Effects of gas pressure on the synthesis and photoluminescence properties of Si nanowires in VHF-PECVD method

Silicon nanowires (SiNWs) were grown on an Au-coated Si(111) substrate at various gas pressures by very high frequency plasma enhanced chemical vapor deposition via the vapor–liquid–solid mechanism. The synthesized SiNWs were characterized by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy and photoluminescence (PL). The SiNWs were sharp needle-shaped and possessed highly crystalline core and oxide amorphous shell. As the gas pressure increases from 70 mtorr to 85 mtorr, the average diameter of the SiNWs decreases from 250 nm to 70 nm. Furthermore, the density of the nanowires increases with the gas pressure. The PL spectra revealed a peak at about 400 nm and a broadband emission at about 700 nm.     

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.     

一种新方法制备硅/聚(3, 4-乙撑二氧噻吩)核/壳纳米线阵列杂化太阳能电池

采用气相聚合法制备了有机/无机杂化的硅/聚3, 4-乙撑二氧噻吩核/壳纳米线阵列(SiNWs/PEDOT)太阳能电池. 相对平面结构Si/PEDOT太阳能电池, SiNWs/PEDOT太阳能电池的能量转换效率提升了7倍, 达到3.23%.对比分析反射光谱、I-V曲线及外量子效率的实验结果, 发现SiNWs/PEDOT太阳能电池性能改进的主要原因可归结为: 气相聚合法能够有效地制备出SiNWs/PEDOT电池的核/壳纳米线阵列结构, 使得器件具有高光捕获、高比结面积和高电荷收集效率. 关键词： Si/PEDOT核/壳纳米线结构 太阳能电池 气相聚合     

Characterization of silicon nanowires grown on silicon, stainless steel and indium tin oxide substrates

Silicon nanowires (SiNWs) have been grown on crystalline silicon (Si), indium tin oxide (ITO) and stainless steel (SS) substrates using a gold catalyst coating with a thickness of 200 nm via pulsed plasma-enhanced chemical vapor deposition (PPECVD). Their morphological, mineralogical and surface characteristics have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman analysis. SiNWs growth is accompanied by oxidation, thus yielding partially (SiO _x ) and fully oxidized (SiO₂) Si sheaths. The mean diameters of these SiNWs range from 140 to 185 nm. Si with (111) and (220) planes exists in SiNWs grown on all three substrates while Si with a (311) plane is detected only for Si and ITO substrates. Computational simulation using density functional theory (DFT) has also been conducted to supplement the experimental Raman analyses for crystalline Si and SiO₂. XPS results reveal that ca. 30 % of the SiNWs have been oxidized for all substrates. The results presented in this paper can be used to aid selection of appropriate substrates for SiNW growth, depending on specific applications.     

Real-time measurement of stress and damage evolution during initial lithiation of crystalline silicon

Crystalline to amorphous phase transformation during initial lithiation in (100) Si wafers is studied in an electrochemical cell with Li metal as the counter and reference electrode. During initial lithiation, a moving phase boundary advances into the wafer starting from the surface facing the lithium electrode, transforming crystalline Si into amorphous Li(x)Si. The resulting biaxial compressive stress in the amorphous layer is measured in situ, and it was observed to be ca. 0.5 GPa. High-resolution TEM images reveal a very sharp crystalline-amorphous phase boundary, with a thickness of ～1 nm. Upon delithiation, the stress rapidly reverses and becomes tensile, and the amorphous layer begins to deform plastically at around 0.5 GPa. With continued delithiation, the yield stress increases in magnitude, culminating in a sudden fracture of the amorphous layer into microfragments, and the cracks extend into the underlying crystalline Si.     

Gibbs free energy and equilibrium states in the Si/Si oxide systems

In this paper, the equilibrium states in the Si/Si oxide systems formed as a result of the phase separation of nonstoichiometric silicon oxide films are studied. The expressions for the Gibbs free energy of Si oxide and Si/Si oxide systems are derived thermodynamically. The transformations of the Gibbs free energy in the amorphous Si/Si oxide and the crystalline Si/Si oxide systems with the change in the amount of separated silicon and the composition of the silicon oxide phase are analyzed. By minimizing the Gibbs free energy of these systems, the equilibrium stoichiometry indices of silicon oxide are calculated as functions of its initial stoichiometry and the temperature. The solubility limits of Si in SiO(2) in equilibrium with amorphous and crystalline Si are determined. The obtained results form the basis for the development of a complete thermodynamic theory of phase separation in nonstoichiometric silicon oxide films with the formation of Si nanoinclusions in the silicon oxide matrix.     

