Formation Of Si Nanowires Research Articles

On the formation of Si nanowires by molecular beam epitaxy

Abstract Silicon nanowires can be successfully grown by applying the vapor – liquid – solid process. In the case of the commonly used chemical vapor deposition technique, a Si containing gas/precursor is cracked at Au droplets acting as seeds. Si adatoms are subsequently dissolved in the liquid metal. Due to a supersaturation within this droplet, Si precipitates predominantly at the liquid – solid interface – a nanowire grows. A different situation occurs if nanowires are grown by molecular beam epitaxy via the vapor– liquid – solid mechanism. The difference consists, for example, of the role of the metal seed, the morphology of the nanowires and their aspect ratio. In particular, surface diffusion including the metal used as well as Si, strongly influences the growth process. This article describes molecular beam epitaxy growth experiments of Si nanowires under ultra-high vacuum conditions and compares the results with other growth techniques.

Study of Si Nanowires Produced by Metal-Assisted Chemical Etching as a Light-Trapping Material in n-type c-Si Solar Cells.

Si nanowires (SiNWs) produced by metal-assisted chemical etching on n-type Si were investigated for their use as a light-trapping material in c-Si solar cells. The nanowires were fabricated before junction formation (on a lightly doped Si substrate) so that their core was bulk and nonporous. The above fabrication process was implemented in solar cell fabrication. The SiNW reflectivity was tested at different steps of solar cell processing and found to be lower than that of conventional random pyramids used in c-Si solar cells. Contact formation on the front side of the cell was investigated by considering metal deposition either directly on the nanowires or on bulk areas in between the nanowire areas. The superiority of this second case was demonstrated. Three different Si nanowire lengths were investigated, namely, 0.5, 1, and 1.5 μm, the case of 1 μm giving better results in terms of solar cell characteristics and external quantum efficiency. The electronic quality of the Si nanowire surface was investigated using the corresponding metal-oxide-semiconductor capacitors with atomic-layer-deposited alumina dielectric. Successful reduction of surface recombination centers at the large Si nanowire surface was achieved by reducing structural defects at their surface through a specific chemical treatment. Finally, using the determined optimized conditions for Si nanowire formation, chemical cleaning, and process implementation in solar cell fabrication, we demonstrated ∼45% increase in solar cell efficiency with 1 μm SiNWs compared to that from a flat reference cell processed under similar conditions. The above study was made on test solar cells without surface passivation.

SiGe vs. Si Selective Wet Etching for Si Gate-all-Around

Gate All-Around (GAA) is considered a key design feature for future CMOS technology. SiGe vs. Si selective etch is required for Si nanowire formation in GAA. It is confirmed the selective SiGe removal with commodity chemical (mixtures of hydrofluoric acid (HF), hydrogen peroxide (H2O2) and acetic acid (CH3COOH, HAc)), however the thick oxidized layer on Si NW was observed after commodity chemical process, which is indicated the significant Si NW loss. On the other hand, the formulated mixture ACT® SG-101, which is focusing on SiGe oxidizer, chemical pH, solvent polarity & corrosion inhibitor for chemical concept, was performed higher selectivity and lower Si loss than commodity chemical. The formulated mixture has also been used to form an inner spacer for cavity etch scheme and confirmed uniform cavity etch and inner spacer filling on topological test structure.

Porous Si nanowires for highly selective room-temperature NO2 gas sensing

We report the room-temperature sensing characteristics of Si nanowires (NWs) fabricated from p-Si wafers by a metal-assisted chemical etching method, which is a facile and low-cost method. X-ray diffraction was used to the the study crystallinity and phase formation of Si NWs, and product morphology was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After confirmation of Si NW formation via the SEM and TEM micrographs, sensing tests were carried out at room temperature, and it was found that the Si NW sensor prepared from Si wafers with a resistivity of 0.001–0.003 Ω.cm had the highest response to NO2 gas (Rg/Ra = 1.86 for 50 ppm NO2), with a fast response (15 s) and recovery (30 s) time. Furthermore, the sensor responses to SO2, toluene, benzene, H2, and ethanol were nearly negligible, demonstrating the excellent selectivity to NO2 gas. The gas-sensing mechanism is discussed in detail. The present sensor can operate at room temperature, and is compatible with the microelectronic fabrication process, demonstrating its promise for next-generation Si-based electronics fused with functional chemical sensors.

Plasma-Assisted Growth of Silicon Nanowires by Sn Catalyst: Step-by-Step Observation.

