Macroporous Silicon Research Articles

Tuning of the Optical Properties in Photonic Crystals Made of Macroporous Silicon

It is well known that robust and reliable photonic crystal structures can be manufactured with very high precision by electrochemical etching of silicon wafers, which results in two- and three-dimensional photonic crystals made of macroporous silicon. However, tuning of the photonic properties is necessary in order to apply these promising structures in integrated optical devices. For this purpose, different effects have been studied, such as the infiltration with addressable dielectric liquids (liquid crystals), the utilization of Kerr-like nonlinearities of the silicon, or free-charge carrier injection by means of linear (one-photon) and nonlinear (two-photon) absorptions. The present article provides a review, critical discussion, and perspectives about state-of-the-art tuning capabilities.

Electrochemical etching of macropores in silicon with grooved etch seeds

Specific features of macropore formation in n-Si with grooved etch seeds on the surface have been studied. In contrast to point nucleation centers, linear centers form a half-ordered lattice of pores: in the course of the self-organization process, pores randomly originate along the grooves and follow a prescribed period a across the grooves. The average distance between the pores along the groove line depends on a and decreases as a grows. By choosing the optimal a and etching photocurrent density, separate pores can be merged into grooves to give rise to a periodic structure of trenches with vertical walls. Some aspects of the pore-pore interaction for different surface profiles and the formation of zones free from pore nucleation are considered.

The fabrication of macroporous polysilicon by nanosphere lithography

The fabrication of ordered macroporous silicon is obtained by exploiting the self-assembly properties of polymer nanospheres. Here, we demonstrate the method by using nanospheres of 200 nm and 500 nm. These self-assemble in monolayers of ordered hexagonal close-packed nanospheres. A controlled reactive ion etch of the assembled nanospheres, subsequent evaporation of metal, followed by ‘lift-off’ of the polymer nanospheres, provides a mask suitable for a further reactive ion etch step to provide macroporous polysilicon. This methodology provides a novel approach for the fabrication of highly ordered macroporous polysilicon; porous silicon substrates with pores of this size (50–500 nm) were previously only fabricated using rather difficult processing methods. The method reported here is straightforward and achieved using fabrication methods that are compatible with those currently used for microelectromechanical systems (MEMS), photonic devices and nanostructured surfaces.

Integrated urea sensor module based on poly(3-methylthiophene)-modified p-type porous silicon substrate

Integrated three-electrode system module composed of a porous silicon (PS)-based sensing electrode, an Ag/AgCl thin film reference electrode (REtf), and a Pt thin film counter electrode (CE) was fabricated for monitoring urea level of artificially-prepared body fluid. After thermal evaporation of the 200 nm thick Ag on the Ti-underlayered planar p-type silicon (p-Si) substrate, the Ag Film was oxidized in a FeCl3 solution to obtain the Ag/AgCl REtf. Multi-layered REtf was clearly shown from results of the auger electron spectroscopy (AES) depth profile. The nernstian slope of the REtf also showed good reproducibility. The PS layer was formed by electrochemical anodization with applying constant current to the p-Si substrate in an ethanolic HF solution and the macro PS (2 μm diameter and 10 μm depth) was obtained. The electrochemical active area (Aea) of the PS-based Pt thin film electrode was determined from the cyclovoltammetric result of redox reactions of K3Fe(CN)6 on the electrode surface and compared with the Aea of the planar silicon (PLS)-based Pt film substrate. After electropolymerization of the conductive poly(3-methylthiophene) (P3MT) on the PS-based Pt thin film, urease (isoelectric point ≈ 4.1) molecules were electrostatically doped into the P3MT film by applying positive bias. Amperometric calibration curves for both PS- and PLS-based sensing electrodes were compared in the range of 0.1–125 mM urea concentrations and the cross-sectional scanning electron microscopy (SEM) image of the PS-based sensing electrode was also shown.

