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Thermal Characterisation of Porous Silicon Membranes
Abstract and Figures
Results of thermal characterisation based on the phase lag of photoacoustic signal for front-rear surface illumination of porous silicon and nitrided porous silicon membranes for gas sensing devices are presented. Thermal conductivity values in good agreement with literature values have been obtained, confirming the usefulness and reliability of photothermal methods in the investigation of new materials for sensors and microsystems. Preliminary results of stokes-antistokes Raman investigations are also reported.
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... To the best of our knowledge, the thermal transport in drop-casted substrate-supported randomly packed Si nanogranular films with defined NP sizes and spherical shapes, has never been studied before compared to other various types of nanostructurally voided Si films ranging from low porosity pressure sintered nanostructured bulk Si (sint-Si) [24][25][26][27][28], crystalline porous Si (c-por-Si) [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], amorphous porous Si (a-por-Si) [49][50][51][52], crystalline porous Si nanowire (c-por-Si NW) films [53][54][55][56] to c-por-Si membranes [57,58]. As opposed to mainly top-down fabrication methods [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], the bottom-up approach, such as dropcasting, used in the present work, has shown to be scalable, simple and cost-effective way to produce nanogranular medium, i.e., new type of nanostructurely voided Si films. ...
... To the best of our knowledge, the thermal transport in drop-casted substrate-supported randomly packed Si nanogranular films with defined NP sizes and spherical shapes, has never been studied before compared to other various types of nanostructurally voided Si films ranging from low porosity pressure sintered nanostructured bulk Si (sint-Si) [24][25][26][27][28], crystalline porous Si (c-por-Si) [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], amorphous porous Si (a-por-Si) [49][50][51][52], crystalline porous Si nanowire (c-por-Si NW) films [53][54][55][56] to c-por-Si membranes [57,58]. As opposed to mainly top-down fabrication methods [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], the bottom-up approach, such as dropcasting, used in the present work, has shown to be scalable, simple and cost-effective way to produce nanogranular medium, i.e., new type of nanostructurely voided Si films. ...
... In general, measured k = 0.19-0.53 W/mK of our Si nanogranular films are in the range of the lowest k values of nanostructurely voided Si films [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], Si nanowire films [53][54][55][56], amorphous porous Si films [49][50][51][52], crystalline porous Si membranes [57,58] and sin-Si NP tablets [24][25][26][27][28]. Additionally, when the size of a nanostructure ≤ l MFP , phonons collide with intergranular boundaries much more often than in single crystals. This additional phonon scattering mechanism suppresses heat flow between NPs and thus reduces the effective k of Si NP films compared to that of the bulk c-por-Si [65,66]. ...
We present results on the photothermal (PT) and heat conductive properties of nanogranular silicon (Si) films synthesized by evaporation of colloidal droplets (drop-casting) of 100 ± 50 nm-sized crystalline Si nanoparticles (NP) deposited on glass substrates. Simulations of the absorbed light intensity and photo-induced temperature distribution across the Si NP films were carried out by using the Finite difference time domain (FDTD) and finite element mesh (FEM) modeling and the obtained data were compared with the local temperatures measured by micro-Raman spectroscopy and then was used for determining the heat conductivities k in the films of various thicknesses. The cubic-to-hexagonal phase transition in Si NP films caused by laser-induced heating was found to be heavily influenced by the film thickness and heat-conductive properties of glass substrate, on which the films were deposited. The k values in drop-casted Si nanogranular films were found to be in the range of lowest k of other types of nanostructurely voided Si films due to enhanced phonon scattering across inherently voided topology, weak NP-NP and NP-substrate interface bonding within nanogranular Si films.
... To the best of our knowledge, the thermal transport in drop-casted substrate-supported randomly packed Si nanogranular films with defined NP sizes and spherical shapes, has never been studied before compared to other various types of nanostructurally voided Si films ranging from low porosity pressure sintered nanostructured bulk Si (sint-Si) [24][25][26][27][28], crystalline porous Si (c-por-Si) [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], amorphous porous Si (a-por-Si) [49][50][51][52], crystalline porous Si nanowire (c-por-Si NW) films [53][54][55][56] to c-por-Si membranes [57,58]. As opposed to mainly top-down fabrication methods [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], the bottom-up approach, such as dropcasting, used in the present work, has shown to be scalable, simple and cost-effective way to produce nanogranular medium, i.e., new type of nanostructurely voided Si films. ...
