Diameter Of Silicon Nanowires Research Articles

Leaky Mode Resonance-Induced Sensitive Ultraviolet Photodetector Composed of Graphene/Small Diameter Silicon Nanowire Array Heterojunctions.

Ultraviolet photodetectors (UVPDs) based on wide band gap semiconductors (WBSs) are important for various civil and military applications. However, the relatively harsh preparation conditions and the high cost are unfavorable for commercialization. In this work, we proposed a non-WBS UVPD by using a silicon nanowire (SiNW) array with a diameter of 45 nm as building blocks. Device analysis revealed that the small diameter SiNW array covered with monolayer graphene was sensitive to UV light but insensitive to both visible and infrared light illumination, with a typical rejection ratio of 25. Specifically, the responsivity, specific detectivity, and external quantum efficiency under 365 nm illumination were estimated to be 0.151 A/W, 1.37 × 1012 Jones, and 62%, respectively, which are comparable to or even better than other WBS UVPDs. Such an abnormal photoelectrical characteristic is related to the HE1m leaky mode resonance (LMR), which is able to shift the peak absorption spectrum from near-infrared to UV regions. It is also revealed that this LMR is highly dependent on the diameter and the period of the SiNW array. These results show narrow band gap semiconductor nanostructures as promising building blocks for the assembly of sensitive UV photodetectors, which are very important for various optoelectronic devices and systems.

Effects of gold film thickness and annealing time on diameter of silicon nanowires grown by VLS process

In this paper, silicon nanowires were prepared by chemical vapor deposition (CVD) in the presence of gold catalyst. We discussed the effects of gold film thickness and annealing time on droplet diameter and silicon nanowire diameter. The study revealed that the diameter of silicon nanowires increases with the increase of the thickness of the gold film. With the increase of annealing time, the number of alloy droplets decreases, and the diameter increases. The average diameter of silicon nanowires decreases first and then increases. The results provide useful information for long-time annealing of silicon nanowires.

Simulation study of surface sputtering and distribution of ions in silicon due to low-energy high-fluence cobalt irradiation

The study of the range and distribution of low-energy heavy ions implanted into lighter substrates is paramount important in terms of three-dimensional device fabrications. In this study, we report the re-distribution of implanted ions due to the sputtering of target atoms, simulated considering dynamic changes in the target due to the high-fluence irradiation of Co and Fe ions at low energy (55 keV) in Silicon substrate and nanowires. The simulations were compared by using various ion-solid interaction codes that are capable of incorporating the targets with both one-dimensional (1D) and three-dimensional (3D) structures. In the 1D simulation codes, the target is generally defined as a stack of sharp interfaced layers that have an infinite lateral extension. Both static code like "Stopping and Range of Ions in Matter (SRIM)" and dynamic code like "Static and Dynamic Trim for Sequential and Parallel computer (SDTrimSP)" were utilized to carry out the simulations in silicon substrates. However, to simulate the distribution of ions with target layers involving nanostructures with a curved surface such as nanowires “Ion range and damage in nanostructures (Iradina)” code was used. Silicon nanowires of different diameters were chosen as the target, and Co ions were implanted at single energy along the surface normal. The fluence of the Co ions was then varied to obtain the optimal distribution of implanted ions in the silicon nanowire. The Si nanowire shows a curved depth profile for the implanted ions with different ranges towards the edge and the center of the nanowire. The surface sputtering in the case of the nanowire was more as compared to the bulk Si substrate. However, in the case of the Si substrate, the atoms are found preferentially sputtered in the forward direction between 30° - 60° angle with a peak around 45° angle to the target surface normal with variation in the angle of incidence.

Modified callaway model calculations for lattice thermal conductivity of a 20 nm diameter silicon nanowire

The modified Callaway model is used to calculate Lattice Thermal Conductivity (LTC) for (20-nm) silicon nanowires diameter in the temperature range from 2K to 800K. Acoustic phonon mode and group velocity in the calculations are modified by spatial confinement of phonons with that of the boundary effects. All important scattering rates such as Umklapp, Mass difference, Resonance, and Boundary are calculated at room temperature. Room temperature LTC for this diameter is about only 10.23% of its bulk value. Numerical evaluation is also investigated and the results are compared to that of the reported experimental as well as theoretical data.

