The oxidation characteristics of silicon nanowires grown with an au catalyst

In this study, we investigated the thermal oxidation of silicon nanowires (SiNWs) grown via the vapor-liquid-solid (VLS) method with an Au catalyst. We systematically analyzed the oxidation mechanism of the SiNWs in both the radial and axial directions and mapped the behavior of the Au atoms on the sidewall and at top of the wire as a function of oxidation time. After thermal oxidation at a temperature of 900 °C, two kinds of oxidation behavior in SiNWs were observed: one was conventional radial oxidation and the other was axial oxidation. In particular, the axial oxidation rate at the Si/Au interface increased dramatically compared with the radial oxidation rate, which can be explained by the reaction between the Si atoms precipitated from the Au tip and the O₂ gas injected in the area surrounding the Au tip. Additionally, we observed that the oxidation rate in the axial direction was inversely proportional to the wire diameter, which is related to the SiO₂ surrounding the Si wire. Moreover, the Au shape changed with respect to the wire diameter, suggesting that both the stress in the Au-Si alloy and the SiO₂ shell thickness of the wire critically affect the growth of SiO₂ on Au.      

Control of surface migration of gold particles on Si nanowires

On the surface of silicon nanowires (SiNWs) synthesized by gold (Au)-catalyzed chemical vapor deposition (CVD), Au particles 5-20 nm in diameter are formed if the growth conditions are within a specific range. We studied the mechanism of Au particle formation by growing SiNWs under different conditions, specifically by dynamically changing the growth parameters during the growth process. We show that insufficient supply of Si source to the Au-Si eutectic on top of the SiNWs enhances the migration of Au atoms on the surface of SiNWs in the form of Au-Si eutectic, which is precipitated on the surface as Au particles during cooling. We also show that using Au-Si eutectic on the surface of SiNWs as a catalyst enables one-step growth of branched SiNWs.     

Origin of diameter-dependent growth direction of silicon nanowires

A nucleation thermodynamic model was developed to clarify the diameter-dependent crystallographic orientation of silicon nanowires (SiNWs) grown via the vapor-liquid-solid (VLS) mechanism with an Au catalyst. The calculated critical energies (E(r*)) and corresponding critical radii (r*) of the SiNWs with <111> and <110> orientations as a function of Au-catalyst size (D(Au)) revealed that the 110-oriented SiNW with r is preferred below D(Au) = approximately 25 nm, but the preferred direction changes to <111> above D(Au) = approximately 25 nm. The model indicated that the nucleated SiNW with a radius (r) above r is stable and continues to grow until the diameter becomes equal to D(Au) but that the crystallographic orientation is maintained. Thus, the predicted growth direction of the final SiNW with a size of D(Au) is <110> for D(Au) < approximately 25 nm and <111> for D(Au) > approximately 25 nm, which is in excellent agreement with reported experimental results.     

Gallium-assisted growth of silicon nanowires by electron cyclotron resonance plasmas

The use of gallium droplets for growing Si nanowires (SiNWs) by electron cyclotron resonance plasmas is investigated. First, the relationship between evaporation time and resultant size of the gallium droplets is studied. Through the use of spectroscopic ellipsometry, the dependence of the surface plasmon resonance (SPR) energy on the droplet size is determined. From these gallium droplets, SiNWs were grown at 300 and 550?°C in electron cyclotron resonance plasmas containing SiH(4), Ar, and H(2). Scanning electron microscopy results show that tapered NWs are obtained for a wide range of growth conditions. Besides, it is found that H(2) plays an important role in the parasitic axial growth of the SiNWs. Namely, H(2) inhibits the radial growth and contributes dramatically to increasing the SiNW defects.     

Focused electron beam induced deposition of gold catalyst templates for Si-nanowire synthesis

A new approach using focused electron beam induced deposition (FEBID) to deposit catalyst particles is reported for the synthesis of single crystalline silicon nanowires (SiNWs) grown by low pressure chemical vapor deposition (LPCVD). The FEBID deposited gold dot arrays fabricated from an acac-Au(III)-Me(2) precursor were investigated by AFM and EDX. The depositions were found to form a sharp tip and a surrounding halo and consist of only 10 at.% Au. However, SiNWs could be synthesized on the deposited catalyst using the vapor-liquid-solid (VLS) method with a mixture of 2% SiH(4) in He at 520?°C. NW diameters from 30 nm up to 150 nm were fabricated and the dependency of the NW diameter on the FEBID deposition time was observed. TEM analysis of the SiNWs revealed a [110] growth direction independent of the NW diameter. This new method provides a maskless and resistless approach for generating catalyst templates for SiNW synthesis on arbitrary surfaces.     

