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Fabrication of antireflective silicon nanowires array
Abstract
In this work, vertical aligned SiNWs array have been fabricated on silicon wafers via metal assisted chemical etching method. This method mainly contains four fabrication steps: wafer cleaning, oxide layer removal, silver catalyst deposition and metal assisted etching. The relationship between the etching time and nanowires length is investigated, and the nanowires can be fabricated in a controllable manner. The optical characteristic of the nanowires array is measured. The average optical reflectance of the SiNWs array is as low as about 3.28% when the etching time is 20 min, implying such prepared SiNWs array possesses an excellent antireflective property. The successful transfer of the SiNWs array from the rigid silicon substrate onto a flexible PDMS film demonstrates its potential in fabricating flexible optoelectronic devices.
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We report a cheap and scalable bottom-up technique for fabricating wafer-scale, subwavelength-structured antireflection coatings on single-crystalline silicon substrates. Spin-coated monolayer colloidal crystals are utilized as shadow masks to generate metallic nanohole arrays. Inverted pyramid arrays in silicon can then be templated against nanoholes by anisotropic wet etching. The resulting subwavelength gratings greatly suppress specular reflection at normal incidence. The reflection spectra for flat silicon and the templated gratings at long wavelengths agree well with the simulated results using a rigorous coupled wave analysis model. These subwavelength gratings are of great technological importance in crystalline silicon solar cells.
Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼ 1.6 mA/cm2 for 1.8 cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼ 12% at 690 nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.
A novel strategy for preparing large-area, oriented silicon nanowire (SiNW) arrays on silicon substrates at near room temperature by localized chemical etching is presented. The strategy is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. The density and size of the as-prepared SiNW's depend on the distribution of the patterned metal particles on the silicon surface. High-density metal particles facilitate the formation of silicon nanowires. Well-separated, straight nanoholes are dug along the Si block when metal particles are well dispersed with a large space between them. The etching technique is weakly dependent on the orientation and doping type of the silicon wafer. Therefore, SiNWs with desired axial crystallographic orientations and doping characteristics are readily obtained. Detailed scanning electron microscopy observations reveal the formation process of the silicon nanowires, and a reasonable mechanism is proposed on the basis of the electrochemistry of silicon and the experimental results.
Broadband wide-angle antireflection characteristics of aluminum-doped zinc oxide (AZO)/silicon (Si) shell/core subwavelength grating (SWG) structures with a hydrophobic surface, together with theoretical prediction using a rigorous coupled-wave analysis simulation, were investigated for Si-based solar cells. The AZO films with different thicknesses were deposited on Si SWGs by rf magnetron sputtering method, which forms a shell/core structure. The AZO/Si shell/core SWGs reduced significantly the surface reflection compared to the AZO films/Si substrate. The coverage of AZO films on Si SWGs improved the antireflective property over a wider incident angle. The AZO/Si shell/core SWG structure with a 200 nm-thick AZO layer deposited at an rf power of 200 W exhibited a water contact angle of 123°. This structure also exhibited a low average reflectance of ~2% over a wide wavelength range of 300-2100 nm with a solar weighted reflectance of 2.8%, maintaining a reflectance of < 9.2% at wavelengths of 300-2100 nm up to the incident angle of θ(i) = 70°. The effective electrical properties of AZO films in AZO/Si shell/core SWGs were also analyzed.
There is great interest in developing rechargeable lithium batteries with higher energy capacity and longer cycle life for applications in portable electronic devices, electric vehicles and implantable medical devices. Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity (4,200 mAh g(-1); ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than various nitride and oxide materials, silicon anodes have limited applications because silicon's volume changes by 400% upon insertion and extraction of lithium which results in pulverization and capacity fading. Here, we show that silicon nanowire battery electrodes circumvent these issues as they can accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances. We achieved the theoretical charge capacity for silicon anodes and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling.
This Communication reports a low-cost solution fabrication of wafer-scale ZnO/Si branched nanowire heterostructures and their high photodetection sensitivity, with an ON/OFF ratio larger than 250 and a peak photoresponsivity of 12.8 mA/W at 900 nm. This reported unique 3D branched nanowire structure offers a generic approach for the integration of new functional materials for photodetection and photovoltaic applications.
