Device fabrication with solid-liquid-solid grown silicon nanowires

High quality, single-crystal silicon nanowires were successfully grown from silicon wafers with a nickel catalyst by utilizing a solid-liquid-solid (SLS) mechanism. The nanowires were composed of a crystalline silicon core with an average diameter of 10?nm and a thick outer oxide layer of between 20 and 30?nm at a growth temperature of 1000?°C. When utilizing the SLS growth mechanism, the diameter of the silicon nanowire is dependent solely upon the growth temperature, and has no relation to either the size or the shape of the catalyst. The characteristics of the silicon nanowires are highly dependent upon the properties of the silicon substrate, such as the crystal phase of silicon itself, as well as the doping type. The possibility of doping of silicon nanowires grown via the SLS mechanism without any external dopant source was demonstrated by measuring the electrical properties of a silicon nanowire field effect transistor.     

Epitaxial growth of InP nanowires on germanium

The growth of III-V semiconductors on silicon would allow the integration of their superior (opto-)electronic properties with silicon technology. But fundamental issues such as lattice and thermal expansion mismatch and the formation of antiphase domains have prevented the epitaxial integration of III-V with group IV semiconductors. Here we demonstrate the principle of epitaxial growth of III-V nanowires on a group IV substrate. We have grown InP nanowires on germanium substrates by a vapour-liquid-solid method. Although the crystal lattice mismatch is large (3.7%), the as-grown wires are monocrystalline and virtually free of dislocations. X-ray diffraction unambiguously demonstrates the heteroepitaxial growth of the nanowires. In addition, we show that a low-resistance electrical contact can be obtained between the wires and the substrate.     

Selective surface functionalization of silicon nanowires via nanoscale joule heating

In this letter, we report a novel approach to selectively functionalize the surface of silicon nanowires located on silicon-based substrates. This method is based upon highly localized nanoscale Joule heating along silicon nanowires under an applied electrical bias. Numerical simulation shows that a high-temperature (>800 K) with a large thermal gradient can be achieved by applying an appropriate electrical bias across silicon nanowires. This localized heating effect can be utilized to selectively ablate a protective polymer layer from a region of the chosen silicon nanowire. The exposed surface, with proper postprocessing, becomes available for surface functionalization with chemical linker molecules, such as 3-mercaptopropyltrimethoxysilanes, while the surrounding area is still protected by the chemically inert polymer layer. This approach is successfully demonstrated on silicon nanowire arrays fabricated on SOI wafers and visualized by selective attachment of gold nanoparticles.     

一种链珠状SiO2纳米线的制备及分析

实验通过硅粉和氯化钙盐高温处理, 以熔融CaCl₂高温下产生的蒸气作为特殊的蒸发载体, 在1300℃条件下通过热蒸发法在石墨基板表面获得了具有草坪状排列的特殊形状的纳米线。系列测试分析表明, 该纳米线的直径为50~400 nm, 长度约为几个微米, 且为面心立方结构。另外, 系统分析显示传统的纳米线生长模型如气-液-固(VLS)生长机制不能很好地解释该二氧化硅纳米线在石墨纸上的生长过程, 本文提出的一种增强的气-液-固生长机制, 可以很好地解释上述纳米线的生长过程。     

The SERS and TERS effects obtained by gold droplets on top of Si nanowires

We show that hemispherical gold droplets on top of silicon nanowires when grown by the vapor-liquid-solid (VLS) mechanism, can produce a significant enhancement of Raman scattered signals. Signal enhancement for a few or even just single gold droplets is demonstrated by analyzing the enhanced Raman signature of malachite green molecules. For this experiment, trenches (approximately 800 nm wide) were etched in a silicon-on-insulator (SOI) wafer along <110> crystallographic directions that constitute sidewalls ({110} surfaces) suitable for the growth of silicon nanowires in <111> directions with the intention that the gold droplets on the silicon nanowires can meet somewhere in the trench when growth time is carefully selected. Another way to realize gold nanostructures in close vicinity is to attach a silicon nanowire with a gold droplet onto an atomic force microscopy (AFM) tip and to bring this tip toward another gold-coated AFM tip where malachite green molecules were deposited prior to the measurements. In both experiments, signal enhancement of characteristic Raman bands of malachite green molecules was observed. This indicates that silicon nanowires with gold droplets atop can act as efficient probes for tip-enhanced Raman spectroscopy (TERS). In our article, we show that a nanowire TERS probe can be fabricated by welding nanowires with gold droplets to AFM tips in a scanning electron microscope (SEM). TERS tips made from nanowires could improve the spatial resolution of Raman spectroscopy so that measurements on the nanometer scale are possible.     

