Article

Porosity and Thickness Characterization of Porous Si and Oxidized Porous Si layers – an ultraviolet-visible-mid Infrared Ellipsometry Study

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Abstract

This paper suggests the evaluation of morphological parameters of porous silicon layers (PSL) using spectroscopic ellipsometry from UV to mid-infrared optical range. PSL were prepared by electrochemical etching of monocrystalline silicon wafers in hydrofluoric acid-based electrolyte. Measuring with an optical and an infrared ellipsometer with a wide spectral range permits an accurate characterization of PSL properties from the top surface to the bottom of the layer with thicknesses from several hundred nanometers up to a few tens of micrometers. Several different optical models for ellipsometric evaluations were developed to determine the thickness, the average porosity, the in-depth porosity gradient, the oxidation level and the surface roughness of the PSL. Porosity was modeled with multiple effective medium layers by varying ratio of crystalline silicon, void and oxidized silicon wherever needed. Thin PSL (<5 μm) shows no impact of current density on porosity and thickness. However, evaluation of thick PSL (20 - 50 μm) highlights the in-depth porosity gradient. Thickness values were also cross-checked with electron microscopy confirming the proposed ellipsometric models. Additionally, different oxidation techniques have also been compared in terms of oxidation level and void content. Volume expansion during PSL oxidation follows exactly the same behavior as that during the oxidation of planar silicon wafers.

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... Since the early 1980s several studies report the SE characterization of PSi layers using models with increasing complexity to obtain the most, physically relevant information complementing other techniques [8][9][10][11][12][13][14][15]. For instance, we have recently demonstrated the characterization of mesoporous Si layer with thickness up to 50 m using infrared spectroscopic ellipsometry [16]. ...
... The electrolyte solution was composed of 30 wt.% hydrofluoric acid and 25 wt.% acetic acid. The electrochemical conditions were set to etch PSi with thicknesses below 5 m to enable optimal UV-NIR ellipsometry characterization [16]. Anodizations were carried out in a single tank electrochemical cell with a surface of silicon exposed to the electrolyte of 5 cm 2 . ...
... In these cases, the dielectric response (and hence the optical properties) of a composite layer is described as a mixture of the dielectric functions of its microscopic constituent parts, as long as they are large enough to keep their bulk like behavior, i.e. there are no quantum confinement or other micro-surface related effects. Many authors have shown, that for PSi layers, EMTs work perfectly [8][9][10][11]13,16,30] and some studies have also shown they work for SiNWs [28,31]. In these cases, as in our method, the nanostructures are described by a varying ratio of monocrystalline silicon (c-Si) and void. ...
Article
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Columnar mesoporous Si thin films and dense nanowire (SiNW) carpets were investigated by spectroscopic ellipsometry in the visible-near-infrared wavelength range. Porous Si layers were formed by electrochemical etching while structural anisotropy was controlled by the applied current. Layers of highly oriented SiNWs, with length up to 4.1 μm were synthesized by metal-assisted chemical etching. Ellipsometric spectra were fitted with different multi-layered, effective medium approximation-based (EMA) models. Isotropic, in-depth graded, anisotropic and hybrid EMA models were investigated with the help of the root mean square errors obtained from the fits. Ellipsometric-fitted layer thicknesses were also cross-checked by scanning electron microscopy showing an excellent agreement. Furthermore, in the case of mesoporous silicon, characterization also revealed that, at low current densities (<100 mA/cm²), in-depth inhomogeneity shows a more important feature in the ellipsometric spectra than anisotropy. On the other hand, at high current densities (>100 mA/cm²) this behaviour turns around, and anisotropy becomes the dominant feature describing the spectra. Characterization of SiNW layers showed a very high geometrical anisotropy. However, the highest fitted geometrical anisotropy was obtained for the layer composed of ∼1 μm long SiNWs indicating that for thicker layers, collapse of the nanowires occurs.
... B-Si is a sponge-like structure of monocrystalline silicon, which became one of the most well-researched silicon structures [3]. B-Si has a wide range of industrial applications (as sensors, as anodes in Lithium-ion battery, photovoltaic applications, biosensors, optoelectronics, self-cleaning coatings, etc) [3][4][5]. This paper deals with applications of b-Si in solar cells. ...
... Thermal oxidation forms an SiO 2 layer on the sample surface. We suppose that the pores were closed in the upper sublayers, which prevents the diffusion of oxygen to the remaining structure [4]. This process is illustrated in Fig. 9. ...
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In this work black silicon (b-Si) samples were prepared by anodic (electrochemical) etching of p-type silicon substrate in solution of hydrofluoric acid (HF). We studied influence of anodic etching conditions (etching time, electrical potential and current) on the spectral reflectance and Raman scattering spectra. Optical properties of b-Si structures were experimentally studied by UV-VIS (AvaSpec-2048) and Raman (Thermo DXR Raman) spectrometers. B-Si layer thickness of formed substrate were determined by using SCOUT software. Effective medium approximation theory (Looyenga) was used in construction of the reflectance model. Influence of the deformation of crystal lattice introduced during the substrate etching was studied by Raman scattering method. Teoretical model of the 1 st order Raman scattering profile was constructed by using pseudo-Voigt function and the profile parameters were extracted. The values of biaxial tensile stress were estimated by using optimized Raman profile parameters. K e y w o r d s: Raman scattering, porous silicon, black silicon, spectral reflectance, anodic etching, electrochemical etching, SCOUT
... The appropriate measurement method depends on the requirements of the application [16]. For instance, spectroscopic ellipsometry (SE) is commonly used for measuring transparent films with thicknesses ranging from nanometers to tens of micrometers [17][18][19]. However, model-based SE thickness calculations require precise modeling and parameter selection to obtain accurate results [20][21][22]. ...
