Metal-assisted Chemical Etching Research Articles

Fabrication of humidity monitoring sensor using porous silicon nitride structures for alkaline conditions

Porous silicon nitride structures were fabricated for a humidity sensor. The porous silicon structures were fabricated by the metal-assisted chemical etching process, and the conformal silicon nitride thin film was deposited by the atomic layer deposition process. The optimized porous sensor with the 10 nm-thick silicon nitride thin film had a hydrophilic surface and compared to other sensors, had an excellent humidity sensing response. Especially, it showed a superior humidity sensing response at 1 kHz with fast response and recovery times of 13.3 s and 12.4 s, respectively, were observed. Based on the electrochemical impedance spectroscopy results, the equivalent circuits and humidity sensing mechanism were discussed. The chemical stability of the silicon nitride was characterized using Tafel analysis in alkaline electrolytes. Additionally, the sensor's humidity sensing capabilities were tested under cement-embedded conditions.

Platinum-Particle-Assisted Etching of Low-, Moderately-, and Highly-Doped p-Type Silicon: Change of Composite Porous Structure

Metal-assisted etching (metal-assisted chemical etching) is an efficient method to fabricate porous silicon (Si). When using platinum (Pt) particles as metal catalysts in metal-assisted etching, a composite porous structure of straight macropores formed beneath the Pt particles and a mesoporous layer formed on the entire surface of Si can be fabricated. The formation mechanism of the composite structure is still open to discussion. We previously demonstrated that the ratio of mesoporous layer thickness to macropore depth showed a large value (approximately 1.1) in the case of highly-doped p-Si. In this study, we investigated the composite structure formation by using p-Si substrates with different doping densities and etching solutions with different concentrations of hydrogen peroxide (H2O2). There was not significant difference in the structures formed on low- and moderately-doped Si, despite the large difference in doping density. The ratio of mesoporous layer thickness to macropore depth increased within the range approximately from 0.1 to 0.4 with increasing the H2O2 concentration in the case of low- and moderately-doped Si, but it did not change in the case of highly-doped Si. We discussed the observation results based on the spatial distribution of hole consumption and the band structures at Pt/Si and Si/electrolyte interfaces.

Controllable Fabrication of Silicon Solar Cells with Inverted or Upright Pyramid Structures by a One-Step Copper Catalysis Method

Silicon has garnered significant attention as the primary material for solar cell preparation. Traditional alkaline etching solutions are limited to creating an upright pyramid structure on monocrystalline silicon surfaces. However, research indicates that an inverted pyramid structure exhibits superior light-trapping properties compared to the upright pyramid structure. In this study, we employed a one-step copper ion metal-assisted chemical etching process to fabricate an inverted pyramid structure on monocrystalline silicon wafers. This method allows for the customization of either inverted or upright pyramid structures by adjusting the concentration of specific solution components. Characterization of the textured silicon wafers reveals that the inverted pyramid structure exhibits lower reflectivity than both the upright pyramid structure and polished silicon. By integrating this texturing technique into the solar cell production line, we successfully produced solar cells with both inverted and upright pyramid structures. Evaluation of various solar cell parameters demonstrates that the inverted pyramid structure outperforms the upright pyramid structure, showcasing lower reflectivity and higher photoelectric conversion efficiency.

Facile synthesis of RGO/TiO2 heterojunction on SiNWs on Si chip for near infrared detector

Here, a ternary nanostructure-based heterojunction was developed for the detection of infrared (IR) radiation in near wavelength range. During materials synthesis process, silicon nanowires (SiNWs) were developed using metal assisted chemical etching (MACE) technique whereas reduced graphene oxide (RGO) sheets and TiO2 nanoparticles were synthesized by chemical vapour deposition (CVD) and solvothermal method, respectively. Thereafter, RGO/TiO2 nanostructure was deposited over SiNWs to form a heterojunction. To analyse the stoichiometry, morphology, and internal structure, various characterization techniques were performed such as scanning electron microscope (SEM), transmission electron microscope (TEM), Energy dispersive spectrometer (EDS), X-Ray Diffractometer (XRD), UV–Vis spectrophotometer and X-Ray Photoelectron Spectrometer (XPS). The RGO/TiO2@SiNWs nanostructure-based heterojunction was used for the detection of near IR radiation where the responsivity (R) was found to be 17.22 × 10−3 A/W and detectivity (D) was measured to be 0.23 × 1011 Jones with response time < 1 s and recovery time < 3 s. These results were found comparatively better than pristine RGO@Si and RGO@SiNWs nanostructure-based sensors used for the near IR detection under same conditions. The photocurrent response was also recorded with 5 ON/OFF cycles which depicted a good reproducibility of the prepared detector. Hence, the SiNWs on Si chip based highly sensitive and selective photodetector can be used to set a new milestone for various near IR applications.

