Catalyst-free growth of amorphous silicon nanowires by laser ablation

Carbon nanotubes (CNTs) appear to be ideal tip materials of atomic force microscopy (AFM) due to their small diameter and high stiffness. In this study, double-walled carbon nanotube (DWCNT) structures with different lengths of inner and outer layers are proposed as AFM tips. Both the vibration response and mode shapes of the tipped nanotubes under axial compression are studied by a theoretical nanobeam model. The results show that the natural frequencies of DWCNTs are significantly affected by the compressive loads and the length difference between the inner and outer nanotubes. The natural frequency associated with certain vibrational modes decreases with increasing compressive loads. This research may provide a useful reference for practical design for AFM tips with CNTs.     

Design of a nanoscale silicon laser

The recent observation of optical gain from silicon nanocrystals embedded in SiO₂ opens an opportunity to develop a nanoscale silicon-based laser. However, the challenge remains to design and develop a laser architecture using CMOS-compatible materials. In this paper we present two designs for a waveguide laser in which silicon nanocrystals embedded in SiO₂ are used as the optical gain media. One design employs a SiO₂ membrane containing encapsulated Si nanocrystals. Preliminary calculations given here show that a highly resonant laser cavity can be produced in a SiO₂ membrane using sub-wavelength structures. This photonic crystal architecture, used to guide and contain the light, can be combined with a gain medium of optically active Si nanocrystals synthesized in the SiO₂ membrane using ion implantation/thermal annealing to produce a Si-based laser. The laser cavity dimensions can be matched to the near-infrared wavelengths where optical gain has been observed from Si nanocrystals. The second design utilizes silicon nanocrystals embedded in a distributed-feedback laser cavity fabricated in SiO₂. Lasing action over a broad wavelength range centered at ∼770 nm should be possible in both of these configurations. Received: 20 December 2002 / Accepted: 7 January 2003 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-434/982-2037, E-mail: supriya@virginia.edu     

Oxide-assisted growth of silicon nanowires by carbothermal evaporation

Silicon nanowires (SiNWs) have successfully been synthesized by carbothermal evaporation method. By ramping-up the furnace system at 20 °C min⁻¹ to 1100 °C for 6 h, the vertically aligned coexist with crooked SiNWs were achieved on the silicon substrate located at 12 cm from source material. The processing parameters such as temperature, heating rate, duration, substrate position and location are very important to produce SiNWs. Morphology and chemical composition of deposited products were investigated by field-emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray analysis (EDX). The existence of small sphere silicon oxide capped nanowires suggested that the formation of SiNWs was governed by oxide-assisted growth (OAG) mechanism.     

Impurity doping in silicon nanowires synthesized by laser ablation

Boron (B) or phosphorus (P) doped silicon nanowires (SiNWs) were synthesized by laser ablation. Local vibrational modes of B were observed in B-doped SiNWs by micro-Raman scattering measurements at room temperature. Fano broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the Si optical phonon peak for B-doped SiNWs. An electron spin resonance signal due to conduction electrons was observed only for P-doped SiNWs. These results prove that B and P atoms were doped in substitutional sites of the crystalline Si core of SiNWs during laser ablation and electrically activated in the sites.     

Generation of single-crystalline domain in nano-scale silicon pillars by near-field short pulsed laser

We observe laser-induced grain morphology change in silicon nanopillars under a transmission electron microscopy (TEM) environment. We couple the TEM with a near-field scanning optical microscopy pulsed laser processing system. This novel combination enables immediate scrutiny on the grain morphologies that the pulsed laser irradiation produces. We find unusual transformation of the tip of the amorphous or polycrystalline silicon pillar into a single crystalline domain via melt-mediated crystallization. On the basis of the three-dimensional finite difference simulation result and the dark field TEM data, we propose that the creation of the distinct single crystalline tip originates from the dominant grain growth initiated at the apex of the non-planar liquid–solid interface. Our microscopic observation provides a fundamental basis for laser-induced conversion of amorphous nanostructures into coarse-grained crystals.     

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.     

