Silicon Walls Research Articles

Tuning the pore wall morphology of mesoporous silicon from branchy to smooth, tubular by chemical treatment

The effect of chemical treatment on physical and chemical properties, i.e., pore diameter, porosity, specific surface area, and chemical bonding of electrochemically formed mesoporous silicon were investigated by using of nitrogen sorption isotherm, scanning electron microscopy, and Fourier transform infrared spectroscopy. The adsorption isotherms measurements show the general behavior found for the porous materials, but at the same time, they exhibit clear differences following different chemical treatments of porous layer. It was clearly observed from Fourier transform infrared spectroscopy that the chemical environment of porous silicon wall changes significantly after chemical treatment. In scanning electron microscopy images, we see that the rough dendritic structure of the pore walls is modified to smooth tubular pore wall structure on chemical treatment. The changes in nanocrystalline porous silicon were also clearly observed by an asymmetric broadening and shift of the optical silicon phonons in Raman spectra. Furthermore, changes are observed in the multiphonon regime due to surface assisted multiphonon processes, which are enhanced in highly porous silicon. The chemically modified porous silicon samples suggest possibilities of use as a porous matrix for fundamental study and technological application.

Bandgap Tuning of Silicon Micromachined 1-D Photonic Crystals by Thermal Oxidation

Fabrication and optical testing of high-aspect-ratio 1D photonic crystals, obtained by electrochemical micromachining of silicon, are discussed in this paper. The devices consist of high-aspect-ratio periodic (P=4 mum) arrays of 1.22-mum-thick silicon walls separated by 2.78-mum-wide air gaps, with 100 mum etching depth. They were designed as hybrid quarter-wavelength reflectors with photonic bandgaps in the near-IR region, one in particular centered at lambda=1.55 mum. The fabrication process was improved to increase structure uniformity and strength. Thermal oxidation of the silicon structures was exploited to tune the wavelength position and width of the bandgaps. Fabricated devices, also with different silicon dioxide thicknesses, were optically tested by measuring their spectral reflectivity in the wavelength range of 1.0-1.7 mum. Experimental results were found in good agreement with the calculated spectra.

A Novel Benzocyclobutene-Based Device for Studying the Dynamics of Heat Transfer During the Nucleation Process

A novel microelectromechanical device has been developed to study the details of the heat transfer mechanisms involved at the nucleation site for the nucleate boiling process. This device enables quantifying the magnitude, time period of activation, and specific areas of influence of different mechanisms of heat transfer from the surface with a resolution several times greater than previously reported. This is achieved through the use of an array of embedded temperature sensors within a carefully designed dual-layer (silicon and benzocyclobutene) wall which allows for the accurate calculation of local heat flux, circumventing difficulties encountered when using existing methods. The sensors are radially distributed around the nucleation site. Heat is supplied to the wall by a thin film heater fabricated on the outer nonwetted surface. Single bubbles are generated at the center of the array while the temperatures and the bubble images are recorded with a sampling frequency of 8 kHz. The temperature data provided the necessary thermal boundary conditions to numerically calculate the surface heat flux with an unprecedented radial resolution of 22-40 mum. Fabrication, characterization, and the ability of the developed device to elucidate the heat transfer aspects of the nucleation process are demonstrated.

Reflection properties of hybrid quarter-wavelength silicon microstructures

The authors present experimental and numerical results relative to the polarization-resolved spectral reflectivity of one-dimensional periodic microstructures, evaluated in the near-infrared region at non-normal incidence. The tested hybrid quarter-wavelength microstructures, fabricated by electrochemical deep etching of silicon, consist of arrays of silicon walls and air gaps, with 3 and 4μm periods and aspect ratio of up to 100. A theoretical Monte Carlo analysis taking into account the presence of a Gaussian statistical distribution for the structure porosity has been carried out and the calculated wavelength dependence of the reflectivity at non-normal incidence has been confirmed by experimental data.

Silicon micromachined periodic structures for optical applications at λ=1.55μm

In this letter, the authors report the design, fabrication, and characterization of a silicon micromachined periodic structure for optical applications at λc=1.55μm. The microstructure, which can be envisioned as a one-dimensional photonic crystal, is composed of a periodic array of 1-μm-thick silicon walls and 2-μm-wide air gaps, each one corresponding to a different odd number of quarter wavelength at λc (hybrid quarter wavelength). The fabrication is based on the electrochemical etching of silicon, yielding parallel trenches with depths up to 100μm. Preliminary reflectivity measurements show the presence of a band gap at λc=1.55μm, as theoretically expected.

Electrical modulation of silicon-based two-dimensional photonic bandgap structures

Electrically tunable photonic band gap (PBG) structures hold the potential to become a versatile and compact backbone for optical signal processing. In this letter we report electrical tuning of silicon-based two-dimensional PBG structures infiltrated with liquid crystals. An improved electrode configuration is used to avoid electric field screening by the conductive silicon walls. Electrical tuning using fields well below 1V∕μm is demonstrated experimentally using both polarized light microscopy and reflectance PBG measurements. The structures can be operated with any electro-optic materials and lead to fast and efficient modulators, routers, and tunable filters.