Structural and photoluminescence investigation on the hot-wire assisted plasma enhanced chemical vapor deposition growth silicon nanowires

High density of silicon nanowires (SiNWs) were synthesized by a hot-wire assisted plasma enhanced chemical vapor deposition technique. The structural and optical properties of the as-grown SiNWs prepared at different rf power of 40 and 80 W were analyzed in this study. The SiNWs prepared at rf power of 40 W exhibited highly crystalline structure with a high crystal volume fraction, X_C of ～82% and are surrounded by a thin layer of SiO_x. The NWs show high absorption in the high energy region (E>1.8 eV) and strong photoluminescence at 1.73 to 2.05 eV (red–orange region) with a weak shoulder at 1.65 to 1.73 eV (near IR region). An increase in rf power to 80 W reduced the X_C to ～65% and led to the formation of nanocrystalline Si structures with a crystallite size of <4 nm within the SiNWs. These NWs are covered by a mixture of uncatalyzed amorphous Si layer. The SiNWs prepared at 80 W exhibited a high optical absorption ability above 99% in the broadband range between 220 and ～1500 nm and red emission between 1.65 and 1.95 eV. The interesting light absorption and photoluminescence properties from both SiNWs are discussed in the text.     

ON THE STUDY OF SILICON NANO-WIRES SELF-ASSEMBLED AS PARTICLES

Two different types of Silicon nano-wires (SiNWs) have been observed by scanning and transmission electron microscopy. One are of free standing SiNWs deposited uniformly on the surface of silicon substrates, and the other are self-assembled into special shaped particles. These SiNWs were synthesized by thermal evaporation of SiO amorphous powders without any metal catalysts in the temperature range of 900-1250℃. Growth history reveals that the self-assembled SiNWs are formed by original nucleation from the surface of amorphous SiO_{x particle matrices through phase separation and silicon precipitation followed by further growth through oxide-assisted vapor-solid reactions. The above results provide a solid experimental support for the oxide-assisted growth model of SiNWs.}     

Coherent anti-Stokes Raman scattering in silicon nanowire ensembles

In this letter, we, for the first time, report on coherent anti-Stokes Raman scattering (CARS) spectroscopy of an ensemble of silicon nanowires (SiNWs) formed by wet chemical etching of crystalline silicon with a mask of silver nanoparticles. The fabricated SiNWs have diameter ranged from 30 to 200 nm and demonstrate both visible and infrared photolumine cence (PL) and spontaneous Raman signal, with their intensities depending on presence of silver nanoparticles in SiNWs. The efficiency of CARS in SiNW ensembles is found to be significantly higher than that in crystalline silicon. The results of CARS and PL measurements are explained in terms of resonant excitation of the electron states attributed to silicon nanoparticles.     

Dynamic nuclear polarization in silicon microparticles

We report record high 29Si spin polarization obtained using dynamic nuclear polarization in microcrystalline silicon powder. Unpaired electrons in this silicon powder are due to dangling bonds in the amorphous region of this intrinsically heterogeneous sample. 29Si nuclei in the amorphous region become polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. Hyperpolarized silicon microparticles have long T1 relaxation times and could be used as tracers for magnetic resonance imaging.     

Silicon nano-wires fabricated by thermal evaporation of silicon wafer

Thin silicon nano-wires (SiNWs) with a diameter of 10–20 nm were fabricated by a simple thermal evaporation of silicon wafer at 1523 K. The gold produced by an electrochemical method was covered on the wafer surface as catalyst. It was found that the SiNWs are amorphous and its Raman peak shifted down maybe due to the effect of laser heating and quantum confinement. Finally, a temperature gradient growth model is suggested to explain the growth direction of SiNWs.     

Au5Si2/Si Heterojunction Nanowires Formed by Combining SiO Evaporation with Vapour--Liquid--Solid Mechanism

Crystalline AusSi2/Si heterojunetion nanowires （AusSi2/SiNWs） are obtained by thermal evaporating SiO pow- ders on thick gold-coated silicon substrates in a low vacuum system. Structure analysis of the produced AusSh/Si heterojunetions is performed by employing a transmission electron microscope （TEM） and a selected area electric diffraetometer. The chemical compositions axe studied by a energy-dispersive x-ray spectroscope attached to the TEM. A two-step growth model is proposed to describe the formation of the AusSi2/SiNWs. During the first step, crystalline SiNWs are formed via a growth mechanism combining the oxide-assisted growth process with the vapour-liquid-solid model at relatively high temperature. In the second step, the temperature decreases and one segment of the preformed SiNWs reacts with the remnant Au to form single crystalline AusSi2 nanowires by a solid-liquid-solid process. The present work should be useful for the future synthesis and research of high-quality gold silicide nanowires and microelectronic devices based on the nanowires.     

Raman spectroscopy for study of interplay between phonon confinement and Fano effect in silicon nanowires

A combined effect of doping (type and species) and size on Raman scattering from silicon (Si) nanowires (NWs) has been presented here to study interplay between quantum confinement and Fano effects. The SiNWs prepared from low doping Si wafers show only confinement effect, as evident from the asymmetry in the Raman line‐shape, irrespective of the doping type. On the other hand SiNWs prepared from wafer with high doping shows the presence of electron–phonon interaction in addition to the phonon confinement effect as revealed from the presence of asymmetry and antiresonence in the corresponding Raman spectra. This combined effect induces an extra asymmetry in the lower energy side of Raman peak for n‐type SiNWs whereas the asymmetry flips from lower energy side to the higher energy side of the Raman peak in p‐type SiNWs. Such an interplay can be represented by considering a general Fano‐Raman line‐shape equation to take care of the combined effect in SiNWs. Copyright © 2015 John Wiley & Sons, Ltd.     

