High-density Silicon Nanowires Research Articles

Deterministic Single-Row-Droplet Catalyst Formation for Uniform Growth Integration of High-Density Silicon Nanowires.

Planar silicon nanowires (SiNWs), grown by using low temperature catalytic approaches, are excellent 1D channel materials for developing high-performance logics and sensors. However, a deterministic position and size control of the metallic catalyst droplets, that lead to the growth of SiNWs, remains still a significant challenge for reliable device integration. In this work, we present a convenient but powerful edge-trimming catalyst formation strategy, which can help to produce a rather uniform single-row of indium (In) catalyst droplets of Dcat = 67 ± 5 nm in diameter, with an exact one-droplet-on-one-step arrangement. This approach marks a significant achievement in self-assembled catalyst formation and offers a foundation to attain a reliable and scalable growth of density SiNW channels, via an in-plane solid-liquid-solid (IPSLS) mechanism, with a uniform diameter down to Dnw = 35 ± 4 nm, and do not rely on high-precision lithography techniques. Prototype SiNW-based field effect transistors (FETs) are also fabricated, with a high Ion/Ioff current ratio and small subthreshold swing of >107 and 262 mV·dec-1, respectively, indicating a reliable new routine to integrate a wide range of SiNW-based logic, sensor, and display applications.

Single Crystalline Higher Manganese Silicide Nanowire Arrays with Outstanding Physical Properties through Double Tube Chemical Vapor Deposition.

In this work, we report a novel and efficient silicidation method to synthesize higher manganese silicide (HMS) nanowires with interesting characterization and physical properties. High density silicon nanowire arrays fabricated by chemical etching reacted with MnCl2 precursor through a unique double tube chemical vapor deposition (CVD) system, where we could enhance the vapor pressure of the precursor and provide stable Mn vapor with a sealing effect. It is crucial that the method enables the efficient formation of high quality higher manganese silicide nanowires without a change in morphology and aspect ratio during the process. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to characterize the HMS nanowires. High-resolution TEM studies confirm that the HMS nanowires were single crystalline Mn27Si47 nanowires of Nowotny Chimney Ladder crystal structures. Magnetic property measurements show that the Mn27Si47 nanowire arrays were ferromagnetic at room temperature with a Curie temperature of over 300 K, highly depending on the relationship between the direction of the applied electric field and the axial direction of the standing nanowire arrays. Field emission measurements indicate that the 20 μm long nanowires possessed a field enhancement factor of 3307. The excellent physical properties of the HMS nanowires (NWs) make them attractive choices for applications in spintronic devices and field emitters.

Silicon nanowires to detect electric signals from living cells

The ability to merge electronic devices with biological systems at the cellular scale is an interesting perspective. Potential applications span from investigating the bio-electric signals in excitable (and non-excitable) cells with an insofar-unreached resolution to plan next-generation therapeutic devices. Semiconductor nanowires are well suited for achieving this goal because of their intrinsic size and wide range of possible configurations. However, production of such nanoscale electrodes can be pricey, time-consuming and affected by poor compatibility with the Complementary Metal-Oxide-Semiconductor integrated circuits (CMOS-IC) process standards. To take a step forward, we introduced a new method to fabricate small, high-density Silicon Nanowires (SiNWs) with a fast, relatively inexpensive and low-temperature (200 °C) process compatible with CMOS-IC standards, thus theoretically allowing on-site amplification of bioelectric signals from living cells in tight contact. Here, we report our preliminary data showing the biocompatibility of SiNWs, as a necessary step to produce a compact device providing super-resolved descriptions of bioelectric waveforms captured from the subcellular to the network level.

Effective Removal of Surface Recombination Centers in Silicon Nanowires Fabricated by Metal-Assisted Chemical Etching

High-density silicon nanowires (SiNWs) were fabricated on p-Si substrates by metal-assisted chemical etching (MACE). A chemical cleaning with HF/piranha/HF was used to remove structural defects from the nanowire surface which are at the origin of surface recombination states. Surface photovoltage (SPV) spectroscopy, capacitance–voltage (C–V), and conductance measurements were used to study the SiNW properties and assess the effectiveness of the cleaning process. For the as-grown samples, the analysis of the SPV amplitude and phase spectra demonstrated high density of positively charged surface recombination centers. These centers increase the surface band bending and decrease the minority carrier lifetime and diffusion length. Si nanostructures at the SiNW surface introduce slow traps, as evidenced by the frequency dispersion at accumulation of the C–V curves and the increase of the diffusion length with frequency. All these features are effectively reduced when the SiNWs are subjected to HF/piranha/HF chemical cleaning. Thus, a cheap and effective way is demonstrated to reduce the surface state density and improve the electronic quality of the surface of SiNWs.

