Metal-assisted Chemical Etching Process Research Articles

Thermal conductivity of silicon nanomaterials measured using the photoacoustic technique in a piezoelectric configuration

In this study, we investigated the thermal transport in porous silicon nanostructures and silicon nanowires fabricated by electrochemical and metal-assisted chemical etching processes, respectively. In particular, the thermal conductivity values for the silicon-based nanomaterials were estimated based on photoacoustic measurements obtained from piezoelectric recordings. The amplitude–frequency and phase–frequency dependencies of the photoacoustic responses of the nanostructured silicon samples were obtained, and simulated successfully with a theoretical model. Correlations were established between the thermal conductivities and etching parameters.

Silicon microstructures through the production of silicon nanowires by metal-assisted chemical etching, used as sacrificial material

A simple, inexpensive and wafer-scale method to obtain Si microstructures is proposed. The method consists in a sequence of steps that include a selective metal-assisted chemical etching process to create regions of Si nanowires that are sacrificed in a post-etching process, leaving microstructures standing. As a proof of concept, Si micropillars with length of 7 µm and diameter of 1.4 µm were fabricated. The advantage of the proposed method is its simplicity, allowing the production of microstructures in a basic chemical laboratory.

Effect of inhomogeneous mesoporosity and defects on the luminescent properties of slanted silicon nanowires prepared by facile metal-assisted chemical etching

Slanted silicon nanowires show an improved optical absorption and better electrical contact than the vertical silicon nanowires. High aspect ratio mesoporous slanted silicon nanowires oriented along the ⟨100⟩ direction are fabricated by a facile two-step metal-assisted chemical etching process. Inhomogeneous porosity with a pore diameter of 2–10 nm is identified by the analysis of transmission electron microscopy, angle dependent Raman spectroscopy, and Brunauer-Emmett-Teller measurements. Slanted silicon nanowires possess a core/shell structure, and the porosity varies from top to bottom of the slanted silicon nanowires. The presence of neutral oxygen defects, self-trapped excitons, and surface defects is identified by photoluminescence spectroscopy, and the results are correlated with Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy data. In addition to mesoporosity, defects such as self-trapped excitons, oxygen vacancies, and surface defects on Si/SiOx interface contribute to the luminescence of slanted silicon nanowires. Red shift in the photoluminescence with increasing etching time is explained using quantum confinement luminescent center model. Understanding the role of defects and porosity in slanted silicon nanowires is highly desirable to increase the efficiency of silicon nanowires based optoelectronic devices.

The Influence of Black Silicon Morphology Modification by Acid Etching to the Properties of Diamond Wire Sawn Multicrystalline Silicon Solar Cells

Black silicon with nanosized textures is very important to the diamond wire sawn (DWS) multicrystalline silicon (mc-Si) solar cell due to its low reflectivity. Normally the nanosized textures can be obtained by metal-assisted chemical etching (MACE) on the DWS mc-Si wafers. Here, we found that in order to improve the conversion efficiency of DWS mc-Si solar cell with nanosized textures, the textures’ morphologies must be modified by acid etching. The acid etching is the defect removal etching process. Our experimental results showed that the acid etching process influences not only the size and the reflectivity of the nanosized textures, but also the recombination velocities of the silicon wafers. A DWS mc-Si solar cell with modified nanosized textures was successfully obtained by MACE and suitable acid etching process, whose reflectivity is lower than that of a DWS mc-Si solar cell prepared by conventional acidic texturizing. The highest conversion efficiency of 19.07% was reached and a 0.6% conversion efficiency gain was obtained for a DWS mc-Si solar cell (15.6 cm × 15.6 cm), as opposed to the DWS mc-Si solar cell prepared by conventional acidic texturizing.