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An astigmatic optical profilometer is a precision instrument with advantages such as high resolution, high bandwidth, a compact size, and low cost. However, current astigmatic optical profilometers measure only surface morphology, and their potential for capturing subsurface information remains underutilized. In this study, we developed a method for measuring the thickness of transparent thin films with an astigmatic optical profilometer. Experimental results demonstrate that the thickness of transparent films tens of micrometers thick can be accurately measured. The maximum thickness measurable through our system is approximately 100 μm, which may be increased to 1.2 mm through the use of a scanner with a greater travel range. A coupling problem occurs for films <25 μm in thickness. However, to solve this problem, we devised a decoupling method, which was experimentally implemented to successfully measure a 18-μm-thick film. Moreover, the ability to obtain 3D images, including of both the upper and lower surfaces, was demonstrated.
... Through the depend of the etching process of the silicon structure and create pores with etching time, we have shown that in Fig. 2, the increase of porosity and thickness of the porous layer with etching time from 2 to 4 min as illustrated in Table 1. That agrees with Fodora et al. reported that the average porosity increases rapidly with thickness silicon [9]. Etching rate or growth rate data of the porous silicon measured by Eq. 3. The thickness T and thereby the growth rate r p . ...
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In this paper, the structural properties of porous silicon layer PSL were reported. Photo-assisted (laser) electrochemical etching PECE technique used to fabrication PSL from n-type wafer silicon as a function of etching time. Optical microscopy OM image is confirmed that the surface topography of porous silicon layer formation was a mud-like structure. The porosity and thickness have been determined gravimetrically are varied from 61% to 82% and 7.2 µm to 9.4µm respectively. The XRD patterns show that one diffraction peak for all PSL through anodization duration and it is assigned to the (400) plane and data confirmed the porous silicon PS was nanocrystalline.
... Since we try to compare compactness of coatings, it is assumed that total content of porosities is proportional to superficial percentage. In another study by Fodor et al. [54] used spectroscopic ellipsometry to measure average porosity and porosity gradient. A simple and direct method to measure porosity content is to measure the porosity through image analysis [13,55]. ...
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Plasma electrolytic oxidation (PEO) was carried out on AA1190 aluminum alloy in mixed silicate-phosphate-based electrolyte in order to fabricate ceramic coating under constant current density. The variations of PEO coating duration with kinetics, surface roughness, amount and size of discharge channels were studied with respect to PEO processing time. The growth mechanism of the ceramic coating was described considering a variation of volume and diameters of discharge channels and pancakes during the PEO. Scanning electron microscope (SEM), atomic force microscope (AFM), and roughness tester were used to study the plasma discharge channels of the PEO coatings. In addition, the effect of alumina nanoparticles in the electrolyte as the suspension was studied on the geometric parameters of discharge channels. It seems that the nanoparticles are adsorbed to the locations of erupted molten oxide, where the dielectric breakdown occurs. Nanoparticles were embedded in the dense oxide layer and were adsorbed at the walls of voids and coatings surface. As a result, they caused significant changes in roughness parameters of the samples containing nanoparticles compared to those without nanoparticles. The obtained results showed that growth kinetics followed a linear trend with respect to PEO coating duration. It was also observed that in the absence of alumina nanoparticles, the average volume of the pancakes is 150% greater than the ones fabricated in the suspension of nanoparticles. Besides, increasing the PEO coating duration leads to adsorbing more nanoparticles on the coating surface, filling the voids, and flattening the surface, and alterations in Rv, Rsk, and Rlo parameters. Correlation between the diameter of discharge channel (dc) and thickness of the pancake (h) also showed a linear relation.
... In the present study, we performed the porosity analysis of this layer material in a nanostructured variant generated by templated sol-gel synthesis [18]. The determination of the porosity of TiO 2 is achieved by independently using SE with an analysis technique similar to that used in [10,19] using BEMA to determine void fractions. ...
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The practical performance of surface coatings in applications like catalysis, water splitting or batteries depends critically on the coating materials’ porosity. Determining the porosity in a fast and non-destructive way is still an unsolved problem for industrial thin-films technology. As a contribution to calibrated, non-destructive, optical layer characterisation, we present a multi-method comparison study on porous TiO2 films deposited by sol-gel synthesis on Si wafers. The ellipsometric data were collected on a range of samples with different TiO2 layer thickness and different porosity values. These samples were produced by templated sol-gel synthesis resulting in layers with a well-defined pore size and pore density. The ellipsometry measurement data were analysed by means of a Bruggeman effective medium approximation (BEMA), with the aim to determine the mixture ratio of void and matrix material by a multi-sample analysis strategy. This analysis yielded porosities and layer thicknesses for all samples as well as the dielectric function for the matrix material. Following the idea of multi-method techniques in metrology, the data was referenced to imaging by electron microscopy (SEM) and to a new EPMA (electron probe microanalysis) porosity approach for thin film analysis. This work might lead to a better metrological understanding of optical porosimetry and also to better-qualified characterisation methods for nano-porous layer systems.
... Schematic overview of CNM-functionalized nanocomposite scaffolds for myogenesis and the promising potential of spectroscopic analysis for examining nanocomposite scaffolds. (8)(9)(10). FTIR spectroscopy can simultaneously collect high-resolution spectra of composite materials over a wide spectral range by measuring an infrared absorption or emission of composite materials. ...