High Density of Aligned Nanowire Treated with Polydopamine for Efficient Gene Silencing by siRNA According to Cell Membrane Perturbation.

High aspect ratio nanomaterials, such as vertically aligned silicon nanowire (SiNW) substrates, are three-dimensional topological features for cell manipulations. A high density of SiNWs significantly affects not only cell adhesion and proliferation but also the delivery of biomolecules to cells. Here, we used polydopamine (PD) that simply formed a thin coating on various material surfaces by the action of dopamine as a bioinspired approach. The PD coating not only enhanced cell adhesion, spreading, and growth but also anchored more siRNA by adsorption and provided more surface concentration for substrate-mediated delivery. By comparing plain and SiNW surfaces with the same amount of loaded siRNA, we quantitatively found that PD coating efficiently anchored siRNA on the surface, which knocked down the expression of a specific gene by RNA interference. It was also found that the interaction of SiNWs with the cell membrane perturbed the lateral diffusion of lipids in the membrane by fluorescence recovery after photobleaching. The perturbation was considered to induce the effective delivery of siRNA into cells and allow the cells to carry out their biological functions. These results suggest promising applications of PD-coated, high-density SiNWs as simple, fast, and versatile platforms for transmembrane delivery of biomolecules.

High-Density Silicon Nanowires Prepared via a Two-Step Template Method

High density ordered Si nanowire arrays can be fabricated from a Fe2O3 template annealed from polystyrene (PS) microsphere layers via a metal-assisted chemical etching method. The metal mesh films, containing position- and density-defined pores that determine the position and density of the remaining structures after etching, are extremely important for achieving high quality Si nanowires. By adding a structural inversion process, a Au metal mesh with arrays of high density nanopores is devised as a catalyst for metal-assisted chemical etching of silicon. The density of Si nanowires can be increased to two times that of the single-layer PS microspheres and further to three times when a double layer of PS microspheres is introduced. The two-step template method for the preparation of high-density Si nanowires shows great potential in the fields of nanofabrication and nanoelectronics.

Growth and characterization of silicon nanowires catalyzed by Zn metal via Pulsed Plasma-Enhanced Chemical Vapor Deposition

High-density silicon nanowires (SiNWs) were grown via Pulsed Plasma-Enhanced Chemical Vapor Deposition at 400°C. Zinc (Zn) metal thin films with varying thickness from 10nm to 100nm were used as a catalyst to synthesize the SiNWs. The surface morphology, crystalline structure, and optical properties of the grown SiNWs were investigated. Results indicated that increasing the Zn thickness from 10nm to 100nm led to an increase in wire diameter from 65nm to 205nm, resulting in a reduction of SiNW density. The wires grown with Zn thicknesses of 10 and 80nm exhibited high crystallinity as shown by the X-ray diffraction patterns. Three emission bands (green, blue, and red) were observed in the photoluminescence spectra of the SiNWs prepared using various Zn catalyst thicknesses. The SiNWs prepared using 10 and 80nm Zn thicknesses displayed a sharp Raman peak that corresponded to the first-order transverse optical phonon mode in contrast to the other samples that produced SiNWs with a broad Raman band.

Ultradense and planarized antireflective vertical silicon nanowire array using a bottom-up technique

The production and characterization of ultradense, planarized, and organized silicon nanowire arrays with good crystalline and optical properties are reported. First, alumina templates are used to grow silicon nanowires whose height, diameter, and density are easily controlled by adjusting the structural parameters of the template. Then, post-processing using standard microelectronic techniques enables the production of high-density silicon nanowire matrices featuring a remarkably flat overall surface. Different geometries are then possible for various applications. Structural analysis using synchrotron X-ray diffraction reveals the good crystallinity of the nanowires and their long-range periodicity resulting from their high-density organization. Transmission electron microscopy also shows that the nanowires can grow on nonpreferential substrate, enabling the use of this technique with universal substrates. The good geometry control of the array also results in a strong optical absorption which is interesting for their use in nanowire-based optical sensors or similar devices.