ZnO nanowires were grown on Si (1 0 0) substrates by oxidation of metallic Zn powder at 600 °C. Sea-urchin-like nanostructures, consisting of straight nanowires of ZnO with blunt faceted ends with a sudden reduction in diameter projecting out, were observed, having diameters of 30–60 nm and lengths of 2–4 μm. TEM analysis and SAED patterns showed that the as-grown nanostructures are highly crystalline in nature and preferably grown along the [0 0 0 1] direction, which is consistent with XRD analysis. Room-temperature photoluminescence (PL) measurements showed a reduced near band-edge emission in the UV region at 380 nm, while a strong deep level emission was observed in the visible region at 500–530 nm. A model for vapor–solid (VS) growth mechanism of ZnO nanowires was presented.
A fluorescence sensor for nitric oxide (NO) was realized by covalently immobilizing reduced fluoresceinamine molecules onto the surface of siliconnanowires (SiNWs). The fluorescence intensity of the sensor can be greatly enhanced by NO. The sensor exhibits excellent selectivity for NO against other reactive species. Facile synthesis, nontoxicity, rapid response and use in a 100% aqueous solution endows the present sensor with suitability for biosystems. As an application, the sensor was used to detect NO released from liver extract, and exhibited high sensitivity and selectivity as well as rapid response. The fluorescence image from a single SiNW-based sensor showed a fine spatial resolution. The present sensor paves a way to detect NO at specific location in a single cell by inserting a single SiNW-based sensor into the cell.
High density vertically aligned and high aspect ratio silicon nanowire (SiNW) arrays have been fabricated on a Si substrate
using a template and a catalytic etching process. The template was formed from polystyrene (PS) nanospheres with diameter
30–50 nm and density 1010/cm2, produced by nanophase separation of PS-containing block-copolymers. The length of the SiNWs was controlled by varying the
etching time with an etching rate of 12.5 nm/s. The SiNWs have a biomimetic structure with a high aspect ratio (∼100), high
density, and exhibit ultra-low reflectance. An ultra-low reflectance of approximately 0.1% was achieved for SiNWs longer than
750 nm. Well-aligned SiNW/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cells
were fabricated. The n-type silicon nanowire surfaces adhered to PEDOT:PSS to form a core-sheath heterojunction structure through a simple and efficient
solution process. The large surface area of the SiNWs ensured efficient collection of photogenerated carriers. Compared to
planar cells without the nanowire structure, the SiNW/PEDOT:PSS heterojunction solar cell exhibited an increase in short-circuit
current density from 2.35 mA/cm2 to 21.1 mA/cm2 and improvement in power conversion efficiency from 0.4% to 5.7%.
In this paper, a vertical-aligned silicon nanowires (Si NWs) array has been synthesized and implemented to the Si NW-array-textured solar cells for photovoltaic application. The optical properties of a Si NWs array on both the plane and pyramid-array-textured substrates were examined in terms of optical reflection property. Less than 2% reflection ratio at 800 nm wavelength was achieved. Using leftover monocrystalline Si (c-Si) wafer (125×125 mm2), a 16.5% energy conversion efficiency, with 35.4% enhancement compared to the pyramid-array-textured c-Si solar cells, was made by the Si NW-array-textured solar cells due to their enhanced optical absorption characteristics. However, without SiNx passivation, the short circuit current reduced due to the increased surface recombination when using Si NWs array as surface texturing, indicating that an optimum surface passivation was prerequisite in high-efficiency Si NW-array-textured solar cells.
Amorphous silicon oxide nanowires (SiOxNWs) were produced on a large scale by simple heat treatment of the Au nanoparticle/Polyimide/Si thin film stack at 1000 °C. The SiOxNWs had diameters ranging from 50 to 80 nm with length extending up to 20 μm. The SiOxNWs exhibited intense blue light emission at 420 nm. It was proposed that the formation of the SiOxNWs was sustained by the oxygen derived from carbonization of the polyimide thin film, while the Au nanoparticles catalyzed the nanowire growth.
This article concerns the detailed investigations on the silver dendrite-assisted growth of single-crystalline silicon nanowires, and their possible self-assembling nanoelectrochemistry growth mechanism. The growth of silicon nanowires was carried out through an electroless metal deposition process in a conventional autoclave containing aqueous HF and AgNO3 solution near room temperature. In order to explore the mechanism and prove the centrality of silver dendrites in the growth of silicon nanowires, other etching solution systems with different metal species were also investigated in this work. The morphology of etched silicon substrates strongly depends upon the composition of the etching solution, especially the metal species. Our experimental results prove that the simultaneous formation of silver dendrites is a guarantee of the preservation of free-standing nanoscale electrolytic cells on the silicon substrate, and also assists in the final formation of silicon nanowire arrays on the substrate surface.