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Skeletal muscle injuries are extremely common because skeletal muscle is quite frequently used in the human body, and cause serious health implications. Currently, grafting and pharmacological therapies are the most common therapeutic methods for treating and repairing the skeletal muscle damages, but both therapeutic methods have significant limitations. Therefore, in recent years, the tissue engineering approaches have attracted more and more attentions in biomedical and bioengineering fields. In particular, up-to-date studies have focused on the novel strategies aimed at promoting and enhancing the regeneration of skeletal muscle tissue by using tissue engineering scaffolds. Although the tissue engineering scaffolds can be readily fabricated with conventional biocompatible materials, such as polymer, ceramic or metallic materials, the carbon nanomaterials (CNMs) are the most fascinating candidates as a scaffold material due to their favorable biocompatibility and extraordinary physicochemical, electronic, mechanical, and thermal properties. The aim of the present review is to summarize some of the recent reports concerning the nanocomposite scaffolds functionalized with CNMs and to highlight promising perspective for the applications of CNMs as skeletal tissue engineering scaffolds. In addition, it is also discussed how the spectroscopic analysis can be employed for analyzing CNMs and nanocomposite scaffolds.
... Within its limit of applicability, the Maxwell Garnet homogenization procedure was extensively validated and tested both using experimental data [27,28] and numerical results obtained from the finite-difference time-domain method [25] or the finite elements method [29]. ...
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We numerically investigated the peculiar spectral properties in the mid- to long-infrared range of a metamaterial composed of silicon carbide subwavelength oriented wires, deposited onto a silicon substrate. We show how anisotropic ellipsoids can be arranged, choosing both shape and orientation, so that the final resulting emissivity could match the emissivity peaks of hazardous chemicals for different polarizations of the emitted light. As an example, we design a metamaterial-based device that matches the emissivity lines of two explosives, such as XRD (cyclotrimethylenetrinitramine) and TNT (trinitrotoluene), providing a double check in polarization that could increase accuracy and versatility of chemical and biological sensors.
... The large internal surface area of microporous and mesoporous silicon leads to a higher mass of dissolved silicon in the HF in comparison with bulk silicon. 32,33 The chemical etching tends to increase pore dimensions and the layer porosity 34,35 from the wafer surface to the PS/Si interface. Furthermore, the cumulative effect of the dihydrogen bubble aggregation at the wafer surface and the limited diffusion in the layer reduced the renewal of the HF into the porous framework. ...
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The formation of thick mesoporous silicon layers in P+-type substrates leads to an increase in the porosity from the surface to the interface with silicon. The adjustment of the current density during the electrochemical etching of porous silicon is an intuitive way to control the layer in-depth porosity. The duration and the current density during the anodization were varied to empirically model porosity variations with layer thickness and build a database. Current density profiles were extracted from the model in order to etch layer with in-depth control porosity. As a proof of principle, an 80 μm-thick porous silicon multilayer was synthetized with decreasing porosities from 55% to 35%. The results show that the assessment of the in-depth porosity could be significantly enhanced by taking into account the pure chemical etching of the layer in the hydrofluoric acid-based electrolyte.
... It is essentially different of earlier works on polarization properties on NWs [38,39,41] that used only polarization filters, that allows the observations of linear polarization states. The presented technique should not be confused to ellipsometry [44][45][46], whose goal is to measure sample properties (optical parameters or thickness of thin films) by combining optical polarization and interference concepts. ...
... Il est possible de mesurer l'indice de réfraction d'une couche poreuse par spectroscopie ou par ellipsométrie [121]. Après sa fabrication, le SiP est constitué à la fois de silicium et d'air, l'indice de réfraction d'une couche de SiP varie donc en fonction des fractions volumiques de chacun des matériaux et permet de couvrir un large intervalle d'indice de réfraction. ...
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Les biocapteurs sont des dispositifs servant à détecter la présence de biomolécules dans un milieu de détection, qui peut être un liquide ou un gaz. L’utilisation de l’optique intégrée permet d’exploiter diverses interactions des biomolécules avec la lumière propagée dans des structures guidantes compactes et facilement réalisables, comme le sont les micro-résonateurs. Dans cette thèse, nous utilisons du silicium poreux dans la fabrication de nos transducteurs optiques composant les biocapteurs. Il s’agit d’un matériau biocompatible présentant une surface spécifique importante sur laquelle peuvent être greffées des molécules. Il permet aussi d’exploiter la détection surfacique de biomolécules directement dans le volume du matériau de par sa nature poreuse. Le matériau a au préalable besoin de subir un procédé de biofonctionnalisation pour permettre l’infiltration de molécules dans les pores qui le compose. En utilisant un procédé de photolithographie, des micro-résonateurs sont fabriqués pour être utilisés comme transducteurs pour la détection surfacique de BSA. La présence de la protéine dans le milieu de détection va induire une modification quantifiable des propriétés des transducteurs et liée à la concentration de la BSA. Une sensibilité de plus de 1000 nm/UIR a pu être obtenue et se révèle meilleure que l’état de l’art. La réalisation d’une structure hybride à base de silicium poreux et de polymères est étudiée. L’avantage de l’utilisation couplée du silicium poreux et des polymères est de permettre la réduction des pertes de propagation tout en améliorant les performances de ce type de biocapteur.
... Porous silicon (pSi) has become a most significant porous material with a wide range of industrial applications, especially in optoelectronics, microelectronics, biomedicine (drug delivery, biosensors) and photovoltaics (high efficiency solar cells) [1][2][3][4]. This paper deals with applications of pSi in photovoltaics. ...