A comprehensive study of the silicon nanowire growth process has been carried out. Silicon nanowires were grown by plasma-assisted-vapor-solid method using tin as a catalyst. We have focused on the evolution of the silicon nanowire density, morphology, and crystallinity. For the first time, the initial growth stage, which determines the nanowire (NW) density and growth direction, has been observed step by step. We provide direct evidence of the merging of Sn catalyst droplets and the formation of Si nanowires during the first 10 s of growth. We found that the density of Sn droplets decreases from ~9000 Sn droplets/μm2 to 2000 droplets/μm2 after just 10 s of growth. Moreover, the long and straight nanowire density decreases from 170/μm2 after 2 min of growth to less than 10/μm2 after 90 min. This strong reduction in nanowire density is accompanied by an evolution of their morphology from cylindrical to conical, then to bend conical, and finally, to a bend inverted conical shape. Moreover, the changes in the crystalline structure of nanowires are from (i) monocrystalline to (ii) monocrystalline core/defective crystalline shell and then to (iii) monocrystalline core/defective crystalline shell/amorphous shell. The evolutions of NW properties have been explained in detail.

Formation of Si nanowires by the electrochemical reduction of SiO2 with Ni or NiO additives.

Various morphologies of silicon nanowires (SiNWs) were successfully prepared by the electrochemical reduction of silica mixed with different additives (Au, Ag, Fe, Co, Ni, and NiO, respectively). Straight SiNWs were extensively obtained by the electro-reduction of porous Ni/SiO2 blocks in molten CaCl2 at 900 °C. The SiNWs had a wide diameter distribution of 80 to 350 nm, and the Ni-Si droplets were found on the tips of the nanowires. The growth mechanism of SiNWs was investigated, which could reveal that the nano-sized Ni-Si droplets formed at the Ni/SiO2/CaCl2 three-phase interlines. Based on the mechanism proposed, NiO particles with sub-micrometer size were selected as the additive, and straight SiNWs with diameters of 60 to 150 nm were also prepared via the electrochemical process.

Formation of Si Nanowires by Electro-reduction of Porous Ni/SiO2 Blocks in Molten CaCl2

Electro-reduction of porous Ni/SiO2 blocks has been investigated in molten CaCl2 at 1173K aiming to control the morphology of the produced silicon nanowires. It was confirmed that silicon nanowires with straight morphology were successfully prepared, and nano-sized nickel-silicon droplets formed at the head of silicon nanowires. The prepared silicon nanowires had a wide diameter distribution ranging from 80 to 350nm, and a length as long as several micrometers. Cyclic voltammograms indicated that nickel additives did not change the mechanism of electrochemical reduction of SiO2.

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 1173K. The SiNWs have diameter distributions ranging from 80 to 350nm, 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.

Early stages of growth of Si nanowires by metal assisted chemical etching: A scaling study

We have studied the kinetic roughening in the growth of Si nanowires (NWs) by metal assisted chemical etching (MACE) process as a function of the etching time using atomic force microscopy imaging. In the early stage of growth of Si NWs by Ag assisted MACE process, we have determined the scaling exponents α, β, and 1/z. In the formation of Si NWs, nascent Ag+ ions play an important role in diffusing through the Si surface by means of excess holes that control the size of the NWs. In this system, kinetic roughening of the growth front within the detectable range of lengths of Si NWs was observed. We have observed an α = 0.74 ± 0.01 at the early stage of growth of Si NWs. Interface width w increases as a power law of etching time (t), w∼tβ, with growth exponent β = 0.30 ± 0.05 and lateral correlation length ξ grows as ξ∼t1/z with 1/z = 0.32 ± 0.05. These exponents α, β, and 1/z determine the universality class in which the system belongs to. The growth mechanism is explained by conventional shadowing instability model, which is common for columnar type of surface growth.

Formation of Si Nanowires By Electroreduction of Porous Ni/SiO2 Blocks in Molten CaCl2

Electroreduction of porous SiO2 in molten CaCl2 is a promising approach to prepare silicon nanowires (SiNWs) in large quantities. However, it is difficult to control the morphologies and surface topologies of the produced SiNWs. In this work, pure nickel metal powders are added into SiO2, and SiNWs with straight morphology are prepared by electroreduction of solid Ni/SiO2 blocks in molten CaCl2 in a wide temperature range. The prepared SiNWs have a diameter distribution of 80 to 350nm, and nickel silicide is found at the tip of the nanowire. Cyclic voltammograms indicate that nickel does not change the mechanism of electrochemical reduction of SiO2. The growth process of SiNWs is investigated by XRD, FESEM, HRTEM, EDS, and SAED. It is confirmed that nano-sized nickel-silicon droplets formed at the Ni/SiO2/CaCl2 three-phase interface lead the growth of SiNWs, and adding nickel metal into SiO2 can promote the growth of straight nanowires during the electroreduction of Ni/SiO2 in molten CaCl2.