An analysis on synthesizing large scales of one-dimensional silicon nano-structures by simple evaporation of sulfur-contained powders

The growth mechanism for synthesizing large scales of one-dimensional silicon nano-structures (silicon nano-wires (SiNWs) or silicon oxide nano-wires (SiO₂-NWs)) by a simple evaporation of sulfur-contained powders on silicon wafer is discussed. A novel sulfide-assisted mechanism referring to oxygen-assisted mechanism is proposed. Amongst this simple method, sulfide or pure sulfur can both assist the formation of SiNWs. The growth is fast and some SiNWs are easily oxidized to be amorphous structure of SiO₂-NWs under the low-vacuum system. The simple method suggests a useful route to achieve plenty of one-dimensional silicon nano-structures for further research.     

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.     

Silicon nano-wires fabricated by a novel thermal evaporation of zinc sulfide

Silicon nano-wires (SiNWs) with diameter of 30 nm and length of tens of micrometers on silicon wafers were synthesized by a novel thermal evaporation of zinc sulfide. After thermal evaporation at 1080°C for 1 h, crystalline SiNWs were produced. It was found that the tip of SiNWs contained sulfur, while the other places of SiNWs did not. It is considered that the decomposition of SiS resulted in the formation of SiNWs. On the basis of the facts, a sulfide-assisted growth model of SiNWs was suggested.     

The effect of differently sized Ag catalysts on the fabrication of a silicon nanowire array using Ag-assisted electroless etching

A simple and low cost method to generate single-crystalline, well-aligned silicon nanowires (SiNWs) of large area, using Ag-assisted electroless etching, is presented and the effect of differently sized Ag catalysts on the fabrication of SiNWs arrays is investigated. The experimental results show that the size of the Ag catalysts can be controlled by adjusting the pre-deposition time in the AgNO₃/HF solution. The optimum pre-deposition time for the fabrication of a SiNWs array is 3 min (about 162.04 ± 38.53 nm Ag catalyst size). Ag catalysts with smaller sizes were formed in a shorter pre-deposition time (0.5 min), which induced the formation of silicon holes. In contrast, a large amount of Ag dendrites were formed on the silicon substrate, after a longer pre-deposition time (4 min). The existence of these Ag dendrites is disadvantageous to the fabrication of SiNWs. Therefore, a proper pre-deposition time for the Ag catalyst is beneficial to the formation of SiNWs.SiNWs were synthesized in the H₂O₂/HF solution system for different periods of time, using Ag-assisted electroless etching (pre-deposition of the Ag catalyst for 3 min). The length of the SiNWs increases linearly with immersion time. From TEM, SAED and HRTEM analysis, the axial orientation of the SiNWs is identified to be along the [001] direction, which is the same as that of the initial Si wafer. The use of HF may induce Si–H_x bonds onto the SiNW array surface. Overall, the Ag-assisted electroless etching technique has advantages, such as low temperature, operation without the need for high energy and the lack of a need for catalysts or dopants.     

Formation of Si nanowires by the electrochemical reduction of porous Ni/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) were prepared by the electrochemical reduction of solid Ni/SiO₂ blocks in molten CaCl₂ at 1173 K. The SiNWs have diameter distributions ranging from 80 to 350 nm, and the nickel–silicon droplets are found on the tips of the nanowires. The growth mechanism of SiNWs was investigated, which confirmed that the nano-sized nickel–silicon droplets formed at the Ni/SiO₂/CaCl₂ three-phase interline. The droplets lead to the oriented growth of SiNWs. Formation of nano-sized nickel–silicon droplets suggests that this method could be a potential way to produce nano-sized metal silicides.     

Plasma-Chemical Deposition of Anti-Reflection and Protective Coating for Infrared Optics

Russian Physics Journal - The films of amorphous hydrogenated carbon doped with Si and O were deposited onto the sample of crystalline silicon by method of plasma-chemical deposition in the mix of...     

Generating Metallic Amorphous Core–Shell Nanoparticles by a Solid‐State Amorphization Process

Metallic crystalline/amorphous core–shell nanoparticles consisting of a crystalline Pd core (c‐Pd) surrounded by an amorphous Fe₂₅Sc₇₅ shell (a‐FeSc) are prepared by inert‐gas condensation. A phase transformation of the c‐Pd by a solid‐state diffusion process resulting in an amorphous core (a‐PdSc) surrounded by an amorphous FeSc shell is observed if the core–shell structure is irradiated at ambient temperature with 300 keV electrons. The amorphization process seems to involve the diffusion of irradiation‐induced defects and is presumably driven by the large negative heat of mixing of Pd and Sc, as well as by the excess enthalpy of the interfaces between the c‐Pd regions and the surrounding a‐FeSc. The structural transformation reported here opens a new way to producing metallic amorphous core–shell nanoparticles of different chemical compositions and probably novel properties.