... To the best of our knowledge, the thermal transport in drop-casted substrate-supported randomly packed Si nanogranular films with defined NP sizes and spherical shapes, has never been studied before compared to other various types of nanostructurally voided Si films ranging from low porosity pressure sintered nanostructured bulk Si (sint-Si) [24][25][26][27][28], crystalline porous Si (c-por-Si) [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], amorphous porous Si (a-por-Si) [49][50][51][52], crystalline porous Si nanowire (c-por-Si NW) films [53][54][55][56] to c-por-Si membranes [57,58]. As opposed to mainly top-down fabrication methods [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], the bottom-up approach, such as dropcasting, used in the present work, has shown to be scalable, simple and cost-effective way to produce nanogranular medium, i.e., new type of nanostructurely voided Si films. ...
... In general, measured k = 0.19-0.53 W/mK of our Si nanogranular films are in the range of the lowest k values of nanostructurely voided Si films [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], Si nanowire films [53][54][55][56], amorphous porous Si films [49][50][51][52], crystalline porous Si membranes [57,58] and sin-Si NP tablets [24][25][26][27][28]. Additionally, when the size of a nanostructure ≤ l MFP , phonons collide with intergranular boundaries much more often than in single crystals. This additional phonon scattering mechanism suppresses heat flow between NPs and thus reduces the effective k of Si NP films compared to that of the bulk c-por-Si [65,66]. ...
We present the results on photothermal (PT) and heat conductive properties of nanogranular silicon (Si) films synthesized by evaporation of colloidal droplets (drop-casting) of 100 ± 50 nm sized crystalline Si nanoparticles (NP) deposited on glass substrates. Finite difference time domain (FDTD) and finite element mesh (FEM) modeling of absorbed light intensity and photo-induced spatial temperature distribution across the Si NP films were well correlated with the local temperatures measured by micro-Raman spectroscopy and used for determination of heat conductivities in the films of various thicknesses. Cubic-to-hexagonal phase transition in these films caused by laser heating was found to be heavily influenced by the film thickness and heat conductive properties of glass substrate, on which the films were deposited. Heat conductivities across the drop-casted Si nanogranular films were found to be in the range of lowest heat conductivities of other types of nanostructurely voided Si films due to enhanced phonon scattering across inherently voided topology, weak NP-NP and NP-substrate interface bonding within nanogranular Si films.
... Thus, the thermal conductivity of bulk silicon is about 150 W/m.K for doping around 10 18 -10 19 cm -3 and can be around 140 W/m.K for higher doping (10 20 cm -3 ) [6]. Values lying in the range 0.1-20 W/m.K have been measured for mesoporous silicon depending on the porosity, the thickness of the porous layer and the measurement technique adopted [7][8][9][10][11][12][13][14][15][16][17][18]. This drastic decrease, of 2 to 3 orders of magnitude of the thermal conductivity of the nanostructured PSi is of interest in the field of thermoelectricity since a good thermoelectric material must exhibit a low the thermal conductivity. ...
In this study, the impact of nanographene incorporation on the thermal properties of mesoporous silicon (PSi) was evaluated using two complementary experimental methods: the temperature gradient (TG) and the photothermal radiometry (MPTR) methods. It is shown that the measured thermal conductivity of the mesoporous silicon (PSi) ranges from 0.10 to 0.68 W/m.K in the case of TG and from 0.37 to 3.02 W/m.K in MPTR and is strongly correlated to the electrochemical etching parameters. These values are much lower than that of crystalline silicon, estimated to be from 100 to 140 W/m.K, depending on the doping rate. They appear to be, however, in the order of magnitude range for the percolation models that also include the in-depth porosity and the crystallite mean radius. This set of experiments on the thermal conductivity was extended to investigate the effect of graphene incorporation in the PSi matrix (G-PSi) as it has seldom been reported in the literature. The results from both methods exhibit significantly higher values (1.7 ± 0.3 W/m.K for TG, and from 0.7 to 2.13 W/m.K for MPTR). This spread of the thermal conductivity values is attributed to the intrinsic working principle of the TG versus the MPTR method as highlighted in the last part of the present paper. Targeting the thermoelectric application of both matrices (PSi, G-PSi), the thermal conductivity remains sufficiently low for them to be considered as very promising materials, keeping in mind the enhancement of the power-factor attributed to the incorporation of graphene.
