Thick Porous Silicon Research Articles

Effect of the composition of electrolyte on separation of porous silicon film by electrochemical etching

Formation and separation of thin porous silicon films from the original substrate in combination with the transfer of such films onto a cheap foreign substrate for the photovoltaic application provides a straightforward way to cost reduction. Separation of the porous silicon film from the reusable p + silicon substrate is achieved by electrochemical etching of silicon under specific HF concentration and current densities. The effect of HF concentration on film separation is studied. It is found that film separation is possible only for HF concentration of 22% and lower. Addition of DI water in the solution results in the formation of a thick porous silicon layer without separation from the substrate during 10 minutes. A theoretical explanation has been given to describe the phenomena of porous silicon film separation.

Characterization of a silicon thermal gas-flow sensor with porous silicon thermal isolation

In this paper, the results of full characterization of a micromachined-silicon thermal gas flow sensor will be presented. The sensor is composed of two series of thermocouples on the right and left side of a polysilicon resistor, used as heater. The resistor and the hot contacts of the thermocouples lie on a thick porous silicon layer, which assures local thermal isolation, while the thermopile cold contacts lie on bulk silicon. Gas flow is parallel to the surface of the sensor and perpendicular to the resistor, which is heated at constant temperature. The power of the heater is stabilized by an external circuit, which provides a feedback current to compensate changes in the resistance of the heater under flow. Characterization of the sensor both under static conditions and under flow of different gases will be presented. The sensor shows high sensitivity [of the order of 175 /spl times/ 10/sup -3/ mV/(m/s)/sup 1/2/ per thermocouple] and very rapid response, below 1 ms, which makes it appropriate for use both under laminar and under turbulent flows.

Effective method for stress reduction in thick porous silicon films

A promising stress control process is demonstrated to achieve near-zero stress levels in thick porous silicon (PS) films. Stress reduction is necessary for thick PS structures to be used for radio-frequency applications such as on-chip inductors and transmission lines that require very thick (>100 μm) insulating materials. This study employs a standard sample structure with 50-μm-thick PS films formed on p+ substrates through anodization under 50 mA/cm2 and 25% HF concentration. Mass spectroscopy analysis shows that the desorption of hydrogen is correlated with the stress evolution in PS films. As-prepared PS films are under compressive stress. Thermal annealing in N2 ambient turns it into tensile stress. Further annealing in an oxidizing ambient restores the compressive stress. Such stress evolution can be explained by the atomic structure changes on the pore walls of PS films: H2-passivated and oxidized Si surfaces result in compression in PS films, whereas reconstructed state results in tension. These findings are used to fabricate thick PS films with negligible stress (<1 MPa) at room temperature.

Thick porous silicon formation using implanted mask technology

Surface micromachining is an established microtechnology. The process is only limited by sacrificial layer thickness and sometimes a disturbing surface topology. This paper describes an innovative surface micromachining technology. Standard surface micromachining allows layer thicknesses of a few micrometer. Using porous silicon as sacrificial layer, it is possible to create any layer thickness up to 100 μm. Thick porous silicon sacrificial layers are used to combine the advantages of standard surface micromachining with the advantages of bulk micromachining. The problems resulting from surface topology are eliminated by using ion implanted masks. Based on different porous silicon formation mechanisms for n- and p-type silicon, it is possible to use n-implanted layers as masking material during the anodization of p-type silicon, resulting in a plane surface without any steps which would be generated using a hard surface mask. For this masking technology, no additional masking layers are required. For free-standing membrane generation it is possible to deposit, e.g. a PECVD-layer on top of the porous silicon layer. A complete process flow has been developed for thick porous silicon layers up to 100 μm. The use of this sacrificial layer technology for thermally isolated gas sensor membrane fabrication and the detailed process parameters will be presented.