Effect of hydrogen plasma pretreatment on the growth of silicon nanowires and their employment as the anode material of lithium secondary batteries

Silicon nanowires were grown from a silane and argon gas mixture directly on a stainless steel substrate by radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) and used without any further treatment as the anode in the fabrication of lithium ion batteries. It was found that suitable pretreatment of the stainless steel substrate was required for the satisfactory growth of the silicon nanowires. In this study, the substrates were polished, etched in HF solution, coated with an aluminum catalyst layer with a thickness of c.a. 10 nm and then treated with a hydrogen plasma before the growth of the silicon nanowires. SEM (Scanning Electron Microscopy) and AFM (Atomic Force Microscopy) analyses showed that the grain size and surface roughness were increased after the hydrogen plasma pretreatment. The electrochemical performance of the silicon nanowires anode was also improved when the aluminum coated stainless steel substrate was exposed to the plasma for 20 min or longer; the initial coulombic efficiency was increased from 69.7% to 82% at a current density of 30 mA cm(-2).     

GaN nanowires grown on silicon substrates engraved using stainless-steel micro-tip

GaN nanostructures have been synthesized on silicon substrates using chemical vapor deposition. Prior to growth silicon substrates were engraved using stainless-steel micro-tips. Straight as well as twisted nanowires were observed along the engraved lines/regions. Straight nanowires were few tens of microns in length and the twisted ones were few microns in length with diameter variation between 30 nm and 100 nm. The electron microscopy analysis indicates that the nanowires were grown parallel to the c-axis and possible growth mechanism is described. Raman scattering indicates good quality of nanowires exhibiting intense E₂(high) mode and A₁(LO) mode and a huge red-shift in the mode position indicates nano-size effects. Such engraved substrates without any explicit catalyst can provide site controlled growth of nanowires and this methodology is extendable for growing nanowires of related materials.     

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.     

Rapid synthesis of aligned zinc oxide nanowires

A solution growth approach for zinc oxide (ZnO) nanowires is highly appealing because of the low growth temperature and possibility for large area synthesis. Reported reaction times for ZnO nanowire synthesis, however, are long, spanning from several hours to days. In this work, we report on the rapid synthesis of ZnO nanowires on various substrates (such as poly(ethylene terephthalate) (PET), silicon and glass) using a commercially available microwave oven. The average growth rate of our nanowires is determined to be as high as 100?nm?min(-1), depending on the microwave power. Transmission electron microscopy analysis revealed a defect-free single-crystalline lattice of the nanowires. A detailed analysis of the growth characteristics of ZnO nanowires as functions of growth time and microwave power is reported. Our work demonstrates the possibility of a fast synthesis route using microwave heating for nanomaterials synthesis.     

Epitaxial insertion of gold silicide nanodisks during the growth of silicon nanowires

Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.     

Carbonization-assisted integration of silica nanowires to photoresist-derived three-dimensional carbon microelectrode arrays

We propose a novel technique of integrating silica nanowires to carbon microelectrode arrays on silicon substrates. The silica nanowires were grown on photoresist-derived three-dimensional carbon microelectrode arrays during carbonization of patterned photoresist in a tube furnace at 1000?°C under a gaseous environment of N(2) and H(2) in the presence of Cu catalyst, sputtered initially as a thin layer on the structure surface. Carbonization-assisted nucleation and growth are proposed to extend the Cu-catalyzed vapor-liquid-solid mechanism for the nanowire integration behaviour. The growth of silica nanowires exploits Si from the etched silicon substrate under the Cu particles. It is found that the thickness of the initial Cu coating layer plays an important role as catalyst on the morphology and on the amount of grown silica nanowires. These nanowires have lengths of up to 100 μm and diameters ranging from 50 to 200 nm, with 30 nm Cu film sputtered initially. The study also reveals that the nanowire-integrated microelectrodes significantly enhance the electrochemical performance compared to blank ones. A specific capacitance increase of over 13 times is demonstrated in the electrochemical experiment. The platform can be used to develop large-scale miniaturized devices and systems with increased efficiency for applications in electrochemical, biological and energy-related fields.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.     

Confinement-guided shaping of semiconductor nanowires and nanoribbons: "writing with nanowires"

To fully exploit their full potential, new semiconductor nanowire building blocks with ab initio controlled shapes are desired. However, and despite the great synthetic advances achieved, the ability to control nanowire's geometry has been significantly limited. Here, we demonstrate a simple confinement-guided nanowire growth method that enables to predesign not only the chemical and physical attributes of the synthesized nanowires but also allows a perfect and unlimited control over their geometry. Our method allows the synthesis of semiconductor nanowires in a wide variety of two-dimensional shapes such as any kinked (different turning angles), sinusoidal, linear, and spiral shapes, so that practically any desired geometry can be defined. The shape-controlled nanowires can be grown on almost any substrate such as silicon wafer, quartz and glass slides, and even on plastic substrates (e.g., Kapton HN).     

One-step growth of curled GaN nanowires using chemical vapor deposition method

The studies of curled GaN nanowires grown on sapphire and silicon substrate using chemical vapor deposition method have been reported in this article. The mean diameters of the nanowires grown on sapphire and silicon were 108.1 nm and 98 nm respectively. A growth model was proposed to describe the growth of nanowires. X-ray diffraction pattern and Raman spectroscopy revealed that the nanowires were hexagonal wurtzite in structure. Gaussian fitting was done on photoluminescence spectra, which revealed two sub-bands that could be attributed to band emission and surface disorder caused by impurities. The absence of yellow luminescence signified undoped case and minimal shallow level defects.     