Investigation into the orientation of the liquid-crystal mixture E7 in composite photonic crystals based on single-crystal silicon

The orientation of the liquid crystal E7 on the surface of silicon wafers and inside the grooves of grooved silicon prepared by anisotropic etching in an alkali is investigated. The orientation of the liquid crystal is determined using IR absorbance and IR reflectance spectroscopy and the capacitance method. The specific features of IR absorption in the liquid crystal introduced into periodic matrices are analyzed. It is shown that the liquid crystal E7 in grooved silicon is characterized by a weakly pronounced planar orientation of the molecules with respect to the silicon walls.

Self-Assembled Silicon Nanotubes under Supercritically Hydrothermal Conditions

Self-assembled silicon nanotubes with one-dimensional structure have been synthesized from silicon monoxide powder under supercritically hydrothermal conditions with a temperature of 470 degrees C and a pressure of 6.8 MPa. The silicon nanotubes were identified by transmission electron microscopy and high-resolution transmission electron microscopy. The results show that the silicon nanotubes (SiNT) have closed caps. The structures of the silicon nanotubes are hollow inner pore, crystalline silicon wall layers with a 0.31 nm interplanar spacing and 2-3 nm amorphous silica outer layers. Pure crystalline silicon nanotubes survive after etching the silicon nanotubes with 5% HF acid for enough time to imply that the self-assembled silicon nanotubes are stable. A possible theoretical reason for the growth of SiNTs from SiO under supercritically hydrothermal conditions was also proposed.

Experimental evidence of photonic band gap extension for disordered 1D photonic crystals based on Si

We have shown theoretically and experimentally the possibility of PBG extension by introducing a silicon wall thickness disorder into the photonic structure. Experiments have been conducted in the infrared spectrum range and calculations have been performed for 1D photonic crystals based on a grooved-silicon structure.

Polarized infrared and Raman spectroscopy studies of the liquid crystal E7 alignment in composites based on grooved silicon

Alignment of liquid-crystal filler molecules and the electro-optical effect in composite photonic crystals based on grooved silicon are studied. It is found that the nematic liquid crystal molecules that fill the grooves are predominantly aligned in a planar configuration with respect to the silicon walls. The liquid crystal molecules are realigned homeotropically with respect to the groove walls under the influence of an electric field. The effect detected can be used to adjust the photonic band gap of a one-dimensional photonic crystal.

A three‐dimensional waveguide substrate for DNA‐microarrays based on macroporous silicon

AbstractIn this paper we present a three‐dimensional waveguide structure with unique optical and fluidic properties and demonstrate its application as a substrate for DNA microarrays. The structure is fabricated by thermal oxidation of a macroporous silicon membrane with a periodic pattern of discrete microchannels running perpendicular through the substrate. Partial oxidation generates compartments with channel walls that are completely converted into SiO2 but leaves a rectangular grid of silicon walls separating the SiO2 compartments. We demonstrate that the SiO2 walls act as optical waveguides and the opaque silicon walls divide the substrate into optically isolated compartments. In DNA microarray experiments, we show that the silicon walls of the compartments prevent cross talk between adjacent DNA spots. The structure is compatible with all conventional read‐out techniques such as fluorescence, chemiluminescence, and precipitation staining. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Membrane supported Yagi–Uda antennae for millimetre-wave applications

This paper addresses for the first time the design, fabrication and 'on wafer' characterisation of membrane supported Yagi-Uda coplanar waveguide fed antenna structures, operating in the 45 GHz frequency range. The antennae were fabricated on 1.4 /spl mu/m thin SiO/sub 2//Si/sub 3/N/sub 4/ membranes obtained by micromachining of high resistivity silicon, using a backside reactive ion etching process. This approach has assured almost free-space operating conditions for the Yagi-Uda antennae and the dry backside etching process has allowed the effect of the bulk silicon wall that supports the membrane in the main radiation direction to be minimised. The design was based on electromagnetic simulations made using the Zeland IE3D software package. The antenna structures, measured 'on wafer', have shown a minimum return loss of 15 dB and a gain of about 7 dBi. Very good agreement has been observed between experimental and simulated results.

Molecular-dynamics study of the local symmetry changes in metallic liquids

It has been conjectured that the local structure of simple monoatomic liquids is mainly of five-fold symmetry. Experimental evidence for such icosahedral building blocks has recently been found in liquid droplets of lead adjacent to a silicon wall and in deeply undercooled melts of metallic elements like bcc iron, fcc nickel and bcc zirconium. We have used molecular dynamics in order to investigate supercooling effects in the melt of another element (aluminum) on the basis of a common neighbour analysis. In the simulation, we have employed an embedded atom method potential optimized with the help of ab initio calculations for aluminum. The simulations confirm the recent experimental results that, independent of the system, the icosahedral short-range order strongly increases with the degree of undercooling.