Conference Paper
Porous silicon (pSi) samples for photovoltaics applications were prepared by the method of electrochemical etching in the hydrofluoric acid (HF) solution. P-type silicon wafers (boron-doped) were used as substrate. Different parameters of the electrochemical etching method (electrical potential and current, etching time) have been used in the production of pSi samples. Optical properties of pSi samples were experimentally studied by UV-VIS spectrometer. The thickness of the porous layer formed on the Si substrate surface was determined by using theoretical model of spectral reflectance. Effective medium approximation theory (Looyenga) was used in construction of this theoretical model.
... Porous silicon (pSi) is a very important material that has a wide range of applications (sensors, as anodes in Lithium-ion battery, photovoltaic applications, biosensors, drug delivery, optoelectronics, self-cleaning coatings, etc.) [1][2][3]. Biocompatibility of pSi is crucial for biomedical applications (exhibits little or no toxic properties, it does not induce an immune response) [4]. Low spectral reflectance (below 1 %) and high light absorption in a wide range of wavelengths (from 300 nm to 100 nm) of pSi is crucial for photovoltaic applications [5]. ...
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The transfer matrix and effective media theories (EMA) were implemented in MATLAB environment. These theories in combination with genetic algorithm were used for optimization of spectral reflectance data of simulated and real porous Si structures. From the spectral reflectance data the thickness and EMA volume fractions were determined. The real porous Si structure was prepared by electrochemical etching of p-type Si substrate. The spectral reflectance of simulated pSi structure was constructed in SCOUT software.
... As the samples were thermally oxidized, it is likely that the pores were closed on the upper layer, which prevents the diffusion of oxygen to the remaining silicon [18]. For this reason, two volume fractions (SiO 2 and c-Si) were used in the Looyenga EMA model. ...
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... In this work we present optical properties of samples prepared by electrochemical etching method. The result of the electrochemical etching method is in most cases a porous structure, which currently belongs to the materials with a wide range of industrial applications (sensors, as anodes in Lithium-ion battery, photovoltaic applications, biosensors, drug delivery, optoelectronics, self-cleaning coatings, etc) [9][10][11][12][13][14][15][16][17][18][19][20]. It is important to note that homogeneous porous structures do not have a sufficiently low reflectivity and are therefore not suitable for photovoltaic applications. ...
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Porous silicon rugate filters are fabricated and investigated for their ability to sense chemical species. The durability of the filter is tested by allowing the structure to undergo many cycles of adsorption and desorption of vapor-phase ethanol molecules. The characteristic reflectivity peak of the structure exhibits a relative blueshift of 2.7% after 86 adsorption∕desorption cycles. The observed shift is ascribed to the formation of silicon dioxide, which has a lower refractive index than that of silicon. In order to stabilize the structure against oxidation expected from cycling and environmental exposure, the filter is subjected to electrochemical oxidation in an aqueous sulfuric acid electrolyte. The treatment dramatically improves stability of the sensor; a relative blueshift of <0.4% is observed after 100 adsorption∕desorption cycles for this sensor. The sensitivity of the sensor is also affected by electrochemical oxidation: the response to saturated ethanol in air changes from Δλ = 100 nm to Δλ = 70 nm, respectively. Theoretical calculations using the Bruggeman effective medium approximation and the characteristic matrix method indicate that up to 15% (by volume) of silicon is transformed to silicon dioxide by the electrochemical oxidation procedure. This volume ratio is close to that estimated from Auger electron spectroscopy measurements.
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Mise au point comportant des definitions generales et la terminologie, la methodologie utilisee, les procedes experimentaux, les interpretations des donnees d'adsorption, les determinations de l'aire superficielle, et les donnees sur la mesoporosite et la microporosite
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The key aspects of porous silicon manufactured by anodization are reviewed, with the following subjects being addressed: anodization of different wafer types, wafer cell design, post-anodization handling requirements (rinsing/drying/storage), parameters affecting layer uniformity, the use of nonaqueous electrolytes and electrolyte additives (surfactants, oxidizers, and other types), methods for tuning porosity, process control and natural variability, different electrode materials, and the requirements for maintaining health and safety. © Springer International Publishing Switzerland 2014. All rights are reserved.
Chapter
The chemical reactions of porous Si, involving formation of Si-O, Si-C, Si-N, or Si-metal surface bonds, is reviewed. The reactivity of as-formed porous Si is dominated by the chemistries of silicon-hydrogen (Si-H) and silicon-silicon (Si-Si) bonds, which are strong reducing agents. Depending on the oxidant, various surface species can be generated in oxidation-reduction reactions of porous Si: in particular metal nanoparticles, silicon oxides, or silicon-carbon species. The oxidation chemistry of porous Si, involving air, water, chemical oxidants, or electrochemical oxidation is discussed. The aqueous stability of these various silicon oxides is quite dependent on the means by which a particular oxide is formed. Si-C bond forming reactions including hydrosilylation, hydrocarbonization, carbonization, and reductive electrochemical grafting, and the chemical method used to confirm Si-C bond formation are presented. Because much interest in the chemistry of porous Si is focused on the generation of functional nanostructures to graft molecules such as drugs, proteins, targeting agents, or biological receptor molecules to porous Si surfaces, the review emphasizes the covalent chemistry of Si-O and Si-C surface species for the attachment of functional species (particularly biomolecules) to porous Si.
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Data and literature are collated that emphasize the high tunability of porous silicon properties, either via manipulation of its structural parameters, via the chemistry of the large internal surface area, or via impregnation of other materials. An overview of quantitative data on more than 30 properties is tabulated and compared to those of nonporous silicon. Where available, the range of values reported to date is given. The properties showing the widest tunability to date include the visible photoluminescence (optical bandgap), mechanical stiffness, thermal conductivity, optical refractive index, electrical resistivity, biodegradability kinetics, optical reflectivity, and surface wettability.