The nature of photocatalytic hydrogen generation on silicon nanowires prepared by MAWC

Highly n-doped silicon nanowires (SiNWs) exhibit excellent hydrogen generation. Our results indicate three hydrogen generation possibilities, i.e. SiNWs oxidation, photocatalysis and the cleavage of dangling H bonds, are jointly responsible for the efficient hydrogen generation. Oxidation accounts for about 80% of total hydrogen, photocatalysis contributes less than 20% of total hydrogen, and about 0.1% of total hydrogen is from cleavage of dangling H bonds. SiNWs were oxidized in water and form SiOx (0 < X < 2) thin layer. Furthermore, the ratio of photo-generated hydrogen to photo-generated oxygen is about 2.4:1. It suggests that photocatalysis appears to be a process of water splitting. This study is significant to reveal the mechanism of hydrogen production on SiNWs prepared by Metal-Assisted Wet Chemical Etching (MAWC).

Growth of β-Ga2O3 nanostructures by thermal oxidation of GaN-on-sapphire for optoelectronic devices applications

Heterostructures of wide-bandgap semiconductors, like β-Ga2O3/ GaN, are being used in many high-power, high-frequency electronic and optoelectronic devices. This paper presents the growth of β-Ga2O3 thin film and nanostructures by thermally oxidizing (at 1000 °C) the planar and nano-structured GaN surfaces respectively. The nano-structures of GaN (having 300–500 nm long vertical nano-kinks) are fabricated by metal-assisted chemical etching of GaN surface for 1–2 hours. The thermally grown β-Ga2O3 samples are polycrystalline. The chemical analysis of the samples using X-ray photoelectron spectroscopy (XPS) showed enhancement of Ga-O peak intensity (at 20.21–20.3 eV) at the expense of the Ga-N peak intensity (at 19.2–19.7 eV) in the thermal oxidation process. The O1s XPS spectra of the oxidized samples exhibited prominent signature of O2- ions (530.7–530.9 eV) representing the formation of Ga2O3 with presence of adsorbed hydroxyl group at the surface (532.2 – 532.4 eV). The valance band XPS spectra disclosed the formation of an additional Ga(4 s)–O(2p) hybridization energy state, and the valence band maxima increased from 2.3 eV to 3.25 eV. The Raman spectra of the oxidized samples exhibited multiple Ag and Bg vibrational modes of β-Ga2O3 along with intense E2(high), A1(LO) modes of GaN. The time-resolved photoluminescence spectra of the samples demonstrated fast transition of charge carriers from excited state to ground state with 460–820 pico-seconds average career lifetime.

Humidity Diode Sensors Based on 1D Nanosized Silicon Structures

Introduction. Humidity measurement is essential in microelectronics, aerospace, biomedical, and food industries, as well as in households for climate control. Currently, various types of devices have been used as humidity sensors: capacitive, resistive, diode, gravimetric, optical structures, field-effect transistors and devices based on surface acoustic waves.Problem Statement. Today, there is a need to develop IC-compatible humidity sensors that have high sensitivityand low cost. To this end, silicon nanowires have been successfully used in resistive and capacitive humidity sensors. However, there is a lack of research on the nanowire effect on device parameters of diode-type humidity sensors.Purpose. To develop diode sensors based on silicon nanowires and to determine the effect of process parameters of synthesis and structural features of nanowires on the performance of humidity sensors.Materials and Methods. The process of sensor fabrication includes several steps: chemical cleaning of silicon wafer, synthesis of silicon nanowires using standard or modified metal-assisted chemical etching, phosphorus diffusion to create a p-n junction, front and back metallization. The surface morphology of the nanostructures has been studied by scanning electron microscopy. The humidity-sensitive characteristics have been studied with theuse of salt hygrostats.Results. It has been shown that the addition of one-dimensional silicon nanostructures to the diode-type sensor signifi cantly improves its characteristics. The rectification ratio increases from 161 to 1807, the response ups from 4.5 to 25, the sensitivity grows from 1.6 to 4.02 (%RH)–1, while the response time and recovery time are reduced from 85/90 to 25/30 s, the hysteresis value goes down from 75 to 16%, the signal deviation after cycling drops from 15to 3%, and the signal fluctuation during continuous device operation decreases from 17 to 15%.Conclusions. The results have shown that the use of a simple and cheap nanowire synthesis technology is effective to produce humidity sensors.

