Effect of temperature and silicon resistivity on the elaboration of silicon nanowires by electroless etching

The morphology of silicon nanowire (SiNW) layers formed by Ag-assisted electroless etching in HF/H₂O₂ solution was studied. Prior to the etching, the Ag nanoparticles were deposited on p-type Si(1 0 0) wafers by electroless metal deposition (EMD) in HF/AgNO₃ solution at room temperature. The effect of etching temperature and silicon resistivity on the formation process of nanowires was studied. The secondary ion mass spectra (SIMS) technique is used to study the penetration of silver in the etched layers. The morphology of etched layers was investigated by scanning electron microscope (SEM).     

The intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes

Estimates of the intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes have been presented from the application of Fermi’s golden rule formula based upon an elastic continuum model for cubic anharmonicity. In particular, results have been presented for the lowest non-zero mode in both nanostructures, and also the breathing mode in the nanotube. Except for the ultrathin nanowire, the lifetime increases with size and decreases with an increase in temperature. Typically, these modes have a lifetime of the order of nanoseconds, almost a thousand times larger than the lifetimes of optical phonon modes in the corresponding bulk materials. Also, at room temperature the lifetime of the lowest non-zero mode is nearly an order of magnitude larger in the (20,20) nanotube than in the nanowire of similar thickness (width 2.2 nm).     

Competition between surface energy and interphase energy in transition region and diameter-dependent orientation of silicon nanowires

A small sandwiched transition region between the Au catalysis droplet and silicon nanowires (SiNWs) is proposed to investigate the diameter-dependent orientation of SiNWs grown by the vapor-liquid-solid (VLS) mechanism. Atomic-scale calculation shows that for a given transition region width, there is always a critical diameter. Below the critical value, surface energy dominates and the 〈1 1 0〉 orientation is preferred, whereas at larger diameters, the interphase energy dominates and SiNWs grow along the 〈1 1 1〉 direction. The variability of the critical diameter is also included in our model by adjusting the transition region width. The theoretical results are in agreement with those from experiments.     

Effects of silver and gold catalytic activities on the structural and optical properties of silicon nanowires

The metal-assisted chemical etching of silicon in an aqueous solution of hydrofluoric acid and hydrogen peroxide is established for the fabrication of large area, uniform silicon nanowire (SiNW) arrays. In this study, silver (Ag) and gold (Au) are considered as catalysts and the effect of different catalysts with various thicknesses on the structural and optical properties of the fabricated SiNWs is investigated. The morphology of deposited catalysts on the silicon wafer is characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). It is shown that the morphology of the fabricated silicon nanostructures remarkably depends upon the catalyst layer thickness, and the catalyst etching time directly affects the structural and optical properties of the synthesized SiNWs. FESEM images show a linear increment of the nanowire length versus time, whereas the etching rate for the Au-etched SiNWs was lower than the Ag-etched ones. Strong light scattering in SiNWs caused the total reflection to decrease in the range of visible light, and this decrement was higher for the Ag-etched SiNW sample, with a longer length than the Au-etched one. A broadband visible photoluminescence (PL) with different peak positions is observed for the Au- and Ag-etched samples. The synthesized optically active SiNWs can be considered as a promising candidate for a new generation of nano-scale opto-electronic devices.     

Si纳米线的固-液-固可控生长及其形成机理分析

以Au膜作为金属催化剂,直接从n-(111)Si单晶衬底上制备了直径为30—60nm和长度从几微米到几十微米的高质量Si纳米线.实验研究了Au膜层厚、退火温度、N2气流量和生长时间对Si纳米线形成的影响.结果表明,通过合理选择和优化组合上述各种工艺条件,可以实现直径、长度、形状和取向可控的纳米线生长.基于固-液-固生长机理,定性阐述了Si纳米线的形成过程.     

Growth and properties of InGaAs nanowires on silicon

Free‐standing ternary InGaAs nanowires (NW) are at the core of intense investigations due to their integration capabilities on silicon (Si) for next‐generation photovoltaics, integrated photonics, tunneling devices, and high‐performance gate all‐round III–V/Si NW transistors. In this review, recent progress on the growth, structural, optical and electrical properties of InGaAs NWs on Si substrate is highlighted. Particular focus is on a comparison between conventional catalyst‐assisted and catalyst‐free growth methods as well as self‐assembled versus site‐selectively grown NW arrays. It will be shown that catalyst‐free, high‐periodicity NW arrays with extremely high compositional uniformity are mandatory to allow un‐ambiguous structure–property correlation measurements. Here, interesting insights into the electronic/optical properties of wurtzite, zincblende and mixed crystal phases of InGaAs will be highlighted based on recent photoluminescence spectroscopy data. Finally, the InGaAs NW‐on‐Si system is also discussed in the realms of heterojunction properties, providing a promising system for steep‐slope tunneling field effect transistors in future low‐power post‐CMOS intergrated microelectronics and broad‐band photoabsorption and detec‐tion devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.     