Crossed carbon skeleton enhances the electrochemical performance of porous silicon nanowires for lithium ion battery anode

As a most promising anode candidate, silicon has the shortcomings of low electron conductivity and high volume expansion of 300%, hindering its applications in lithium ion batteries (LIBs). In this study, one-dimension porous silicon nanowires (pSi-NWs) were prepared through a simple metal-assisted chemical etching process by using metallurgical silicon as raw material. To consolidate the structural integrity of pSi-NWs, a crossed carbon skeleton (c-Cs) was introduced into pSi-NWs via in-situ polymerization and carbonization process. The resultant pSi-NWs@c-Cs composite delivered the high capacity of 1253 mAh g−1 with good cycling stability as well as the notable rate capability (476 mAh g−1 at 4 A g−1), much superior to those of pSi-NWs and pSi-NWs@reduced graphene oxide composite. The enhanced electrochemical performance of pSi-NWs@c-Cs composite is attributed to the crossed carbon skeleton in constructing the advanced Si/C interface to more effectively improve the electron conductivity of pSi-NWs and acting as the protective shell to keep the structure integrity of pSi-NWs. As the additive of commercial graphite anode, 28% of capacity augment (460 mAh g−1 at 0.2 A g−1) was realized by adding 20 wt% of pSi-NWs@c-Cs composite into graphite, demonstrating its promising applications in LIBs.

Conformal Antireflective Surface Formed onto 3-D Silicon Structure.

This letter reports on the fabrication, simulation and characterization of conformal antireflective black-silicon (BSi) nanowires on a 3D silicon structure. The BSi nanostructures were formed on various facets of a 3D Si structure including sharp tips and sidewalls using a metal-assisted chemical (MAC) etching process. The conformal BSi design was simulated using FDTD Lumerical software. The antireflection capability was indicated by the quantified reduction in normalized intensity after image processing of diffraction patterns. An optical iris of 1.00-mm circular aperture with conformal BSi nanowires was fabricated and characterized to demonstrate the anti-reflectivity capability at two visible wavelengths of 532 and 633 nm. The iris showed a significant reduction in glare around its Airy disc, up to 3× smaller than the same one but without the BSi nanostructures.

Texturization of diamond-wire-sawn multicrystalline silicon wafer using Cu, Ag, or Ag/Cu as a metal catalyst

In this work, Cu, Ag, or Ag/Cu was used as a metal catalyst to study the surface texturization of diamond-wire-sawn (DWS) multi-crystalline silicon (mc-Si) wafer by a metal-assisted chemical etching (MACE) method. The DWS wafer was first etched by standard HF-HNO3 acidic etching, and it was labeled as AE-DWS wafer. The effects of ratios of Cu(NO3)2:HF, AgNO3:HF, and AgNO3:Cu(NO3)2 on the morphology of AE-DWS wafer were investigated. After the process of MACE, the wafer was treated with a NaF/H2O2 solution. In this process, H2O2 etched the nanostructure, and NaF removed the oxidation layer. The Si {1 1 1} plane was revealed by etching the wafer in a mixture of 0.03 M Cu(NO3)2 and 1 M HF at 55 °C for 2.5 min. These parallel Si {1 1 1} planes replaced some parallel saw marks on the surface of AE-DWS wafers without forming a positive pyramid or an inverted pyramid structure. The main topography of the wafer is comprised of silicon nanowires grown in <1 0 0> direction when Ag or Ag/Cu was used as a metal catalyst. When silicon is etched in a mixed solution of Cu(NO3)2, AgNO3, HF and H2O2 at 55 °C with a concentration ratio of [Cu2+]/[Ag+] of 50 or at 65 °C with a concentration ratio of [Cu2+]/[Ag+] of 33, a quasi-inverted pyramid structure can be obtained. The reflectivity of the AE-DWS wafers treated with MACE is lower than that of the multiwire-slurry-sawn (MWSS) mc-Si wafers treated with traditional HF + HNO3 etching.

Surface Nanostructures Forming during the Early Stages of the Metal-Assisted Chemical Etching of Silicon. Optical Properties of Silver Nanoparticles

In this two-part work, nanostructures formed in a three-step process of metal-assisted chemical etching of silicon are investigated. In the first part (present publication), the process of the chemical deposition of a layer of self-assembled silver nanoparticles on the surface of a silicon wafer (the first stage of metalassisted chemical etching) is studied. This layer, on the one hand, serves as a catalyst for the subsequent etching of silicon, and, on the other hand, represents a kind of mask for the formation of a certain topology of the emerging Si nanowires. The morphology of the obtained 40- to 60-nm-thick silver nanoparticle layers is investigated by scanning electron microscopy. The spectral dependences of the ellipsometric angles Ψ and Δ are measured using spectroscopic ellipsometry (λ = 250–900nm), and the complex dielectric function of the silver nanolayers is determined from these spectra. The dielectric function features a characteristic plasmon resonance peak in the ultraviolet spectral range. The study of the optical properties of Si nanofilament layers which form during the early stages of metal-assisted chemical etching will be reported as the second part of this work in a separate publication.