Electrodeposition of Noble Metals into Ordered Macropores in p-Type Silicon

The electrodeposition of noble metals, i.e., platinum, palladium, and gold, into macroporous p-type silicon was examined. For platinum and palladium, the electrodeposition proceeded preferentially from the pore bottom to the opening when sodium chloride was used as a supporting electrolyte. When sodium sulfate was used as a supporting electrolyte, the electrodeposition mainly proceeded at pore openings, leading to plugging. For gold electrodeposition, a condition for achieving pore filling from the bottom was not found in either the NaCl or solutions. Pore depth was another key factor to achieve continuous filling by electrodeposition. As the depth of pores became deeper, the electrodeposition proceeded preferentially from the bottom. The effect of mass transfer in pores was also investigated by changing the concentration of metal ions and applied potential.

Macroporous Silicon as a Model for Silicon Wire Array Solar Cells

Macroporous silicon samples have been investigated in photoelectrochemical cells, and their behavior has been compared to that of conventional, planar, Si/liquid junctions. The liquid electrolyte junction provided a conformal contact to the macroporous Si and allowed assessment of the trade-offs between increased surface area and decreased carrier collection distances in such systems relative to the behavior of planar semiconductor/liquid photoelectrochemical junctions. The electrolyte contained the dimethylferrocene/dimethylferrocenium redox system in methanol because this system has been shown previously to produce bulk recombination-diffusion-limited contacts at planar Si(100) electrodes under 100 mW cm-2 of simulated air mass 1.5 illumination. Introduction of a network of ∼2−3 μm diameter, ∼80 μm long pores into the Si was found to slightly reduce the short-circuit photocurrent density and the open-circuit voltage of the system, but energy-conversion efficiencies in excess of 10% were nevertheless obtained from such samples. This system therefore validates the concept of using interpenetrating networks to produce efficient solar energy conversion devices in systems that do not have long carrier collection distances.

First use of macroporous silicon loaded with catalyst film for a chemical reaction: A microreformer for producing hydrogen from ethanol steam reforming

Macroporous silicon with millions of parallel microchannels per square centimeter can be used as a micromonolithic support for high-speed catalysis in miniaturized systems, opening a new and exciting field of research and application. We used macroporous silicon membranes with >10 6 parallel microchannels with a diameter of 3.3 μm and depth/width ratio >60 as microreactors for the generation of hydrogen through the steam reforming of ethanol. The microchannels walls were coated with a thin layer of Co 3O 4–ZnO catalyst by a complexation–decomposition method, resulting in a specific inner surface of 10 5–10 6 m 2 m −3. A constant flow of 20 μL s −1 H 2 (0.054 mmol min −1 H 2) was obtained from a liquid water–ethanol mixture at 773 K with a steam-to-carbon ratio of S/C = 3 and LHSV = 2 × 10 4 h −1 (0.1 mL min −1). The turnover frequency under these conditions was ca. 1.1 s −1, based on surface Co atoms.

Photoluminescence of polydiacetylene membranes on porous silicon utilized for chemical sensors

Langmuir–Blodgett (LB) films of the conjugated polydiacetylene (PDA) exhibit spectroscopic behavior, which is dependent on the type of the supporting substrate. While on polished silicon surfaces the photoluminescence (PL) of PDA is quenched, it is preserved on top of 2D patterned macro-porous silicon (2D-MPS). 2D-MPS, prepared by electrochemical etching of photo-lithographically pre-patterned silicon, is a 2D array of ca. 10 μm deep pores with lateral 2–4 μm repeating unit cells in orthogonal or hexagonal arrangements. LB films of PDA on such surfaces form membranes with continuous domains of the size sufficient to cover laterally many cell units. Apparently, the PL from this film results exclusively from the portion of the PDA membrane which is suspended over pore openings, while portions of the film which are attached to the silicon on top of the pores walls does not exhibit PL at all. We have used these membranes in different configurations and exposed them to different chemical and biological agents and followed the PL intensity change. This report demonstrates the effectiveness of the combined system: LB films of PDA on top of 2D-MPS as sensing probe for a variety of chemicals including, Cd ions and TNT explosives. In addition, the use of films of PDA, in which glycol-lipid were embedded, for binding and recognition of lectin protein, mimicking the cell membrane interaction with its environment, is also demonstrated.

