Macroporous Silicon Layers Research Articles

Kinetics of charge carriers in bilateral macroporous silicon

The kinetics of charge carriers in bilateral macroporous silicon with macroporous layers of equal thicknesses is calculated by the finite difference method. A diffusion equation for a monocrystalline substrate and macroporous layers is solved. The boundary conditions are defined at the boundaries between the monocrystalline substrate and the macroporous silicon layers on both sides. Stationary distribution of excess charge carriers in the bilateral macroporous silicon with the macroporous layers of equal thicknesses calculated by the finite difference method is set as the initial condition. Under stationary conditions, excess charge carriers are generated by light with the wavelengths of 0.95 µm and 1.05 µm. It is shown that at the counting times much longer than the relaxation time, all the distributions of the concentration of excess minority carriers generated by light with any wavelength approach the same distribution with exponentially decreasing value.

Elastic properties of porous silicon layer of hybrid SiC/Si substrates

Elastic properties of porous silicon layer of hybrid SiC/Si substrates grown by the atomic substitution method are investigated. The feature of the growth method is the formation of the macroporous silicon layer at the SiC/Si interface during growth. The elastic properties of the layer are studied using the finite element method. The biaxial modulus of the porous silicon is obtained as a function of porosity considering the different shape of the pores and presence of thin SiC boundary layer. The presence of the pores in the silicon layer adjacent to SiC results in the decrease of the elastic moduli by about 35%. However, this leads to a negligibly small change of the substrate curvature.

Different Electrode Configurations for NH3 Gas Sensing Based on Macro Porous Silicon Layer

Different Electrode Configurations for NH3 Gas Sensing Based on Macro Porous Silicon Layer

Efficient, fast response, and low cost sensor for NH3 gas molecules based on SnO2: CuO/macroPSi nanocomposites

In the present study, a procedure of the inserting of SnO2:CuO nanoparticles with different CuO nanoparticles contents within a macroporous silicon layer (macroPSi) gas sensor was prepared and successfully investigated. The macroPSi was effectively fabricated by laser assisted etching process, and CuO nanoparticles loaded with SnO2 with a high value of surface area were successfully synthesized by the spray pyrolysis method. Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FE-SEM) manifested a novel morphology for CuO Bucky particles inside the pores and a nano nail like structure for SnO2 with a small average grain size of CuO Bucky particles with 30% content. This morphology of nanocomposites improved the sensing performance for NH3 gas. A higher sensitivity with a very swift response and recovery times of 4 s and 55 s, respectively, was obtained with 150 ppm of NH3 gas at the room temperature. This improvement in gas sensor performance is strongly related to the higher specific surface areas and smaller particle size with a higher surface roughness of SnO2 and CuO nanoparticles within the nanocomposites.

Fast response and recovery of nano-porous silicon based gas sensor

Fusion of the transduction mechanism in micro and macro nanopores of porous silicon (PS) was employed to fabricate an MEMS-based aliphatic alcohol impedance sensor. The presence of a nanopore network on silicon was confirmed by the SEM image. The morphology of the PS nanopores was roughly distributed in a uniform manner. The performance of the sensor was studied using Impedance spectroscopy at room temperature. Electrochemical impedance spectroscopy and an equivalent circuit analysis of the small amplitude (± 10 mV) AC impedance measurements (frequency range 0.1 Hz–1 kHz) at ambient temperature were carried out. The Sensing layer consists of nanopores (45.30–71.13 nm), micropores (0.95–5 µm), and comb type alumina electrodes with the micro PS layer having a thickness of about 0.2 µm and the macro PS layer having a thickness of about 4 µm. These results were used to assess the effect of the micro PS and macro PS of the particulate layer on the conductivity of the given aliphatic gases. The measured impedance was approximately 2.3e5 for the micro PS, and 3.22e5 macro PS for 8 ppm of gas injected into the gas chamber. The grain boundary resistance increases with an increase in the concentration of butane, benzene, and methane, which ranges from 2 to 16 ppm.