Height reversal of Si micro dot and well patterns during Si nanowire formation by Ag-assisted chemical etching

The Si nanowire (Si-NW) formation process has been studied to develop a new anode material for lithium ion batteries. Si-NWs were prepared by a combination of electroless metal deposition (EMD) of Ag and metal-assisted chemical etching (Mac etching) on Si substrates that have micro dot and well patterns with respect to their height. During the fabrication of Si-NWs, height reversal occurred in the micro dot and well patterns, and Si micro- and nano-hybrid systems were fabricated. We also observed structural changes of the Si micro- and nano-hybrid structures during the electrochemical charge/discharge process of lithium ions, and showed that this hybrid system is robust for the volume change accompanied with the lithiation/delithiation reaction.

Au transport in catalyst coarsening and Si nanowire formation.

The motion of Au between AuSi liquid eutectic droplets, both before and during vapor-liquid-solid growth, is important in controlling tapering and diameter uniformity in Si nanowires. We measure the kinetics of coarsening of AuSi droplets on Si(001) and Si(111), quantifying the size evolution of droplets during annealing in ultrahigh vacuum using in situ transmission electron microscopy. For individual droplets, we show that coarsening kinetics are modified when disilane or oxygen is added: coarsening rates increase in the presence of disilane but decrease in oxygen. Matching droplet size measurements on Si(001) with coarsening models confirms that Au transport is driven by capillary forces and that the kinetic coefficients depend on the gas environment present. We suggest that the gas effects are qualitatively similar whether transport is attachment limited or diffusion limited. These results provide insight into manipulating nanowire morphologies for advanced device fabrication.

High Density Growth of Indium seeded Silicon Nanowires in the Vapor phase of a High Boiling Point Solvent

Herein, we describe the growth of Si nanowires (NWs) in the vapor phase of an organic solvent medium on various substrates (Si, glass, and stainless steel) upon which an indium layer was evaporated. Variation of the reaction time allowed NW length and density to be controlled. The NWs grew via a predominantly root-seeded mechanism with discrete In catalyst seeds formed from the evaporated layer. The NWs and substrates were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The suitability of the indium seeded wires as anode components in Li batteries was probed using cyclic voltammetric (CV) measurements. The route represents a versatile, glassware-based method for the formation of Si NWs directly on a variety of substrates.

Morphology-tunable assembly of periodically aligned Si nanowire and radial pn junction arrays for solar cell applications

Large-area periodically aligned Si nanowire (PASiNW) arrays have been fabricated on Si substrates via a templated catalytic chemical etching process. The diameter, length, packing density, and even the shape of Si nanowires (SiNWs) could be precisely controlled and tuned. A local coupling redox mechanism involving the reduction of H2O2 on silver particles and the dissolution of Si is responsible for formation of SiNWs. With the as-prepared SiNWs as templates, three kinds of PASiNW radial pn junction structures were fabricated on Si substrates via a solid-state phosphorous diffusion strategy and their applications in solar cells were also explored. The PASiNW radial pn junction-based solar cell with big diameter and interspace shows the highest power conversion efficiency (PCE) of 4.10% among the three kinds of devices. Further optimization, including surface passivation and electrode contact, is still needed for the higher efficiency PASiNW radial pn junction-based solar cells in the future.

Fabrication and optical characterization of Si nanowires formed by catalytic chemical etching in Ag2O/HF solution

An etchant system, Ag2O–HF–H2O, was used to fabricate vertically aligned Si nanowire (SiNW) arrays on Si wafers. The synthesis was based on catalytic etching and produced large-area brushlike SiNWs on Si wafers. The Ag2O concentration was varied from 0.001 to 0.1mol/L and various synthesis conditions were optimized. The synthesized SiNWs were investigated by Fourier-transform infrared spectroscopy analysis, optical absorption, and contact-angle measurements. Spectroscopic ellipsometry was also used to assess the surface roughness produced in the early stage of etching. The optical measurements revealed that the SiNWs have high optical absorbance from the far infrared to ultraviolet regions. Passive HF etching of the SiNWs changed their wettability from superhydrophilic (∼0°) to highly hydrophobic (∼135°). The effect of sulfuric peroxide mixture (SPM) cleaning on the SiNW formation properties was also examined.