Pore formation in aluminum films on macroporous silicon during high-temperature annealing

The surface of an aluminum metallization coating applied on a macroporous silicon layer and annealed for 10–60 min at 550°C has been studied by atomic-force and electron microscopy techniques. It is established that the annealing results in the formation of through (open) macropores with lateral dimensions within 400–1300 nm in the aluminum film and in the growth of large hillocks with heights of up to 1.2μ m on the surface, which are located at the edges of these macropores. The geometric parameters of large hillocks are independent of the duration of annealing.

Photoconductivity in macroporous silicon with regular structure of macropores

The effects of the increase of photoconductivity in periodic macroporous silicon structures depending on the size and period of cylindrical macropores are investigated. It is obtained that the ratio of macroporous silicon photoconductivity to bulk silicon photoconductivity achieves a maximum at the distance between macropores equal to two thicknesses of the Schottky layer, which corresponds to the experimental data. The increase of photoconductivity is due to both the large total surface area of macropores and the existence of Schottky layers in the near-surface region of cylindrical macropores..

Structural and luminescent characteristics of macro porous silicon

The results of structural and luminescent study of porous Si prepared by electrochemical etching and containing the pores of micron sizes are presented. The samples demonstrate bright luminescence with external quantum efficiency of 15–20%. Raman scattering spectra contain only one line corresponding to bulk silicon. Atomic force microscopy image shows the structural elements essentially larger than quantum confinement crystallites. However, X-ray diffraction measurements demonstrate the presence of strained Si quantum confinement nanowires. It is shown that the recombination through interface oxide-related centers of the carriers excited due to light absorption in quantum confinement nanowires gives an essential contribution to emission spectra.

High Performance Macroporous Silicon Chemical Sensor With Improved Phase Detection Electronics

Abstract In this paper, a high sensitive, stable and reproducible electrical chemical sensor system based on macroporous silicon has been developed. Screen printed metal contacts have been taken from macroporous silicon layer which result in extremely stable, quasi ohmic and reproducible contacts. Such a sensor structure has been characterized for different organic and ionic solutions commonly used for biochemical applications. The sensitivity of the reported sensor at a particular frequency has been found to be almost ten times compared to previous reports of macroporous silicon sensor where contacts are taken from the backside and also in comparison to interdigited electrode array structure. The improvement and variation of sensitivity with frequency for different solutions has been explained taking into account the dependence of double layer impedance with frequency, distributed RC networks of the macroporous silicon structure, ionic conductivity of the solution and effects of ion sizes. However, from the impedance measurement it has been observed that the sensor capacitance with such stable contacts has an ultra low-Q value and to exploit such sensor for portable field use systems, an embedded signal conditioning unit has been realized to display the measured capacitance value after offset compensation and reduction of parasitic effects.

Processing and Mechanical Properties of Macroporous Silicon Carbide Ceramics Fabricated by Carbothermal Reduction

Processing and Mechanical Properties of Macroporous Silicon Carbide Ceramics Fabricated by Carbothermal Reduction

Study and estimation of the residual stress in porous silicon layer formed on the surface of a crystalline silicon substrate

Recently, it has been suggested that the structural evolution of pores in porous silicon during high temperature annealing depends strongly on the value of the initial residual stress in the film. In this work, the residual stress in porous silicon is calculated from the surface stress acting on the lateral surface of the cylindrical pores. Such a stress is directly related to the strain caused by desorption of hydrogen and rearrangement of Si–Si bonds during annealing. One dimensional distributions as well as 2-D contours of the residual stress are calculated according to the model described here and used in 2-D COMSOL FEMLAB simulations to predict the final pore distribution and shape after annealing. Simulation results are in excellent agreement with the post annealed pore shape and distribution of experimental macro-porous silicon samples annealed at 1150 °C in argon.

Study of pore reorganisation during annealing of macroporous silicon structures for solar cell application

Thin layers of silicon for solar cells can be lifted-off from wafers by taking advantage of the property of macropores to reorganise at high temperature and, in specific conditions, to form a high-quality layer on top of an empty space. Those conditions need to be identified and this work focuses in particular on the annealing ambient composition. Its influence was investigated by scanning and transmission electron microscopy and by Fourier-transform infrared spectroscopy. Apart from hydrogen, which has been used in previous studies, argon was found to be also compatible with the reorganisation. This result points out that the presence of oxygen impurities may not be as deleterious as expected, but that those impurities complicate the set of reactions taking place during the high-temperature process.