Enhanced Morphological Properties of Macroporous Silicon with the Incorporation of Au-Ag Bimetallic Nanoparticles for Improved CO2 Gas Sensing

In this work, the effects of incorporating bare macroporous silicon (macro-Psi) layer with bimetallic nanoparticles Au-Ag on the performance Psi CO2 gas sensor synthesized by a laser-assisted etching (LAE) process were investigated. Well-controlled and simple immersion process of bare macro-Psi layer in the 10−3 M of silver nitrate (AgNO3) and chloroauric acid (HAuCl4) individually and in the mixing solution with ratio 1:1 was employed to synthesize monometallic AuNPs/macro-Psi, AgNPs/macro-Psi, and bimetallic Au-AgNPs/macro-Psi hybrid structures of CO2 gas sensors. Morphological properties of bare macro-Psi layer, monometallic, and bimetallic hybrid structures were investigated using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and energy-dispersive X-ray analysis (EDS). Electrical characteristics (I–V)of the sensor were measured primary in a vacuum case and with CO2 at 0.2, 0.5, 1, and 1.5 mbar gas pressures. The result showed a significant enhancement in sensitivity and temporal response of the bimetallic hybrid structures compared with that of monometallic and bare macro-Psi layer and an enhanced performance of the sensors due to the high value of integrated specific surface area of the bimetallic nanoparticles and the resulting Schottky barrier height.

Formation of Porous Silicon by Nanopowder Sintering

The technique of the electrochemical and photoelectrochemical etching of single-crystal silicon wafers commonly used to fabricate macroporous silicon layers is inefficient and costly. An alternative method for forming bulk macroporous silicon is the high-temperature sintering of Si powder. The process of nanopowder sintering preliminarily subjected to dry cold compression (without binding additives) is investigated. The properties of the sintered material, including its microstructure, density, and electrical conductivity, are studied at different annealing temperature and time. Techniques for changing the porosity of the sintered samples and for determining of the interior surface area are discussed.

Improved cycling performances of binder-free macroporous silicon Li-ion negative electrodes using room temperature ionic liquid electrolyte

The present article describes the innovative combination of freestanding macroporous silicon layers on copper foil collectors and room temperature ionic, liquid 1-propyl-1-methylpyrrolidinium bis(trifluosulfonyl)imide (Pyr13 FSI)-containing electrolyte (Pyr13 FSI mixed with 1 M LiTFSI) to produce highly stable negative electrodes for Li-ion batteries. A 20-μm-thick macroporous silicon layer was synthesized by anodization in hydrofluoric acid (HF)-based electrolyte followed by the deposition of a thick and mechanically stable copper layer acting as current collector. After peeling-off the parent substrate, the freestanding macroporous silicon layer was characterized as a negative electrode in a half-cell configuration. The electrode performances were determined under constant charge conditions (500, 750, and 1000 mA h g−1) and exhibited significantly higher stability for over 1800 charge and discharge cycles in 1 M LiTFSI dissolved in Pyr13 FSI.

Photoluminescence of Hybrid Structure Base in ZnO@SiO<sub>2</sub> Core-Shell Nanoparticles inside Porous Silicon

In this work, core-shell ZnO@SiO2nanoparticles (NPs) were infiltrated into a macro/meso-porous silicon (PS) structure, to study its luminescent properties. The core-shell ZnO@SiO2NPs were obtained by colloidal synthesis. The core-shell ZnO@SiO2NP was 5 nm in diameter. The macro/meso-PS structure was made in two steps: we obtained the macroporous silicon (macro-PS) layer fist and the mesoporous silicon (meso-PS) layer second. This process was conducted using different electrolyte solutions, and the change of electrolyte led to a decrease in the special charge region over the wall macro-PS layer; this allowed the building of the meso-PS layers on the walls and the bottom of the macro-PS layer. The SEM results show the cross-section of the macro/meso-PS structure with and without core-shell ZnO@SiO2NPs. These SEM images show that the core-shell ZnO@SiO2NPs that infiltrated into macro/meso-PS structure were more efficiently bonded over all the porous walls. The core-shell ZnO@SiO2PL interacted with the macro/meso-PS structure, modifying its PL intensity and controlling a shift toward a lower wavelength.