Effects of hydroxyl groups and hydrogen passivation on the structure,electrical and optical properties of silicon carbide nanowires

The effects of hydrogen and hydroxyl passivation on the structure, electrical and optical properties of SiCNWs were investigated. The passivation performance of different atoms (groups) were discussed by analyzing the distribution of electronic states and the polarity of chemical bonds. The results show that passivation can improve the stability of SiCNWs structure, and the effect of hydroxyl is better than hydrogen passivation. And hydrogen and hydroxyl passivation both increase the band gap of SiCNWs, and the changing trend of band gap is relevant to the polarity of the covalent bond formed by the passivation of surface atoms. Moreover, passivation enhances the stability of the optical properties of SiCNWs, resulting in narrowing of light absorption, photoconductivity and other spectra, and the response peak shifts to the deep ultraviolet region, which means that hydrogen or hydroxyl passivation of SiCNWs is likely to be a candidate material for deep ultraviolet micro-nano optoelectronic devices.     

Nature of the band gap of silicon and germanium nanowires

Nanowires of both Si and Ge have been predicted to have band gaps that are either direct or indirect depending upon the crystallographic direction along which the nanowire is oriented. We use a sp³d⁵s^* tight binding model to calculate the band structures for both Ge and Si nanowires oriented along the (1 0 0), (1 1 0) and (1 1 1) directions. We show that the nature of the band gap and the variation of the zone centre band gap with nanowire width depends upon the nanowire stacking direction for both Si and Ge nanowires. We then show, by considering bulk unit cells along the (1 0 0), (1 1 0) and (1 1 1) directions, that it is possible to accurately predict whether a nanowire stacked in the same direction as one of these bulk unit cells has a direct or indirect band structure.     

Etching of silicon nanowires on Ag(1 1 0) by atomic hydrogen

Scanning Tunnelling Microscopy, High Resolution Electron Energy Loss Spectroscopy and High Resolution X-Ray Photoelectron Spectroscopy have been used to study the adsorption of atomic hydrogen onto Si nanowires grown on Ag(1 1 0). We demonstrate that the hydrogen strongly interacts with the Si nanowires modifying their structural and electronic properties. Hydrogen atoms etch the Si nanowires and eventually lead to their complete removal from the Ag(1 1 0) surface.     

分子动力学研究表面缺陷对硅纳米线杨氏模量的影响

硅纳米线因受量子尺寸效应与表面效应的影响而具有奇特的力、电及其耦合特性,成为了纳米电子器件的核心构件.然而在硅纳米线的制备过程中,表面产生缺陷不可避免.因此本文采用分子动力学方法着重研究了表面缺陷浓度对不同横截面形状(正方形、六角形和三角形)的[110]晶向和[111]晶向硅纳米线杨氏模量的影响.研究结果表明,当硅纳米线仅有单一表面缺陷时,不同晶向硅纳米线的杨氏模量均随表面缺陷浓度增加而迅速单调减小.当表面缺陷浓度为10%时,杨氏模量的减小幅度在10%-20%之间,减小幅度的差异与硅纳米线的晶向以及横截面形状密切相关.当存在多个表面缺陷时,杨氏模量随着缺陷浓度的增加表现出了不同程度的波动趋势.三角形截面硅纳米线的杨氏模量波动幅度最大,正方形截面的波动较小,即表面缺陷分布的不同对正方形截面硅纳米线的杨氏模量影响较小,这表明表面缺陷的影响与其分布及硅纳米线的横截面形状密切相关.通过与实验结果对比,本文的研究结果揭示了表面缺陷是导致硅纳米线杨氏模量实验值变小的重要因素,因此在表征硅纳米线的力学性能时,需要考虑表面缺陷的影响.     

Fabrication and characterization of polycrystalline silicon nanowires with silver-assistance by electroless deposition

Single crystal silicon wafers are widely used as the precursors to prepare silicon nanowires by employing a silver-assisted chemical etching process. In this work, we prepared polycrystalline silicon nanowire arrays by using solar-grade multicrystalline silicon wafers. The chemical composition and bonding on the surface of silicon nanowire arrays were characterized by Fourier Transform Infrared spectroscope, and X-ray photoelectron spectroscope. The photoluminescence spectra of silicon nanowires show red light emissions centered around 700 nm. Due to the passivation effect of Si dangling bonds by concentrated HNO₃ aqueous solution, the photoluminescence intensities are improved by 2 times. The influences of surface chemical states on the wettability of silicon nanowire arrays were also studied. We obtained a superhydrophobic surface on the as-etched silicon nanowire arrays without surface modification with any organic low-surface-energy materials, and realized the evolution from superhydrophobicity to superhydrophilicity via surface modifications with HNO₃ solutions.     

Influence of pre-surface treatment on the morphology of silicon nanowires fabricated by metal-assisted etching

Herein we demonstrate an improved metal-assisted etching method to achieve highly dense and uniform silicon nanowire arrays. A pre-surface treatment was applied on a silicon wafer before the process of metal-assisted etching in silver nitrate and hydrogen fluoride solution. The treatment made silver ion continuously reduce on silver nuclei adherence on the silicon surface, leading to formation of dense silver nanoparticles. Silver nanoparticles acting as local redox centers cause the formation of dense silicon nanowire arrays. In contrast, an H-terminated silicon surface made silver ion reduce uniformly on the silicon surface to form silver flakes. The silicon nanowires fabricated with a pre-surface treatment reveals higher density than those fabricated without a pre-surface treatment. The volume fraction improves from 18 to 38%. This improvement reduces the solar-weighted reflectance to as low as 3.3% for silicon nanowires with a length of only 0.87 μm. In comparison, the silicon nanowires fabricated without a pre-surface treatment have to be as long as 1.812 μm to achieve the same reflectance.     

