Effect of the etching time on the morphological, optical and electronic properties of silicon nanowires

ABSTRACT: Owing to their interesting electronic, mechanical, optical and transport properties, silicon nanowires (SiNWs) have attracted much attention, giving opportunities to several potential applications in nanoscale electronic, optoelectronic devices and silicon solar cells. For photovoltaic (PV) application, a superficial film of SiNWs could be used as an efficient antireflection coating (ARC). In this work, we investigate the morphological, optical and electronic properties of SiNWs fabricated at different etching time. Characterizations of the formed SiNWs films were performed using a Scanning Electron Microscope (SEM), UV-Vis-NIR spectrophotometer and Light-Beam-Induced-Current (LBIC) technique. The later technique was used to determine the effective diffusion length in SiNWs films. From LBIC investigations, we deduce that the homogeneity of the SiNWs film play a key role on the electronic properties.     

Fabrication and photocatalytic properties of silicon nanowires by metal-assisted chemical etching: effect of H2O2 concentration

In the current study, monocrystalline silicon nanowire arrays (SiNWs) were prepared through a metal-assisted chemical etching method of silicon wafers in an etching solution composed of HF and H₂O₂. Photoelectric properties of the monocrystalline SiNWs are improved greatly with the formation of the nanostructure on the silicon wafers. By controlling the hydrogen peroxide concentration in the etching solution, SiNWs with different morphologies and surface characteristics are obtained. A reasonable mechanism of the etching process was proposed. Photocatalytic experiment shows that SiNWs prepared by 20% H₂O₂ etching solution exhibit the best activity in the decomposition of the target organic pollutant, Rhodamine B (RhB), under Xe arc lamp irradiation for its appropriate Si nanowire density with the effect of Si content and contact area of photocatalyst and RhB optimized.     

Synthesis of Highly Crystalline Needle-Like Silicon Nanowires for Enhanced Field Emission Applications

Needle-like silicon nanowires (SiNWs) were successfully synthesized on gold-coated silicon substrates using a very high frequency plasma enhanced chemical vapor deposition method (VHF-PECVD). The prepared samples were characterized by field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDX), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and photoluminescence (PL). XRD analysis confirmed formation of single crystalline SiNWs along (111) crystalline planes and microscopic studies revealed formation of NWs with diameters ranging from 10 to 100 nm and lengths of a few micrometers. Furthermore, the presence of gold nanoparticles on the tip of the NWs verified the vapor–liquid–solid growth mechanism of SiNWs. It was also demonstrated that SiNWs are composed of well-crystallized silicon cores and an amorphous shell. The obtained results verified potential application of such structures in field emission displays.     

Silicon nanowires prepared by electron beam evaporation in ultrahigh vacuum

ABSTRACT: One-dimensional silicon nanowires (SiNWs) were prepared by electron beam evaporation in ultrahigh vacuum (UHV). The SiNWs can be grown through either vapor-liquid-solid (VLS) or oxide-assisted growth (OAG) mechanism. In VLS growth, SiNWs can be formed on Si surface, not on SiO2 surfaces. Moreover, low deposition rate is helpful for producing lateral SiNWs by VLS. But in OAG process, SiNWs can be grown on SiO2 surfaces, not on Si surfaces. This work reveals the methods of producing large-scale SiNWs in UHV.     

a-Si:H/SiNW shell/core for SiNW solar cell applications

Vertically aligned silicon nanowires have been synthesized by the chemical etching of silicon wafers. The influence of a hydrogenated amorphous silicon (a-Si:H) layer (shell) on top of a silicon nanowire (SiNW) solar cell has been investigated. The optical properties of a-Si:H/SiNWs and SiNWs are examined in terms of optical reflection and absorption properties. In the presence of the a-Si:H shell, 5.2% reflection ratio in the spectral range (250 to 1,000 nm) is achieved with a superior absorption property with an average over 87% of the incident light. In addition, the characteristics of the solar cell have been significantly improved, which exhibits higher open-circuit voltage, short-circuit current, and efficiency by more than 15%, 12%, and 37%, respectively, compared with planar SiNW solar cells. Based on the current–voltage measurements and morphology results, we show that the a-Si:H shell can passivate the defects generated by wet etching processes.     