Article
With a view to producing thick and very high surface area microporous silicon layers (and subsequently powders) by electrochemical anodisation, the incorporation of various types of chemical additives has been investigated, these in combination with hydrofluoric acid electrolyte and high-resistivity p-type parent substrates. Comparison under constant charge conditions shows that anodisation using 50 wt% hydrofluoric acid, or inclusion of the additives hydrochloric acid, sulphuric acid, or ammonium dodecylsulfate with lower concentration hydrofluoric acid, can facilitate powders with internal surface areas of up to 864 m²/g, average pore sizes in the region of 2.8-3.2 nm, and pore volumes in excess of 0.8 ml/g - all as determined using nitrogen gas adsorption and associated isotherm analysis.
Article
For most thin film structures, by changing the wavelength range to fit ellipsometric spectra, the values of the fitted parameters also change to a certain extent. The reason is that compared with the ellipsometric sensitivity many thin films are vertically non-uniform. In absorbing films with significant dispersion in the used wavelength range, the penetration depth of probing light can show large variations depending on the wavelength. Consequently, the value of a fitted parameter for a certain wavelength range is a weighted sum of structural information over different depth ranges corresponding to the different wavelengths. By changing the wavelength range, the range of penetration depths can be adjusted, and the fitted values can be plotted as a function of the probed depth range calculated directly from the determined or tabulated extinction coefficients. We demonstrate the results on deposited polycrystalline thin films. The advantage of this approach over the parameterization of structural properties as a function of depth is that the wavelength scan approach requires no parameterized depth distribution model for the vertical dependence of a layer property. The difference of the wavelength scan method and the vertical parameterization method is similar to the difference between the point-by-point and the parameterized dielectric function methods over the used wavelength range. The lateral structures strongly influence the ellipsometric response, as well. One of the most remarkable effects is when the lateral feature sizes approach the wavelength of the probing light. In this case the effective medium method is not valid any more, since scattering and depolarization occurs. By scanning the wavelength range, the limit wavelength of the onset of scattering can be found, and used for the determination of the corresponding critical lateral period length.
Article
Herein, a mesoporous silicon film (5 μm thick, diameter of pores ranging from 60 to 70 nm) was prepared through an electrochemical etching of a silicon wafer, and its performance for lithium-ion microbatteries was investigated. A sluggish penetration of the electrolyte into the pores of the material along its depth was clearly observed thanks to cyclic voltammetry measurements, as in fact, the lithiation and delithiation peaks raise during scanning. The penetration of the electrolyte in the mesoporous layer was monitored by elemental analysis and by energy-dispersive X-ray spectroscopy coupled with scanning electron microscopy. Herein, it is clearly reported that after 50 voltammetric cycles, electrochemical reactions take place in the whole depth of the porous silicon layer. In contrast, after only 10 cycles, the bottom part of the silicon pores seems to not be affected. Galvanostatic cycling at a rate of 300 μA cm-2 was performed for two different lower cut-off voltages. A charge limitation of 0.1 V resulted in a stable specific capacity of 1910 mAh g-1. For a deeper charge with a potential limitation of 0.07 V, a higher specific capacity of 2480 mAh g-1 was reached but, unfortunately, this was accompanied by a severe fading of the performances. This phenomenon was attributed to the strong mechanical damages in the porous structure of the silicon negative electrode.
Article
We present a systematic study on ultrathin porous silicon (PS) layers (40–120nm) of different porosities, formed by electrochemical etching and followed by thermal oxidation treatment (300°C and 600°C) and by electrochemical oxidation. The oxidised and non-oxidised PS layers have been analysed by spectroscopic reflectometry (SR), spectroscopic ellipsometry (SE) and secondary ion mass spectroscopy (SIMS). The SR and SE spectra were fitted by a multiparameter fit program and the composition and the thickness of the PS layers were evaluated by different optical models. PS layers, formed electrochemically in the outermost layer of a p/n+ monocrystalline silicon junction were successfully evaluated using a gradient porosity optical model. The non-oxidised PS, formed in p-type silicon, can be well described by a simple optical model (one-layer of two-components, silicon and voids). The spectra of the oxidised PS layers can be fitted better using an optical model with three interdependent components (crystalline-silicon, silicon-dioxide, voids). The SIMS results give a strong support for the optical model used for SR and SE.
Article
Spectroscopic ellipsometry (SE) has proven to be a very powerful diagnostic for thin film characterization, but the results of SE experiments must first be compared with calculations to determine thin film parameters such as film thickness and optical functions. This process requires four steps. (1) The quantities measured must be specified and the equivalent calculated parameters identified. (2) The film structure must be modeled, where the number of films is specified and certain characteristics of each layer specified, such as whether or not the film is isotropic or anisotropic, homogeneous or graded. (3) The optical functions of each layer must be specified or parameterized. (4) The data must be compared with the calculated spectra, where a quantifiable figure of merit is used for the comparison. The last step is particularly important because without it, no “goodness of fit” parameter is calculated and one does not know whether or not the calculated spectrum fits the data.
Article
Thick porous silicon layers were prepared by electrochemical etching on n- and p-type single-crystalline silicon substrate, and characterised by spectroscopic ellipsometry and complementary techniques. The layers were thicker than the penetration depth of light for all wavelengths, so the samples were assumed to be bulk porous material. Best fit was obtained using two-layer models taking into account the surface inhomogeneity. The microstructure of the porous material was characterised using effective medium models with dielectric function reference spectra from the literature or calculated by model dielectric functions (MDF). Using the MDF model, reference dielectric function spectra were generated with different broadening parameters. A good agreement was found between the porosity obtained by ellipsometry and Brillouin scattering.