High throughput nanofabrication of silicon nanowire and carbon nanotube tips on AFM probes by stencil-deposited catalysts

A new and versatile technique for the wafer scale nanofabrication of silicon nanowire (SiNW) and multiwalled carbon nanotube (MWNT) tips on atomic force microscope (AFM) probes is presented. Catalyst material for the SiNW and MWNT growth was deposited on prefabricated AFM probes using aligned wafer scale nanostencil lithography. Individual vertical SiNWs were grown epitaxially by a catalytic vapor-liquid-solid (VLS) process and MWNTs were grown by a plasma-enhanced chemical vapor (PECVD) process on the AFM probes. The AFM probes were tested for imaging micrometers-deep trenches, where they demonstrated a significantly better performance than commercial high aspect ratio tips. Our method demonstrates a reliable and cost-efficient route toward wafer scale manufacturing of SiNW and MWNT AFM probes.     

Large-area silicon nanowires from silicon monoxide for solar cell applications

Large-area upstanding silicon nanowires (SiNWs) were synthesized by hot-filament chemical vapor deposition (HFCVD) using silicon monoxide (SiO) powder as Si source under high vacuum (1.2 x 10(-5) Torr). Gold nanoparticles (AuNPs) were employed as catalyst, which were formed on Si substrate by in-situ reduction of gold chloride (AuCl3). The size and distribution of the Au nanoparticles can be easily controlled through chemical reaction conditions. Consequently, the diameter, length and density of SiNWs could be varied in certain range. The SiNWs obtained are single crystalline with growth directions predominantly along [01-1]. Silicon nanowires in large-scale and diameter less than 10 nm can be grown on different Si substrates with this method. Organic inorganic hybrid solar cells based on SiNWs arrays have been demonstrated.     

Quantifying surface roughness effects on phonon transport in silicon nanowires

Although it has been qualitatively demonstrated that surface roughness can reduce the thermal conductivity of crystalline Si nanowires (SiNWs), the underlying reasons remain unknown and warrant quantitative studies and analysis. In this work, vapor-liquid-solid (VLS) grown SiNWs were controllably roughened and then thoroughly characterized with transmission electron microscopy to obtain detailed surface profiles. Once the roughness information (root-mean-square, σ, correlation length, L, and power spectra) was extracted from the surface profile of a specific SiNW, the thermal conductivity of the same SiNW was measured. The thermal conductivity correlated well with the power spectra of surface roughness, which varies as a power law in the 1-100 nm length scale range. These results suggest a new realm of phonon scattering from rough interfaces, which restricts phonon transport below the Casimir limit. Insights gained from this study can help develop a more concrete theoretical understanding of phonon-surface roughness interactions as well as aid the design of next generation thermoelectric devices.     

Bismuth-catalyzed and doped silicon nanowires for one-pump-down fabrication of radial junction solar cells

Silicon nanowires (SiNWs) are becoming a popular choice to develop a new generation of radial junction solar cells. We here explore a bismuth- (Bi-) catalyzed growth and doping of SiNWs, via vapor-liquid-solid (VLS) mode, to fabricate amorphous Si radial n-i-p junction solar cells in a one-pump-down and low-temperature process in a single chamber plasma deposition system. We provide the first evidence that catalyst doping in the SiNW cores, caused by incorporating Bi catalyst atoms as n-type dopant, can be utilized to fabricate radial junction solar cells, with a record open circuit voltage of V(oc) = 0.76 V and an enhanced light trapping effect that boosts the short circuit current to J(sc) = 11.23 mA/cm(2). More importantly, this bi-catalyzed SiNW growth and doping strategy exempts the use of extremely toxic phosphine gas, leading to significant procedure simplification and cost reduction for building radial junction thin film solar cells.     

Growth of germanium nanowires on silicon(111) substrates by molecular beam epitaxy

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.     