Directed Self-Assembly of Block Copolymers for the Fabrication of Functional Devices.

Directed self-assembly of block copolymers is a bottom-up approach to nanofabrication that has attracted high interest in recent years due to its inherent simplicity, high throughput, low cost and potential for sub-10 nm resolution. In this paper, we review the main principles of directed self-assembly of block copolymers and give a brief overview of some of the most extended applications. We present a novel fabrication route based on the introduction of directed self-assembly of block copolymers as a patterning option for the fabrication of nanoelectromechanical systems. As a proof of concept, we demonstrate the fabrication of suspended silicon membranes clamped by dense arrays of single-crystal silicon nanowires of sub-10 nm diameter. Resulting devices can be further developed for building up high-sensitive mass sensors based on nanomechanical resonators.

Enhanced photocatalytic activities of silicon nanowires/graphene oxide nanocomposite: Effect of etching parameters

Homogeneous and vertically aligned silicon nanowires (SiNWs) were successfully fabricated using silver assisted chemical etching technique. The prepared samples were characterized using scanning electron microscopy, transmission electron microscopy and atomic force microscopy. Photocatalytic degradation properties of graphene oxide (GO) modified SiNWs have been investigated. We found that the SiNWs morphology depends on etching time and etchant composition. The SiNWs length could be tuned from 1 to 42 µm, respectively when varying the etching time from 5 to 30 min. The etchant concentration was found to accelerate the etching process; doubling the concentrations increases the length of the SiNWs by a factor of two for fixed etching time. Changes in bundle morphology were also studied as function of etching parameters. The SiNWs diameter was found to be independent of etching time or etchant composition while the size of the SiNWs bundle increases with increasing etching time and etchant concentration. The addition of GO was found to improve significantly the photocatalytic activity of SiNWs. A strong correlation between etching parameters and photocatalysis efficiency has been observed, mainly for SiNWs prepared at optimum etching time and etchant concentrations of 10 min and 4:1:8. A degradation of 92% was obtained which further improved to 96% by addition of hydrogen peroxide. Only degradation efficiency of 16% and 31% has been observed for bare Si and GO/bare Si samples respectively. The obtained results demonstrate that the developed SiNWs/GO composite exhibits excellent photocatalytic performance and could be used as potential platform for the degradation of organic pollutants.

Fabrication and Spectral Characteristics of Silicon Nanowires for Efficient Solar Energy Harvesting

Solar cell utilizes a small portion of solar spectrum leaving higher energy (> band gap, Eg) as thermalization loss and lower energy (< band gap, Eg) as absorption loss. Wavelength-sensitive engineered absorbing layer such as nanometric absorber holds huge potential in this context. Here in this work, a simple and hands-on strategy was devised to grow silicon nanowires (Si-NWs) on silicon wafer. Nanoparticles were achieved in the first step and used as seeds to directed growth of Si-NWs. As-grown Si-NWs with coverage ca. 6.5 × 108/cm2 were characterized through scanning electron microscope. To realize such Si-NWs as nanometric absorber in nanowire solar cell, a three-dimensional finite-difference time-domain simulation has been carried out. Considering the possibility of Si-NWs of different diameters as observed in experimental investigations, Si-NW of 50-, 100-, and 150-nm diameters was chosen in simulation. Two specific wavelengths, 700 and 1100 nm, were in prime focus to understand the characteristics of exciton generation within Si-NW. Confinement in exciton generation rate distribution at 700-nm solar spectrum for Si-NW of 150 nm was found to be most effective, whereas at 1100-nm wavelength Si-NW of 100 nm showed higher exciton generation rate distribution with the nanowire. Exciton generation line profiles along the center and edge were extracted, and comparative analysis was carried out for different diameters of Si-NW at 700- and 1100-nm wavelengths. Such experimental and correlated simulation is indispensable not only to reduce costs but also to understand and improve the cell efficiency using the light-trapping technique.

