A New Route to Large‐Scale Synthesis of Silicon Nanowires in Ultrahigh Vacuum

An approach for the large‐scale synthesis of high‐purity silicon nanowires (SiNWs) in ultrahigh vacuum is presented. A mixture of Si and SiO₂ is evaporated by an electron beam, and the growth temperature is 700 °C, which is much lower than those used for other oxide‐assisted growths. A new type of single‐crystal SiNWs, with [221] orientation, is thus synthesized. Moreover, it is experimentally demonstrated that SiO intermediates are formed in the process, and the nanowires are obtained via a disproportionation reaction of 2SiO → Si + SiO₂. A growth mechanism is proposed and the critical factors for the formation of 1D nanowires are also determined. The approach is particularly compatible with the mature Si‐based technology, and is favorable for device integration and practical applications.     

Growth of silicon nanowires on H-terminated Si {111} surface templates studied by transmission electron microscopy

We have studied the growth of silicon nanowires (SiNWs) by means of transmission electron microscopy. SiNWs are grown from nanocatalysts via the Vapor-Liquid-Solid (VLS) mechanism using silane (SiH4) gas as a source gas. The nanocatalysts are prepared on a hydrogen (H)-terminated Si surface. We have examined the formation mechanism of nanocatalysts on H-terminated surface and have observed several structural variants of SiNWs. According to the study we have suggested that many structural variations of SiNWs are possible, which modify the structural properties of SiNWs to great extents.     

A systematic study of the impact of etching time to the sensitivity of SiNW sensor fabricated by MACEtch process

In this paper, silicon nanowires (SiNWs) was fabricated by a combination of metal-assisted chemical etching (MACEtch) and nanosphere lithography. We get the silicon nanowires with different specific surface area by changing the etching time. The microscopic structure of the silicon nanowires is observed by field emission scanning electron microscope (FESEM). The gas sensing performances of the SiNWs with different specific surface area have been systematically examined by measuring the resistance change towards the concentrations of NO₂ in the range of 1–5 ppm at room temperature (RT, 300 K), the gas sensor composed of SiNWs showed perfect gas sensitive property and possessed a short response–recovery time. The main reason of these excellent attributes is quite likely that high specific surface area of the SiNWs, and NO₂ sensing mechanism of the SiNWs was also further explained, which can be attributed to the oxygen in the air and detected NO₂ extract electrons from the surface of the SiNWs, and the resistivity of SiNWs changed with the changing of space-charge layer under the of SiNWs surface.     

Nitrogen-doped ZnO/n-Si core–shell nanowire photodiode prepared by atomic layer deposition

Photodiodes made from core–shell nanowires (NWs) comprising n-type silicon (n-Si; core) and nitrogen-doped ZnO (ZnO:N; shell) were fabricated by atomic layer deposition of ZnO:N on vertically aligned Si NWs. The device properties were investigated as functions of nitrogen content of the ZnO:N shell. The electron-carrier concentration of ZnO:N was modulated by adjusting the concentration of the reactant, diluted ammonium hydroxide, from 0 to 30%. The rectification ratio and the reverse-current density of the ZnO:N/n-Si planar heterojunction were evaluated under dark condition for various NH₄OH concentrations. The ZnO:N/n-Si heterojunction prepared with NH₄OH 15% was found to have the lowest reverse-current density with a moderate resistivity. In order to realize an effective ZnO:N/n-Si photodiode, a ZnO:N layer prepared with 15% NH₄OH was deposited on well-aligned Si nanowires. The core–shell NW photodiode showed more sensitive photodetecting performance in UV light than the planar photodiode. Also, the significantly enhanced performances of the core–shell NW photodiode were evaluated by examining its spectral responsivity.     

Fabrication of Single‐Crystalline Silicon Nanowires by Scratching a Silicon Surface with Catalytic Metal Particles

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 SiNWs 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.     

Integration of SiC-1D nanostructures into nano-field effect transistors

We report on the integration and the electrical transport properties of silicon carbide-based one-dimensional nanostructures into field effect transistors. Different kinds of SiC-based 1D nanostructures have been used: 3C– and 4H–SiC nanowires obtained by a plasma etching process, Si–SiC core–shell nanowires and SiC nanotubes both obtained by a carburization route of silicon nanowires.     