Structural and photoluminescent properties of carbon structures on thick porous silicon

In this work, we studied the properties of thin nanoporous carbon coatings on very thick porous silicon (PS) layers of very high porosity. The PS was obtained by etching pores into crystalline Si wafers in a process controlled to prevent crack formation during etching and drying. The nanoporous carbon was made by dipping the PS samples in diluted polyvinyl alcohol and submitting the coating to pyrolysis at different temperatures (in the 513–843 K range). The porous carbon coatings obtained on porous silicon using this method presented good adherence, uniform thickness and good optical transparency, the greatest luminescence intensity being obtained at a pyrolysis temperature of 513 K. Such a composite nanoporous structure of two type IV materials may find a large number of future applications, ranging from luminescent devices to molecular sieves.

Micro-Raman analysis of polysilicon membranes deposited on porous silicon channels

Local stress distribution is determined in polycrystalline silicon using micro-Raman spectroscopy. The microstructures are thin membranes over a thick porous silicon channel locally grown on bulk silicon. The effect of the width of the membrane on the distribution of the local stress is measured. The results show that annealing reduces the residual stress by an order of magnitude and that the tensile stress in the membrane has a sinusoidal dependence for the relatively narrow channels.

X-ray scattering study of porous silicon layers

X-ray reflectivity is used to study the mesoscopic structure of porous silicon layers. For a porous silicon thin film, we present measurements of the specular and the diffuse scattering together with their best fits. The analysis of the scattered intensity within the distorted-wave Born approximation based on a model of rough fractal interface yields new structural information. Furthermore, an overview of experimental results obtained for several thick porous silicon samples (either as–formed or etched) is presented. The diffuse scattering exhibits characteristic effects due to large surface roughness and the porous structure.

Chopping Frequency Dependence of Photoacoustic Spectrum in Porous Silicon

The dependence of photoacoustic (PA) spectra on the chopping frequency in the range of 10 to 180 Hz was studied in porous silicon (PS) samples of varying thicknesses the PS layer. The enhancement of PA signals over those of bulk Si, reported in our previous work, is ascribed to the combined effects of interstitial gas “pressure” and intrinsic change of absorption. Although the enhancement seen in the long wavelengths, above 600 nm in the thick PS samples, is mainly caused by the “pressure" effect, the enhancement in the short wavelengths, below 600 nm, appears to be mainly due to the intrinsic effects related to the quantum confinement effect because it increases as the porosity increases and seems to remain at higher chopping frequencies (f=180 Hz). We could also estimate the thermal diffusivity of the PS samples from the crossover frequency of the change of slope in the log PA signal vs the log chopping frequency plot. The dependencies of the PA spectral edges on porosity and chopping frequency are shown and discussed in reference to the double layered structure model proposed in our previous work.

A Novel Experimental Approach to PLE Spectral Measurement in PS: Results and Simulation

The detailed study of photoluminescence excitation (PLE) spectra in thick porous silicon (PS) layers is carried out using modified experimental technique. Instead of monochromatic detection usually used, a PLE signal is recorded over the whole emission band from a PS layer. For the first time, this technique allowed us to obtain the reliable PLE data up to far UV range of excitation energy Eexc = 5.4 eV. The PLE spectra showed increasing region below the maximum at Eexc ≈ 4.3 eV, then PLE intensity decreased monotonically. The absorption spectrum for PS layers is extracted from the same PLE data using depth profiling method. Low-energy region of the PLE spectra is satisfactorily simulated in terms of quantum confinement model assuming a constant quantum efficiency and log-normal size distribution of silicon nanocrystallites. The PLE decrease above Eexc ≈ 4.3 eV is discussed assuming that hot electrons may be injected from silicon crystallites to adjacent oxide phase in PS. In this way, leaving electrons give rise to nonradiative losses.

Microcrystalline diamond deposition on a porous silicon host matrix

Porous silicon (PS) is a nanostructured material obtained by etching pores into crystalline Si wafers. In this paper, we report on the nucleation and growth of diamond on very thick PS films (130–220 μm) of very high porosity (10–50%). The edges of the pores were in the form of small crosses, which followed the original directions of the 〈100〉 c-Si. The diamond coating was made by chemical vapor deposition (CVD) in a hot-filament reactor. We observed that the diamond nucleation occurs mainly at the edges of the pores but relatively few nuclei follow a preferential orientation axis. As the nucleation density is very low, coalescence does not occur even after 11 h and 30 min of deposition. Using a pre-deposition ‘seeding’ process with diamond grains, it was possible to produce a complete diamond CVD coating. A cross-section analysis of the diamond/PS/c-Si structure by scanning electron microscopy (SEM), micro-Raman and photoluminescence spectroscopies revealed interesting results: the luminescence of the PS under the diamond layer is preserved. There is no diamond deposition inside of the pores, but a small permeation of carbon was identified which forms diamond-like phases at the bottom of the pores. The Raman analyses indicated also a small contamination of the diamond layer by Si nano-crystals.