Morphological and electrical study of p-type silicon nanowires synthesised by Ag-assisted electroless chemical etching

Fabrication of Silicon nanowires (SiNWs) by electroless etching of p-type (100) Silicon wafer using silver (Ag) particles as catalyst at room temperature has been reported in this paper. Dependence of SiNWs formation on Ag electroplating and etching time was investigated. Surface morphology of SiNWs has been studied using scanning electron microscope (SEM) for different samples. The length of SiNWs increased with time of etching and decreased with electroplating time. A systematic study regarding the surface topography of fabricated SiNWs was performed using atomic force microscope (AFM). A 3D profile of the surface on a nanoscale were analysed and compared with SEM images. The average diameter of 73.9 nm for SiNWs was measured using as-obtained AFM images. Electroplating and etching time had minimum effect on the diameter of as-fabricated SiNWs. Fourier transform infrared spectroscopy was used to extract the bonding information for SiNWs. Electrical properties were evaluated using two-probe source measuring unit. Resistivity of 0.11 Ω-cm was obtained for synthesised SiNWs. A cost-effective, environment friendly electroless metal assisted chemical etching process was used to successfully develop SiNWs from the bulk Silicon wafer.

Spatial delocalization of absorption and emission process in silicon nanowires

Dependence of optical properties of porous silicon nanowires on their size has been investigated here. Based on the experimental evidence, a new model to explain the process of absorption and photoluminescence in these Si nanowire samples has been proposed. Three different samples, with different nanowire diameters, have been prepared using metal-induced etching of silicon wafers. These wafers have different doping type and doping concentration which results in silicon nanowires of different diameters embedded with Si nanostructures of sizes around the Bohr's exciton radius. The absorption properties of these different types of Si nanostructures show a strong size dependence. However, the photoluminescence spectrum does not show any direct dependence on the size of the nanostructures, doping levels and type of silicon wafer used for fabrication of silicon nanostructures. It is also observed that the photoluminescence life time from these structures inversely depends on the size of the nanostructures whereas directly depends on the porosity, thus defects, in the samples. Based on these results it has been shown that the absorption of photons in these porous silicon nanowires happens in the silicon nanostructures embedded in the nanowires while the photoluminescence emission originates due to the surrounding porous SiOx.

Challenges and advances of positronium Bose-Einstein condensation

While scientists in the USA have accomplished Bose-Einstein Condensation (BEC) of inert gas and alkaline atoms successfully, a challenging question comes into the other’s mind why not BEC of Positronium (Ps), even though, its lowest lifetime (142 ns)? A few auspicious methods of having BEC were invented and successfully applied in the atoms of infinite lifetime. But in case of Ps-BEC laser cooling is considered to be the best method provided we can meet the requisite parameters. Using the medium range e+ beam intensity we explored some solid surfaces for the laser cooling of Ps but Ps-BEC could not be achieved. Because the intensity of pulsed beam was not enough to have the phase space density, outgoing Ps from the surface with thermal motion could not be controlled inside the scattering chamber and laser-Ps interaction was without synchronization. Hence, scientists of different laboratories have been searching materials of high density Ps production, intense pulsed e+ beam, advanced detector and image reconstruction techniques. Recently we have developed calumnious silicon nano-wires of various diameters and found the signature of a long-life component of Ps and estimated the Ps production density about 1015 Ps/cm3. It is an ideal case for the achievement of the Ps-BEC. The details of these systems are explained in the following sections.