Novel Type‐II InAs/AlSb Core–Shell Nanowires and Their Enhanced Negative Photocurrent for Efficient Photodetection

The control of optical and transport properties of semiconductor heterostructures is crucial for engineering new nanoscale photonic and electrical devices with diverse functions. Core–shell nanowires are evident examples of how tailoring the structure, i.e., the shell layer, plays a key role in the device performance. However, III–V semiconductors bandgap tuning has not yet been fully explored in nanowires. Here, a novel InAs/AlSb core–shell nanowire heterostructure is reported grown by molecular beam epitaxy and its application for room temperature infrared photodetection. The core–shell nanowires are dislocation‐free with small chemical intermixing at the interfaces. They also exhibit remarkable radiative emission efficiency, which is attributed to efficient surface passivation and quantum confinement induced by the shell. A high‐performance core–shell nanowire phototransistor is also demonstrated with negative photoresponse. In comparison with simple InAs nanowire phototransistor, the core–shell nanowire phototransistor has a dark current two orders of magnitude smaller and a sixfold improvement in photocurrent signal‐to‐noise ratio. The main factors for the improved photodetector performance are the surface passivation, the oxide in the AlSb shell and the type‐II bandgap alignment. The study demonstrates the potential of type‐II core–shell nanowires for the next generation of photodetectors on silicon.     

Electrical characteristics of nickel silicide–silicon heterojunction in suspended silicon nanowires

Electronic characteristics of silicide/silicon interface were studied in the suspended, chemically synthesized silicon nanowires (SiNWs). Step-by-step intrusion of a silicide/Si interface along the axial direction of a suspended silicon nanowire was performed by repeated thermal annealing cycles, and the current-voltage (I-V) characteristics of the annealed silicide/SiNW/silicide structure were measured at each cycle. The intruded length of the silicide was found to be directly proportional to the total annealing time, but the rate of silicidation was much smaller than previous works on similar silicide/SiNWs. A structural kink with Ni atoms diffused along the sidewall created a secondary source of silicidation, resulting in anomalous I-V characteristics. The measured I-V including this unintentional silicidation in the Si channel was explained by various combinations of Schottky barriers and resistors.     

ZnSe–Si Bi‐coaxial Nanowire Heterostructures

We report on the fabrication, structural characterization, and luminescence properties of ZnSe/Si bi‐coaxial nanowire heterostructures. Uniform ZnSe/Si bi‐coaxial nanowire heterostructures are grown on silicon substrates by the simple one‐step thermal evaporation of ZnSe powder in the presence of hydrogen. Both ZnSe and silicon are single‐crystalline in the bi‐coaxial nanowire heterostructures, and there is a sharp interface along the nanowire axial direction. Furthermore, secondary nanostructures of either ZnSe nanobrushes or a SiO_x sheath are also grown on the primary bi‐coaxial nanowires, depending on the ratio of the source materials. The experimental evidence strongly suggests that bi‐coaxial nanowires are formed via a co‐growth mechanism, that is, ZnSe terminates specific surfaces of silicon and leads to anisotropic, one‐dimensional silicon growth, which simultaneously serves as preferential nucleation sites for ZnSe, resulting in the bi‐coaxial nanowire heterostructures. In addition, the optical properties of ZnSe/Si nanowires are investigated using low‐temperature photoluminescence spectroscopy.     

Atomistic simulations of the tensile and melting behavior of silicon nanowires

Molecular dynamics simulations with Stillinger-Weber potential are used to study the tensile and melting behavior of single-crystalline silicon nanowires (SiNWs). The tensile tests show that the tensile behavior of the SiNWs is strongly dependent on the simulation temperature, the strain rate, and the diameter of the nanowires.For a given diameter, the critical load significantly decreases as the temperature increases and also as the strain rate decreases. Additionally, the critical load increases as the diameter increases. Moreover, the melting tests demonstrate that both melting temperature and melting heat of the SiNWs decrease with decreasing diameter and length, due to the increase in surface energy. The melting process of SiNWs with increasing temperature is also investigated.     