... The thermal conductivity of porous Si depends on its structure and morphology, which in turn depend on the Si wafer type and resistivity and the electrochemical conditions used for porous Si formation (1997). Since the thermal conductivity of porous Si depends on its structure, morphology, and porosity, the experimental values obtained by different groups in the literature (Drost et al. 1995;Benedetto et al. 1997;Gesele et al. 1997;Lysenko et al. 1999;Périchon et al. 1999Périchon et al. , 2000Bernini et al. 1999Bernini et al. , 2001Amato et al. 2000;Lysenko et al. 2002;Shen and Toyoda 2003;Lettieri et al. 2005;Wolf and Brendel 2006;Gomes et al. 2007;Siegert et al. 2012) show a significant scattering. For mesoporous Si fabricated from p-type Si with resistivity in the range of 5-15 Ω cm, the reported values of room temperature (RT) thermal conductivity range from 0.18 to 1.2 W/m K. On the other hand, for porous Si from (100) p + -type Si wafers with resistivity in the range of 10-20 mΩ cm, the reported values of RT thermal conductivity range from 0.3 to 20.8 W/m K for the grown material, depending on material porosity, the thickness of the studied porous Si layer, and the measurement technique. ...
The exceptionally low thermal conductivity of highly porous silicon has led to its use as a thermal insulator within microsystems. A comprehensive review of thermal conductivity literature is provided, together with examples of its use in microsensing and microphotonic systems. © Springer International Publishing AG, part of Springer Nature 2018. All rights are reserved.
... K. Several additional studies have been performed on the thermal conductivity of electrochemically etched porous Si [114][115][116][117][118][119][120]. Song and Chen [121] reported temperature dependent (50-300 K) in-plane thermal conductivity of single crystal Si with periodically fabricated micropores, and a more than three-fold reduction in (e.g. ...
This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lays in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.
Template‐directed self‐assembly of solidifying eutectics results in emergence of unique microstructures due to diffusion constraints and thermal gradients imposed by the template. Here, the importance of selecting the template material based on its conductivity to control heat transfer between the template and the solidifying eutectic, and thus the thermal gradients near the solidification front, is demonstrated. Simulations elucidate the relationship between the thermal properties of the eutectic and template and the resultant microstructure. The overarching finding is that templates with low thermal conductivities are generally advantageous for forming highly organized microstructures. When electrochemically porosified silicon pillars (thermal conductivity < 0.3 Wm⁻¹K⁻¹) are used as the template into which an AgCl‐KCl eutectic is solidified, 99% of the unit cells in the solidified structure exhibit the same pattern. In contrast, when higher thermal conductivity crystalline silicon pillars (≈100 Wm⁻¹K⁻¹) are utilized, the expected pattern is only present in 50% of the unit cells. The thermally engineered template results in mesostructures with tunable optical properties and reflectances nearly identical to the simulated reflectances of perfect structures, indicating highly ordered patterns are formed over large areas. This work highlights the importance of controlling heat flows in template‐directed self‐assembly of eutectics.
Here the thermal transport properties of low thermal conductivity porous silicon thin films attached to high thermal conductivity silicon substrates are studied using the 3ω method implemented over the 100 Hz to 33 kHz frequency range. The thermal conductivity and thermal diffusivity of the films are extracted using temperature impedance monitoring of electrical contacts deposited on films, combined with an extended-frequency, multi-layer thermal model. From the extracted thermal conductivity and diffusivity of the films, the heat capacity could be determined. Validation of the approach is performed using the known properties of thick substrate glass and SU-8 layers spun on silicon substrates, the latter ranging in thickness from 1.35 to 12.5 µm. The technique was then applied to porous silicon films with porosities ranging from 45% to 77%. The extracted thermal properties for as-fabricated films show a reduction of thermal conductivity and diffusivity from 1.7 to 0.15 W/mK and 1.9 to 0.2 mm²/s, respectively as the porosity increases. After passivation by annealing in nitrogen and at 600 °C, the same films exhibited higher values of thermal conductivity and diffusivity ranging from 2.7 to 0.7 W/mK and 2.5 to 0.65 mm²/s. The ability to extract both thermal conductivity and thermal diffusivity for these films removes the need to make assumptions around specific heat capacity, commonly made during analysis of porous media. These results show for the first time a monotonic increase in specific heat capacity of porous silicon films as a function of porosity.