Synthesis of gold-silica composite nanowires through solid-liquid-solid phase growth

Nanoscale wires of silicon oxide, and silicon oxide with embedded gold-silicide nanospheres, are synthesized by heating of a gold-coated silicon wafer at temperatures of 1000 degrees C or above, with the resulting wires having diameters ranging from 30 to 150 nm and lengths of approximately 1 mm. This simple fabrication process should make possible economical bulk production of nanowires. Studies indicate that the growth of these gold-silica composite nanowires occurs directly on the silicon wafer by a solid-liquid-solid mechanism.     

Taper-free and kinked germanium nanowires grown on silicon via purging and the two-temperature process

We investigate the growth procedures for achieving taper-free and kinked germanium nanowires epitaxially grown on silicon substrates by chemical vapor deposition. Singly and multiply kinked germanium nanowires consisting of <111> segments were formed by employing a reactant gas purging process. Unlike non-epitaxial kinked nanowires, a two-temperature process is necessary to maintain the taper-free nature of segments in our kinked germanium nanowires on silicon. As an application, nanobridges formed between (111) side walls of V-grooved (100) silicon substrates have been demonstrated.     

Catalytic-free growth of ZnGa2O4 nanowires on amorphous carbon layers

Zinc gallate (ZnGa₂O₄) nanowires were directly grown on the amorphous carbon-coated silicon substrates using a facile chemical vapor deposition method without any metal catalysts. The growth mechanism can be attributed to a self-organization vapor-liquid-solid (VLS) process. The amorphous carbon layer plays an important role in the nucleation and growth process of the ZnGa₂O₄ nanowires. The photoluminescence (PL) of the nanowires shows a broad, strong green emission band centered at 532 nm and a weak UV emission band at 381 nm, which can be attributed to a large amount of ionized oxygen vacancies and the combination of Ga³⁺ ions with free electrons in coordinated oxygen vacancies, respectively.     

Selective growth of silica nanowires in silicon catalysed by Pt thin film

Selective growth of amorphous silica nanowires on a silicon wafer deposited with Pt thin film is reported. The mechanism of nanowire growth has been established to follow the vapour liquid solid (VLS) model via the PtSi phase acting as the catalyst. Nanowires grow with diameters ranging from 50 to 500?nm. These bottom-up grown nanowires exhibit photoluminescence with a stable emission of blue light at 430?nm under excitation. The effect of varying the seed layer thickness (Pt film) from 2 to 100?nm has been studied. It is observed that, above 10?nm thickness, a continuous layer of Pt(2)Si re-solidifies on the surface, inhibiting the growth of nanowires. The selectivity to the Pt thickness has been exploited to create regions of nanowires connected to conducting silicide (Pt(2)Si) simultaneously in a single furnace treatment. This novel approach has opened the gateways for realizing hybrid interconnects in silicon for various nano-optical applications such as the localization of light, low-dimensional waveguides for functional microphotonics, scanning near-field microscopy, and nanoantennae.     

Manufacturing aspects of oxide nanowires

Exploring the mass manufacturing aspects of nanostructures can enable the transition from laboratory-based research into a commercial product. Among the several one-dimensional nanostructures, oxide nanomaterials have a wide variety of applications including energy harvesting, photonics and biosensing applications. In this article, mass manufacturing aspects of bottom-up grown silica nanowires on silicon (Si) by metal thin film catalysis have been detailed. The investigation reports on (a) a growth model derived from studying nanowire nucleation as a function of heating time, (b) nanowire growth rate estimation via weight differential of the Si substrate before and after growth, and (c) reusability of the Si substrate for nanowire growth.Silica nanowires were found to grow on Pd coated Si substrate in an open tube furnace at 1100 °C with Ar as a carrier gas and a Si support wafer. Nanowires nucleated following a combination of Vapor Liquid Solid (VLS) and Oxide Assisted Growth (OAG) mechanisms conducive for mass manufacturing. The role of SiO vapor was found to be critical in the growth of the wires. Further, five distinct growth regimes were identified while estimating the growth rate. Experimental observations indicated the non-reusability of the Si substrate after one time growth due to depletion of catalyst.     

Large-area silicon nanowires from silicon monoxide for solar cell applications

Large-area upstanding silicon nanowires (SiNWs) were synthesized by hot-filament chemical vapor deposition (HFCVD) using silicon monoxide (SiO) powder as Si source under high vacuum (1.2 x 10(-5) Torr). Gold nanoparticles (AuNPs) were employed as catalyst, which were formed on Si substrate by in-situ reduction of gold chloride (AuCl3). The size and distribution of the Au nanoparticles can be easily controlled through chemical reaction conditions. Consequently, the diameter, length and density of SiNWs could be varied in certain range. The SiNWs obtained are single crystalline with growth directions predominantly along [01-1]. Silicon nanowires in large-scale and diameter less than 10 nm can be grown on different Si substrates with this method. Organic inorganic hybrid solar cells based on SiNWs arrays have been demonstrated.