Anisotropy of the conductivity of silicon nanowires

ABSTRACT This study reports the findings from an investigation into charge carrier transport within a silicon nanowire layer. The samples were prepared using the metal-assisted chemical etching method applied to crystalline silicon wafers with a resistance of 10–20 Ω·cm. The resulting silicon nanowires had a diameter of about 100 nm with a resistance of approximately 15 kΩ·cm. Electrical conductivity measurements were performed in both planar and sandwich configurations, revealing analogous conductivity mechanisms across different geometries. Frequency-dependent conductivity studies unveiled the presence of hopping conductivity. A hypothesis is proposed regarding the existence of a potential barrier at the interface between the nanowire layer and the substrate.

Demystifying metal-assisted chemical etching of GaN and related heterojunctions

GaN and related semiconductors have become an increasingly prominent material for a wide range of active and passive devices from optoelectronics to high frequency and power electronics as well as photocatalysis. Regardless of the application, anisotropic etching is required for micro and nano structuring, currently performed by reactive ion etching (RIE). Alternately, metal-assisted chemical etching (MacEtch) is an open-circuit plasma-free anisotropic etching method that has demonstrated high aspect ratio device structures devoid of plasma-induced damage found in RIE. This paper presents an in-depth study of the ensemble electrochemical mechanisms that govern the photo-enhanced MacEtch process of GaN and related heterojunctions. Through in-depth experimental investigations, modeling and simulations, the effects of local cathode and anode design, energy-band alignments, and solution chemistry on MacEtch are correlated with the underlying electronic mechanisms of carrier generation, annihilation, transport, and extraction, establishing a fundamental framework for parametrized prediction of system behavior. These findings carry profound implications for tailored design of photoelectrochemical processes employed not just for uniformly etching wide/ultrawide bandgap materials but more broadly for semiconductor-based photocatalytic reactions in general. One-pot photo-enhanced MacEtching of AlInGaN multi-heterojunction device structures including superlattices and multi-quantum wells are demonstrated.

Synthesis of Well-Ordered Functionalized Silicon Microwires Using Displacement Talbot Lithography for Photocatalysis.

Metal-assisted chemical etching (MACE) is a cheap and scalable method that is commonly used to obtain silicon nano- or microwires but lacks spatial control. Herein, we present a synthesis method for producing vertical and highly periodic silicon microwires, using displacement Talbot lithography before wet etching with MACE. The functionalized periodic silicon microwires show 65% higher PEC performance and 2.3 mA/cm2 higher net photocurrent at 0 V compared to functionalized, randomly distributed microwires obtained by conventional MACE at the same potentials.

Wetting Properties of Black Silicon Layers Fabricated by Different Techniques

The wettability of black silicon (BSi) layers fabricated by reactive ion etching (RIE), metal‐assisted chemical etching (MACE), and laser‐induced etching (LIE) techniques is studied. The contact angles of wetting on the samples with deionized water and methylammonium iodide‐based perovskite solutions are determined. It is found that the element composition and the enlargement area factor of BSi layers have a significant effect on their wettability. When tested with water, the RIE and MACE BSi layers exhibit hydrophobic properties, while the LIE BSi layer demonstrates hydrophilic properties due to the SiOx‐rich surface structures. It is also shown that aging leads to a decrease in the water contact angle. Upon exposure to perovskite solution droplets, BSi layers become highly lyophilic. Based on the Wenzel and Cassie–Baxter models, the mechanisms responsible for the wetting states of the fabricated samples are identified. In the results obtained, valuable insights are provided into the potential of using these layers in tandem perovskite/silicon solar cells.

