Porous Silicon Process Research Articles

Novel approach of SERS sensors AgNPs/PSi by incorporated MWCNT

In this study, two types of nano photonics sensors silver nanoparticles/porous silicon (AgNPs/PSi) and silver nanoparticles/multi wall carbon nanotube/porous silicon (AgNPs/MWCNT/PSi) were formed and tested extensively. The porous silicon (PSi) substrate was formed by laser assisted chemical etching process at laser intensity 150 mW/cm2 and wavelength 630 nm respectively. The formation of these types of nano photonics SERS sensors were carried out by a simple and low cost dipping process of porous silicon (PSi) in silver nitrate (AgNO3) and multi wall carbon nanotube (MWCNT) solution fixed concentration of AgNO3 about (10−3 M). These sensors were tested at different concentrations of difenoconazole, 10−5, 10−7, 10−9 and 10−10 M at room temperature. The research results showed an excellent sensing behavior of silver nanoparticles/multi wall carbon nanotube/porous silicon (AgNPs/MWCNT/PSi) comparing with the silver nanoparticles/porous silicon (AgNPs/PSi) nanoparticles sensors. Higher enhancement factor with lower limit of detection with multi wall carbon nanotube (MWCNT) were improved by two orders of magnitude, due to the high value of the sensing area after incorporating multi wall carbon nanotube (MWCNT) to nano photonics sensors.

Research on micro-cavity structure processing technology based on porous silicon

Aiming at the problem of heterogeneity of sealed cavity in silicon microstructure processing technology, the technology of preparing micro-cavity by using porous silicon sacrificial layer is proposed. The effect of current density on the preparation of porous silicon and the effect of porous silicon with different porosity on the formation of micro-cavity in the preparation process of porous silicon were studied. Different process parameters were selected for experiments and the prepared micro-cavities were tested and analyzed. According to the test results, the suitable electrochemical corrosion process parameters were selected to prepare porous silicon, and the micro-cavity was realized by changing the process parameters, which greatly increased the application fields of micro-sensors and micro-actuators.

Influence of Anodizing Parameters on the Electrochemical Characteristics and Morphology of Highly Doped P-type Porous Silicon

Controlling pore structure, e.g. pore diameter, size and distribution of porous silicon (PS) is highly needed for technological applications such as microfluidic operation and filtration in which the pore size and shape can block the entrance of some species or increase its concentration or purity by separation. In the present study, highly doped p-type silicon is utilized to prepare PS with different pore dimensions by electrochemical etching (anodization) process in HF and C2H5OH mixture. The anodization process was performed under various conditions such as different electrolyte types, temperature, and concentrations, etching potential, current density, and etching duration. This research was to further study the influence of electrochemical etching parameters on pore formation kinetics including pore size, layer thickness and mechanistic aspects of silicon etching. The study offers a fundamental understanding of the fabrication process of porous silicon that may be exploited in nanotechnology applications. The present study confirms that the anodization rate increases with increasing the steady-state current at all the presented conditions. It is demonstrated that adjusting the anodizing potential is very important as it determines whether the anodization will lead to PS formation and/or PS dissolution.

Detachment yield statistics for kerfless wafering using the porous silicon process

The porous silicon (PSI) process is a wafering method to fabricate high quality kerfless crystalline Si wafers by epitaxial wafer growth on porous Si and subsequent detachment from a reusable substrate wafer. The process yield is a key parameter for the economic viability of the PSI process. We experimentally demonstrate the detachment of 59 out of 62 PSI wafers with a size of 10 × 10 cm2, and separation layer etch current densities of 105–120 mA/cm2 for electrochemically etching the porous Si, and for substrate wafers with a resistivity of 15.7–16.9 mΩcm. We discuss the statistics of how to deduce a detachment probability from this. From our experiments, we determine a detachment yield of at least 88% with an error probability of 5%. The demonstration of a 99% detachment yield with an error probability of 5% would require at least 300 successfully detached wafers with no failed detachment. Samples have a minority carrier density ranging from 1 to 1.7 ms before any external gettering, which demonstrates the high electric quality of the PSI wafers.