Article
In this study, we report on structural variation of ultrathin of p-type Porous Silicon (PS) films during the early stage growth process using X-Ray Reflectometry (XRR). The PS samples were firstly characterized by Fourier Transform Infrared spectroscopy (FTIR) measurements. The homogeneity and surface roughness of the PS films, which enable characterizing the PS films by XRR, were investigated by UV-visible spectroscopy and atomic force microscopy (AFM) measurements. The PS layers were then analyzed by specular X-Ray Reflectivity (XRR). XRR reveals a linear thickness behavior as a function of etching time, while the porosity increases before reaching a constant value around 72%. These results were compared to those obtained from other characterization. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Article
To study the effect of various n-type substrates on high-frequency inductor performances, several devices were inte- grated on porous silicon (PS), silicon (Si), and glass. Both n-type mesoporous Si and mesoporous/macroporous Si bilayers were fabricated. The analysis further shows that PS reduces signifi- cantly the substrate losses. Indeed, higher quality factors have been obtained for the inductors integrated on PS than on the Si substrate and particularly in the case of bilayer structures. These original results can be added to p-type PS performances already shown in the literature. Then, this work demonstrates that PS can also be a promising candidate for the integration of passive and active devices on n-type silicon. Index Terms—Electrochemical etching, on-chip inductor, porous silicon (PS), quality factor, radio frequency (RF).
Article
Oxidation behavior of porous silicon under various environments of dry and wet air, and solution with and without appropriate oxidant at mild temperatures has been investigated. The progress of oxidation was followed by infrared spectroscopy. The presence of water vapor greatly accelerates the oxidation rate in comparison with the rate in dry air. The oxidized states are clarified with the help of oxidation experiments of partially hydrogen-desorbed porous silicon, which does not contain SiH2 and SiH3 as the hydride species. An oxidation mechanism is proposed to explain that oxidation is accelerated in the presence of water vapor and at the partially hydrogen-desorbed porous silicon. Further, oxidation behavior of porous silicon in solution containing appropriate oxidant is also investigated. The rate is very rapid and the oxidation does not produce the back-bond oxidized state of OySiHx in contrast to the oxidation in air.
Article
Vacuum annealed and oxidized porous silicon layers (PSL) were investigated by in situ spectroscopic ellipsometry (SE). The nominal porosity of the layers was between 60 and 77% and the nominal thickness was 500 nm. The annealing was performed by direct ohmic heating (R.T. < 450 degrees C) in 5 x 10(-10) Torr vacuum. The oxidation was performed in two steps, the first step at 5 x 10(-5) Torr, the second at 10 Torr. Two optically different types of silicon compounds, a bulk-type silicon (c-Si) and a fine-grain polycrystalline silicon with enhanced absorption due to extensive grain-boundary regions (p-Si) were mixed with voids in the appropriate ratio to fit the spectra of as-prepared PSL. For the annealed PSL amorphous silicon (a-Si) was needed in conjunction with p-Si. The oxidized PSL could be fitted with a reduced a-Si content. We can interpret the annealing effect as a depassivation process of the inner surfaces of the PSL (a-Si fraction). At the same time, oxidation leads to a repassivation process. (C) 1998 Elsevier Science S.A.
Article
Electrochemical pore formation in silicon electrodes is a well-known phenomenon. While micropore formation is commonly understood as due to quantum size effects, the formation of larger pores is dominated by the electric field of the space charge region. In contrast to the macropore regime which is well understood, little is known about the morphology and formation mechanism of mesopores. In this report mesopore morphology and its dependence on formation parameters, such as HF concentration, current density, bias, and substrate doping density, is investigated in detail. In addition, a simulation of the breakdown conditions at the pore tip is performed which shows that mesopore formation is dominated by charge carrier tunneling, while avalanche breakdown is found to be responsible for the formation of large etchpits.
Article
Spectroscopic ellipsometry has become an essential metrology tool for the semiconductor industry. It is widely used where precise film thicknesses and optical constants are required. Faster measurement speeds are opening new doors for ellipsometry in thin-film processing environments, primarily due to its ability to maintain high precision when measuring very thin or multilayered films (such as gate oxides). The technique works well for all varieties of films, including semiconductors, dielectrics, metals, and polymer coatings.
Article
We present a technique involving the use of pulsed anodization for porous silicon (PS) thin-film fabrication on low doped substrates, for which the interface roughness and porosity gradients usually observed in such films can be eliminated. The work presented includes a detailed characterization of the effects of duty cycle and frequency during pulsed anodization. The study spans pulsing frequencies of 0.1-1000 Hz and duty cycles of 5-50%. The combination of low frequency (0.1 Hz) and low duty cycle (5%) for the pulse train used for anodization produces PS thin films, displaying no measurable interface roughness or porosity gradient. The mechanisms behind the wide variation in available PS thin-film properties with pulsed anodization parameters are analyzed using a galvanostatic technique. The analysis indicates that the inhomogeneity and roughness observed in PS films fabricated on low doped starting wafers are both due to the unstable multistep dissolution kinetics of silicon during PS film formation.