Получение пористого кремния путем спекания нанопорошка

AbstractThe technique of the electrochemical and photoelectrochemical etching of single-crystal silicon wafers commonly used to fabricate macroporous silicon layers is inefficient and costly. An alternative method for forming bulk macroporous silicon is the high-temperature sintering of Si powder. The process of nanopowder sintering preliminarily subjected to dry cold compression (without binding additives) is investigated. The properties of the sintered material, including its microstructure, density, and electrical conductivity, are studied at different annealing temperature and time. Techniques for changing the porosity of the sintered samples and for determining of the interior surface area are discussed.

The role of aluminum in the formation of macropores, microtubes of silicon and nickel made from porous silicon

Silicon tubes with small dimensions show to be a suitable material for sensors application as well as to other areas of nanotechnology, but its production is not trivial, because of the influence of different formation parameters and the most important of them could be pointed out to the substrate pre-conditioning. In this paper is reported the investigation about the influence of aluminum treatment of crystalline substrates upon the electrical and morphological features of macroporous silicon layers, as well as upon the microtubes yielded by selective corrosion in etchant based in ammonium fluoride solution mixed with nickel ions, as function of the sintering time. The results point out that solely for sample sintered for 4.0 hours nickel microtubes are formed, whereas for other sintered times, silicon microtubes formation are achieved. This fact is strongly linked to the amount of aluminum and oxygen diffused inside the crystalline silicon substrate, which remains even after pores formation. These diffused impurities promote strong chemical modification of the substrate, reason by which its open circuit potential is sharply shifted toward more negative regions. The Tafel plot shows a highest increment of the cathodic current of this sample, which is compatible with the sharp shift of the open circuit potential and enhancement of the exchange anodic current giving rise to the condition for the formation of nickel microtubes through the substitution of silicon atoms by nickel owing to the galvanic displacement effect. The effect of Al-treatment upon the pore's morphology of macroporous layers are shown, as well as the possible mechanism for the formation of nickel microtubes from these macroporous structures are also discussed in this paper.

Physical and chemical characterization of PANI/SiO2/MPS heterostructure to be used as high sensitivity chemosensor for naphthalene

Macroporous silicon layer was passivated by chemical oxidation and then polyaniline was deposited chemically within the porous structure in order to fabricate a heterostructure to be used as chemical sensor. The structural characterization of this device by scanning electron microscopy reveals the effectiveness of the chemical method for polyaniline deposition in oxidized porous matrix. The analysis by AT-FTIR and Raman spectroscopy shows that polyaniline was deposited in its emeraldine state, which is the most conductive phase, forming a well-defined replica of the porous structure. However, according to the EDS analysis, this polymer layer is non-homogeneous in depth. This feature was also confirmed by electrical measurements of current-voltage and electrochemical impedance at different points between the polymer surface and silicon substrate. In spite of this fact, the immersion of this structure in an ethanolic solution containing low amounts of naphthalene shown that this structure is excellent to be used as chemical sensor for the detection of low concentrations of naphthalene in the order of 30 ppb.

In English

Relaxation of the excess minority carrier distribution in macroporous silicon structure was calculated using the finite difference method. The initial distribution of the excess minority carriers has two maxima after carrier generation by electromagnetic wavelength 0.95 μm with small absorption depth. The first maximum of the initial distribution function is in macroporous layer, the second one is in monocrystalline substrate. Surface recombination leads to the diffusion of excess charge carriers to recombination centers and creates the non-homogeneity of the excess charge carrier distribution. A rapid maximum decrease of the excess carrier distribution function in a macroporous layer and near the boundary of a macroporous layer and monocrystalline substrate is found. The slow decrease of the distribution function in a monocrystalline substrate is evaluated. We observed one maximum of the excess minority carrier distribution after homogeneous carrier generation by electromagnetic wavelength 1.05 μm with big absorption depth. The rate of change in the concentration of excess minority carriers decreases in time in macroporous silicon layer due to high recombination and increases due to diffusion to the surface of silicon substrate after generation by the electromagnetic wave 0.95 μm with small absorption depth. The rate of change in the concentration of excess minority carriers decreases in time in the entire structure after generation by the electromagnetic wave 1.05 μm with big absorption depth and small non-homogeneity.