Review of the Preparation and Structures of Si Nanowires, Ge Quantum Dots and Their Composites

Because the motion of charge carriers in nanowires and quantum dots is restricted within nanoscale in two and three dimensions, respectively, both nanowires and quantum dots exhibit many excellent optoelectronic properties. Particularly, with the advantage of being compatible with Si integrated circuits, Silicon nanowires (SiNWs) and germanium quantum dots (GeQDs) have been extensively studied in the past few decades. In order to explore novel physical properties, the integration of SiNWs and GeQDs has attracted great attention recently. In this paper, recent researches on the preparation methods and structures of SiNWs, GeQDs and their composites are reviewed, respectively. The synthesis of SiNWs with random distribution and ordered arrays by using vapor–liquid–solid growth mechanism and metal-assisted chemical etching technique is firstly summarized. Some special structures of SiNWs are also discussed. Furthermore, the development of some novel structures of GeQDs for further improving their optical properties is reviewed. Finally, the growth mechanism and structure evolution of SiNWs/GeQDs composites are illustrated from the view of theory and experiment. The strain in Ge shell layers and SiNWs, the relationship between Ge growth mode and SiNW diameter, and the distribution of GeQDs on the radial and axial directions of SiNWs are discussed in detail. The research about the growth of SiNWs/GeQDs composite structures is in its early stage, so there are many questions that need to be resolved in future.

Investigation of Effects of Diameter, Doping and Vacancy Defects on the Band Structure and Transport Properties of Silicon Nanowires for Potential Applications in Field-Effect Transistors

Silicon nanowires (SiNWs) with unique band structure and transport properties are considered potential candidates for future nanoelectronics devices such as field-effect transistors (FETs). We present a model of a SiNW-FET comprising $$\langle100\rangle$$ silicon atomic wires with a cylindrical-shaped metallic gate wrapped around the wires. For this purpose, we report on the energy band structure and density of states of SiNWs of diameters 5.93 A, 9.71 A and 13.55 A with $$\langle100\rangle$$ cleavage orientation by employing generalized gradient approximation and meta-generalized gradient approximation as well as the semi-empirical extended-Huckel model. Moreover, the transmission and transport properties of doped and undoped SiNWs of diameter 5.93 A with and without vacancy defects are explored using a non-equilibrium green function approach with self-consistent calculations. The corresponding I–V characteristics of the proposed cylindrical-shaped metallic-gate SiNW-FET under a specific gate voltage are presented. Our results show that the undoped SiNWs with vacancy defects on the surface are more suitable candidates for nanoelectronic device applications such as FETs in contrast to their counterparts with vacancies at the center.

(Invited) High-Index Dielectric Nanoantennas for Light Management, Nonlinear Optics, and Controlled Photoluminescence of Quantum Emitters

Over the last twenty years, nanoscale optics was mainly driven by plasmonics since noble metal nanostructures, thanks to localized surface plasmon resonances (LSPR), can be used to manipulate, concentrate or redirect visible light. Such properties have led to numerous actual or potential applications in integrated optics, sensors, nonlinear optics, field-enhanced spectroscopies, or photovoltaics. Recently, an alternative to plasmonics emerged with high refractive index dielectric nanostructures (e.g. silicon, n≈4 in the visible range), which offer the same range of applications as plasmonics by manipulating Mie optical resonances instead of LSPRs. The Mie resonances can be adjusted by modifying the size, shape, and material of those nanostructures. Furthermore, high index dielectric nanostructures offer several key advantages: absorption losses are far weaker than in metals for wavelengths longer than the direct band gap (near UV to near IR for Si), presence of strong magnetic resonances much more difficult to obtain in metals, and access to semiconductor (CMOS) technology for nanostructure fabrication (essential for easier transfer towards applications). We present three examples describing the interest of using silicon nanoantennas: enhancement and directionality control of light scattering, enhancement and polarization control of second harmonic generation (SHG), and emission rate modification of quantum emitters placed in the silicon nanoantenna near-field. We focus also on the important role of both electric and magnetic components of light. First, we show that silicon nanospheres and nanowires are able to scatter and/or absorb very efficiently visible light. Furthermore, due to a kind of Fano process involving the interference between a broad resonance and a sharp higher mode simultaneously excited, the direction of light scattering can be very different (either dominating forward or backward scattering). Second, we investigate the nonlinear emission of silicon nanowires (Si-NWs). We show that SHG is strongly increased if a Mie optical mode corresponding to the fundamental (laser) wavelength is supported by the nanowire. According to nonlinear scattering theory and the so-called Miller's rule, the highest SHG is predicted for an optimum overlap of resonant modes and linear susceptibilities, at both the fundamental and harmonic wavelengths, respectively. Contrary, no SHG can be detected in absence of any resonance. This behavior, and experimental SHG mappings of individual Si-NWs, are well reproduced by numerical simulations (Green Dyadic Method - GDM). We also show that, depending on the Si-NW diameter, different surface of bulk-like contributions to the SHG are addressed. This also results in different polarizations of SHG as function of both the Si-NW diameter and incident light polarization. Third, we show the effect of a high index dielectric nanoantenna on the spontaneous emission of quantum emitters placed in its vicinity. The emission rate corresponds to the local density of photonic states (LDOS), which is modified by the nanostructure. Thus, it can be controlled (enhancement or quenching) by properly designing the nanoantenna. For application in field-enhanced spectroscopy and single molecule detection, the goal is to obtain the highest enhancement. We discuss results of photoluminescence enhancement of molecules or quantum dots placed around simple Si nanoantennas (nanowires, nanodisk dimers). We also present preliminary results of the photoluminescence of Eu3+-doped clusters deposited on top of Si nanoantennas. These rare earth ions exhibit both electric dipole (ED) and magnetic dipole (MD) transitions of nearly equal strength. We show that these ED and MD transitions are very sensitive to the electric and magnetic LDOS, respectively. This can be evidenced using Si nanoantennas, which support both electric and magnetic resonances of comparable strength. Due to the very high index, they also give the unique opportunity to separate and redistribute in the nearfield the energy of the magnetic and electric parts of the electromagnetic field, otherwise inextricably connected in the farfield.