Article
We propose to analyze ellipsometry data by using effective medium approximation (EMA) models. Thanks to EMA, having nanocrystalline reference dielectric functions and generalized critical point (GCP) model the physical parameters of two series of samples containing silicon nanocrystals, i.e. silicon rich oxide (SRO) superlattices and porous silicon layers (PSL), have been determined. The superlattices, consisting of ten SRO/SiO2 layer pairs, have been prepared using plasma enhanced chemical vapor deposition. The porous silicon layers have been prepared using short monopulses of anodization current in the transition regime between porous silicon formation and electropolishing, in a mixture of hydrofluoric acid and ethanol. The optical modeling of both structures is similar. The effective dielectric function of the layer is calculated by EMA using nanocrystalline components (nc-Si and GCP) in a dielectric matrix (SRO) or voids (PSL). We discuss the two major problems occurring when modeling such structures: (1) the modeling of the vertically non-uniform layer structures (including the interface properties like nanoroughness at the layer boundaries) and (2) the parameterization of the dielectric function of nanocrystals. We used several techniques to reduce the large number of fit parameters of the GCP models. The obtained results are in good agreement with those obtained by X-ray diffraction and electron microscopy. We investigated the correlation of the broadening parameter and characteristic EMA components with the nanocrystal size and the sample preparation conditions, such as the annealing temperatures of the SRO superlattices and the anodization current density of the porous silicon samples. We found that the broadening parameter is a sensitive measure of the nanocrystallinity of the samples, even in cases, where the nanocrystals are too small to be visible for X-ray scattering. Major processes like sintering, phase separation, and intermixing have been revealed as a function of annealing of the SRO superlattices.
Article
A laser reflection method has been developed and tested for analyzing the etching of porous silicon (PS) films. It allows in situ measurement and analysis of the time dependency of the etch rate, the thickness, the average porosity, the porosity profile, and the interface roughness. The interaction of an infrared laser beam with a layered system consisting of a PS layer and a substrate during etching results in interferences in the reflected beam which is analyzed by the short-time Fourier transform. This method is used for analysis of samples prepared with etching solutions containing different concentrations of HF and glycerol and at different current densities and temperatures. Variations in the etch rate and porosity during etching are observed, which are important effects to account for when optical elements in PS are made. The method enables feedback control of the etching so that PS films with a well-controlled porosity are obtainable. By using different beam diameters it is possible to probe interface roughness at different length scales. Obtained porosity, thickness, and roughness values are in agreement with values measured with standard methods.
Article
Porosities of porous silicon layers formed on different types of substrates and under different experimental conditions are compared with and related to the pore size distribution determined by gas adsorption experiments. Results show that porous layers formed on lightly P‐doped silicon exhibit a network of very narrow pores, of radii less than 2 nm. Porous films formed on heavily doped silicon present larger radii, ranging between 2 and 9 nm according to the experimental conditions. Larger porosities and larger pore sizes are obtained by increasing the forming current density or by decreasing the concentration. Heavily P‐doped porous silicon layers are homogeneous in depth and generally present a quite sharp pore size distribution. With heavily N‐doped silicon, an increase in porosity with increasing thickness is found, which corresponds to an increase in pore size, leading to a broadening of size distributions. This porosity gradient is attributed to a chemical dissolution of the layer occurring during anodization. In addition, a strong dependence of porosity with small variations in doping level is found.
Article
A thermoresponsive hydrogel, poly(N-isopropylacrylamide) (poly(NIPAM)), is synthesized in situ within an oxidized porous Si template, and the nanocomposite material is characterized. Infiltration of the hydrogel into the interconnecting nanoscale pores of the porous SiO2 host is confirmed by scanning electron microscopy. The optical reflectivity spectrum of the nanocomposite hybrid displays Fabry-Pérot fringes characteristic of thin film interference, enabling direct, real-time observation of the volume phase transition of the confined poly(NIPAM) hydrogel. Reversible optical reflectivity changes are observed to correlate with the temperature-dependent volume phase transition of the hydrogel, providing a new means of studying nanoscale confinement of responsive hydrogels. The confined hydrogel displays a swelling and shrinking response to changes in temperature that is significantly faster than that of the bulk hydrogel. The porosity and pore size of the SiO 2 template, which are precisely controlled by the electrochemical synthesis parameters, strongly influence the extent and rate of changes in the reflectivity spectrum of the nanocomposite. The observed optical response is ascribed to changes in both the mechanical and the dielectric properties of the nanocomposite.
Article
The results of multiparameter fitting of spectroscopic ellipsometric (SE) spectra on porous silicon layers (PSL) were connected with the processing parameters (oxidation, etching time, porosity, argon implantation dose).Two optically different types of silicon forms, a bulk-type silicon (c-Si) and polycrystalline-like silicon with enhanced absorption in the grain boundaries (p-Si) needed to be mixed with voids in the appropriate ratio, and the PSL had to be divided in depth in several different sections in order to obtain the best fit. The sectioning reflects the effect of upper and lower interfaces or inhomogeneity in depth. The effective porosity and the sublayer thicknesses are determined with high precision.In the case of argon implantation, we used the dielectric function of c-Si and implanted amorphous silicon (a-Si) mixed with voids. The regression analysis of SE spectra clearly shows the effect of different doses of implantation. The sectioning reflects that to the full range of argon ions the porous silicon became amorphous and denser. The overall thickness of the originally porous layer also significantly reduced (from 670 nm to 320 nm) due to the argon implantation.
Article
Electrochemically prepared porous silicon (PS) layers were oxidized thermally and investigated by spectroscopic ellipsometry (SE). The SE spectra were measured in the range of 270–850 nm with a rotating polarizer ellipsometer. The PS was modelled as a mixture of void and crystalline silicon or fine-grained polycrystalline silicon with enhanced absorption due to extensive grain-boundary regions, i.e. the complex refractive index of the layer was calculated by Bruggeman effective medium approximation. The dielectric function of the fine-grained polycrystalline silicon was taken from the work published by G.E. Jellison, Jr., M.F. Chisholm, S.M. Gorbatkin, Appl. Phys. Lett. 62 (1993) 3348. The porosity, the layer thickness and the composition of the oxidized PS layers were determined. Oxidation at 900°C was performed after a stabilizing heat treatment at 320°C. The oxidation at 900°C for 10 min generated only a few nm silicon dioxide on single crystalline Si while in the case of PS with 57% porosity nearly complete oxidation was found. For PS with 68% porosity complete oxidation was observed.