Hydrogen gas sensing properties of platinum decorated silicon carbide (Pt/SiC) Nanoballs

In the present study, platinum decorated silicon carbide (Pt/SiC) nanoballs (NBs) were directly grown on Ag coated porous Si substrates via DC/RF magnetron sputtering. Porous Si substrates were prepared by the metal-assisted chemical etching process at room temperature. The structural and morphological properties of as-grown SiC nanoballs (NBs) were studied using various techniques such as X-ray diffraction (XRD), Raman spectroscopy and field scanning electron microscopy (FESEM) respectively. Hydrogen gas sensing properties along with the sensing mechanism of Pt/SiC nanoball-based sensor within low detection limit (5–1000 ppm) at the high operating temperature range (30–480 °C) were investigated in detail. The sensor exhibits high sensing response (44.48%) with very fast response time (15 s) toward 100 ppm hydrogen gas at 330 °C. These above results suggest the feasibility of Pt decorated SiC nanoballs for highly sensitive and selective hydrogen gas sensor at the high operating temperature and harsh environment conditions.

Effect of Surface Structure on Electrical Performance of Industrial Diamond Wire Sawing Multicrystalline Si Solar Cells

We report industrial fabrication of different kinds of nanostructured multicrystalline silicon solar cells via normal acid texturing, reactive ion etching (RIE), and metal-assisted chemical etching (MACE) processes on diamond wire sawing wafer. The effect of different surface structure on reflectivity, lifetime, and electrical performance was systematically studied in this paper. The difference between industrial acid, RIE, and MACE textured multicrystalline silicon solar cells to our knowledge has not been investigated previously. The resulting efficiency indicates that low reflectivity surface structure with the size of 0.2–0.8 μm via RIE and MACE process do not always lead to low lifetime compared with acid texturing process. Both RIE and MACE process is promising candidate for high efficiency processes for future industrial diamond wire sawing multicrystalline silicon solar cells.

Поверхностные наноструктуры, формирующиеся на ранних стадиях металл-стимулированного химического травления кремния. Оптические свойства наночастиц серебра

AbstractIn this two-part work, nanostructures formed in a three-step process of metal-assisted chemical etching of silicon are investigated. In the first part (present publication), the process of the chemical deposition of a layer of self-assembled silver nanoparticles on the surface of a silicon wafer (the first stage of metalassisted chemical etching) is studied. This layer, on the one hand, serves as a catalyst for the subsequent etching of silicon, and, on the other hand, represents a kind of mask for the formation of a certain topology of the emerging Si nanowires. The morphology of the obtained 40- to 60-nm-thick silver nanoparticle layers is investigated by scanning electron microscopy. The spectral dependences of the ellipsometric angles Ψ and Δ are measured using spectroscopic ellipsometry (λ = 250–900nm), and the complex dielectric function of the silver nanolayers is determined from these spectra. The dielectric function features a characteristic plasmon resonance peak in the ultraviolet spectral range. The study of the optical properties of Si nanofilament layers which form during the early stages of metal-assisted chemical etching will be reported as the second part of this work in a separate publication.

Cyclic Voltammetry Peaks Due to Deep Level Traps in Si Nanowire Array Electrodes

When metal-assisted chemical etching (MACE) is used to increase the effective surface area of Si electrodes for electrochemical capacitors, it is often found that the cyclic voltammetry characteristics contain anodic and cathodic peaks. We link these peaks to the charging–discharging dynamics of deep level traps within the nanowire system. The trap levels are associated with the use of Ag in the MACE process that can leave minute amounts of Ag residue within the nanowire system to interact with the H2O layer surrounding the nanowires in a room temperature ionic liquid. The influence of the traps can be removed by shifting the Fermi level away from the trap levels via spin-on doping. These results in lower capacitance values but improved charge–discharge cycling behavior. Low-frequency noise measurements proof the presence or absence of these deep level traps.