Large-scale synthesis and characterization of SiC nanowires by high-frequency induction heating

SiC nanowires were synthesized in large quantity by heating SiO and activated carbon fibers without metal catalysts. The morphologies of product were studied by SEM and TEM and it indicated that the SiC nanowires obtained at the bottom of the activated carbon fibers surfaces were coated by an amorphous silicon oxide layer and the core of SiC nanowires were measured, typically as about 8-20 nm in diameter, while in situ formed nanotree-bud network, Y-junction and the “S” shape nanostructure were observed in those SiC nanowires obtained at the up of the activated carbon fibers surfaces. These SiC/SiO₂ heterostructure, particularly novel 2-D heterostructure, would be expected as important blocks in building nanodevices and as reinforcement in advanced composites.     

Growth process and mechanism of a multi-walled carbon nanotube nest deposited on a silicon nanoporous pillar array

A large scale nest array of multi-walled carbon nanotubes (NACNTs) was grown on silicon nanoporous pillar array (Si-NPA) by thermal chemical vapor deposition. Through observing its macro/micromorphology and structure, ascertaining the catalyst component and its locations at different growth time by hiring field emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and selected area electron diffraction, the growth process was deduced. Its thermal properties were also investigated by using a thermogravimetric analyzer. Our experiments demonstrated that the CNTs growth by means of root-growth mechanism at the initial growth stage, then a continuous growth process with its tip open is suggested, finally, a schematic growth model of NACNT/Si-NPA was presented.     

Preparation and atomic oxygen erosion resistance of silica film formed on silicon rubber by sol-gel method

Based on the characteristic that silicon coupling agents have the capability to develop ‘molecular bridge’ in the interface of organic materials and inorganic materials, silica films were prepared on the surface of flexible silicon rubber by sol-gel method and the optical transmittance of the sample before and after atomic oxygen irradiation was tested. The surface morphology and structure of silica films were investigated by scanning electronic microscope (SEM) and Fourier transformed infrared spectroscopy (FTIR). The results indicated that the silica sol could easily form a uniform thin film on the surface of silicon rubber pretreated by high concentration silicon coupling agents, and the inorganic silica films could combine with organic silicon rubber without obvious delamination on the interface.     

合成参数对气-液-固法低温生长MgO纳米线的影响

将Mg(C11H19O2)2(即双(2,2,6,6,-四甲基-3,5-庚二酮酸)镁)作为反应前驱体,用脉冲液滴注入式金属有机物化学气相沉积法,在较低温度(T=600℃)下合成MgO纳米线.纳米线沿着[001]方向生长且Au催化剂位于纳米线顶端,这表明纳米线是由气-液-固机制诱导生长的.通过改变前驱体注入的脉冲周期或周期注入剂量能够控制纳米线的生长模式,使之垂直或平行于样品表面生长.     

Chemical vapor deposition synthesis of Ge doped ZnO nanowires and the optical property investigation

Ge-doped ZnO nanowires with high quality were prepared with simple chemical vapor deposition method. The morphology, crystal structure and elemental distribution were characterized using SEM (scanning electron microscopy), HRTEM (high resolution transmission electron microscopy) and EDS (energy dispersive spectrometry) mapping approaches. The optical properties were investigated with combination of room-temperature CL (cathode ray luminescence) and variable temperature micro-PL methods. It is found for the first time that a tapered Ge doped nanowire exhibits a thickness-dependent spectrum shift under a cathode-ray excitation, which was interpreted by the electron-lattice interaction model and the band-filling effect of semiconductors. The exciting power dependent emission spectra (lux-luminescent characteristics) at varied temperatures reveal a varied radiative recombination mechanism. At 83 K, free excitons contribute to the emission spectrum, while at 300 K while at 300 K multi-levels related non-radiative transition process largely contributed. The optical property indicates an excellent crystallization of the nanowires with a small amount of defects.     

Catalyst formation at various temperatures by hydrogen radical treatment and synthesis of silicon nanowires

Metal nanocrystals as catalyst from a metal oxide film were fabricated at various temperatures after hydrogen radical treatment and great quantities of silicon nanowires (SiNWs) were successfully synthesized using the hydrogen microwave afterglow deposition method. Indium (In) metal nanocrystals with size of about 12 nm were obtained from indium oxide film after hydrogen radical pre-treatment for 5 min at 400 °C and their quantity reached approximately 3 × 10¹⁰ cm⁻². Subsequently, a numerous SiNWs were grown with the crystal diffraction of (1 1 1), (2 2 0) and (3 1 1). The diameters of the SiNWs mainly ranged from 5 to 120 nm and their lengths extended to about 8.5 μm.