Voltage- and Metal-assisted Chemical Etching of Micro and Nano Structures in Silicon: A Comprehensive Review.

Sculpting silicon at the micro and nano scales has been game-changing to mold bulk silicon properties and expand, in turn, applications of silicon beyond electronics, namely, in photonics, sensing, medicine, and mechanics, to cite a few. Voltage- and metal-assisted chemical etching (ECE and MaCE, respectively) of silicon in acidic electrolytes have emerged over other micro and nanostructuring technologies thanks to their unique etching features. ECE and MaCE have enabled the fabrication of novel structures and devices not achievable otherwise, complementing those feasible with the deep reactive ion etching (DRIE) technology, the gold standard in silicon machining. Here, a comprehensive review of ECE and MaCE for silicon micro and nanomachining is provided. The chemistry and physics ruling the dissolution of silicon are dissected and similarities and differences between ECE and MaCE are discussed showing thattheyare the two sides of the same coin. The processes governing the anisotropic etching of designed silicon micro and nanostructures are analyzed, and the modulation of etching profile over depthis discussed. The preparation of micro- and nanostructures with tailored optical, mechanical, and thermo(electrical) properties is then addressed, and their applications in photonics, (bio)sensing, (nano)medicine, and micromechanical systems are surveyed. Eventually, ECE and MaCE are benchmarked against DRIE, and future perspectives are highlighted.

Experimental study on flow boiling heat transfer performance of nanowire-printed substrates with porous-like structures

In this study, ZnO nanowires (NWs) were demonstrated to have considerable potential for heat transfer enhancement in flow boiling. Unlike the conventional NW fabrication method using metal-assisted chemical etching, we use the microcontact printing (µCP) method comprising solution-based growing methodologies to create ZnO NWs on different substrates, including bare Si-, Cu-, and Al-coated surfaces. This ZnO NW fabrication approach successfully overcomes the substrate limitation of typical NWs, such as Si- and Cu-NWs, which can be only formed on its intrinsic material following the high-cost fabrication approaches required by the metal catalyst. Additionally, a high pore-connected structure was observed on the interfacial area of the ZnO NWs, with the nanocore distribution area considerably enhanced by lengthening the NW length, beneficial to bubble nucleation and surface coolant supply during nucleate boiling. The length of the ZnO NW substrates is controllable by varying the growing period during the hydrothermal growth procedure. Further, the ZnO NW substrates reveal a remarkable enhancement in the flow boiling heat transfer regardless of the substrate variation, thereby exhibiting high variability in the substrate selection and potential of being used as the cooling approach for the thermal management of the high heat flux applications. We conducted a series of experiments related to the surface wettability, wicking, and bubble characteristics for different test substrates under saturated flow boiling conditions to explicitly determine the underlying mechanism in the heat transfer enhancement of the ZnO NWs.

Fabrication of Metal Contacts on Silicon Nanopillars: The Role of Surface Termination and Defectivity.

The application of nanotechnology in developing novel thermoelectric materials has yielded remarkable advancements in material efficiency. In many instances, dimensional constraints have resulted in a beneficial decoupling of thermal conductivity and power factor, leading to large increases in the achievable thermoelectric figure of merit (ZT). For instance, the ZT of silicon increases by nearly two orders of magnitude when transitioning from bulk single crystals to nanowires. Metal-assisted chemical etching offers a viable, low-cost route for preparing silicon nanopillars for use in thermoelectric devices. The aim of this paper is to review strategies for obtaining high-density forests of Si nanopillars and achieving high-quality contacts on them. We will discuss how electroplating can be used for this aim. As an alternative, nanopillars can be embedded into appropriate electrical and thermal insulators, with contacts made by metal evaporation on uncapped nanopillar tips. In both cases, it will be shown how achieving control over surface termination and defectivity is of paramount importance, demonstrating how a judicious control of defectivity enhances contact quality.