Detachment folding, fold amplification, and diapirism in thrust wedge experiments

The relations between detachment folding, fold amplification, and salt diapirism in contractional settings have been investigated by means of scaled analogue models. The viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak (type 1 models) and strong (type 2 models) décollements. Deformation patterns in the roof sequence exhibited two contrasting styles, (1) outward propagation of detachment folding along the décollement (OFP mode) and (2) passive roof duplex (PRD mode). In type 2 models, detachment folding propagated away from the most external thrust in the floor sequence, while in type 1 models, long‐lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization. A silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and OFP models. Best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both OFP‐PRD and amplified detachment folds‐box folds transitions. A quantitative comparison of model results with nature has been attempted by plotting salt‐based fold‐and‐thrust belts data points on the scaled transition curves obtained from the modeling. Such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core. By reducing the roof sequence strength, pore fluid pressure λb is inferred to shift the equilibrium of fold‐and‐thrust belts toward the field of OFP and diapirism.

One-dimensional photonic crystal obtained by vertical anisotropic etching of silicon

The potentialities of vertical anisotropic etching of (110) silicon for the fabrication of one-dimensional photonic crystal with a high refractive index contrast have been studied. It is shown that advances toward the near-IR spectral range are limited by the mechanical strength of thin silicon walls. The device structures obtained consist of 50 trenches, 114 µm deep, with 1.8-µm-thick Si walls (structure period 8 µm). Their reflectance spectra in the wavelength range 2.5–16.5 µm show good agreement with calculation results, although the main photonic band gap at λ≈28±10 µm remained outside the spectral region of measurements.

Dimensional Constraints on High Aspect Ratio Silicon Microstructures Fabricated by HF Photoelectrochemical Etching

The fabrication of macropores in crystalline silicon by photoelectrochemical etching in a hydrofluoric acid electrolyte is investigated. It is shown that the dimensional constraints on the pore diameters, which, in previous literature, are considered to depend on substrate doping, can be significantly relaxed. We show that it is possible to fabricate arrays of square section macropores with sides ranging from 2 to 15 μm using the same n-doped (2.4-4 Ω cm) silicon substrate. Moreover, we demonstrate that macropore arrays with pitch variation up to 100% (8 and 16 μm) can be simultaneously grown on the same silicon sample. The same process is used to fabricate arrays of silicon walls with different spacing and pitch as well. A simple model, based on the coalescence in a single pore of multiple stable pores, is proposed to explain the experimental data. © 2002 The Electrochemical Society. All rights reserved.

Observation of five-fold local symmetry in liquid lead.

The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.

AEROSOL FORMATION UNDER HETEROGENEOUS/HOMOGENEOUS THERMAL DECOMPOSITION OF SILANE: EXPERIMENT AND NUMERICAL MODELING

Experimental and numerical study of aerosol formation under heterogeneous/homogeneous silane thermal decomposition is carried out. Experimental exploration included monitoring of the following parameters during silane decomposition: silane conversion degree and concentrations of disilane and trisilane; aerosol concentration; size and morphology of aerosol particles; number of monohydride SiH and polyhydride (SiH 2) n groups in aerosol particles; number of dangling bond active centers in aerosol particles. To explain the experimental results, a numerical model was developed. It covered homogeneous reactions (involving 11 gaseous species), heterogeneous reactions of silicon wall deposition from gaseous species, aerosol formation and aerosol particle wall deposition. The modeling results are in reasonable agreement with the experimental data.

Adsorption in Ordered Porous Silicon: a Reconsideration of the Origin of the Hysteresis Phenomenon in the Light of new Experimental Observations

AbstractPorous silicon formed in p+ single crystal silicon is an interesting material for the study of the behavior of a fluid confined in mesoporous media, because it is a simple system of non interconnected straight pores, all perpendicular to the substrate and it can be well characterized by different methods. Moreover, the pores may be closed at one end when the porous layer is supported by the substrate or opened at both ends when the layer is removed from the substrate. Surprising results have been obtained. A hysteresis loop of type H2 (IUPAC classification), which is generally obtained for highly interconnected porous material, is also observed. Furthermore, a hysteresis of same type is also observed in pores closed at one end which is in contradiction with the Cohan model. The steep desorption process does not account for the large pore size distribution extracted from transmission electronic microscopy study. It is believed that, during the desorption process, the presence of an adsorbed layer on the external surface of the pores and/or the coupling between the pores via the silicon walls must be taken into account.

Optical Applications of Macroporous Silicon

AbstractTwo promising optical applications of macroporous silicon are presented. Due to the high contrast in dielectric constant between the air filled pores and the silicon walls the porous structure exhibits a photonic band gap for infrared radiation perpendicular to the pore axis. By photolithograpic patterning waveguides and optical cavities can be realized in this two-dimensional photonic crystal. Along the pore axis a short-pass filter characteristic is observed for ultraviolet and visible light. Such macropore filters are of high optical quality and may replace conventional filters in imaging systems.