Structure and properties of diamond films deposited on porous silicon

Thick porous silicon (PS) layers, made by anodic etching of crystalline Si wafers, have been coated with diamond films using the hot-filament chemical vapor deposition (CVD) technique. Ethanol diluted in hydrogen was used as the carbon source for diamond deposition. We observed that diamond nucleation occurs predominantly on the top of the PS spikes, creating individual islands of growth. Nucleation is apparently homogeneous over the PS surface and the islands grow independently until the complete coalescence of the diamond film. Although the diamond film is polycrystalline, it does not have the usual columnar structure of diamond deposited on c-Si. A good adherence between diamond and PS was observed in diamond–PS and diamond–PS–diamond structures. The problems of the intrinsic and thermal stresses have been addressed. Samples were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Raman and photoluminescence spectroscopy.

Heteroepitaxy of PbS on porous silicon

Epitaxial PbS films were grown by molecular-beam epitaxy (MBE) on the surface of porous silicon (PS) formed on the silicon substrate of (111) orientation. The uniform PS layers 5μm thick were produced by electrochemical anodic treatment of n+-silicon in HF solution. The structure of both PbS and buffer PS was studied by X-ray diffraction analysis as well as scanning electron microscopy. The PbS layer has been demonstrated to have a columnar structure at the early stages of growth, while a solid structure of the grown layer has been observed with increasing a thickness of the epitaxial layer up to 0.5–1.0μm. PbS epitaxial films grown on the substrates with the 2–5μm thick PS layers of 20–40% porosity were comparable with the films grown on the BaF2 substrates.

Thick oxidised porous silicon layers for the design of a biomedical thermal conductivity microsensor

Porous silicon (PS) offers new possibilities to be applied as thermal insulating material for microsensor design due to its low thermal conductivity (TC) value compared with TC of SiO 2. A biomedical TC microsensor based on differential thermoelectric measurements has been designed using a PS substrate. In order to ensure an efficient thermal isolation in the microsensor, main thermal and geometrical characteristics of the PS layers as well as of the whole microsensor have been numerically simulated. PS layers with low TC have to be thick and mechanically stable under further processing. To form thick (50–200 μm) and stable PS layers, a new approach based on progressive changing of anodisation current density (from 100 to 25 mA/cm 2) during PS formation has been elaborated. To find a suitable compromise between low TC and mechanical stability of thick PS layers, an adapted thermal oxidation recipe at moderate temperatures (500–600°C) in dry oxygen atmosphere has been applied. It leads to 20–50% oxidation fraction in PS layers (measured by Energy Dispersive Spectroscopy) corresponding to SiO 2 TC value. A test device has been realised and characterised. A Seebeck coefficient of 400 μV/°C per junction has been measured for a Poly-Si/Al thermopile deposited on the PS layer.

Inelastic light scattering and X-ray diffraction from thick free-standing porous silicon films

The results of Raman scattering and X-ray diffraction studies of thick, free-standing, porous Si layers with thickness up to 500 μm are presented. The Raman scattering spectra have a distinctive difference from previous data for porous Si films on Si substrate and for thin, free-standing, porous Si layers. The experimental data can be explained by a modified phonon confinement model that accounts for a comprehensive strained Si nanocrystal. The comprehensive strain is a tensile one, and the value of stress can be up to 3 GPa. This interpretation is supported by data of X-ray diffraction measurements.