Enhancing formation rate of highly-oriented silicon nanowire arrays with the assistance of back substrates

Facile, effective and reliable etching technique for the formation of uniform silicon (Si) nanowire arrays were realized through the incorporation of back substrates with metal-assisted chemical etching (MaCE). In comparison with conventional MaCE process, a dramatic increase of etching rates upon MaCE process could be found by employing the conductive back substrates on p-type Si, while additionally prevented the creation of nanopores from catalytic etching reaction. Examinations on the involving etching kinetics, morphologies, wetting behaviors and surface structures were performed that validated the role of back substrates upon MaCE process. It was found that the involved two pathways for the extraction of electrons within Si favored the localized oxidation of Si at Si/Ag interfaces, thereby increasing the etching rate of MaCE process. This back-substrate involved MaCE could potentially meet the practical needs for the high-yield formation of Si nanowire arrays.

Cantilever with High Aspect Ratio Nanopillars on Its Top Surface for Moisture Detection in Electronic Products

This work reports the patterning silicon pillars by metal‐assisted chemical etching (MACE) process as a post process on a silicon cantilever for a moisture detection. Although the cantilever is very fragile, the patterning of the pillar structures on the cantilever has been successfully demonstrated. The cantilever coated with a material absorbing water (such as polyimide and mesoporous silica) can use as a humidity sensor. Its bending is due to the surface stress change from water molecule absorption. However, the bending of the cantilever is usually at a small value. Here, the silicon cantilever with high aspect ratio pillars on its surface is proposed, which is expected for a larger bending of the cantilever during the water molecule absorption. The moisture detection utilizes the principle that the pillars stack together based upon the condensation behavior of a water vapor on their surfaces.

Vertically Aligned Silicon Nanowire Array Decorated by Ag or Au Nanoparticles as SERS Substrate for Bio-molecular Detection

This review article summerises preparation techniques of vertically aligned silicon nanowire (Si NW) arrays through metal-assisted chemical etching (MacEtch) process and plasmonic nanoparticles (Ag and Au) with the perspective of the fabrication of surface-enhanced Raman scattering (SERS)-active substrates which are highly efficient for bio-molecular detection. At first, basic methods and mechanisms for SERS have been introduced and size and shape effects of the nanoparticles (NPs) on plasmonic vibration have been discussed. Comparative discussions on optical and plasmonic characteristics of Ag and Au NPs have also been presented in this section. Potential techniques for the synthesis of Ag and Au NPs with different sizes and shapes have been reported in the following section. Basic processes and mechanism for the fabrication of vertically aligned Si NW arrays on Si by MacEtch of Si wafer have been discussed. Template-assisted fabrication techniques for the vertically aligned Si NW arrays with controlled diameter and number density have also been reported. Finally, multifarious ways for the fabrication of SERS-active substrates by assembling noble metal NPs onto the NW surface have been discussed and their performance for bio-molecular detection has also been reported.

Antireflection subwavelength structures based on silicon nanowires arrays fabricated by metal-assisted chemical etching

In this paper, we have obtained a series of large-area and different diameters nanosphere lithography (NSL) to obtain the required silicon nanowires (SiNWs) arrays. The single-crystalline SiNWs have been presented by combining nanosphere lithography (NSL) and metal-assisted chemical etching (MACE). The period of SiNW arrays can be controlled by adjusting the original diameter of polystyrene nanosphere (PSs) and the etching time during the NSL process. The special SiNWs structure obtained can be demonstrated to be significant for improving the antireflection properties of silicon substrate. The results show that SiNW arrays with various parameters, such as diameter, distance and height can be obtained by controlling the key etching parameter during the MACE process, which are important to obtain the structures of different parameters to adapt an appropriate value to decrease the light scattering. For a wide wavelength range of 300–1200 nm, the reflectance is below 10% or less, which is due to an ultra-high surface area. Especially, the reflectance of antireflection structure (ARS) surface reduces below 1% over a wavelength range of 300–400 nm. Furthermore, the silicon nanowire (SiNW) arrays with highly efficient antireflection obtained by MACE exhibit different surface roughness from the bottom to the top part of SiNWs by high resolution images, which is benefit for further improving the ARS of SiNWs.

