Thermal stability of silicon nanowires: atomistic simulation study

Using the Stillinger--Weber (SW) potential model, we investigate the thermal stability of pristine silicon nanowires based on classical molecular dynamics (MD) simulations. We explore the structural evolutions and the Lindemann indices of silicon nanowires at different temperatures in order to unveil atomic-level melting behaviour of silicon nanowires. The simulation results show that silicon nanowires with surface reconstructions have higher thermal stability than those without surface reconstructions, and that silicon nanowires with perpendicular dimmer rows on the two (100) surfaces have somewhat higher thermal stability than nanowires with parallel dimmer rows on the two (100) surfaces. Furthermore, the melting temperature of silicon nanowires increases as their diameter increases and reaches a saturation value close to the melting temperature of bulk silicon. The value of the Lindemann index for melting silicon nanowires is 0.037.     

Y掺杂Si纳米线的光致发光特性

以n型单晶Si(111)为衬底,利用Au作为催化剂,在温度、N₂流量和生长时间分别为1 100 ℃,1.5 L·min-1和60 min的条件下,基于固-液-固生长机制,生长了直径为60～80 nm、长度为数十微米的高密度Si纳米线。随后,以Y₂O₃粉末为掺杂源,采用高温扩散方法对Si纳米线进行了钇(Y)掺杂。利用扫描电子显微镜、X射线衍射仪和荧光分光光度计对不同掺杂温度(900～1 200 ℃)、掺杂时间(15～60 min)和N₂流量(0～400 sccm)等工艺条件下制备的Y掺杂Si纳米线的形貌、成分、结晶取向以及激发光谱和发射光谱特性进行了详细的测量和表征。结果表明,在掺杂温度为1 100 ℃,N₂流量为200 sccm、掺杂时间为30 min和激发波长为214 nm时,Y掺杂Si纳米线样品表现出较好的发光特性。样品分别在470～500和560～600 nm范围内出现了两条发光谱带。560～600 nm的发光带由两个发光峰组成,峰位分别为573.6和583.8 nm,通过结构分析可以推测,这两个发光峰是由Y3+在Si纳米线的带隙中引入的杂质能级引起的。而470～500 nm较宽的发光带同样来源于Y离子在Si纳米线带隙中引入的与非晶SiO_x壳层中氧空位能级十分接近的杂质能级。     

Thin film silicon nanowire photovoltaic devices produced with gold and tin catalysts

Silicon nanowires produced using pulsed plasma-enhanced chemical vapor deposition have been used as part of a thin film photovoltaic device. Nanowires of differing morphologies were produced by using both gold and tin thin films as a catalyst for growth. A prototype silicon nanowire-based thin-film photovoltaic device was produced by using doped silicon nanowires as the p-layer. Amorphous silicon was used as the intrinsic and n-layers of the device. The nanowires used in the photovoltaic devices had an average diameter of 420 nm after the deposition and coating of amorphous silicon intrinsic and n-layers. The nanowires were deposited in bulk as films of 3 to 42 μm in thickness. The resulting device, although of low efficiency, had a demonstrable photocurrent. Tin-catalyzed nanowires were found to produce a thin-film device with a measurable photocurrent whereas gold-catalyzed silicon nanowires did not. This was attributed to the length of the nanowires and thickness of the p-layer produced when using gold-catalyzed nanowires.     

Oxide reduced silicon nanowires

Core crystalline silicon nanowires with a heavily reduced amorphous shell have been successfully synthesised using palladium as a metal catalyst. We present two approaches to reduce the oxidation of the nanowires during the thermal annealing growth. The ratios of the amorphous shell to crystalline core of the nanowires produced, from the two methods, are compared and show a remarkable drop (hence thinner oxide) compared to wires fabricated using currently available techniques. In addition, a focused ion beam was utilised to contact the oxide-reduced nanowires for transport measurements, without first removing the thin oxide shell. The oxygen-reduced core-shell silicon nanowires showed a very low electrical resistivity (4 × 10⁻¹ Ω cm). Our novel approach presents a new alternative to the production of low cost, high yield, highly conducting silicon nanowires offering a wide range of opportunities for semiconductor based technology.     