Ultra Responsive NO2 silicon nanowires gas sensor

Nitrogen dioxide (NO2) is a hazardous gas emitted primarily from burning fossil fuels. When inhaled, it can have severe health implications, particularly for individuals with respiratory conditions such as asthma. It contributes to ground-level ozone formation, which can cause reduced lung function and increased susceptibility to respiratory infections. When in contact with water moisture in the air, NO2 is easily oxidized into nitric acid (HNO3), causing acid rain. Detection of NO2 levels is therefore important for monitoring air quality and protecting public health. Among the possible technical solutions for gas sensing, silicon nanowires (SiNWs) have proven to be a promising candidate, partly due to their high sensitivity, low manufacturing cost, and ability to perform in various air qualities. This paper presents using SiNWs, fabricated by metal-assisted chemical etching, as a fast, accurate, sensitive, robust, and cost-effective NO2 gas sensor. Our SiNWs gas sensor can experimentally detect 20 parts per billion (ppb) of NO2 under a wide range of relative humidity. Our sensor shows ultra-high sensitivity with fast response and recovery time.

Highly sensitive photoelectrochemical sensors for multichannel detection and uniform bioimaging based on fireworks-like silicon nanowires and its heterojunctions fabricated by MACE and sputtering processes

Photoelectrochemical (PEC) sensors have emerged as a promising candidate for biochemical detection. However, PEC sensors based on bulk semiconductors are limited by sensitivity and detection varieties, though they can provide uniform imaging; PEC sensors based on nanomaterials can meet the requirements of various analytes and sensitivity, but the inhomogeneity of their surface poses a challenge to imaging uniformity. In this work, metal-assisted chemical etching (MACE) and sputtering processes were used to directly fabricated PEC sensors with fireworks-like silicon nanowires (F-SiNWs) and their heterojunctions with excellent performance. Through regulating the etching and sputtering times, the structure and nanomaterials attachment of F-SiNWs can be well controlled. The photoelectric and sensing properties of F-SiNWs with different morphologies were investigated and optimized. Then, the sensor fabricated by optimal F-SiNWs can directly detect H2O2 and indirectly detect glucose under negative bias, while directly detect dopamine (DA) under positive bias. The detection of these three analytes do not interfere with each other, contributing to an excellent multichannel detection system. Subsequently, the multichannel detection of various secretions (H2O2, DA and glucose) of PC-12 cells was achieved by the sensor. Finally, the imaging uniformity of the sensor was demonstrated in different solutions. PEC bioimaging was successfully applied to monitor enzyme activity by bioimaging gradients and cell survival by bioimaging distribution. These results demonstrate the high sensitivity in multichannel detection and excellent stability and homogeneity in bioimaging of the sensor. Therefore, MACE and sputtering process can be a promising tool for constructing PEC sensor.

Modulating the density of silicon nanowire arrays for high-performance hydrovoltaic devices

Hydrovoltaic devices (HDs) based on silicon nanowire (SiNW) arrays have received intensive attention due to their simple preparation, mature processing technology, and high output power. Investigating the impact of structure parameters of SiNWs on the performance of HDs can guide the optimization of the devices, but related research is still not sufficient. This work studies the effect of the SiNW density on the performance of HDs. SiNW arrays with different densities were prepared by controlling the react time of Si wafers in the seed solution (t seed) in metal-assisted chemical etching. Density of SiNW array gradually decreases with the increase of t seed. HDs were fabricated based on SiNW arrays with different densities. The research results indicate that the open-circuit voltage gradually decreases with increasing t seed, while the short-circuit current first increases and then decreases with increasing t seed. Overall, SiNW devices with t seed of 20 s and 60 s have the best output performance. The difference in output performance of HDs based on SiNWs with different densities is attributed to the difference in the gap sizes between SiNWs, specific surface area of SiNWs, and the number of SiNWs in parallel. This work gives the corresponding relationship between the preparation conditions of SiNWs, array density, and output performance of hydrovoltaic devices. Density parameters of SiNW arrays with optimized output performance and corresponding preparation conditions are revealed. The relevant results have important reference value for understanding the mechanism of HDs and designing structural parameters of SiNWs for high-performance hydrovoltaic devices.