Ultra high-density silicon nanowires for extremely low reflection in visible regime

We fabricated large-area, vertically aligned silicon nanowire (SiNW) arrays on Si substrates employing catalytic etching on a polystyrene nanosphere template. The density of SiNWs was as high as 1010/cm2, and the bottom radii of SiNWs ranged between 30 and 60 nm. The reflection from the SiNW layer was approximately 0.1% over the spectral range of 300–800 nm for SiNWs longer than 750 nm. Effective medium theory was applied to explain this extremely low reflection, and it was confirmed that the incident light scatters randomly inside cone-like SiNWs, which lengthens the actual traveling path of light.

Organized porous alumina membranes for high density silicon nanowires growth

In this paper, we present results on the fabrication of porous alumina membranes on silicon substrates with a long-range order induced by nanoimprint lithography. Fabricated porous alumina matrices present a perfect triangular array of vertical cylindrical pores on areas of 500 × 500 μm 2 corresponding to the imprinted surfaces. Also, we demonstrate that it is possible to have a directed density multiplication during the pore formation, compared to the nanoimprint mold, by the initial indentation of only one third of the expected alumina pores. The gold catalyst, needed for nanowires growth, is deposited at the bottom of each pore by electrochemistry. The proposed process is scalable to wafer-scale areas, compatible with microelectronics fabrication standards and is not limited to non-fragile substrates like direct bulk aluminum nanoindentation.

Confined growth of silicon nanowires as a possible process for third generation solar cells

AbstractA generic method for the growth of high density silicon nanowires in nano channels of alumina is presented. All fabrication steps are shown. Using nanoimprint lithography, fabricated porous alumina matrices present a perfect triangular array of vertical cylindrical pores. The gold catalyst was deposited at the bottom of the pores by an electrochemical way and catalytic growth of silicon nanowires was performed in a Chemical‐Vapour‐Deposition reactor. Scanning Electron Microscopy images are presented and detailed. We show that this system is the first step for the elaboration of a solar cell based on silicon core‐shell nanowires. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of Silicon Nanowires with Precise Diameter Control Using Metal Nanodot Arrays as a Hard Mask Blocking Material in Chemical Etching

We report on a simple and cost-effective method to fabricate high density silicon nanowires (SiNWs) through catalytic chemical wet etching. Metallic chromium/gold (Cr/Au) nanodots were first deposited onto the silicon wafer using an anodic aluminum oxide (AAO) template. The AAO template was then removed before a thin blanket layer of gold catalyst was evaporated onto the sample surface. The gold-assisted chemical wet etching was carried out in a solution consisting of deionized water, hydrogen peroxide, and hydrofluoric acid to produce well-aligned silicon nanowires of uniform diameters. We demonstrate that the diameter of the silicon nanowires can be precisely controlled to a precision of 10 nm in the range of 40 to 80 nm through fine-tuning of the pore diameter of the AAO template. The reported fabrication procedure therefore gives a highly repeatable method to form well-aligned, uniform, and crystalline SiNWs of high density with controllable diameters below 100 nm. The use of Cr/Au as a hard mask blocking material will also be of great interest for the fabrication of other Si nanostructures using the catalytic etching process.

Selective Growth of Silicon Nanowires on Glass Substrate with an Ultrathin a-Si:H Layer

High density silicon nanowires (SiNWs) were selectively grown on a Cr/glass template with a very thin 5 nm thick a-Si:H layer at 500°C by a vapor-liquid-solid process using Au-Si nanoparticles as catalysts. SiNWs cannot be grown without the a-Si:H layer. Field emitters using these SiNWs were also fabricated. The threshold field of the fabricated field emitters was 15 V/μm. Furthermore, the field enhancement factor, β, of the fabricated SiNW field emitter was around 1700.

Extended Arrays of Vertically Aligned Sub-10 nm Diameter [100] Si Nanowires by Metal-Assisted Chemical Etching

Large-area high density silicon nanowire (SiNW) arrays were fabricated by metal-assisted chemical etching of silicon, utilizing anodic aluminum oxide (AAO) as a patterning mask of a thin metallic film on a Si (100) substrate. Both the diameter of the pores in the AAO mask and the thickness of the metal film affected the diameter of SiNWs. The diameter of the SiNWs decreased with an increase of thickness of the metal film. Large-area SiNWs with average diameters of 20 nm down to 8 nm and wire densities as high as 10 (10) wires/cm (2) were accomplished. These SiNWs were single crystalline and vertically aligned to the (100) substrate. It was revealed by transmission electron microscopy that the SiNWs were of high crystalline quality and showed a smooth surface.