Probing of Free Charge Carriers in Nanostructured Silicon Layers by Attenuated Total Reflectance Technique

Infrared spectroscopy in attenuated total reflection (ATR) mode is used to evaluate the concentration of free charge carriers (electrons) in n‐type macroporous silicon (Si) layers formed by metal‐assisted chemical etching of crystalline silicon (c‐Si) followed by additional doping with phosphorus. The ATR spectra are fitted by considering an effective medium approximation and Drude model, which is modified to describe an additional scattering of the charge carriers on the surface of Si nanocrystals. The electron concentration is of the order of 1019 cm−3 and it is slightly dependent on the layer thickness, while the surface scattering of charge carriers increases with layer thickness. The obtained results are discussed in view of possible applications of the ATR method for an express diagnostics of doping efficiency of Si nano‐ and microstructures for photonic and thermoelectrical applications.

Formation of cross-cutting structures with different porosity on thick silicon wafers

Three-layered pass-through structures of two types have been obtained on 500μm thick single crystal silicon wafers by electrochemical etching in a 48% solution of hydrofluoric acid without using additional single crystal layer removal operations. The first type of the pass-through structures comprises two outermost 220–247.5μm thick macroporous silicon layers with a pore diameter of 7–10μm and an intermediate 5–60μm thick mesoporous silica layer with a pore diameter of 100–150nm. For the formation of the first type structures we used two-stage etching without cell disassembly:–in a 48% water solution of hydrofluoric acid (H2O: HF = 1: 1 vol.) for 140–180min with a current density of 40mA/cm2;–in a 48% water/alcohol solution of hydrofluoric acid (H2O: HF: С2Н5ОН = 1: 1: 1 vol.) for 60–90min with a current density of 10mA/cm2.The second type pass-through structures include a macroporous silicon layer with a thickness of 250μm which interlock in the depth of the silicon wafer to form a cavity with a size of 4–8μm. For the formation of the second type structures we only used the first one of the abovementioned stages, the etching time being longer, i.e. 210min. All the etching procedures were carried out in a cooling chamber at 5°C. The developed technology will provided for easier and more reliable formation of the monolithic structures of membrane-electrode assembly micro fuel cells.

Silicon microtubes made by immersing macroporous silicon into ammonium fluoride solution

In this paper, we report the fabrication of silicon microtubes (SMT) by electrochemical dissolution of macroporous silicon layers immersed into aqueous solution of NH4F mixed with Na2SO4, at different pH levels (4.5–9.5). Well-defined SMT have been obtained at pH 6.5, 7.5 and 9.5, whereas for other pH levels, the surface of the porous structure becomes passivated or formation of needle-like structure occurs. The mean inner diameter and the thickness of walls of the tubes shown to be depended on the pH level of the solutions. In this investigation, we obtained silicon tubes with inner diameter from 0.2 to 2.0 μm with thickness wall from about 0.23 to 0.50 μm. The investigation of these structures indicates that for tube formation the pH level and predominance of the anisotropic dissolution are necessary conditions. The tube formation cannot be explained regarding the silicon dissolution mechanism. For this aim, the role of the space charge region must be regarded. The structural characterization of silicon tubes was achieved by X-ray diffraction (XRD), X-ray energy dispersive Spectroscopy (EDS) and Raman spectroscopy and the results pointed out that these tubes are constituted by crystalline silicon structures covered by a very thin silicon oxide film. In this paper, the formation mechanism of the SMT is also discussed, highlighting the role of the space charge region (SCR) and the pH level for tube formation.

Anodes Based on Porous Silicon Films Using Polymer Electrolyte for Lithium-Ion Microbatteries