Synthesis of gallium-catalyzed silicon nanowires by hydrogen radical-assisted deposition method

Gallium-catalyzed silicon nanowires (SiNWs) were synthesized by the hydrogen radical-assisted deposition method. The voluminous quantities of SiNWs with various crystal growth directions were synthesized and their characteristics were estimated by using XRD, FE-SEM, TEM and EDX analyses. Most of the Ga-capped SiNWs were directly grown with some smoothly curved SiNWs. Large quantities of small-SiNWs with diameters of 20–80 nm were tree-likely grown on the large-SiNWs surface. The diameters of large-SiNWs were approximately 200 nm–2 μm. Furthermore, a simple model of growth mechanism for sub-grown silicon nanowires by the hydrogen radical-assisted deposition method was proposed.     

Silicon-MnO2 core-shell nanowires as electrodes for micro-supercapacitor application

The aim of the present work is to develop a novel electrode material for micro-supercapacitor (μ-SC) application, which must be compatible with the fabrication of Si microelectronic. A simple, cost-effective and scalable approach has been proposed to enhance the electrochemical capacitance of silicon nanowires (SiNWs). SiNWs grown using hot wire chemical vapor process (HWCVP) at low temperature were encapsulated with manganese dioxide (MnO₂) nanoparticles film. The prepared hybrid nanostructure of SiNW@MnO₂ core-shell showed stable electrochemical performance in an aqueous electrolyte of 1 M Na₂SO₄. The MnO₂ shell was deposited on SiNWs by electrophoretic process (EPD) at room temperature. The synthesized SiNW@MnO₂ core-shell structure exhibited a specific capacitance of 2.1 mF/cm², which is larger than to other Si-based materials reported in literature. The scalability and simplicity of the process make it useful to fabricate SiNW@MnO₂ electrodes based on-chip μ-SC with enhanced electrochemical performance.     

Optical properties of silicon nanowire arrays formed by metal-assisted chemical etching: evidences for light localization effect

We study the structure and optical properties of arrays of silicon nanowires (SiNWs) with a mean diameter of approximately 100 nm and length of about 1–25 μm formed on crystalline silicon (c-Si) substrates by using metal-assisted chemical etching in hydrofluoric acid solutions. In the middle infrared spectral region, the reflectance and transmittance of the formed SiNW arrays can be described in the framework of an effective medium with the effective refractive index of about 1.3 (porosity, approximately 75%), while a strong light scattering for wavelength of 0.3 ÷ 1 μm results in a decrease of the total reflectance of 1%-5%, which cannot be described in the effective medium approximation. The Raman scattering intensity under excitation at approximately 1 μm increases strongly in the sample with SiNWs in comparison with that in c-Si substrate. This effect is related to an increase of the light-matter interaction time due to the strong scattering of the excitation light in SiNW array. The prepared SiNWs are discussed as a kind of ‘black silicon’, which can be formed in a large scale and can be used for photonic applications as well as in molecular sensing.     

Growth of low temperature silicon nano-structures for electronic and electrical energy generation applications

This paper represents the lowest growth temperature for silicon nano-wires (SiNWs) via a vapour-liquid–solid method, which has ever been reported in the literature. The nano-wires were grown using plasma-enhanced chemical vapour deposition technique at temperatures as low as 150°C using gallium as the catalyst. This study investigates the structure and the size of the grown silicon nano-structure as functions of growth temperature and catalyst layer thickness. Moreover, the choice of the growth temperature determines the thickness of the catalyst layer to be used.The electrical and optical characteristics of the nano-wires were tested by incorporating them in photovoltaic solar cells, two terminal bistable memory devices and Schottky diode. With further optimisation of the growth parameters, SiNWs, grown by our method, have promising future for incorporation into high performance electronic and optical devices.     

The effects on the field emission properties of silicon nanowires by different pre-treatment techniques of Ni catalysts layers

Silicon nanowires (SiNWs) were synthesized directly from silicon substrates via a catalytic reaction by thermal CVD system. Ni catalyst layers were deposited on silicon substrates by RF sputtering and electroless-plating pre-treatment techniques, respectively. It was found that the average diameters, lengths, and growth densities of SiNWs by the RF sputtering pre-treatment technique was larger than that by the electroless-plating pre-treatment technique. Furthermore, a better and more stable emission property of the SiNWs was also observed by the RF sputtering pre-treatment technique. Therefore, it was then concluded that the growth mechanism and the emission properties were both strongly influenced by the Ni catalyst layers of different pre-treatment techniques.     