Uniformly Sized Gold Nanoparticles Derived from PS-b-P2VP Block Copolymer Templates for the Controllable Synthesis of Si Nanowires

A monolayer of gold-containing surface micelles has been produced by spin-coating solution micelles formed by the self-assembly of the gold-modified polystyrene-b-poly(2-vinylpyridine) block copolymer in toluene. After oxygen plasma removed the block copolymer template, highly ordered and uniformly sized nanoparticles have been generated. Unlike other published methods that require reduction treatments to form gold nanoparticles in the zero-valent state, these as-synthesized nanoparticles are in form of metallic gold. These gold nanoparticles have been demonstrated to be an excellent catalyst system for growing small-diameter silicon nanowires. The uniformly sized gold nanoparticles have promoted the controllable synthesis of silicon nanowires with a narrow diameter distribution. Because of the ability to form a monolayer of surface micelles with a high degree of order, evenly distributed gold nanoparticles have been produced on a surface. As a result, uniformly distributed, high-density silicon nanowires have been generated. The process described herein is fully compatible with existing semiconductor processing techniques and can be readily integrated into device fabrication.

High-density silicon nanowire growth from self-assembled Au nanoparticles

We report a method to grow high-density silicon nanowires (SiNWs) from self-assembled Au nanoparticles on SiO 2 surfaces. A high-vacuum electron-beam metal evaporation system was utilized for the deposition of the particles. The size of the particles is about 5–10 nm with a relatively tight distribution and a homogeneous separation. The Au nanoparticles were used as growth mediating impurities for the vapor–liquid–solid growth of SiNWs utilizing a low pressure chemical vapor deposition system and silane as Si source. The diameter of the SiNWs can be varied between 20 and 250 nm by changing the growth conditions. It was found that the SiNW diameter increases with increasing silane pressure and also with increasing temperature. Local growth of SiNWs is demonstrated using optical and electron-beam lithography. The CVD-based SiNW growth from self-assembled nanoparticles in combination with top-down lithography may form the bridge between top-down and bottom-up processing for future devices.

Synthesis of High Density, Size-Controlled Si Nanowire Arrays via Porous Anodic Alumina Mask

Porous anodic alumina (PAA) masks are employed for the template synthesis of high density silicon nanowire (SiNW) arrays on a silicon substrate by the vapor−liquid−solid method. The uniform-sized ordered arrays of nanopores in the PAA mask anchored to Si(111) substrate are shown to enable the realization of vertically aligned epitaxial SiNWs with uniform diameter and spacing. The average diameter of the wires is 72 nm while the density is 60 wires/μm2. The high packing density and tightly controlled dimensions of SiNWs obtained by this nonlithographic method allow their effective integration into nanodevices for mass production.

Growth and use of metal nanocrystal assemblies on high-density silicon nanowires formed by chemical vapor deposition

In this paper, we describe the growth and potential application of metal nanocrystal assemblies on metal-catalyzed, CVD-grown silicon nanowires (SiNWs). The nanowires are decorated by chemical assembly of closely spaced (1–5 nm) Ag (30–100 nm diameter) and Au (5–25 nm diameter) nanocrystals formed from solutions of AgNO3 and NaAuCl4·2H2O, respectively. The formation and growth of metal nanocrystals is believed to involve the galvanic reduction of metal ions from solution and the subsequent oxidation of available Si-hydride sites on the surfaces of the nanowires. A native oxide layer suppresses formation of metal nanocrystals; adding HF to the ionic solutions significantly increases the density of nanocrystals on the surfaces of the nanowires. The nanocrystals coating the nanowires were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction. Ag nanocrystals on the nanowires afford sensitive detection of Rhodamine 6G (R6G) molecules in the 100 picomolar–micromolar range by surface enhanced Raman spectroscopy. In addition, Au nanocrystals formed on selected surfaces of a substrate of arbitrary shape can serve as effective nuclei for localized nanowire growth.