Nanoscale nature and low thermal conductivity of porous silicon layers

Recently discovered phenomenon of low thermal conductivity of porous silicon (PS) layers is discussed in detail. A proposed theoretical model explains the considerable decrease of the thermal conductivity of nanoscale PS in comparison with meso-PS and bulk silicon. The thermal conductance dependence of Si/porous Si structures on the formation conditions of PS layers has been studied. By varying the values of anodisation current density and anodisation time it is possible to optimize the thermal conductance values of the Si/porous Si structures. In this way, an efficient thermal isolation can be obtained by forming thick PS layers.

Formation and Luminescent Properties of Oxidized Porous Silicon Doped with Erbium by Electrochemical Procedure

AbstractThe present work is concerned with Er-doped oxidized porous silicon (PS). The characteristic feature of the work is that PS doping has been realized by an electrochemical procedure followed by a high temperature treatment. 5-μm thick PS layers were formed on p-type Si of 0.3-Ohm-cm resistivity. Er incorporation was performed by a cathodic polarization of PS in a 0.1 M Er(NO3)3 aqueous solution. A high temperature treatment in an oxidizing ambient at 500-1000°C was utilized to provide either partial or total oxidation of PS:Er layers. X-ray microanalysis was used to study chemical composition of the samples. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were investigated. After the partial oxidation (in the temperature range of 600-800°C), weak Er3+-related PL at 1.53 ptm was observed. A high temperature anneal in Ar atmosphere at the temperature of 1100°C caused a significant increase in the Er3+-related PL intensity. Resonant features were observed in PLE spectra of fully oxidized PS. Five peaks at 381, 492, 523, 654 and 980 nm were revealed. The strongest excitation occurred at 381 and 523 nm. The excitation of different Er3+ energy levels, cross-relaxation interactions and emission due to the 4I13/2→4I15/2 transitions were considered. Application of the Er-doped oxidized PS for integrated optical waveguides is presented.

Reflection characteristics of porous silicon surfaces

Advanced solar cells and other optical detection devices incorporate surface texturing to reduce optical reflection and enhance absorption. Using electrochemical etching techniques, porous silicon surfaces are fabricated. Thin (pore depths less than 1 ?m) and thick (pore depths greater than 5 mm) porous silicon surfaces are optically characterized in terms of their ability to reduce optical reflectance. Three regions of the optical spectrum were investigated: visible (500 to 900 nm), near-IR (1.25 to 2.5 ?m), and mid-IR (2.5 to 12.5 ?m). A highly polished, single-crystal silicon wafer was used as a reference surface. It was determined that thick porous silicon manifested superior results compared to the thin porous silicon, demonstrating a 91% total reflectance reduction in the visible region, a 7% reduction in the near-IR range, and a 53% reduction over the mid-IR wavelengths.

Relationship of Processing Parameters to Photoluminescence Intensity and Mechanical Failure in Thick Porous Silicon Layers

The surface passivation of porous silicon has received much attention. While nuclear magnetic resonance may be able to provide insight into the surface passivation of porous silicon, relatively thick (>40 μm) layers are desired. However, mechanically stable layers could only be produced within identifiable ranges of HF concentration, current density, and anodization time. This information allowed sample preparation for nuclear magnetic resonance characterization of porous silicon. In addition, an inverse correlation between mechanical stability and photoluminescence intensity was observed. In a pairwise comparison, similar porosity and surface passivation were maintained as anodization time increased under otherwise constant conditions. However, at the longer anodization time, photoluminescence intensity and surface area increased. To account for these observations, we hypothesize that the thickness of the silicon wall between pores decreases as electrochemical etching proceeds. Decreasing wall thickness could lead to both mechanical failure and increase the probability of creating regions of quantum confinement. A larger number of quantum confined regions, having the same surface passivation, could account for the observed increased photoluminescence intensity.

厚いポーラスシリコンの光化学エッチングによる発光領域の深さ方向への移動

厚いポーラスシリコンの光化学エッチングによる発光領域の深さ方向への移動

Application to optical components of dielectric porous silicon multilayers

Narrow-band color filters have been integrated on porous silicon layers. Distributed Bragg reflectors and Fabry–Pérot interference filters based on layered porous silicon samples are demonstrated. The effects of narrowing and tuning the porous silicon emission band are shown in structures composed by Fabry–Pérot filters integrated on top of thick porous silicon layers.