A novel morphology of SiOx nanowires with a modified diameter

A novel morphology of SiO_x (x=1–2) nanowires was fabricated via a thermal evaporation method by heating pure silicon powder at 1373 K followed by depositing silicon vapor on a quartz-glass substrate which was coated with a catalyst precursor from a 0.005 M Fe(NO₃)₃ aqueous solution. At the deposition temperature, the catalyst particles aggregated and formed quasi-plates, from which a short Si rod grew outward on one side and a great deal of SiO_x nanowires projected outward on the other side, forming comet-like objects with long tails. The diameters of the nanowires gradually decreased with an increase in the distance from the catalyst plate. Received: 9 April 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Synthesis of silicon nanowires and novel nano-dendrite structures

We report a study on the effects of various parameters on the synthesis of silicon nanowires (5--50 nm in diameter) by pulsed laser ablation. A novel silicon nanodendrite structure is observed by changing some of the growth parameters abruptly. This growth mechanism is explained by a qualitative model. These nanodendrites show a promise of being used as a template in fabricating nanocircuits. Thermal quantum confinement effects were also observed on the silicon nanowires and have been reported.     

Synthesis of thin silicon nanowires using gold-catalyzed chemical vapor deposition

Silicon nanowires were synthesized using chemical vapor deposition catalyzed by gold nanoparticles deposited on silicon substrates. Silicon nanowires grew epitaxially in 111 directions on (100)-oriented silicon substrates. For a particular set of process parameters, we observed a critical thickness of the nucleating gold film, below which nanowires could not be grown. We studied the dependence of the Au-Si alloy droplet size and size distribution on the starting gold film thickness and the annealing conditions. Increasing the Cl:Si ratio in the gas phase allowed nanowires to grow on smaller Au-Si alloy droplets. We used a modified heating sequence that deconvoluted the effect of silicon substrate consumption and gas-phase silicon supply on the Au-Si alloy formation and allowed growth of nanowires with diameters less than 20 nm. The modified heating sequence was also used to demonstrate the growth of bridging silicon nanowires with diameters less than 20 nm, which is a significant step in producing electronic devices. PACS 81.07.b; 81.15.Gh     

Ni掺杂浓度对硅纳米线光电性质的影响

利用基于密度泛函理论的第一性原理,对不同直径和浓度Ni掺杂硅纳米线的形成能、能带结构、态密度和光学性质进行了计算,结果表明：杂质Ni的形成能随硅纳米线直径的减小和掺杂浓度的降低而下降,这说明直径越大的硅纳米线掺杂越困难,杂质浓度越高的硅纳米线越不稳定. Ni掺杂在费米能级附近及带隙中引入杂质能级,其主要来自Ni的3d轨道,杂质能级扩展成杂质带,改变Ni的掺杂浓度可改变硅纳米线的带隙,改善其导电性. 另外,还发现掺杂浓度明显改变了硅纳米线的吸收强度和宽度.     

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.     

硅纳米线拉曼光谱的研究

声子限制效应会引起本征硅纳米线拉曼光谱红移及不对称宽化,但研究发现其并非引起硅纳米线拉曼光谱改变的主要因素。研究表明,由于在拉曼光谱测量中,通常使用的入射激光功率都在5 mW以上,激光加热会导致很高的局部温度,从而引起拉曼光谱大幅度红移并对称宽化,这是硅纳米线拉曼光谱红移的主要影响因素。另外,激光功率很高时,由激光激发的载流子会与声子发生Fano型干涉,从而使硅纳米线拉曼光谱发生Fano型红移和不对称宽化。除此之外,对小直径本征硅纳米线,声子限制效应导致波矢选择定弛则弛豫,使不在布里渊区中心的声子也可以参与拉曼散射,因而其拉曼光谱中除常见的几个拉曼峰外还会出现新拉曼峰。     

碳纳米管包裹的硅纳米线复合结构的热稳定性研究

采用经典分子动力学方法模拟一定直径[111]晶向的硅纳米线填充不同扶手椅型单壁碳纳米管复合结构的加热过程, 通过可视化和能量分析的方法判断复合结构中硅纳米线和碳纳米管的热稳定性. 通过讨论碳纳米管的空间限制作用和分子间相互作用力的关系, 对碳纳米管和硅纳米线的热稳定性变化进行初步解释. 研究发现碳纳米管中硅纳米线的热稳定性和碳纳米管的直径关系密切: 当管径较小时, 硅纳米线的热稳定性有所提高, 当管径增大到一定大小时, 硅纳米线的热稳定性会突然显著地下降, 直到硅纳米线与管壁不存在分子间相互作用力, 硅纳米线的热稳定性才会恢复. 而硅纳米线填充到碳纳米管中对碳纳米管的热稳定性有着明显的降低作用.     

From Si nanowires to porous silicon: the role of excitonic effects

We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modeling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.     