The specific thermal properties of anodized porous silicon (PSi) are discussed in relation to the possible applications. Based on the summary of the experimental approach required for nanomaterials, it is shown that both the thermal conductivity and the volumetric heat capacity of PSi are dramatically decreased in comparison to conventional thermal insulators. In high-porosity PSi, the thermal constants become close to the limit of solid-state materials. The potential usefulness of induced thermal functions has been demonstrated in many application studies. Thermo-acoustic emission effect, in particular, has some advantageous features over the conventional transducers, including resonance-free and broad-band dynamic response with a significantly low distortion. The PSi device is useful as a digital speaker, for object sensing in air, as a noncontact actuator, for analyses of bio-acoustic communications between mice, and as a chemical reactor array.
A technological process aimed at realizing passivated porous silicon (PS) layers as the thermo-insulating material for thin as well as thick film gas sensor applications is reported and discussed. Oxidized PS (OPS) layers (5 to 35 μm thick) have been realized on p-Si substrates using the Si anodization process followed by PS thermal oxidation. The thick SiO 2 layer obtained from PS has the same stoichiometry as standard thermal silicon dioxide; unfortunately, stress introduced by the oxidation process induces significant wafer warpage, which inhibits further technological processing. Passivation of the PS layer can be achieved by a nitridation process executed in a rapid thermal system (RTS) in ammonia. In this case, the Si rods are covered by a thin oxynitride layer, which stabilizes the PS structure without introducing large stress. Nitrided PS membranes (25 - 30 μm thick) that are coplanar with the surrounding bulk Si and have good mechanical stability have been obtained.
Porous silicon as a fool for the structuring of thermal sensors is presented. The special requirements of thermal structures are discussed and microstructuring techniques are reviewed. Then the technology of porous silicon and its etch and etch stop techniques are described. Examples of thermal sensor applications of porous silicon technology are given.
The amplitude of the photoacoustic signal is measured as a function of chopping frequency for thin benzophenone polycrystalline samples in the temperature range 100–300 K. Analysis of experimental results, obtained by rear surface illumination, allows us to determine the thermal diffusivity of benzophenone crystals in the whole temperature range.
Porous silicon layers of different morphologies and porosities have been produced and characterized by means of a photoacoustic technique, showing in some cases a very low thermal conductivity with respect to crystalline bulk silicon. The results confirm other experimental evidence and suggest in particular strong similarities between highly porous silicon layers and silica aerogel systems.
When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.
The formation of oxide layers from porous silicon layers formed on P substrates which present different and well‐controlled porous textures is studied in detail. It is shown that the overall process leading to oxides with properties similar to standard thermal oxides consists of two steps: the oxidation of silicon in the porous layer and the densification of silica. The first step does not depend upon the initial texture of the layer, but is mostly determined by the porosity of the sample and the temperature of the oxidation sequence. The second step, oxide densification, which takes place at temperatures higher than 1000 °C, is very sensitive to the porous texture of the layer, the densification times increasing sharply with the average pore size. The densification process has a high activation energy, which can be related to the activation energy for the viscous flow of silica.
Sensors, actuators and microsystems can be realized by silicon micromachining. The structures are typically three-dimensional. To realize them, the methods known from microelectronics are not suffucient but new structuring methods which allow deep etching are applied. For that reason, the microstructing of silicon is one of the characteristic and essential tasks of the technology of microelectromechanical systems (MEMS). In this review four important microstructing techniques are described: bulk micromachining by anistropic etching, reactive ion etching, surface micromachining and porous silicon technology. The basic principles are described, the technological realization is discussed and examples for applications in the field of microsensors are given.
A simple method is demonstrated for obtaining the thermal diffusivity of solids, by measuring the phase lag between a front and rear illumination, at a single chopping frequency. The method is tested using some semiconductor and glass samples. Pages: 1316-1318
Steady-state laser heating of silicon disk microstructure on
focused silica and sapphire is examined using the 4880 and 5145 Å
lines from an argon-ion laser to heat the disk at the center and to
probe it with submicrometer spatial resolution. The Stokes shift and
line broadening of the Raman microprobe spectra are compared to
simulations of the Raman spectra, which utilize temperature profiles
calculated by a finite-difference analysis of the heat flow equation.
The Raman simulation model incorporates the effects of temperature
inhomogeneities, strain, and photon-created free carriers within the
probed volume; temperature nonuniformity is found to be the most
important factor and to be quite significant
Properties of Silicon
- M N Wybourne