Article
Ultrathin oxides formed on p-type (100) Si using anodic oxidation in dilute aqueous NH4OH solution have been characterized by Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and x-ray reflectometry. The aim of the work was to optimize the growth and annealing conditions for fabrication of ultrathin gate oxides. Two alternate growth conditions (potentiostatic and galvanostatic) could be used to grow oxides of thickness between 3 and 16 nm. There was very little difference between the two types of oxides; however, the FTIR asymmetric stretch maximum νm was at slightly higher frequencies and this band was slightly narrower for potentiostatic oxides compared to galvanostatic oxides of the same thickness. For both types of films, νm increased with film thickness, while the corresponding full width at half-maximum decreased. As-grown ∼11-nm-thick films of both types contain 3.8±0.3% -OH (bound as isolated silanol) and 5.0±0.4% -OH (bound as H2O and/or associated silanol) by mass, and have a density of 2.05±0.03 g cm -3 compared with a density of 2.27±0.03 g cm-3 measured for thermal oxides. Thus, the composition of the as-grown anodic oxides can be written as SiO1.93(OH)0.14·0.18H 2O. Discounting the H content, this converts to an O/Si ratio of 2.25±0.02, which can be compared to the O/Si ratio of 2.27±0.06 measured for as-grown films by XPS. Potentiostatically grown ∼11-nm-thick films were annealed at temperatures between 300 and 900°C in forming gas. Two different stages were observed as a function of anneal temperature. At temperatures below 500°C, water and/or associated silanol was ejected from the films. This resulted in a maximum in the stress and/or disorder in the oxides at anneal temperatures of 500°C. At temperatures above 500°C, the remainder of the silanol was removed from the films; some kind of stress relief occurred. The oxides became stoichiometric at temperatures 700°C and above.
Article
Stress was determined in oxidized porous silicon layers using x‐ray diffraction to measure the substrate curvature. Stress is always compressive and its magnitude depends on oxide quality. The maximum value is reached when the oxide obtained from porous silicon is densified by a high‐temperature process and is equivalent to standard thermal silicon dioxide. The stress magnitude decreases when oxide porosity increases. An increase in layer thickness is always observed when oxidation conditions lead to a porous oxide or when the initial porosity of the layer is lower than 56%, due to volumic expansion of silica relative to silicon, otherwise the thickness decreases.
Article
The internal surface of porous silicon (PS) nanostructures was chemically modified by octadecyl and carboxylic acid groups by applying the hydrosilylation reaction as well as by aminopropyl and vinyl groups applying the silanization reaction. Concentrations of the chemically grafted groups and thicknesses of grafted layers were determined by measurements of PS refractive index in the infrared spectral range. The Landau-Lifshitz- Looyenga effective media model was used to relate the measured refractive index values to a volume fraction and then to the concentration of the grafted groups. The described quantitative method was applied to determine the sensitivity limits of PS-based sensing devices.
Article
We thoroughly and critically review studies reporting the real (refractive index) and imaginary (absorption index) parts of the complex refractive index of silica glass over the spectral range from 30 nm to 1000 microm. The general features of the optical constants over the electromagnetic spectrum are relatively consistent throughout the literature. In particular, silica glass is effectively opaque for wavelengths shorter than 200 nm and larger than 3.5-4.0 microm. Strong absorption bands are observed (i) below 160 nm due to the interaction with electrons, absorption by impurities, and the presence of OH groups and point defects; (ii) at aproximately 2.73-2.85, 3.5, and 4.3 microm also caused by OH groups; and (iii) at aproximately 9-9.5, 12.5, and 21-23 microm due to Si-O-Si resonance modes of vibration. However, the actual values of the refractive and absorption indices can vary significantly due to the glass manufacturing process, crystallinity, wavelength, and temperature and to the presence of impurities, point defects, inclusions, and bubbles, as well as to the experimental uncertainties and approximations in the retrieval methods. Moreover, new formulas providing comprehensive approximations of the optical properties of silica glass are proposed between 7 and 50 microm. These formulas are consistent with experimental data and substantially extend the spectral range of 0.21-7 microm covered by existing formulas and can be used in various engineering applications.
Correlation between the independently measured and expected volume fractions (see eqs. (2a) and (2b) for the calculation of the expected values)
  • Fig
Fig. 10. Correlation between the independently measured and expected volume fractions (see eqs. (2a) and (2b) for the calculation of the expected values).
Porous Silicon (2014) 1e6
  • L Canham
  • Handb
L. Canham, Handb. Porous Silicon (2014) 1e6, http://dx.doi.org/10.1007/9783-319-04508-5_19-1.
Porous Silicon in Practice: Preparation, Characterization and Applications
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M.J. Sailor, Porous Silicon in Practice: Preparation, Characterization and Applications, 2012, http://dx.doi.org/10.1002/9783527641901. Wiley.com.
  • L Desplobain
  • T Ventura
  • G Defforge
  • Gautier
Desplobain, L. Ventura, T. Defforge, G. Gautier, in: A. Cantarero, E. Matveeva (Eds.), Porous Semicond. e Sci. Technol. Alicante-Benidorm, 2014, pp. 404e405.