Study the effect of plasma power density and gold catalyst thickness on Silicon Nanowires growth by Plasma Enhanced Chemical Vapour Deposition

Gold (Au) metal catalysed tapered Silicon Nanowires (SiNWs) of diameter 50–150 nm at the base and length of 0.8–2.5 µm are synthesised by VLS mechanism using Silane (SiH4) as a precursor gas in Plasma Enhanced Chemical Vapour Deposition (PECVD) reactor at a substrate temperature of 650 °C. In this work, the effect of plasma power on the growth morphology of SiNWs is studied with two different thicknesses of Au catalyst layer. The catalyst layers are deposited using thermal evaporation system. Field Emission Scanning Electron Microscopy (FESEM) images confirm the growth of randomly oriented tapered nanowires. From the FESEM studies, it is clear that the density and the length of SiNWs increases with the increase of plasma power. High-resolution Transmission Electron Microscopy (HRTEM) demonstrated that the SiNWs are highly crystalline with (1 1 1) and (2 2 0) plane orientation. Raman spectra of SiNWs have been analysed and the first order optical phonons for SiNWs is observed at 519 cm−1 with 1 nm and 514.82 cm−1 with 2 nm Au catalyst.

Experimental and Simulation Study of Silicon Nanowire Transistors Using Heavily Doped Channels

The experimental results from 8 nm diameter silicon nanowire junctionless field-effect transistors with gate lengths of 150 nm are presented that demonstrate on-currents up to 1.15 mA/μm for 1.0 V and 2.52 mA/μm for 1.8 V gate overdrive with an off-current set at 100 nA/μm. On- to off-current ratios above 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> with a subthreshold slope of 66 mV/dec are demonstrated at 25 °C. Simulations using drift-diffusion which include density-gradient quantum corrections provide excellent agreement with the experimental results. The simulations demonstrate that the present silicon-dioxide gate dielectric only allows the gate to be scaled to 25 nm length before short-channel effects significantly reduce the performance. If high-K dielectrics replace some parts of the silicon dioxide then the technology can be scaled to at least 10 nm gatelength.

One dimensional transport in silicon nanowire junction-less field effect transistors

Junction-less nanowire transistors are being investigated to solve short channel effects in future CMOS technology. Here we demonstrate 8 nm diameter silicon nanowire junction-less transistors with metallic doping densities which demonstrate clear 1D electronic transport characteristics. The 1D regime allows excellent gate modulation with near ideal subthreshold slopes, on- to off-current ratios above 108 and high on-currents at room temperature. Universal conductance scaling as a function of voltage and temperature similar to previous reports of Luttinger liquids and Coulomb gap behaviour at low temperatures suggests that many body effects including electron-electron interactions are important in describing the electronic transport. This suggests that modelling of such nanowire devices will require 1D models which include many body interactions to accurately simulate the electronic transport to optimise the technology but also suggest that 1D effects could be used to enhance future transistor performance.