In situ-grown hexagonal silicon nanocrystals in silicon carbide-based films

Silicon nanocrystals (Si-NCs) were grown in situ in carbide-based film using a plasma-enhanced chemical vapor deposition method. High-resolution transmission electron microscopy indicates that these nanocrystallites were embedded in an amorphous silicon carbide-based matrix. Electron diffraction pattern analyses revealed that the crystallites have a hexagonal-wurtzite silicon phase structure. The peak position of the photoluminescence can be controlled within a wavelength of 500 to 650 nm by adjusting the flow rate of the silane gas. We suggest that this phenomenon is attributed to the quantum confinement effect of hexagonal Si-NCs in silicon carbide-based film with a change in the sizes and emission states of the NCs.     

Superhydrophobic PDMS/SiNPs/T-ZnOw coating with reduced adhesion of Streptococcus mutans for dental caries prevention

The prevention of dental caries is based mainly on killing the cariogenic bacteria Streptococcus mutans. Prevention of S. mutans adhesion through the development of physical structures is rarely utilized. In this study, a superhydrophobic PDMS/SiNPs/T-ZnOw (PST) coating was prepared for use on a bovine tooth by mixing polydimethylsiloxane (PDMS), silicon dioxide nanoparticles (SiNPs), and tetrapod-like zinc oxide whiskers (T-ZnOw) using one-pot solution and spray methods. The results showed that the superhydrophobicity and roughness of the coating, affected by the PDMS content, were positively correlated with the anti-adhesive effect on S. mutans. The PST coating with PDMS, SiNPs, and T-ZnOw at a ratio of 2.5:1:1 exhibited the highest water contact angle (161°) and the best anti-adhesion effect (97.2% at 4 h and 98.1% at 12 h). The anti-adhesion property towards S. mutans was attributed to its needle-like structure, and the biofilm live-dead staining test showed that the coating had no bactericidal effect. In addition, the coating exhibited favorable durability and biocompatibility, providing a solid foundation for application in the human oral cavity. Thus, this study provides an effective method for caries prevention.     

An easy and scalable approach to synthesize three-dimensional sandwich-like Si/Polyaniline/Graphene nanoarchitecture anode for lithium ion batteries

A new three-dimensional (3D) sandwich-like Si/Polyaniline/Graphene nanoarchitecture anode for lithium ion batteries (LIBs) is successfully fabricated through an easy approach. In this nanoarchitecture, the in-situ polymerized electronic conductive polyaniline (PAni) hydrogel, acting as “glue”, agglutinates tightly to both the silicon nanoparticles (SiNPs) and graphene sheets, forming efficient conductive networks with high elastic modulus and high tensile strength. This mechanically robust nanoarchitecture can endure the great volume change of silicon and retain structural stability during Li-ion insertion/extraction. The electrodes consisting of this 3D sandwich-like Si/Polyaniline/Graphene nanoarchitecture reveal excellent electrochemical performance. The progress made in this work provides an easy and scalable route for preparing Si-based anode materials with high performance for advanced LIBs.     

On the origin of emission and thermal quenching of SRSO:Er3+ films grown by ECR-PECVD

Silicon nanocrystals embedded in a silicon-rich silicon oxide matrix doped with Er³⁺ ions have been fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition. Indirect excitation of erbium photoluminescence via silicon nanocrystals has been investigated. Temperature quenching of the photoluminescence originating from the silicon nanocrystals and the erbium ions has been observed. Activation energies of the thermally activated quenching process were estimated for different excitation wavelengths. The temperature quenching mechanism of the emission is discussed. Also, the origin of visible emission and kinetic properties of Er-related emission have been discussed in details.     

Thermal conductivity in porous silicon nanowire arrays

The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.     

Ultrathin polypyrrole films on silicon substrates

The electrochemical deposition of polypyrrole (PPy) on p-Si(1 0 0) electrodes was investigated. The electrodeposition was performed in aqueous electrolyte solutions utilising cyclic voltammetry. Thin, adhesive, uniform PPy films were successfully deposited on p-Si(1 0 0) electrodes. The Si/PPy interface was characterised with infrared spectroscopic ellipsometry (IR-SE) and photoluminescence (PL) measurements to obtain information of a possible oxidation of the Si interface and charge carrier recombination at the interface, respectively. Very small amounts of interfacial silicon oxides have been found at the Si/PPy interface. PL measurements lead to the assumption that electrodeposition of PPy onto the Si electrodes generated only very few additional non-radiative recombination-active (nr) defects. Hence, polypyrrole is an excellent passivation of nr defects at the silicon surface.     