Nanostructuring of silicon surface by femtosecond laser pulse mediated with enhanced near-field of gold nanoparticles

High-energy synchrotron X-ray diffraction and imaging experiments were performed at the Advanced Photon Source on two ancient Chinese bronzes from the Art Institute of Chicago with the goal to nondestructively study their microstructure. The first object, a bronze fragment from an early Western Zhou dynasty vessel (Hu, 11th/10th century B.C.), was investigated with spatially-resolved diffraction to reveal the depth and composition of the surface corrosion layer as well as the composition and grain size of the underlying bronze core. The second object, a bronze dagger-axe (Ge, 3rd/2nd century B.C.) with a silver-inlaid sheath, was studied under both diffraction and imaging conditions. It was found to have been cast as a single object, answering longstanding scholars’ questions on whether the ceremonial object concealed an interior blade. PACS 81.00; 01.75.+m     

MBE growth of ZnSe nanowires on oxidized silicon substrate

We report the growth of single crystalline ZnSe nanowires on oxidized Si(100) substrates by molecular-beam epitaxy using Au nano-particles as the catalysts. It was found that average length decreased while the average diameter increased as we increased the temperature from 230 to 320 ^°C. It was also found that crystal quality of the ZnSe nanowires prepared at 320 ^°C was poorer than the ZnSe nanowires prepared at 230 ^°C and 280 ^°C.     

Modulation of millimeter-waves using diode laser illumination of a silicon waveguide

A quantitative study of the phase and/or amplitude modulation of millimeter-waves by a diode laser is reported. Using millimeter-waves as a probe, the transport properties of an optically induced electron-hole plasma in a bulk silicon waveguide may be measured. A theoretical model has been developed to predict the temporal behavior of the plasma in the case of low level optical injection. This model agrees well with the measured data.     

衬底对化学气相沉积法制备氧化硅纳米线的影响

通过化学气相沉积法在不同衬底上制备了大量的氧化硅纳米线.选用衬底为Si片、带有约100nm厚SiO2氧化层Si片和石英片.利用场发射扫描电子显微镜(SEM)和透射电镜(TEM，配备有能谱仪)对样品的表面形貌、结构和成分进行研究.结果表明：这些纳米线都为非晶态，但在不同衬底上生长的纳米线形貌、尺寸和化学成分不同.讨论了各种衬底对不同特征氧化硅纳米线生长的影响. 关键词： 化学气相沉积 纳米线 纳米颗粒     

Silicon micro-hemispheres with periodic nanoscale rings produced by the laser ablation of single crystalline silicon

We describe the fabrication of silicon micro-hemispheres by adopting the conventional laser ablation of single crystalline silicon in the vacuum condition without using any catalysts or additives. The highly oriented structures of silicon micro-hemispheres exhibit many periodic nanoscale rings along their outer surfaces. We consider that the self-organized growth of silicon micro-structures is highly dependent on the laser intensity and background air medium. The difference between these surface modifications is attributed to the amount of laser energy deposited in the silicon material and the consequent cooling velocity.     

A model for growth directional features in silicon nanowires

Long silicon nanowires (SiNWs) grown by laser ablation or by thermal evaporation of monoxide source materials are primarily oriented in the <112> direction, and some in the <110> direction, but rarely in the <100> or <111> directions. We propose a model to explain these SiNW growth directional features. The model consists of two parts. Part one is concerned with mechanism-based criteria and part two with applying these criteria to explain the experimental results. Four criteria are considered: (i) the stability of a Si atom occupying a surface site; (ii) the Si {111} surface stability in the presence of oxygen; (iii) the stepped Si {111} surface layer lateral growth process; and (iv) the effect of dislocations in providing perpetuating {111} steps to facilitate SiNW growth. Analyses of SiNW growth in accordance with these criteria showed that <112> and <110> are the preferred SiNW growth directions, and that <111> and <100> are not. Received: 12 November 2001 / Accepted: 20 November 2001 / Published online: 23 January 2002     

Controlling growth density and patterning of single crystalline silicon nanowires

This study examines the usage of well-patterned Au nanoparticles (NPs) as a catalyst for one-dimensional growth of single crystalline Si nanowires (NWs) through the vapor-liquid-solid (VLS) mechanism. The study reports the fabrication of monolayer Au NPs through the self-assembly of Au NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modified silicon substrate. Results indicate that the spin coating time of Au NPs plays a crucial role in determining the density of Au NPs on the surface of the silicon substrate and the later catalysis growth of Si NWs. The experiments in this study employed optical lithography to pattern Au NPs, treating them as a catalyst for Si NW growth. The patterned Si NW structures easily produced and controlled Si NW density. This approach may be useful for further studies on single crystalline Si NW-based nanodevices and their properties.     