Hierarchical Multi-Diameter Single-Crystal Silicon Nanowires by Successive Wet Chemical Etching.

A simple and low cost top–down wet chemical etching method was developed to fabricate hierarchical multi-diameter well-ordered single-crystal silicon nanowires. The procedure starts with forming single diameter silicon nanowire arrays by using nanosphere lithography and metal-assisted chemical etching of single crystal silicon wafer, which is followed by anisotropic radial etching of the wires. Successive repetitions of these etching steps result in arrays of multi-diameter single crystal nanowires. This technique can allow engineering nanowires in a hierarchical three-dimensional geometry for the development of advanced nano devices.

Study of indium catalyst thickness effect on PECVD-grown silicon nanowires properties

In this work, we focus on the elaboration at low temperature of metal-catalyzed silicon nanowires (SiNWs) obtained by vapor–liquid–solid (VLS) process. In particular, the effect of the metal thickness on SiNWs properties is reported. SiNWs are formed on indium (In) coated Si substrates using SiH4 as a precursor gas in plasma enhanced chemical vapor deposition (PECVD) reactor at a low substrate temperature of 400 °C. Morphological characterization has shown that increasing the thickness of indium layer leads to the increase of the (In) catalysts diameter, the SiNWs density, length and diameter. The grown SiNWs are randomly oriented and have a tapered form with an average length up to 7 µm for a deposition time of 15 min. According to X-ray diffraction patterns, SiNWs are highly crystalline with (111) (220) and (311) plane orientation. The Raman spectra show a downshift of the first-order optical phonon from 520 cm−1 for the c-Si to 517 and 513 cm−1 for SiNWs samples attributed essentially to the confinement effect in silicon nanowires. We notice also that SiNWs are composed essentially of amorphous and crystalline silicon. Increasing the indium thickness leads to the disappearance of the amorphous component and the presence of peaks assigned to nanocrystalline grains induced by the crystallization of the amorphous layer catalyzed by the indium particles.

Ultralow thermal conductivity of silicon nanowire arrays by molecular dynamics simulation

We investigate the thermal conductivities of silicon nanowires (SiNWs) and their arrays based on molecular dynamics simulations. It is found that diminishing diameter, roughing surface and doping impurity of SiNWs can reduce their thermal conductivities by two or three orders of magnitude compared with that of bulk silicon crystals due to the strong phonon boundary and phonon impurity scattering. The simulated thermal conductivities of SiNW arrays demonstrate that arraying nanowires can further lower the thermal conductivity owing to the laterally-coupled effect, and the thermal conductivity of arrays decreases notably with the increased nanowire volume fraction, resulting in an ultralow thermal conductivity for the doped SiNW arrays with rough surfaces, which provides theoretical guidance of thermal management for semiconductor nanowire based microelectronic and thermoelectric devices.

Controlling the Size of Interior Core of Silicon Nanowires

As the physical and electrical properties of silicon nanowires (SiNWs) are determined by their dimension, it is necessary to control their dimension to integrate them in a device. SiNWs were synthesized via Vapor-Liquid-Solid (VLS) mechanism in hot-wire chemical vapor process (HWCVP) technique using silane as a Si source and Sn as a catalyst. Different sizes of nano-template have been made by depositing of different amount of Sn using thermal evaporation method. The size of nano-template is found to be increased with the quantity of Sn. The diameter of resulted SiNWs depends on the size of the nano-template and it increases with the nano-template size. However, the diameter of SiNWs is found to be much larger than the used nano-template which is due to the deposition of silicon film on the sidewalls of the growing SiNWs. It is demonstrated here that the diameter of the interior core of SiNWs can be controlled desirable by adjusting the size of the nano-template.