Growth mechanism and thermoelectric properties of β-FeSi2 matrix with Si nanowires

Abstract A novel composite material consisted of β -FeSi 2 and in situ formed Si nanowires (SiNWs) with the diameter of 50–120 nm, is prepared by mechanical alloying and annealing process. The effects of ball-milling time on the formation of SiNWs are investigated systematically. The growth way of SiNWs is named as autocatalytic growth, in which no catalyst is intentionally added to the system. The possible formation mechanisms of SiNWs are discussed in detail. Due to the increase of boundary scattering, the thermal conductivity of the composite sample with SiNWs decrease significantly. Thus, the composite sample shows excellent thermoelectric performance.

Formation of Si Nanowires by Direct Electrolytic Reduction of Porous SiO2 Pellets in Molten CaCl2

The effects of starting material and catalyst on the morphology of produced Si have been investigated for the direct electrolytic reduction of porous SiO2 pellets in molten CaCl2 at 1123 K. The Si nanowires (SiNWs) were produced using amorphous SiO2 pellets with a tetrapod-like microstructure as the starting material, whereas plate-like Si was obtained from pellets made of spherical fumed SiO2. The SiNWs showed irregular branching and a wide distribution of diameters as the electrolysis proceeded. On the other hand, elongated SiNWs were formed during the electrolysis of pellets comprised of Au nanoparticles (AuNPs) and tetrapod-like microstructured SiO2 powder. This suggests that AuNPs work as catalysts for longitudinal nanowire growth in the present electrolytic process.

Strategies for controlling Si nanowire formation during Au-assisted electroless etching

Dramatic changes in the morphologies of Si nanowires (NWs) ranging from vertical, to hooked, to ground-collapsed shapes are presented depending on the HF/H2O2 ratio in the etch solution during Au-assisted electroless etching. It is shown that the deviations from vertical NWs are not due to capillary action but correlated with the development of nanoporous structures, which is attributed to excess hole (h+) diffusion toward the NWs' sidewalls during the etching process. The role of hole diffusion is made more evident by observation of NWs with the neck formation when transitioning to different etching regimes during SiNW formation. Furthermore, the hooked and ground-collapsed SiNW can be suppressed at the cost of enhanced tapering by adding a strong electrolyte in solution, which is correlated to reduced etching rate resulting from electric screening of the charged Au surfaces. We provide an important finding that common salts such as NaCl, KCl, and CaCl2 can be useful retardants for the metal-assisted electroless etching. The new insights into structural transformation presented in this study afford greater control over the structure of SiNWs produced by metal-assisted electroless etching.

Galvanic Deposition of Pt Clusters on Silicon: Effect of HF Concentration and Application as Catalyst for Silicon Nanowire Growth

We report on the galvanic deposition of Pt on Si from solutions containing PtCl(2) and different concentrations of HF. The results show that for low [HF]/[Pt] ratios (<or=26), only a thin layer of PtSi is formed. The deposition rate of Pt increases with [HF] in the plating solution, up to [HF]/[Pt] approximately 530; after this ratio, the morphology of the Pt film changes: larger clusters are formed, which cover the Si substrate less densely. Detailed atomic force microscopy and X-ray photoelectron spectroscopy analyses show that the deposited Pt layers do not completely cover the Si substrate. The Pt and PtSi films formed are able to catalyze the formation of Si nanowires (Si NWs) arrays formed via vapor-liquid-solid (VLS) process. By changing the immersion time in the Pt plating solution, Si NWs arrays with different density, diameter, and orientation are obtained.

Formation of silicon-fullerene-linked nanowires inside carbon nanotubes: A molecular-dynamics and first-principles study

We study the formation of Si nanowires inside carbon nanotubes by using a combination of empirical molecular-dynamics and first-principles approaches. Molecular-dynamics simulations demonstrate that liquid Si encapsulated into a (13,0) nanotube crystallizes into a nanowire composed of linked ${\mathrm{Si}}_{16}$ fullerene cages. On the other hand, a nanowire composed of linked ${\mathrm{Si}}_{20}$ fullerene cages forms inside a (14,0) nanotube. The stabilities of these nanowires are further confirmed by first-principles calculations. We also find that the freestanding ${\mathrm{Si}}_{16}$-linked nanowire is a metal, while the ${\mathrm{Si}}_{20}$-linked nanowire is a semiconductor. The present findings suggest that the choice of the nanotube size allows us to control the structure of Si nanowires, and therefore to tailor the material properties.