Photoluminescent porous silicon nanowires as contrast agents for bioimaging

Porous silicon nanowires (pSi NWs) have attracted considerable interest due to their unique structural, optical properties and biocompatibility. The most common method for their top-down synthesis is metal-assisted chemical etching (MACE) of crystalline silicon (c-Si) wafers using silver nanoparticles as a catalyst. However, the replacement of silver with bioinert gold nanoparticles (Au NPs) markedly improves the efficiency of pSi NWs in biomedical applications. The present study demonstrates the fabrication of porous pSi NWs arrays using Au NPs as the catalyst in MACE of c-Si wafers with a resistivity of 1–5 mOhm·cm. Using scanning electron microscopy (SEM), the formation of arrays of porous nanowires with a diameter of 50 nm that consist of small silicon nanocrystals (nc-Si) and pores was observed. Raman spectroscopy analysis determined the size of nc-Si is about 4 nm. The pSi NWs exhibit effective photoluminescence (PL) with a peak in the red spectrum, which is attributed to the quantum confinement effect occurred in small 4 nm nc-Si. In addition, the pSi NWs exhibit low toxicity towards MCF-7 cancer cells, and their PL characteristics allow them to be used as contrast agents for bioimaging

Tip-based Lithography with a Sacrificial Layer.

The fabrication of a highly controlled gold (Au) nanohole (NH) array via tip-based lithography is improved by incorporating a sacrificial layer-a tip-crash buffer layer. This inclusion mitigates scratches during the nano-indentation process by employing a 300nm thick poly(methyl methacrylate) layer as a sacrificial layer on top of the Au film. Such a precaution ensures minimal scratches on the Au film, facilitating the creation of sub-50nm Au NHs with a 15nm gap between the Au NHs. The precision of this method exceeds that of fabricating Au NHs without a sacrificial layer. Demonstrating its versatility, this Au NH array is utilized in two distinct applications: as a dry etching mask to form a molybdenum disulfide hole array and as a catalyst in metal-assisted chemical etching, resulting in conical-shaped silicon nanostructures. Additionally, a significant electric field is generated when Au nanoparticles (NPs) are placed within the Au NHs. This effect arises from coupling electromagnetic waves, concentrated by the Au NHs and amplified by the Au NPs. A notable result of this configuration is the enhancement factor of surface-enhanced Raman scattering, which is an order of magnitude greater than that observed with just Au NHs and Au NPs alone.

Nanoengineered micro-supercapacitors based on graphene nanowalls for self-powered wireless sensing system

This work presents a comprehensive investigation of micro-supercapacitors (MSCs) based on graphene nanowalls (GNWs), covering core material synthesis, device fabrication, evaluation, and application demonstrations. Three-dimensional hierarchical GNWs structures are grown successfully by a microwave plasma-enhanced chemical vapor deposition (MPECVD), with detailed descriptions of the synthesis process and underlying mechanisms. The structural and morphological properties of the GNWs are thoroughly analyzed. Several types of MSCs are reported, ranging from thin-film to three-dimensional (3D) MSCs, as well as symmetric to asymmetric MSCs. Thin-film MSC is developed by a novel micromachining process for one-step fabricating GNWs-Ni core-shell composite microstructures. A solid-state MSC utilizing 3D hierarchical electrode is created by a fully micro electro mechanical system (MEMS)-compatible process. This involves depositing GNWs-RuOx hybrid nanostructures onto microstructure scaffolds formed using deep reactive ion etching (deep RIE). Additionally, another 3D MSC utilizes GNWs-PANI electrode material deposited on silicon nanowires generated through metal-assisted chemical etching (MACE) method. To enhance MSCs performance, asymmetric MSC is proposed and realized, incorporating GNWs-MnO2 acting as the positive electrode and carbon black serving as the negative electrode. Furthermore, the successfully combination of the fabricated MSCs with an energy harvesting system is demonstrated. The self-powered sensing system driven by solar energy, with the harvested energy stored in MSCs, is effectively showcased, opening up new possibilities for futuristic MSCs to accumulate and store harvested energy as usable electricity. This demonstration has significant potential in fields such as self-powered wireless IoT sensing systems, offering valuable contributions towards sustainable and autonomous energy solutions.