Fabrication of silicon nanowires

2 at 750 °C and 850 °C. The oxide and interface morphology are characterized by cross-sectional scanning electron microscope images. It is found that the oxidized nanowire following oxidation at 750 °C still keeps its pentagon shape even if it has been oxidized for 19 h. However, the oxidized samples at 850 °C become circular in shape. The oxidation-temperature dependence of the sample shapes is discussed. Our results should be useful in generating silicon nanowires coated with SiO₂ in microelectronic technology with careful selection of the SiO₂ growth temperatures. Received: 26 September 1997/Accepted: 8 December 1997     

Scanning probe oxidation of Si3N4 masks for nanoscale lithography,micromachining, and selective epitaxial growth on silicon

For studying the physical, chemical, and electronic properties of ultrasmall man-made structures, the major challenge is to fabricate highly uniform structures and control their positions on the nanometer length scale. Local oxidation of metals and semiconductors using a conductive-probe atomic force microscope (AFM) or other scanning probe microscopes in air at room temperature has emerged as a simple and universal method for this purpose. Here the uses of scanning probe oxidation of Si₃N₄ masks for performing nanolithography, nanomachining, and nanoscale epitaxial growth on silicon are reviewed. The three most unique features of this approach are presented: (1) exceptionally fast oxidation kinetics using silicon nitride masks (∼30 μm/s at 10 V for a ∼5-nm-thick film); (2) selective-area anisotropic etching of Si using a Si₃N₄ etch mask; and (3) selective-area chemical vapor deposition of Si using a SiO₂/Si₃N₄ bilayer growth mask.     

Designing arbitrary nanoscale patterns by a nanocavity waveguide with omnidirectional illumination

We report one- and two-dimensional arbitrary patterns which are achieved by nanocavity waveguides made of a quasi-metal?Cdielectric-metal heterostructure with omnidirectional illumination. This proposed heterostructure supports the surface plasmon polaritons whose phase and group velocities have opposite sign for given frequency. Negative refraction and reflection in the waveguide result in imaging nanolithography and the omnidirectional property can be well understood by the dispersive relation of the nanocavity waveguide. Numerical results demonstrate that such an omnidirectional nanolithography scheme is feasible for arbitrary 1D gratings and 2D linearly chirped gratings with TM and circular polarized incidence, respectively, at 365?nm.     

基于近场均匀照明的LED阵列的优化设计

基于几何光学与辐射照度理论,对菱形、环形和蜂窝状等3种典型LED阵列光源在近场上的照度分布进行研究,推导了不同阵列光源照射到目标面上的总辐射照度表达式,并依据斯派罗法则确定了LED间的最优化距离。进而根据照度公式,对LED阵列进行了仿真和对比分析,得出了不同阵列的光照度分布特点。菱形阵列可以得到较大范围的平坦度,环形阵列的平坦范围较小,能量集中分布在一个圆形范围内,有良好的集光效果,蜂窝状阵列的照度比较集中且占用的面板空间较小,可在一定程度上降低设计成本。     

基于近场均匀照明的LED阵列的优化设计

基于几何光学与辐射照度理论,对菱形、环形和蜂窝状等3种典型LED阵列光源在近场上的照度分布进行研究,推导了不同阵列光源照射到目标面上的总辐射照度表达式,并依据斯派罗法则确定了LED间的最优化距离。进而根据照度公式,对LED阵列进行了仿真和对比分析,得出了不同阵列的光照度分布特点。菱形阵列可以得到较大范围的平坦度,环形阵列的平坦范围较小,能量集中分布在一个圆形范围内,有良好的集光效果,蜂窝状阵列的照度比较集中且占用的面板空间较小,可在一定程度上降低设计成本。