法制备了多孔氧化铝模板(AAO),并利用AAO结合用电化学诱导溶胶-凝胶法制备了锐钛矿型的TiO2纳米线阵列,探讨了形成TiO2纳米线阵列的生长机理.发现TiO2纳米线是由Au基底沿着孔洞逐步向上生长,直接生成纳米线,因而可以通过控制氧化铝模板的尺寸和电化学沉积的时间来控制纳米线的长径比.