Vertically aligned ZnO nanowires were synthesized on the p+ silicon chip by modifying the CVD process with a vapor trapping design. Scanning electron microscopy was used to investigate the morphology of as-obtained nanowires. X-ray diffraction showed that the obtained nanowires were ZnO crystalline. The rectifying characteristics of the p–n heterojunction composed of ZnO nanowires and a p+ silicon chip were observed. The positive turn-on voltage was 0.5 V and the reverse saturation current was 0.01 mA. These vertically aligned ZnO nanowires showed a low field emission threshold of 4 V/μm at a current density of 0.1 μA/cm2. The dependence of emission current density on the electric field followed Fowler–Nordheim relationship.
This article describes for the first time the direct observation of the nucleation and growth process of CdS nanowires in a typical vapor-solid synthetic route. Thermal evaporating of CdS nanosized powders at 1173 K for various time durations corresponds to different growth stages and leads to varied product morphologies. Initially, CdS appeared as amorphous spherical particles, followed by nucleation of nanorods from cusps on the particle surface. Subsequently, nanorods developed to nanowires with matrix particles being consumed. Strategies could be optimized to grow CdS nanowires of varied diameters and lengths for a variety of applications.
Metal-assisted etching of silicon in HF/H2O2/H2O solutions with Ag nanoparticles as catalyst agents was investigated. SEM observations and etch rate measurements were carried out as a function of the etching solution composition. Depending on the relative amount of HF and H2O2, different regimes of dissolution take place and a strong similarity with the etching in HF–HNO3 solution is evidenced, for the first time. Formation of meso- and macroporous Si, etched craters and polished Si are observed as the HF/H2O2 ratio decreases. The dissolution mechanisms are discussed on the basis of a localized hole injection from the Ag nanoparticles into Si and in terms of the well known chemistry of Si dissolution in HF-based chemical and electrochemical systems. At high HF/H2O2 ratio, there is no formation of oxide at the surface. Hole injection and Si dissolution occur at the level of the Ag nanoparticle only, resulting in the formation of meso and macropores depending on the Ag nanoparticle size. At low HF/H2O2 ratio, the Si surface is oxidized, the injected holes are homogeneously distributed and thus polishing occurs. There is an intermediate range of composition in which injected holes diffuse away from the Ag nanoparticles and cone-shaped macropores, several tens of nm in diameter are formed.
Silicon antireflection is realized with vertical-aligned SiNWs by using improved metal-induced etching technique. The spectral responses of the transmission, reflection, and absorption characteristics for the SiNWs of different lengths are investigated. In order to realize short SiNWs to provide sufficiently low reflection, a post chemical etching process is developed to make the nanowires have a larger length fluctuation and/or tapered structure. The use of short SiNWs can allow a faster process time and avoid the sub-bandgap absorption that frequently occurs in long nanowires. Short SiNWs can also provide more compatible material structure and fabrication procedures than long ones can for applying to make optoelectronic devices. Taking the applications to solar cells as examples, the SiNWs fabricated by the proposed technique can provide 92% of solar weighted absorption with about 720 nm long wires because of the resultant effective graded index and enhanced multiple optical scattering from the random SiNW lengths and tapered wires after KOH etching.
The development of display scan drivers is an essential step in the effort to develop
transparent and flexible display devices based on nanowire transistors. Here we report a
transparent nanowire-based shift register that functions as the standard logic circuit of a
display scan driver. To form the shift register circuits using only n-type nanowire
transistors, a novel circuit structure was introduced to avoid the output voltage drop
typical of purely n-type circuits. A circuit simulation based on the measured nanowire
transistor characteristics was developed in the planning phase to verify the circuit
operation of the shift register. The shift register successfully produced an output of
0–3 V without an output voltage drop while applying an input of 3 V peak to
peak. In addition, the shift register was designed to have multiple channels with a
randomly oriented nanowire placement method to enhance the operation yield.