Evaluation of optical and electronic properties of silicon nano-agglomerates embedded in SRO: applying density functional theory

In systems in atomic scale and nanoscale such as clusters or agglomerates constituted by particles from a few to less than 100 atoms, quantum confinement effects are very important. Their optical and electronic properties are often dependent on the size of the systems and the way in which the atoms in these clusters are bonded. Generally, these nanostructures display optical and electronic properties significantly different to those found in corresponding bulk materials. Silicon agglomerates embedded in silicon rich oxide (SRO) films have optical properties, which have been reported to be directly dependent on silicon nanocrystal size. Furthermore, the room temperature photoluminescence (PL) of SRO has repeatedly generated a huge interest due to its possible applications in optoelectronic devices. However, a plausible emission mechanism has not been widely accepted in the scientific community. In this work, we present a short review about the experimental results on silicon nanoclusters in SRO considering different techniques of growth. We focus mainly on their size, Raman spectra, and photoluminescence spectra. With this as background, we employed the density functional theory with a functional B3LYP and a basis set 6-31G* to calculate the optical and electronic properties of clusters of silicon (constituted by 15 to 20 silicon atoms). With the theoretical calculation of the structural and optical properties of silicon clusters, it is possible to evaluate the contribution of silicon agglomerates in the luminescent emission mechanism, experimentally found in thin SRO films.     

Synthesis of colloidal solutions with silicon nanocrystals from porous silicon

In this work, we have obtained colloidal solutions of Si nanocrystals (Si-ncs), starting from free-standing porous silicon (PSi) layers. PSi layers were synthesized using a two-electrode Teflon electrochemical cell; the etching solution contained hydrogen peroxide 30%, hydrofluoric acid 40% (HF), and methanol. The anodizing current density was varied to 250 mA cm^-2, 1 A cm^-2, and 1.2 A cm^-2. Thus obtained, PSi was mechanically pulverized in a mortar agate; then, the PSi powders were poured into different solutions to get the final Si-ncs colloidal solutions. The different optical, morphological, and structural characteristics of the colloidal solutions with Si-ncs were measured and studied. These Si-ncs colloidal solutions, measured by photoluminescence (PL), revealed efficient blue-green or violet emission intensities. The results of X-ray diffraction (XRD) indicate that the colloidal solutions are mainly composed of silicon nanocrystallites. The result of UV–vis transmittance indicates that the optical bandgap energies of the colloidal solutions varied from 2.3 to 3.5 eV for colloids prepared in methanol, ethanol, and acetone. The transmission electron microscopy (TEM) images showed the size of the nanocrystals in the colloidal solutions. Fourier transform infrared spectroscopy (FTIR) spectra showed different types of chemical bonds such as Si-O-Si, Si-CH₂, and SiH _x , as well as some kind of defects.

PACS

61.46Df.-a; 61.43.Gt; 61.05.cp; 78.55.-m; 81.15.Gh     

Synthesis and characterisation of functional manganese doped ZnS quantum dots for bio-imaging application

Fluorescent quantum dots (QDs) are potential candidates for bio-imaging but major problems in using them for bio-imaging applications are either bandgap lying in UV region or the cytotoxicity of the visible light-excited cadmium-based QDs. Keeping these things in mind, glutathione (GLT) functionalised Mn-doped ZnS QDs has been studied to make a desired fluorescent system for bioimaging application. XRD measurements show that grain size decreases at higher concentration of GLT capping on Mn-doped quantum dots. UV–visible studies show that the band gap shows a blue shift at higher GLT concentration. FTIR studies confirm GLT functionalisation on the surface of ZnS QDs. Both UV excited and IR excited photoluminescence spectrum of samples exhibited a tunable orange emission intensity with increase in GLT concentration however multiphoton infrared excited spectra was used to show the feasibility of GLT functionalised ZnS:Mn quantum dots for a real application in a biological window of 650–950?nm.     

Conductive-probe atomic force microscopy characterization of silicon nanowire

The electrical conduction properties of lateral and vertical silicon nanowires (SiNWs) were investigated using a conductive-probe atomic force microscopy (AFM). Horizontal SiNWs, which were synthesized by the in-plane solid-liquid-solid technique, are randomly deployed into an undoped hydrogenated amorphous silicon layer. Local current mapping shows that the wires have internal microstructures. The local current-voltage measurements on these horizontal wires reveal a power law behavior indicating several transport regimes based on space-charge limited conduction which can be assisted by traps in the high-bias regime (> 1 V). Vertical phosphorus-doped SiNWs were grown by chemical vapor deposition using a gold catalyst-driving vapor-liquid-solid process on higly n-type silicon substrates. The effect of phosphorus doping on the local contact resistance between the AFM tip and the SiNW was put in evidence, and the SiNWs resistivity was estimated.