The Influence of Anodization Time With The Electrochemical Cell Design on The Fabrication Process of Porous Silicon Nanostructures

The influence of anodization time with the electrochemical cell design on the fabrication process of porous silicon (PS) nanostructures based on two electrochemical anodization cells (designed single tank cell and double tank cell) with two anodization times (10 and 30 minutes) was studied. Atomic force Microscopy (AFM) characterization had revealed three types of pores, mesopores, mesopore fill of mesopores, and macropore fill of mesopores were obtained from designed single tank cell with (10 and 30 minutes) of anodization time, whilst for double tank cell has not revealed precise information about the size and type of pores. Pores formation have been further approved by current-voltage (I-V) measurement and photoluminescence emission by ultraviolet-lamp (254-366) nm for both electrochemical cells.

Optimizing of Gold Nanoparticles on Porous Silicon Morphologies for a Sensitive Carbon Monoxide Gas Sensor Device

A set of carbon monoxide (CO) gas sensors based on porous silicon (PSi)/gold nanoparticle (AuNP) hetro structures were fabricated. Different forms of PSi surface morphologies were studied as a substrate for growth of AuNPs. Simple dipping process of PSi in hydrogen tetrachloroaurate (III) solution (HAuCl4) at fixed concentrations of 10−2 M/3.5 HF was used to synthesize AuNPs. The n-type PSi was equipped through photo-electrochemical etching process at current density value of 10 mA/cm2 under illumination condition of 530-nm wavelength and laser illumination intensity of 20 to 80 mW/cm2. Three different forms of PSi morphology, meso, macro, and double layers with pore shapes and sizes, were prepared. The structural and surface morphology properties of PSi-based substrate before and after deposition of AuNPs were investigated through studying of scanning electron microscopy (SEM), photoluminescence (PL), and X-ray diffraction (XRD). The electrical property (J-V) was carried out in primary vacuum and CO at low pressure. The results show that PSi surface morphologies strongly influenced the AuNP sizes and hence the sensor performance. It was found that decrease the AuNP sizes could be occasioned in high and fast current response.

Modifying and Fine Controlling of Silver Nanoparticle Nucleation Sites and SERS Performance by Double Silicon Etching Process

Different forms of modified and well-controlled plasmonic silver nanoparticles (AgNPs) were synthesized by silver ion reduction process of porous silicon (PS). Fine control of PS surface morphology was accomplished by employing two etching processes: light-induced etching (LIE) and photo electrochemical etching (PECE). The idea was to prepare excellent and reproducible surface-enhanced Raman scattering (SERS) substrates with high enhancement performance. PS surface modification was employed to create efficient and nearly uniformly distributed AgNP hotspot regions with very high specific surface areas. Reproducibility deviation of no more than 5% and enhancement factor of 1.2 × 1014 were obtained by SERS measurements at very low, rhodamine 6G (R6G) dye, concentration 10−15 M. The PS morphology SERS substrate was well discussed and analyzed using field emission scanning electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), and Raman measurements.

Lifetime Analysis for Defect Characterization in Kerfless Epitaxial Silicon from the Porous Silicon Process

Kerfless epitaxial silicon from the porous silicon (PSI) process is a promising alternative for standard wafers. They allow the reduction of PV costs by combining high material quality at reduced production costs. We evaluate the minority carrier lifetime of p-type and n-type epitaxial silicon layers fabricated with the PSI process by means of photoconductance decay measurements. For p-type layers we observe a strong injection dependence of the lifetime that we attribute to bulk Shockley-Read-Hall (SRH) recombination. We determine two limiting defects K3.6 and K157 that describe the injection dependence of 9 samples grown in one batch. Defect K3.6 has a symmetry factor of k=3.6 and is similarly concentrated in all 9 investigated samples. Its concentration decreases upon high temperature processing with and without phosphorous diffusion. The defect K157 has a symmetry factor of k=157 and a higher concentration in samples with a higher porosity in the starting layer. As a consequence of the k-factors being larger than unity the identified defects are less detrimental in n-type silicon than p-type silicon. Accordingly, we fabricate n-type epitaxial layers for which we measure effective lifetimes up to 1330±130μs at Δp = 1015 cm –3.

Optimizing of porous silicon morphology for synthesis of silver nanoparticles

A group of porous silicon (PSi) samples with different surface morphologies prepared at different etching current densities were investigated as a substrate for formation of silver nanoparticles (AgNPs). Simple dipping process of PSi in silver nitrite (AgNO3) with concentrations of 10−3, 10−2, and 5 × 10−2 M was employed to synthesize AgNPs.The p-type PSi was prepared by electrochemical etching process at different values of current densities. Three different forms of PSi morphology; meso, macro, and pillar with different roughness values 1.21, 177, 89.5, and 38.7 nm were prepared with varying the current densities 30, 60, 90, and 120 mA/cm2 respectively. The structural and surface morphology properties of samples before and after dipping in AgNO3 were studied through analyzing of scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The results show that the AgNPs sizes depend on the surface morphology and roughness of PSi. The AgNPs prepared by dipping process does not depend on the porosity of the porous layer. For meso pore-like structures the sizes of AgNPs were in the range from 0.52 to 4.09 μm for a low surface roughness, and from 0.52 to 6.83 nm for high surface roughness. For the case of macro pore-like structure which possesses the highest pore sizes and surface roughness, the AgNPs sizes vary from 0.61 to 3.42 μm. The minimum AgNPs size of about 190 nm was obtained from porous surface of pillar form.

Synthesis and Functionalization of L, DL-Iso, Nor, Leucine on Porous Silicon for Sensing Application

The etching process of Porous Silicon has been carried out using an electrochemical oxidation technique. Oxidized samples were immobilized with -Leucine, DL-Iso Leucine and DL-Nor Leucine solutions. Surface morphology and optical properties of the samples have been analyzed using scanning electron microscopy and optical absorption analysis techniques respectively. The bonding characteristics on the surface before and after amino acid deposition have been studied using fourier transform infrared spectroscopy. Photoluminescence spectroscopy was employed to determine the enhancement in wavelength shift of the etched porous silicon.

Fabrication of Porous Silicon Based Humidity Sensing Elements on Paper

A roll-to-roll compatible fabrication process of porous silicon (pSi) based sensing elements for a real-time humidity monitoring is described. The sensing elements, consisting of printed interdigitated silver electrodes and a spray-coated pSi layer, were fabricated on a coated paper substrate by a two-step process. Capacitive and resistive responses of the sensing elements were examined under different concentrations of humidity. More than a three orders of magnitude reproducible decrease in resistance was measured when the relative humidity (RH) was increased from 0% to 90%. A relatively fast recovery without the need of any refreshing methods was observed with a change in RH. Humidity background signal and hysteresis arising from the paper substrate were dependent on the thickness of sensing pSi layer. Hysteresis in most optimal sensing element setup (a thick pSi layer) was still noticeable but not detrimental for the sensing. In addition to electrical characterization of sensing elements, thermal degradation and moisture adsorption properties of the paper substrate were examined in connection to the fabrication process of the silver electrodes and the moisture sensitivity of the paper. The results pave the way towards the development of low-cost humidity sensors which could be utilized, for example, in smart packaging applications or in smart cities to monitor the environment.

Electrochemical impedance spectroscopy of oxidized porous silicon

We present a study of the electrochemical oxidation process of porous silicon. We analyze the effect of the layer thickness (1.25–22μm) and of the applied current density (1.1–11.1mA/cm2, values calculated with reference to the external samples surface) on the oxidation process by comparing the galvanostatic electrochemical impedance spectroscopy (EIS) measurements and the optical specular reflectivity of the samples. The results of EIS were interpreted using an equivalent circuit to separate the contribution of different sample parts. A different behavior of the electrochemical oxidation process has been found for thin and thick samples: whereas for thin samples the oxidation process is univocally related to current density and thickness, for thicker samples this is no more true. Measurements by Energy Dispersive Spectroscopy using a Scanning Electron Microscopy confirmed that the inhomogeneity of the electrochemical oxidation process is increased by higher thicknesses and higher currents. A possible explanation is proposed to justify the different behavior of thin and thick samples during the electrochemical process.

High efficiency back-contact back-junction thin-film monocrystalline silicon solar cells from the porous silicon process

This work demonstrates the fabrication of a 45 μm thick back-contact back-junction thin-film monocrystalline silicon solar cell from the porous silicon process with an energy conversion efficiency of 18.9%. We demonstrate an efficiency improvement of 5.4% absolute compared to our prior record of 13.5% for back-contact back-junction thin-film monocrystalline silicon solar cells. This increase in efficiency is achieved by reducing the recombination at the base contact using a back surface field and by increasing the generation with a front texture. We investigate the loss mechanisms in the cell using finite element simulations. A free energy loss analysis based on experiments and simulations determines the dominating loss mechanisms. The efficiency loss by base recombination is 0.8% absolute and the loss by base contact recombination is 0.5% absolute in the 18.9% efficiency cell.