Phonon heat transport in silicon nanowires

Thermal conductivity of silicon and porous silicon nanowires based on the equation of phonon radiative transport is theoretically evaluated. The thermal conductivities of silicon nanowires with square cross-sections are found to match molecular dynamics simulation results reasonably well. It is shown that the results of meso-porous silicon nanowires are about two orders of magnitude lower than that of silicon nanowires in a wide range of temperature (50 K-300 K). Received 24 April 2001 and Received in final form 23 December 2001     

Synthesis of 6H-SiC single-crystal nanowires in a flow of carbon-silicon high-frequency arc plasma

Silicon carbide 6H-SiC nanoparticles and nanowires were obtained in carbon-silicon high-frequency arc plasma plasma in a helium atmosphere at a pressure of 0.1–0.6 MPa. It was shown that 6H-SiC nanowires grow from the arc plasma, as well as from the vapor, according to the known mechanism of vapor-solid condensation on a cold surface covered with single-crystal silicon carbide nuclei. The content of silicon carbide nanowires in the condensate reached 60 wt %. The obtained single-crystal silicon 6H-SiC nanowires had the diameter of 15–18 nm and length of 200–600 nm.     

硅纳米线的PEOVD生长研究

本文以硅烷（SiH4）为反应气体,利用等离子体化学气相沉积（PECVD）方法在硅（100）衬底上生长硅纳米晶体、纳米线。应用扫描电镜观察不同条件下生长的样品表面,发现衬底条件对硅纳米结构的影响十分显著。在温度、压强等其它条件相同的情况下,对硅衬底应用Fe^3＋催化剂处理后,呈纳米线状结构生长,而无Fe催化剂涂覆情况下,基本呈纳米晶体状生长,说明催化剂对si纳米线的生成起了重要的促进生长作用。通过进一步研究硅纳米晶体、纳米线的等离子增强化学气相生长机理,发现它们以气-液-固（VLS）机制生长。     

X射线光电子能谱法研究硅纳米线的能带结构

制备了氧辅助热分解法,以一氧化硅为原料,以氩气为载气,维持管内压强为1000Pa,在高温炉中于1250℃下反应5 h后得到硅纳米线。硅纳米线经5%氢氟酸水溶液处理5 min后,与1×10-3 mol·L-1的氯化金溶液中反应5 min,在硅纳米线的表面上修饰了金纳米粒子,用X射线粉末衍射表征了产物的结构,同时观察到单质硅和金的XRD图谱;用电子扫描和透射显微镜观察了产物的形貌,表明氧辅助方法可制得大量均匀的硅纳米线,修饰在硅纳米线上的金纳米点形状整齐,尺寸均匀,平均直径约8 nm;并用X射线光电子能谱分析了修饰过程中能带结构的变化。结果表明,金纳米粒子表面带负电,它在施主能级和受主能级上都有电子存在;由于氧杂质的存在,硅纳米线的费米能级移向价带顶。     

Synthesis and room-temperature NO_2 gas sensing properties of a WO_3 nanowires/porous silicon hybrid structure

We report on the fabrication and performance of a room-temperature NO2 gas sensor based on a WO3 nanowires/porous silicon hybrid structure. The W18O49 nanowires are synthesized directly from a sputtered tungsten film on a porous silicon （PS） layer under heating in an argon atmosphere. After a carefully controlled annealing treatment, WO3 nanowires are obtained on the PS layer without losing the morphology. The morphology, phase structure, and crystallinity of the nanowires are investigated by using field emission scanning electron microscopy （FESEM）, X-ray diffractometer （XRD）, and high-resolution transmission electron microscopy （HRTEM）. Comparative gas sensing results indicate that the sensor based on the WO3 nanowires exhibits a much higher sensitivity than that based on the PS and pure WO3 nanowires in detecting NO2 gas at room temperature. The mechanism of the WO3 nanowires/PS hybrid structure in the NO2 sensing is explained in detail.     

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.     

Control of ZnO nanowire arrays by nanosphere lithography (NSL) on laser-produced ZnO substrates

Nanosphere lithography (NSL) is a successful technique for fabricating highly ordered arrays of ZnO nanowires typically on sapphire and GaN substrates. In this work, we investigate the use of thin ZnO films deposited on Si by pulsed laser deposition (PLD) as the substrate. This has a number of advantages over the alternatives above, including cost and potential scalability of production and it removes any issue of inadvertent n-type doping of nanowires by diffusion from the substrate. We demonstrate ordered arrays of ZnO nanowires, on ZnO-coated substrates by PLD, using a conventional NSL technique with gold as the catalyst. The nanowires were produced by vapor phase transport (VPT) growth in a tube furnace system and grew only on the areas pre-patterned by Au. We have also investigated the growth of ZnO nanowires using ZnO catalyst points deposited by PLD through an NSL mask on a bare silicon substrate.