Distribution of active impurities in single silicon nanowires

The distribution of electrically active B concentration in single SiNWs (nanowires) grown by a vapor-liquid-solid (VLS) process was studied by analyzing Fano resonance in Raman spectra. We found a gradient of active B concentration along the growth direction; the B concentration was the largest at the substrate side and the smallest at the catalyst side. The observed concentration gradient suggests the conformal growth of a high B concentration layer during a VLS process. To confirm this effect, we grew SiNWs with controlled impurity profiles, that is, p-type/intrinsic ( p-i) and intrinsic/ p-type ( i-p) SiNWs, by controlling the supply of B source during SiNWs growth. We found that p-i SiNWs can be grown by just stopping the supply of B source in the middle of the growth, while i-p SiNWs were not realized; that is, the whole region of nominal " i-p" SiNWs was B-doped even if we started the supply of B source in the middle of the growth. These results confirm the above doping model. We also found that the distribution of active B concentration was significantly modified by high temperature annealing. By annealing at 1100 degrees C for 1 min, B concentration became almost uniform along 10 mum long SiNWs irrespective of initial B profiles. This suggests very efficient diffusion of B atoms in a defective high B concentration surface layer of SiNWs.     

Radial junction amorphous silicon solar cells on PECVD-grown silicon nanowires

Constructing radial junction hydrogenated amorphous silicon (a-Si:H) solar cells on top of silicon nanowires (SiNWs) represents a promising approach towards high performance and cost-effective thin film photovoltaics. We here develop an all-in?situ strategy to grow SiNWs, via a vapour-liquid-solid (VLS) mechanism on top of ZnO-coated glass substrate, in a plasma-enhanced chemical vapour deposition (PECVD) reactor. Controlling the distribution of indium catalyst drops allows us to tailor the as-grown SiNW arrays into suitable size and density, which in turn results in both a sufficient light trapping effect and a suitable arrangement allowing for conformal coverage of SiNWs by subsequent a-Si:H layers. We then demonstrate the fabrication of radial junction solar cells and carry on a parametric study designed to shed light on the absorption and quantum efficiency response, as functions of the intrinsic a-Si:H layer thickness and the density of SiNWs. These results lay a solid foundation for future structural optimization and performance ramp-up of the radial junction thin film a-Si:H photovoltaics.     

Vapor-solid-solid growth of crystalline silicon nanowires using anodic aluminum oxide template

Silicon nanowires (SiNWs) were grown at low temperatures close to metal silicon eutectic point on a silicon substrate using gold catalyst coupled with assistance of the aluminum anodic oxide template. Either a vapor-solid-solid (VSS) growth process below metal silicon eutectic temperature or a vapor-liquid-solid (VLS) process at slightly higher temperatures was observed. The transmission electron microscopy coupled with both the X-ray energy dispersive spectroscopy and the selected area electron diffraction was adopted to characterize the SiNWs. Although the mechanism triggering the VSS process is still not clear, both the geometric and morphological characteristics of the SiNWs grown by the VSS process are discussed and compared with the SiNWs grown by the VLS process. The VSS SiNWs have a much slower growth rate (less than 100 nm/h), a smaller and more uniform diameter (in the range of 15.22 nm) due to a much slower rate of silicon diffusion and much smaller amount of silicon (6.8 wt.%) dissolved in the solid nanocatalyst.     

Extended arrays of vertically aligned sub-10 nm diameter [100] Si nanowires by metal-assisted chemical etching

Large-area high density silicon nanowire (SiNW) arrays were fabricated by metal-assisted chemical etching of silicon, utilizing anodic aluminum oxide (AAO) as a patterning mask of a thin metallic film on a Si (100) substrate. Both the diameter of the pores in the AAO mask and the thickness of the metal film affected the diameter of SiNWs. The diameter of the SiNWs decreased with an increase of thickness of the metal film. Large-area SiNWs with average diameters of 20 nm down to 8 nm and wire densities as high as 10 (10) wires/cm (2) were accomplished. These SiNWs were single crystalline and vertically aligned to the (100) substrate. It was revealed by transmission electron microscopy that the SiNWs were of high crystalline quality and showed a smooth surface.     

Facile and Clean Release of Vertical Si Nanowires by Wet Chemical Etching Based on Alkali Hydroxides

A simple method to release Si nanowires (SiNWs) from a substrate, with their original length almost intact, is demonstrated. By exploiting the unique chemistry involved for the fabrication of vertical arrays of SiNWs in metal‐assisted chemical etching (MaCE) based either on HF/AgNO₃ or HF/H₂O₂ chemistries, wet etching with alkali hydroxides such as NaOH or KOH preferentially attacks the bottom part of the vertical SiNWs. A protective layer of Si oxide is found to exist on the outer wall of the SiNWs and to play the key role of etch mask during the release‐etching by alkali hydroxides. The clean release of SiNWs also enables the repeated use of the Si substrate for the fabrication of vertical SiNW arrays by MaCE. The released SiNWs are further used for the fabrication of field‐effect transistors on a flexible plastic substrate. The method developed here, when combined with a suitable assembling technique, can be very useful in implementing flexible electronics, or in the fabrication of SiNW composites with other functional materials.     