Graphene/porous silicon Schottky-junction solar cells

Porous silicon (PSi) is highly attractive for the solar cell applications due to its unique properties such as efficient antireflection, band gap widening, broad range of optical absorption/transmission, and surface passivation/texturization effect. We first report PSi Schottky-type heterojunction solar cells by employing graphene transparent conductive electrodes doped with silver nanowires (Ag NWs). The PSi is formed based on metal-assisted chemical etching process, and its porosity is controlled by varying the deposition time (td) of Ag nanoparticles used for the etching. The Ag NWs-doped graphene/PSi solar cells show a maximum power-conversion efficiency (PCE) of 4.03% at td = 3 s/concentration (nA) of Ag NWs = 0.1 wt percent (wt%). As td increases, the diode ideality factor and the light absorption increase. As nA increases, the work function (thus the open circuit voltage) and the transmittance decrease whilst the light absorption increases/the sheet resistance decreases. These trade-offs explain why the PCE is maximized at td = 3 s/nA = 0.1 wt%.

MoS2/TiO2/SiNW surface as an effective substrate for LDI-MS detection of glucose and glutathione in real samples

Here, we report for the first time, the use of molybdenum disulfide/titanium oxide/silicon nanowires (MoS2/TiO2/SiNW) surfaces for SALDI-MS detection as alternative to MALDI-MS method. Silicon nanowires were fabricated by the well-known metal-assisted chemical etching process followed by the deposition of TiO2 by atomic layer deposition. MoS2 deposition was achieved through hydrothermal treatment. The MoS2/TiO2/SiNW substrate has shown high performance for the detection of small compounds of different molecular weights, including glutathione, glucose, amino acids, antibiotics to name a few. All of the tested compounds, in pure or in mixed solutions were successfully detected in positive ion mode. Therefore, we have also attempted quantitative measurements of GSH and glucose in human blood serum.

How the Oxidation Stability of Metal Catalysts Defines the Metal-Assisted Chemical Etching of Silicon

Metal-assisted chemical etching (MACE) is done with different metal species. The resulting silicon nanostructures appear strongly dependent on the choice of metal, but a deeper understanding of the MACE process is still missing. We report here direct evidence that the etching solution composition plays a major role in the chemical stability of the metal catalyst used. We show from an elemental analysis of post-MACE etch baths that dissolved silver is found in the bath with concentrations up to 3 orders of magnitude larger than when gold is used. Furthermore, the dissolved silver content also correlates with the amount of H2O2, either in different initial conditions, or as would be expected from its decomposition over time. We also show that silver dissolution leads to unintended etching elsewhere on the substrate. This species-dependent behavior of the metal catalyst is responsible for the different kinds of control possible over the nanostructures produced with silver- and gold-based MACE.

Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm Nanopillars from Phase Separation of Cross-Linkable Polysiloxane/Polystyrene Blend.

We demonstrated a simple and effective approach to fabricate dense and high aspect ratio sub-50 nm pillars based on phase separation of a polymer blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to obtain the phase-separated domains with nanoscale size, a liquid prepolymer of cross-linkable polysiloxane was employed as one moiety for increasing the miscibility of the polymer blend. After phase separation via spin-coating, the dispersed domains of liquid polysiloxane with sub-50 nm size could be solidified by UV exposure. The solidified polysiloxane domains took the role of etching mask for formation of high aspect ratio nanopillars by O2 reactive ion etching (RIE). The aspect ratio of the nanopillars could be further amplified by introduction of a polymer transfer layer underneath the polymer blend film. The effects of spin speeds, the weight ratio of the polysiloxane/PS blend, and the concentration of polysiloxane/PS blend in toluene on the characters of the nanopillars were investigated. The gold-coated nanopillar arrays exhibited a high Raman scattering enhancement factor in the range of 108-109 with high uniformity across over the wafer scale sample. A superhydrophobic surface could be realized by coating a self-assembled monolayers (SAM) of fluoroalkyltrichlorosilane on the nanopillar arrays. Sub-50 nm silicon nanowires (SiNWs) with high aspect ratio of about 1000 were achieved by using the nanopillars as etching mask through a metal-assisted chemical etching process. They showed an ultralow reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm.