Porous Si Layer Research Articles

Porous Gold Structures Templated by Porous Silicon

In this work, we describe the synthesis of porous Au structures by galvanic displacement reaction using porous Si exhibiting arrays of parallel pores with diameters near as template material. Two metal ions were used as Au precursors: tetrachloroaurate(III) and bis(ethylenediamine)Au(III). Fluoride was employed to etch the Si oxides that form as a result of the displacement reaction. The porous metal layer that forms with tetrachloroaurate(III) is an agglomerate of spherical particles with ca. diameter. Metallization with bis(ethylenediamine)Au(III) preserves the morphology of the original porous Si layer, creating a porous Au layer containing an array of parallel pores with ca. diameter. The displacement process is under diffusion control for both reagents. The expansion rate of the porous metal layer into the substrate agrees well with a simple one-dimensional reaction-diffusion model entailing the displacement of Si in a large extent. The electrochemical behavior of the synthesized porous Au layers was studied by voltammetry.

Characterization of Nanoporous Silicon Layer to Reduce the Optical Losses of Crystalline Silicon Solar Cells

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated using SEM. The formation of a nanoporous Si layer on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900 nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.

Characterization of Nanoporous Silicon Layer to Reduce the Optical Losses of Crystalline Silicon Solar Cells

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated using SEM. The formation of a nanoporous Si layer on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900 nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.

Porous Gold Structures Built on Silicon Substrates

In this work, we describe the synthesis of porous Au structures by galvanic displacement reaction using porous Si as template material. Two metal ions were used as Au precursors: tetrachloroaurate(III) and bis(ethylenediamine)Au(III). Fluoride was employed to etch the Si oxides that form as a result of the displacement reaction. The porous Si layer employed as template displays an array of parallel cylindrical pores with diameters near 100 nm. The porous metal layer that forms with tetrachloroaurate(III) is an agglomerate of spherical particles with ca. 100 nm diameters. On the other hand, metallization with bis(ethylenediamine)Au(III) preserves the honeycomb morphology of the original porous Si layer.

Effects of hydrogenation and aging on the optical properties in porous Si layers

The effects of hydrogenation and aging on the optical properties in porous Si (PS) layers were investigated by using photoluminescence (PL) measurements. When the hydrogenated PS layers were aged in air, the intensity of the PL spectrum increased. The emission peak for the hydrogenated PS layers shifted to higher energy with decreasing H2/N2 ratio. The relation of the dehydrogenized states in the as-formed PS surface to the quantum states of Si nanoparticles with relatively small sizes is discussed. These results indicate that the optical properties of PS layers are significantly affected by hydrogenation and aging.

Effect of form anisotropy of silicon nanocrystals on birefringence and dichroism in porous silicon

AbstractArtificial anisotropic optical media based on nanostructured silicon formed by electrochemical porosifying of Si substrates are investigated. Strong birefringence and dichroism of surface silicon‐hydrogen bonds vibrations are found in the infrared spectral range. A comprehensive analysis of the absorption bands corresponding to the deformation and stretching modes is performed. The experimental results are discussed in terms of an effective medium model, taking into account the morphological anisotropy of Si nanocrystals in porous Si layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Different deposition behavior of copper on silicon and porous silicon surfaces during immersion plating

AbstractThe deposition behavior of Cu during immersion plating was investigated on Si and porous Si (PS) surfaces. The deposition of Cu could be observed on the PS surface, but not on the native Si surface. In the electrochemical characterization of the two surfaces in HF solution, a plateau of anodic current was observed at the PS structure, but not at the Si surface. The potential region of the plateau was more negative than the potential at which Si dissolution or PS formation takes place. The electrochemical measurements of the PS samples in HF solution revealed that oxidation of hydrogen remaining in the PS layer can not participate in the appearance of the anodic plateau. The different deposition behavior on the Si and PS surfaces enables selective deposition of Cu on a PS pattern drawn on n ‐type Si by laser scanning. A new patterning method of Cu on n ‐type Si by utilizing this behavior was successfully achieved. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence properties of the porphyrin/porous silicon composites

AbstractA study of luminescence properties of water‐soluble and hydrophobic porphyrin and chlorin molecules immobilized in oxidized (i.e. non‐luminescent) and as‐prepared (i.e. luminescent) porous silicon (porSi) layers is carried out. It is found that the oxidized porSi matrix itself does not introduce any quenching effect on the porphyrin fluorescence, but the fluorescence is subjected to the concentration quenching. At maximum porphyrin/chlorin concentration used (ca. 10–3 M in impregnation solutions) the quenching is strong in case of hydrophobic porphyrins and sufficiently moderate for water‐soluble ones, especially for cationic porphyrin TMPyP4. A concentration of the latter in porSi layer is determined by direct measurements. Both hydrophobic and water‐soluble porphyrins immobilized on the surface of as‐prepared porSi are found to quench the luminescence of the latter. In case of hydrophobic porphyrins the quenched porSi luminescence is regained almost to the initial value on the time scale of tens of hours, whereas in case of water‐soluble porphyrins the fluorescence spectrum of the latter builds up in course of time. Possible mechanisms responsible for these effects are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Integrated inductors on porous silicon

AbstractThe cover picture illustrates the effective use of a thick porous silicon layer as an integrated micro‐plate for RF isolation on a silicon substrate, proposed by Harry Contopanagos and Androula Nassiopoulou in their Original Paper [1] in the current issue. What is plotted is the magnitude of the current distribution (colour coded from blue (low) to high (red) values) on the metallization and on a screen 50 µm underneath the bottom oxide layer of a 2‐metal integrated CMOS‐compatible inductor on bulk silicon (lower right) and on a 50 µm thick porous silicon layer (upper left) for a frequency of 2.5 GHz.Inductors were designed in a standard 0.13 µm CMOS technology. Efficient RF isolation is produced by the porous Si layer, as evidenced by the virtual elimination of surface currents relative to the case of standard CMOS, indicating virtually complete substrate shielding by a 50 µm thick porous Si layer for the relevant size scale. The quality factor of the inductor with the use of the porous Si layer is increased by 100%, reaching a maximum value of 33 for the design shown.The first author of the article is a visiting senior researcher at the Institute of Microelectronics (IMEL), National Center for Scientific Research “Demokritos” (Athens, Greece). His research focuses on electromagnetics and microwave engineering, artificial materials and photonic crystals, wireless front ends, antennas and high‐frequency analog integrated circuits.

Integrated inductors on porous silicon

AbstractWe examine porous Si as a thick RF‐isolation layer for on‐chip inductors and separate the loss sources that downgrade the Q factors into their material components, namely Si substrate loss and metal types common to standard CMOS technologies. First we validate theoretical designs with measurements in standard 0.18 μm CMOS technology. Then we examine the effect of a porous Si substrate on a fixed metal layout of a 2‐metal optimized inductor design compatible either with 0.18 μm CMOS technology using Aluminum or with 0.13 μm CMOS technology using Copper metallization. For typical on‐chip inductor layouts of (200–300 μm)2, we show that a 50 μm thick porous Si layer produces negligible current distributions inside the underlying lossy Si substrate, compared to the ones without it, which maintain high values throughout a large lateral extent of the die. In Al technology, a fixed inductor layout produces Q ‐factor enhancements of the order of 50%, compared to the case where no porous Si layer is used. In a 0.13 μm‐compatible CMOS technology using Cu on porous Si, the resulting Q ‐factors increase by a factor of 2 and reach values of 30 or more in the 3 GHz frequency range. For porous‐Si‐isolated inductors we compare the effect of metal losses and show that the resulting Q ‐factors scale with the metal conductivity. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gas sensor applications of porous Si layers

The porous silicone (PSi) is a relatively new and promising semiconductor material with special physical and chemical properties which somewhat differ from the properties of single crystal Si. Some of these properties are valuable in the field of gas sensor technology, but a lot of questions arise in connection with its application. Do we really need porous semiconductor material for proper gas sensing function? How can electrical properties of the PSi layer be measured if the electrical contacting is problematic? Is it possible to activate the PSi with catalytic noble metal layers or particles? What about the Fermi-level pinning in the PSi layer? The main target of this article is to seek answers to questions listed above and to give a short, but still comprehensive review of the application of the PSi layers on the field of the gas sensor technology, with special care on electrical output signal giving sensors.

Influence of pH solution on photoluminescence of porous silicon

The radiative lifetime of as-prepared and modified layers of porous silicon (por-Si) were studied in liquid solutions with different pH. It was observed that por-Si photoluminescence (PL) intensity and decay lifetime strongly depend on pH value. This phenomenon is explained by competition of the following processes: UV-induced hydrogen effusion, hydrogen adsorption from the buffer solution, and por-Si oxidation. Regarding the phenomenon a pH change sensor can be proposed. Por-Si layer degradation can be decreased somehow by protective PEDOT layer.

PREDICTED AND REALIZED IMPROVED PERFORMANCES OF PZT FILM–SILICON BASED STRUCTURES FOR INTEGRATED PYROSENSORICS

ABSTRACT The comparative investigation of pyroelectric response of “polar film–Si substrate” structures based on RF-sputtered PZT and spin coated P(VDF/TrFE) films and “PZT film–porous Si layer–Si substrate” structures has been performed. By the thermowave pyroelectric under-surface probing amplitude-to-frequency and phase-to-frequency dependences of pyroelectric response of investigated systems in the voltage and current modes were obtained. Theoretical and experimental data are in a good agreement and prove that por-Si layer is suitable for effective thermal decoupling of film sensitive element and heat removing Si-substrate which realized in improved performances of pyroelectric detectors of radiation.

Microwave detectors based on porous silicon

Two types of structures comprising porous silicon (por-Si) layers between metal electrodes were prepared, which possessed nonlinear (A type) and linear (B type) current-voltage characteristics. The exposure to microwave radiation leads to the appearance of an emf between electrodes. The B-type structures exhibit high voltage responsivity and can be used as microwave radiation sensors.

Influence of temperature–pressure treatment on heavily hydrogenated silicon surface

Microstructure and related properties of hydrogenated silicon samples, Si:H, treated at high-temperature (HT) up to 1270 K under hydrostatic argon pressure (HP) up to 1.1 GPa are investigated. To prepare Si:H, Czochralski grown 0 0 1 oriented single crystalline Si wafer with 50 nm thick surface SiO 2 layer was heavily implanted with hydrogen using the immersion plasma source of hydrogen ions with energy 24 keV. The surface of HT–HP treated Si:H was characterised by scanning electron microscopy. Reflectivity pattern measurements in the wavelength range of 350–2000 nm have been performed to analyse their surface and bulk properties. The volume averaging method for a model of layer-like structure has been used to simulate the HT–HP treated Si:H. The analysis of Si:H samples suggests the multi-layer structure composed of Si, Si:H, SiO, SiO 2, and of porous Si layers in the sub-surface region. The porous Si:H samples model is in good consistency with experimental data from reflectance measurements.

Conduction mechanisms in porous Si LEDs

Abstract Diode-like structures were prepared from p- and n-type Si, in which porous Si layers were sandwiched between the crystalline Si and the evaporated Al top contact. The current–voltage characteristics were investigated in that bias polarity, at which electroluminescence occurs. It was found that the characteristics follow the Fowler–Nordheim tunneling process for both type of devices. The tunneling occurs through the heterojunction barrier at the crystalline-porous interface. The leakage current experienced at low biases in the p-type structure is attributed to trap-assisted tunneling; its saturation character was pointed out by low-frequency noise measurements.

Fast exothermic processes in porous silicon

It is found that rapid oxidation of porous Si (por-Si) layers in air may occur in the form of combustion or explosion. Combustion occurs in por-Si layers thinner than 60 μm and impregnated with potassium nitrate, while explosion is observed in thicker porous layers. It is suggested that explosion develops by a thermal mechanism resulting from an exponential increase in the reaction rate with temperature.

Biosensing Using Porous Silicon Double-Layer Interferometers: Reflective Interferometric Fourier Transform Spectroscopy

A simple, chip-based implementation of a double-beam interferometer that can separate biomolecules based on size and that can compensate for changes in matrix composition is introduced. The interferometric biosensor uses a double-layer of porous Si comprised of a top layer with large pores and a bottom layer with smaller pores. The structure is shown to provide an on-chip reference channel analogous to a double-beam spectrometer, but where the reference and sample compartments are stacked one on top of the other. The reflectivity spectrum of this structure displays a complicated interference pattern whose individual components can be resolved by fitting of the reflectivity data to a simple interference model or by fast Fourier transform (FFT). Shifts of the FFT peaks indicate biomolecule penetration into the different layers. The small molecule, sucrose, penetrates into both porous Si layers, whereas the large protein, bovine serum albumin (BSA), only enters the large pores. BSA can be detected even in a large (100-fold by mass) excess of sucrose from the FFT spectrum. Detection can be accomplished either by computing the weighted difference in the frequencies of two peaks or by computing the ratio of the intensities of two peaks in the FFT spectrum.

Rayleigh scattering in multilayered structures of porous silicon

The possibility of growing high-quality porous silicon (por-Si) multilayered structures[1,2], which can consist of hundreds of layers with different, controllable porosity [3], revealed variousnew ways of using this material in optoelectronics and photonics. Apart from obvious applications of por-Si multilayered structures as mirrors, optical filters, etc., the most attractive goal is to combine the photonicmanipulation with the por-Si luminescence, which, in principle, would allow the fabrication of Si-basedlasers with adjustable properties [4].The photon transport through the por-Si multilayered structure is usually described using the transfermatrix method [5]. In this approximation it is assumed that the individual por-Si layers can be characterizedby a well defined refractive index, which depends on porosity. The general features of the reflection andtransmission coefficients are reproduced by this method (see, e.g., the recent observation of the photonicBloch oscillations [6]). However, even in the case when the absorption is negligible, not all interferencepeaks predicted by the transfer matrix calculations are seen experimentally, and the observed peaks arenoticeable suppressed [6,7]. To get more insight in this phenomenon it is convenient to study some simplemultilayered structures, in particular those containing a single microcavity, where the resonant transmit-tance peak is well separated and could be analyzed in details. Below we present the experimental data onthe transmittance spectra of several structures of this sort. The observed suppression of the resonant peaksis then interpreted by taking into account the Rayleigh scattering of light on the microscopic structuredisorder of por-Si—the effect, which is not allowed for in the transfer matrix calculations.

Anisotropy of infrared absorption in (110) porous silicon layers

In-plane birefringent porous Si (PSi) layers formed from heavily boron-doped (110) Si wafers are investigated by using polarization-resolved infrared absorption (IR) spectroscopy. The absorption by free charge carriers and by Si-Hx (x = 1, 2, 3) surface bond vibrations are found to exhibit strong anisotropy (dichroism), which originates from the form anisotropy of Si nanocrystals assembling (110) PSi layers. The free carrier absorption dichroism is explained by using the effective medium approximation and classical Drude model and considering additional carrier scattering in anisotropically shaped Si nanocrystals. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)