Applications of porous silicon formed by electrochemical etching using an electrolyte based on HF:formaldehyde

In this work, we report the experimental results on the formation of porous silicon (PSi) monolayers by electrochemical etching using a formaldehyde based electrolyte. The results were compared with PSi monolayers obtained with the traditional electrolyte (HF:ethanol). Both electrolytes facilitate the removal of H2 generated as a subproduct during the electrochemical etching process in the surface of the c-Si substrate. Formaldehyde presents a good affinity to surfaces and interfaces and the excess of water in the electrolyte reduces the pore sizes of PSi samples. The porosity and etching rate values are similar than those obtained using HF:et solutions. The refractive index values are the same in both cases at the same porosity in the visible range. The results have shown that the chemical characteristics of the ethanol and formaldehyde can give some different advantages to the PSi process and its applications.

Failure Stress of Epitaxial Silicon Thin Films

Ultra-thin silicon wafer have to withstand forces and stresses during handling procedures without breakage. Here we investigate the failure stresses of ∼30μm thick monocrystalline silicon films produced with the porous silicon process by use of a three line bending setup. We use a finite element simulation in order to evaluate the experiments and conclude that the porous silicon layers break at stresses comparable to those of silicon wafers with standard thickness. The edge preparation has a large impact on the failure stress. For samples with manually cleaved edges the failure stress surpasses 600MPa, which is the largest stress that is accessible with our testing setup.

Lift-off of Free-standing Layers in the Kerfless Porous Silicon Process

We discuss the lift-off of free-standing epitaxially grown silicon layers from the porous silicon (PSI) process, which is a kerfless wafering technology. The lift-off is a crucial step in the PSI cycle. A high-porosity layer serves as a mechanically weak layer for lift-off and consists of widely spaced silicon bridges with thicknesses of 40-100nm. The low width leads to a 33-fold stress enhancement in the bridges, making them break when a force is applied while the epitaxial layer and the substrate remain intact. We perform the free-standing lift-off with a curved vacuum chuck. A vacuum pressure of 0.2bar is sufficient for controlled peeling off of the 30-50μm thick silicon layers. We simulate the stresses and the displacements of the epitaxial layer in the lift-off process close to the first non-broken bridge. We demonstrate the defect-free lift-off of 8 of 9 of 9 x 9cm2 layers from 6” substrates.

Preparation of Porous Silicon and Effect of Gettering on the Resistivity

The porous silicon layer was fabricated by electrochemical etching process using an aqueous HF-based electrolyte. The characterizations of porous silicon layer were investigated by Emission-type scanning electron microscope (SEM), Raman spectra and X-ray diffraction (XRD). With the current density increasing, the pore diameter and density become much bigger. This result also was confirmed by Raman spectra and XRD result of samples, which revealed the decreasing of grain size of silicon. The resistivity of crystalline silicon increased when the porous layer was removed after heat treatment at 850°C for 2.5h, which should be attributed to the gettering process of porous silicon.

Investigation of MeV-Cu implantation and channeling effects into porous silicon formation

P-type (1 1 1) silicon wafers were implanted by copper ions (2.5 MeV) in channeling and random directions using ion beam accelerator of the Atomic Energy Commission of Syria (AECS). The effect of implantation direction on formation process of porous silicon (PS) using electrochemical etching method has been investigated using scanning electron microscope (SEM) and photoluminescence (PL) techniques. SEM observations revealed that the size, shape and density of the formed pores are highly affected by the direction of beam implantation. This in turn is seen to influence the PL behavior of the PS.

Numerical Simulation of the Galvanic Displacement Process of Porous Silicon by Metal Ions

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Variability on Raman Shift to Stress Coefficient of PorousSilicon

Porous silicon film is a capillary-like medium, which is able toreveal different meso-elastic modulus with porosity. During thepreparation of porous silicon samples, the capillary force is anon-classic force related to the liquid evaporation which directlyinfluences the evolution of residual stress. In this study, anon-linear relation of Raman shift to stress coefficient and theporosity is obtained from the elastic modulus measured withnano-indentation by Bellet et al. [J. Appl. Phys. 60 (1996) 3772]Dynamic capillarity during the drying process of porous silicon isinvestigated using micro-Raman spectroscopy, and the results revealthat the residual stress resulted from the capillarity increasedrapidly. Indeed, the dynamic capillarity has a close relationship witha great deal of micro-pore structures of the porous silicon.