Today, the development of efficient storage systems is an important topic for electronic manufacturers. Indeed, with the arrival of new portable autonomous systems such as RFID Taggs, SmartCards, autonomous sensors, it’s important to develop a new energy storage compatible with these new technologies. Current Li-ion technologies are not compatible with these new technologies. This is due to the oversize in comparison with electronic device. Moreover, classical Li-ion batteries use liquid electrolyte. Liquid electrolyte is an important problem for safety (flammable, leakage). In this context, the development of low flammability and no volatile electrolytes to replace classical liquid electrolytes is then highly popular. One alternative electrolyte was proposed: polymer based electrolytes. Polymer electrolyte composed with PVdF/HFP and Pyrrolidinium bis(fluorosulfonyl)imide (P13FSI) was chosen for their good electrochemical, mechanical and transport properties. Concerning microbatteries, due to its high theoretical gravimetric capacity of 3579 mAh.g-1 at room temperature, silicon (Si) is a candidate to use in microbatteries. Bulk single crystalline silicon wafers are not suitable for a reliable negative electrode material because of strong volume changes that Si undergo during lithiation and delithiation reactions [1]. Nanostructured Si materials would be stable towards volumetric expansion and would limit crackings and pulverization during cycling, and hence improve the capacity retention [2]. We present an inexpensive and industrially scalable procedure to prepare porous silicon (pSi) based anodes. Meso or macroporous Si is prepared by electrochemical etching of a Si wafer to form a pSi layer, which is then simultaneous peeled-off and transferred onto a metallic current collector. Free standing and flexible films of porous silicon are thus achieved. Transferring a macroporous silicon layer onto a copper current collector follows these steps : 1) Electrochemical etching of Si wafer to achieve pores (Fig.1a) 2) Copper nanoparticles seeding by electroless deposition (Fig.1b) 3) Electroplating of a copper film 4) Mechanical peeling-off of the pSi/Cu film. SEM observation after peeling shows that the obtained pSi/Cu film surface is homogeneous (Fig.1c) and at higher magnification it is possible to descry the Cu seeding film at the bottom of the pores (inset of Fig.1c). The photograph of the final device (Fig.1d) shows, at a macroscopic scale, the homogeneity of the integrated pSi on the Cu current collector and the flexibility of the pSi/Cu film. Microbatteries were assembled by hot pressed polymer electrolyte on the pSi/Cu film. After that, lithium foil was put on it and sealed in button cell. In this communication, our results on the characterization and the use of polymer electrolyte with anodes based on porous silicon will be exposed. Thermal properties, ionic conductivity, diffusion coefficient of the electrolyte and electrochemical studies (galvanostatic cycling) will be exposed. A comparative study between polymer electrolyte and the corresponding ionic liquid (P13FSI) as ionic liquid electrolyte and their impact on the silicon morphology will be presented. 1. Y. He, X. Yu, G. Li, R. Wang, H. Li, Y. Wang, H. Gao, X. Huang, Journal of Power Sources, 216 131 (2012). 2. M. Ge, J. Rong, X. Fang and C. Zhou, Nanoletters, 12, 2318 (2012). Figure 1

FORMATION OF CROSS-CUTTING STRUCTURES WITH DIFFERENT POROSITY ON THICK SILICON WAFERS

Through three−layered structure has been formed on silicon wafers 500 microns thick by electrochemical etching in a solution of hydrofluoric acid without using additional deletions monocrystalline layers. The resulting structures are divided into two types. The first type pass−through structure comprises two outermost macroporous silicon layers 220—247,5 microns thick with a pore diameter 7—10 microns and an average mesoporous silica layer 5—60 microns thick with a pore diameter of 100—150 nm. The second type pass−through structure includes macroporous silicon layer 250 microns in thickness, interlocking in the depth of the silicon wafer to form a cavity the size of 4—8 microns. The developed technology will allow forming monolithic structures of membrane−electrode assembly microfuel elements in an easier and more reliable manner.

Effect of the duration of electrochemical anodization on the microhardness of macroporous silicon

The dependence of the microhardness H of macroporous silicon layers formed on the duration tan of electrochemical anodization at a constant current density of J = 20 mA/cm2 is studied. It is shown that the nonlinear run of the dependence H(tan) is due to the influence exerted on the microhardness not only by the surface density of pores, but also by the single-crystal base at L < 10h, where L and h are the thickness of the porous layer and the depth of indenter penetration, respectively.

Lambertian light trapping in thin crystalline macroporous Si layers

AbstractLambertian light trapping is a benchmark for efficient light trapping. In this Letter we experimentally quantify the degree of Lambertian light trapping in a macroporous silicon layer. The optical absorption of the effective (26.7 ± 5.5) µm thick sample with randomly arranged pores yields a photogeneration corresponding to a maximum current density of (40.8 ± 0.4) mA cm–2 and thus achieves a fraction of 0.985 ± 0.012 of the current density expected from a Lambertian light trapping scheme. The measured spectrum of the escape reflectance is well described with an analytic model assuming a complete randomization of the directions of light propagation. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)