热蒸发铜粉法制备硅纳米线的研究

研究发现,热蒸发铜粉即可在硅衬底上直接生长出硅纳米线.场发射电子扫描电镜和透射电镜分析表明,纳米线的形貌、结构及生长机制,随沉积区域的不同而变化.在高温沉积区,硅纳米线高度弯曲且相互缠绕,按气-液-固机制生长;在低温沉积区,高度定向生长的直硅纳米线,规整地排列在硅衬底表面,其生长机制是氧化辅助生长机制.     

Platinum nanoparticle decorated silicon nanowire arrays for photoelectrochemical hydrogen production

Wafer-scale high density aligned p-type silicon nanowire (SiNW) arrays decorated with discrete platinum nanoparticles (PtNPs) have been fabricated by metal assisted electroless etching followed by an electroless platinum deposition process, and systematic investigations of photoelectrochemical behavior of Pt/SiNW were also reported in this study. Coating of PtNPs on SiNW sidewalls yielded a more positive onset potential (Vos), which enhances the photoelectrochemical hydrogen generation performance of the photoelectrodes, though excessive PtNPs deposition leads to a decreased photocurrent. Additionally, we have demonstrated that the photoelectrode consisting of longer SiNWs yielded a higher limiting current. However, when the length of SiNWs was increased further to >4 μm, the limiting current dramatically reduced, which is presumably because an increased interface recombination and scattering resulting from the increased surface area of SiNWs begin to play a dominant role. The results demonstrate Pt/SiNW to be a promising hybrid system for photoelectrochemical water splitting, and device performance may be further improved via optimal conditions of PtNPs deposition time and SiNWs length.     

Reversible electrowetting on superhydrophobic silicon nanowires

This paper reports for the first time on the reversible electrowetting of liquid droplets in air and oil environments on superhydrophobic silicon nanowires (SiNWs). The silicon nanowires were grown on Si/SiO2 substrates using the vapor-liquid-solid (VLS) mechanism, electrically insulated using 300 nm SiO2, and hydrophobized by coating with a fluoropolymer C4F8. The resulting surfaces displayed liquid contact angle (Theta) around 160 degrees for a saline solution (100 mM KCl) in air with almost no hysteresis. Electrowetting induced a maximum reversible decrease of the contact angle of 23 degrees at 150 VTRMS in air.     

Synthesis, morphology and compositional evolution of silicon nanowires directly grown on SnO(2) substrates

We here propose an all-in situ method for growing vapor-liquid-solid (VLS) silicon nanowires (SiNWs) directly on SnO(2) substrates in a plasma-enhanced chemical vapor deposition system. The tin catalysts are formed by a well-controlled H(2) plasma treatment of the SnO(2) layer. The lowest temperature for the tin-catalyzed VLS SiNWs growth in a silane plasma is ～250?°C. The effects of substrate temperature and H(2) dilution of silane on the morphology and compositional evolution of the SiNWs were systematically investigated. The catalyst content in the SiNWs can be effectively controlled by the deposition temperature. Moreover, enhanced absorption (down to ～1.1?eV) is achieved due to the strong light trapping and anti-reflection effects in the straight and long tapered SiNWs.     

The effects of Au surface diffusion to formation of Au droplets/clusters and nanowire growth on GaAs substrate using VLS method

Using VLS method with the separated 220 nm thick Au catalyst circles/stripes configurations sputtered onto GaAs substrate surface, this paper investigated the effects of the Au droplets/clusters formation as well as the nanowires growth process inside and outside the Au circles/stripes configurations. The Au surface outward diffusion from the Au layer edge up to several tens of micrometers has strongly dominated. The effects of Au surface diffusion to formation of Au droplets/cluster and to the nanowires growth on GaAs semiconductor substrate in the region outside the Au layers have been shown. The mechanism of the droplets/clusters formation outside the Au layer could explained by the surface cluster diffusion, meanwhile the nanowires have grown simultaneously during the Au outward diffusion. The growth could explain by the diffusion of Ga and As atoms into the diffusing Au droplets/clusters via dissociative mechanism to form nanowire seeds inside for nanowires growth. The Au droplets/clusters formation and nanowires growth on GaAs substrate outside Au layer could be applied for making nanodevices blocks outside the Au layer. Unfortunately if this Au surface diffusion phenomenon is occurring on the GaAs semiconductor containing the Au stripes interconnections in micro/nanocircuits this could also cause the short-circuits phenomenon, even at thin Au layer.