硅纳米线用于纳米器件前的准备工序

硅纳米线是一种新型半导体光电材料,具有量子限制效应并且能与目前的硅芯片相兼容,是一种很有前途的适用于纳米器件的材料,未经处理的硅纳米线存在大量的晶体缺陷以及表面氧化物保护层,直接将这样的硅纳米线应用于纳米器件中时,表面氧化层的保护作用使硅纳米线在电子器件中应用时不能有效地实现欧姆接触,因此对硅纳米线使用前的前期处理是非常必要的。本文主要针对硅纳米线应用于电子器件的准备工序包括为制备完整硅纳米线晶体结构而进行的减少缺陷处理、避免硅纳米线团聚而进行的分散处理,以及使硅纳米线具有有效欧姆接触而进行的表面金属离子处理等作系统的阐述。     

TEM characterization of Si nanowires grown by CVD on Si pre-structured by nanosphere lithography

Silicon nanowires (SiNWs) were grown on Si(1 0 0) and Si(1 1 1) substrates by chemical vapour deposition (CVD) via the vapour–liquid–solid (VLS) mechanism with small gold particles used as seeds. In order to control the diameter of nanowires, their density on the substrate and their orientation we controlled the size and the distribution of Au seed particles. This was accomplished using nanosphere lithography (NSL) by which regular arrays of Au nanoparticles can be generated. This allowed us to grow single-crystalline SiNWs perpendicular to the surface of Si(1 1 1) substrates. The SiNWs and their Au caps were studied with respect to their morphology and composition using TEM, HREM and EFTEM methods. Clusters of Au are observed along the surface of SiNWs and the existence of a thin Si film on gold particles capping the SiNWs is demonstrated.     

Electrochemical Properties of Si‐Ge Heterostructures as an Anode Material for Lithium Ion Batteries

Si‐Ge composites have recently been explored as an anode material for lithium‐ion batteries due to their stable cycle performance and excellent rate capability. Although previous reports show the benefits of Si‐Ge composites on electrochemical performance, the specific mechanism and structural effects have been overlooked. Here, the structural effect of Si‐Ge heterogeneous nanostructures on both mechanics and kinetics is systematically studied through theoretical analysis and detailed experimental results. Si‐Ge and Ge‐Si core–shell nanowires are employed for this study. The Si‐Ge core–shell nanowires show a much improved electrochemical performance, especially cycle performance and rate capability, when compared to those of the Ge‐Si core–shell nanowires electrode. On the basis of the detailed experimental results and associated theoretical analysis, its is demonstrated that the strain distribution and Li diffusivity and/or diffusion path are significantly affected by the Si‐Ge heterostructure, which induce different mechanics and kinetics associated with lithium.     

硅纳米线拉伸与熔化行为的原子模拟

Molecular dynamics simulations with Stillinger-Weber potential are used to study the tensile and melting behavior of single-crystalline silicon nanowires（SiNWs）.The tensile tests show that the tensile behavior of the SiNWs is strongly dependent on the simulation temperature,the strain rate,and the diameter of the nanowires.For a given diameter,the critical load significantly decreases as the temperature increases and also as the strain rate decreases.Additionally,the critical load increases as the diameter increases.Moreover,the melting tests demonstrate that both melting temperature and melting heat of the SiNWs decrease with decreasing diameter and length,due to the increase in surface energy.The melting process of SiNWs with increasing temperature is also investigated.     

Epitaxial, high-K dielectrics on silicon: the example of praseodymium oxide

We show the first results for crystalline growth of praseodymium oxide on Si as a potential high-K dielectric with very promising electrical properties. All layer growth experiments were performed using solid source molecular beam epitaxy. The initial growth phase was studied using scanning tunneling microscopy. On Si(0 0 1) oriented surfaces, crystalline Pr₂O₃ grows as (1 1 0)-domains, with two orthogonal in-plane orientations. Epitaxial silicon overgrowth seems to be impossible. We obtain perfect epitaxial growth on Si(1 1 1). These layers can also be overgrown epitaxially with silicon. Finally, we show that the structural quality of epitaxial grown Pr₂O₃ on Si(0 0 1) degrades when the film is exposed to air due to silicon oxide formation at the interface based on oxygen indiffusion. However, it can be stabilized by capping with Si.     

Improving the performance of radial n-i-p junction Si nanowire solar cells by catalyst residue removal

It is hoped that silicon nanowire (SiNW)-based solar cells will provide the basis for a new generation of photovoltaics. However, metal-catalyzed SiNWs contain metal residues (such as indium) which may degrade the performance of solar cells. In this study, we prepared SiNW solar cells by plasma-enhanced chemical vapor deposition using indium as the catalyst to grow the SiNWs. The SiNWs were treated with hydrochloric acid to reduce the indium contamination at different concentrations, C_HCl (1–5%). We found the decreasing the indium contamination improved the performance of the solar cells at optimum C_opHCl. However, the performance of the solar cells decreased when C_HCl exceeded C_opHCl. This was attributed to the variation in the conduction-band offset ΔE_c between the n type amorphous silicon layer (E_c n-a-Si) and the n type crystalline silicon nanowires (E_c n-c-SiNWs). Finally, a conversion efficiency (Eff) improvement from 2.11% to 6.18% was obtained with the optimized C_HCl.     