Development of Porous-Silicon-Based Active Microfilters

With a goal of preparing silicon-based interactive filters in the micropore range, we have developed the first single-step etch-liftoff procedure (one-step separation) based on the formation and removal of macroporous silicon layers. With silicon wafers whose resistivities are in the range from , we are able to create, in a surprisingly controlled manner, films whose pore diameters range from and whose thickness ranges from . These silicon-based films (filters), which carry a polarizing negative charge, represent an alternative to porous alumina (films) filters (pore diameter not yet in the range) both in terms of their size range and potential interaction-reaction at elevated temperature. Preliminary experiments demonstrate that platinum and copper can be introduced to these filters using electroless solutions with a goal to creating an effective reductive surface. Using these and alternate material combinations, interactive-reactive filters in the size range that operate at temperatures well in excess of porous polymer films can be realized.

63Ni-based β-electric current source

The possibility of developing a β-electric atomic battery with microwatt power based on 63Ni with a lifetime of at least 30 years is analyzed. A method is proposed for large-scale commercial production of 63Ni with concentration 80–90% in the final product. A design of a β-electric transducer based on macroporous silicon with optimized configuration and macropore depth (microchannels) is presented and analyzed. 63Ni is deposited on the inner surface of the microchannels.

Porous silicon surfaces – A candidate substrate for reverse protein arrays in cancer biomarker detection

This paper introduces a new substrate for reverse-phase protein microarray applications based on macroporous silicon. A key feature of the microarray substrate is the vastly surface enlarging properties of the porous silicon, which simultaneously offers highly confined microarray spots. The proof of principle of the reverse array concept was demonstrated in the detection of different levels of cyclin E, a possible cancer biomarker candidate which regulates G1-S transition and correlates with poor prognosis in different types of human cancers. The substrate properties were studied performing analysis of total cyclin E expression in human colon cancer cell lines Hct116 and SW480. The absence of unspecific binding and good microarray quality was demonstrated. In order to verify the performance of the 3-D textured macroporous surface for complex biological samples, lysates of the human tissue spiked to different levels with cell extract overproducing cyclin E (Hct116) were arrayed on the chip surface. The samples were spotted in a noncontact mode in 100 pL droplets with spots sizes ranged between 50 and 70 mum and spot-to-spot center distances 100 mum, allowing microarray spot densities up to 14 000 spots per cm(2). The different sample types of increasing complexities did not have any impact on the spot intensities recorded and the protein spots showed good homogeneity and reproducibility over the recorded microarrays. The data demonstrate the potential use of macroporous silicon as a substrate for quantitative determination of a cancer biomarker cyclin E in tissue lysates.

Infrared thermal emission in macroporous silicon three-dimensional photonic crystals

In this paper we investigate the infrared thermal emission properties of macroporous silicon with modulated pore diameter. Samples with different pore modulation periodicities but fixed in-plane lattice constant are fabricated. Normal emission of these samples is measured between 373 and 673K. Room-temperature normal-incidence reflectance and transmission spectra are also measured and compared with the photonic band structure simulation. It is shown that thermal emission is suppressed due to photonic band gap effect along the pore axis in excellent agreement with the numerical calculations.

Nanophotonic Materials: Photonic Crystals, Plasmonics and Metamaterials

AbstractFollowing the successful launch of the book on 'Photonic Crystals' in 2004 [1], this special issue of physica status solidi again comprises the work of research groups from the corresponding Priority Programme of the German Research Foundation and some of their close collaborators. Three years on, the understanding of photonic crystal properties has improved across numerous materials systems, applications were devised and developed approaching the industrial stage, and attention extended to the new areas of plasmonic crystals and metamaterials. The current issue presents a collection of Feature Articles reporting on the progress in this active field.In the tradition of the first publication, the Cover Picture shows an overlay of a 3D photonic crystal made from macroporous silicon, a simulation of a waveguide splitter, and the lasing modes of a planar photonic crystals laser made from III–V‐compound semiconductors (Picture courtesy of conImago, http://www.conimago.de). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)