In recent years metal-assisted chemical etching (MaCE) of silicon, in which etching is confined to a small region surrounding metal catalyst templates, has emerged as a promising low cost alternative to commonly used three-dimensional (3D) fabrication techniques. We report a new methodology for controllable folding of 2D metal catalyst films into 3D structures using MaCE. This method takes advantage of selective patterning of the catalyst layer into regions with mismatched characteristic dimensions, resulting in uneven etching rates along the notched boundary lines that produce hinged 2D templates for 3D folding. We explore the dynamics of the folding process of the hinged templates, demonstrating that the folding action combines rotational and translational motion of the catalyst template, which yields topologically complex 3D nanostructures with intimately integrated metal and silicon features.
Arrays of oriented, crystalline Si wires are transferred into flexible, transparent polymer films. The polymer-supported Si wire arrays in liquid-junction photoelectrochemical cells yield current-potential behavior similar to the Si wires attached to the brittle growth substrate. These systems offer the potential for attaining high solar energyconversion efficiencies using modest diffusion length, readily grown, crystalline Si in a flexible, processable form. (Figure Presented)
Semiconductor nanowires have potential applications in photovoltaics, batteries, and thermoelectrics. We report a top-down fabrication method that involves the combination of superionic-solid-state-stamping (S4) patterning with metal-assisted-chemical-etching (MacEtch), to produce silicon nanowire arrays with defined geometry and optical properties in a manufacturable fashion. Strong light emission in the entire visible and near infrared wavelength range at room temperature, tunable by etching condition, attributed to surface features, and enhanced by silver surface plasmon, is demonstrated.
Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, <0001> oriented
zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process.
These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and
lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with
an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them
ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical
computing, information storage, and microanalysis.
Homogeneous and dense arrays of ZnO nanowires were synthesized on silicon wafers (and many other substrates) using a mild solution process at 90°C. Uniform ZnO nanocrystals were deposited to act as seeds for subsequent hydrothermal nanowire growth, which yielded single-crystalline ZnO nanowires grown along the [0001] direction and oriented perpendicular to the water surface (see picture; scale bar = 1 μm). The photoluminescence and lasing behavior of the arrays has been studied as a function of annealing treatment conditions.
Silicon nanowires were synthesized, in a controlled manner, for their practical integration into devices. Gold colloids were used for nanowire synthesis by the vapor-liquid-solid growth mechanism. Using SiCl4 as the precursor gas in a chemical vapor deposition system, nanowire arrays were grown vertically aligned with respect to the substrate. By manipulating the colloid deposition on the substrate, highly controlled growth of aligned silicon nanowires was achieved. Nanowire arrays were synthesized with narrow size distributions dictated by the seeding colloids and with average diameters down to 39 nm. The density of wire growth was successfully varied from approximately 0.1-1.8 wires/microm2. Patterned deposition of the colloids led to confinement of the vertical nanowire growth to selected regions. In addition, Si nanowires were grown directly into microchannels to demonstrate the flexibility of the deposition technique. By controlling various aspects of nanowire growth, these methods will enable their efficient and economical incorporation into devices.
Nanowires have attracted considerable interest as nanoscale interconnects and as the active components of both electronic and electromechanical devices. Nanomechanical measurements are a challenge, but remain key to the development and processing of novel nanowire-based devices. Here, we report a general method to measure the spectrum of nanowire mechanical properties based on nanowire bending under the lateral load from an atomic force microscope tip. We find that for Au nanowires, Young's modulus is essentially independent of diameter, whereas the yield strength is largest for the smallest diameter wires, with strengths up to 100 times that of bulk materials, and substantially larger than that reported for bulk nanocrystalline metals (BNMs). In contrast to BNMs, nanowire plasticity is characterized by strain-hardening, demonstrating that dislocation motion and pile-up is still operative down to diameters of 40 nm. Possible origins for the different mechanical properties of nanowires and BNMs are discussed.
We have developed a new technique to fabricate an antireflection surface using silicon nanotips for use on a micro Sun sensor for Mars rovers. We have achieved randomly distributed nanotips of radii spanning from 20 to 100 nm and aspect ratio of approximately 200 using a two-step dry etching process. The 30 degrees specular reflectance at the target wavelength of 1 microm is only about 0.09%, nearly 3 orders of magnitude lower than that of bare silicon, and the hemispherical reflectance is approximately 8%. When the density and aspect ratio of these nanotips are changed, a change in reflectance is demonstrated. When surfaces are covered with these nanotips, the critical problem of ghost images that are caused by multiple internal reflections in a micro Sun sensor was solved.
The future of the video display is both flexible and transparent. Finding a material for the attendant electronics that is small-scale, bendy and see-through is a tall order - but a promising candidate is emerging.