Silicon in functional epitaxial oxides: A new group of nanostructures

The ability to integrate low-dimensional crystalline silicon into crystalline insulators with high dielectric constant (high-k) can open the way for a variety of novel applications ranging from high-k replacement in future nonvolatile memory devices to insulator/Si/insulator structures for nanoelectronic applications. We will present an approach for nanostructure fabrication by incorporation of crystalline silicon into epitaxial oxide that is based on a solid-phase epitaxy of Si. In dependence on the preparation conditions we obtained nanostructures containing an either ultra-thin single-crystalline Si quantum-well buried in single-crystalline oxide matrix with sharp interfaces or Si-nanocrystals (ncs) embedded into single-crystalline oxide layer. As an example, we demonstrate the growth of Si buried in Gd₂O₃ and the incorporation of epitaxial Si clusters into single-crystalline Gd₂O₃ on silicon as well as silicon carbide substrates using molecular beam epitaxy. The leakage current of the obtained nanostructures exhibited negative differential resistance at lower temperatures. For structures containing Si-ncs a large hysteresis in capacitance–voltage measurements due to charging and discharging of the Si-ncs was obtained.     

Formation of Luminescent Silicon Nanowires and Porous Silicon by Metal-Assisted Electroless Etching

Metal-assisted etching of silicon in HF/H₂O₂ aqueous solutions has been used to fabricate luminescent silicon nanowires (SiNWs) and porous silicon. The impact of the gold catalyst layer thickness and the etching solution on the morphology of the synthesized nanostructures and the diameter of the obtained nanowires were systematically investigated. Scanning electron microscopy (SEM) analyses reveal that the morphology of the fabricated structures strongly depends on the composition of the solution and the thickness of the catalyst layer. It has been observed that SiNWs are formed in solutions with H₂O₂ ratios (ξ) below 10 %; increasing the H₂O₂ concentration above this critical value leads to mesoporous (10 % < ξ < 14 %) and macroporous (14 % < ξ < 17 %) structures. Photoluminescence measurements show that SiNWs emit light at about 430 nm. Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) analyses were utilized to determine the origin of the emission in the silicon nanostructures. TEM imaging demonstrates that SiNWs are covered by a thin layer of porous silicon, which is assumed to be responsible for their light emission.     

Au@pNIPAM Thermosensitive Nanostructures: Control over Shell Cross‐linking,Overall Dimensions,and Core Growth

Thermoresponsive nanocomposites comprising a gold nanoparticle core and a poly(N‐isopropylacrylamide) (pNIPAM) shell are synthesized by grafting the gold nanoparticle surface with polystyrene, which allows the coating of an inorganic core with an organic shell. Through careful control of the experimental conditions, the pNIPAM shell cross‐linking density can be varied, and in turn its porosity and stiffness, as well as shell thickness from a few to a few hundred nanometers is tuned. The characterization of these core–shell systems is carried out by photon‐correlation spectroscopy, transmission electron microscopy, and atomic force microscopy. Additionally, the porous pNIPAM shells are found to modulate the catalytic activity, which is demonstrated through the seeded growth of gold cores, either retaining the initial spherical shape or developing a branched morphology. The nanocomposites also present thermally modulated optical properties because of temperature‐induced local changes of the refractive index surrounding the gold cores.     

非晶SiO_2纳米线的合成及其显微结构和光学性质的研究

本研究以硅片为衬底,热蒸发一氧化硅粉末在较低温度下合成了大量直径均匀的非晶SiO2纳米线.这些纳米线直径分布在15 nm～40 nm之间,长度几十微米.选区电子衍射(SAED)、能谱(EDS)、电子能量损失谱(EELS)分析结果表明这些纳米线为非晶SiO2纳米线.光致发光(PL)谱测试结果显示纳米线在波长550 nm处存在一个较强的PL峰.本文进一步指出了蒸发源SiO粉末的颗粒度和蒸发温度对纳米线生长有强烈的影响.