Silicon, macroporous mesoporous

With electrochemically etched porous silicon structures, quantum size effects are only important in microporous and mesoporous silicon. Macroporous silicon can for many properties be modeled as bulk silicon with voids therein, provided the macropores therein do not contain mesoporosity, for example. 

The etch rate is further increased if H202 is added to the solution, as shown in Fig. 2.5 b. At such low rates the reaction is controlled by the kinetics of the reaction at the interface and not by diffusion in the solution. This etching solution is therefore found to be perfect to remove micro- and mesoporous silicon selectively from a bulk silicon substrate or to increase the diameter of meso- or macropores in an well-controlled, isotropic manner [Sa3],... 

An electric field in the semiconductor may also produce passivation, as depicted in Fig. 6.1c. In semiconductors the concentration of free charge carriers is smaller by orders of magnitude than in metals. This permits the existence of extended space charges. The concept of pore formation due to an SCR as a passivating layer is supported by the fact that n-type, as well as p-type, silicon electrodes are under depletion in the pore formation regime [Ro3]. In addition a correlation between SCR width and pore density in the macroporous and the mesoporous regime is observed, as shown in Fig. 6.10 [Thl, Th2, Zh3, Le8]. 

If the pore density is plotted versus the doping density of the silicon electrode, it can be seen that the micropore density is independent of doping, while the macropore and mesopore densities increase linearly with doping density, as shown in Fig. 6.10. This is a consequence of the QC formation mechanism being independent of doping, while the SCR-related mechanisms are not, as discussed in Section 6.2. 

A specific feature of macropore formation in n-type silicon is the possibility of controlling the pore tip current by illumination and not by applied bias. This adds another degree of freedom that is not available for mesopore or macropore formation on p-type substrates. The dark current density of moderately doped n-type Si electrodes anodized at low bias is negligible, as shown in Fig. 4.11, therefore all macropore structures discussed below are formed using illumination of the electrode to generate the flux of holes needed for the dissolution process. Illumination, however, is not the only possible source of holes for example, hole injection from a p-doped region is expected to produce similar results. 

In contrast to the micro- and mesoporous regimes, for which only a few empirical laws for the growth rate and porosity are available, the detailed pore geometry for macropore arrays in n-type silicon can be pre-calculated by a set of equations. This is possible because every pore tip is in a steady-state condition characterized by = JPS [Le9]. This condition enables us to draw conclusions about the porosi-... 

An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

Bulk silicon constitutes an optical long-pass filter, as shown in Fig. 7.6. The same is true for micro- and mesoporous silicon, for which the effective medium approximation (EMA) is valid in the visible regime. The dimensions of macroporous silicon are in the visible regime and the EMA becomes invalid. 

Relatively few studies on the synthesis of mesoporous alumina have been reported to date [8]. One of the limitations of the reported synthetic strategies is that the rate of hydrolysis (and condensation) reaction of aluminum alkoxide are much faster than that of silicon alkoxide. In this study, we proposed a novel method to prepare bimodal porous aluminas with meso- and macropores with narrow pore size distribution and well-defined pore channels. The fiamewoik of the porous alumina is prepared via a chemical templating method using alkyl caiboxylates. Here, self-assemblied micelles of carboxylic acid were used as a chemical template. Mesoporous aluminas were prepared through carefiil control of the reactants pH, while the procedures are reported elsewhere [9]. 

A considerable gain in the catalytic reactivity was found to be obtained by using mesoporous silicon in comparison with the macroporous electrode. 

Monocrystalline, macro- and mesoporous silicon were used for the electrochemical deposition of Pt. A 10 pm thick macroporous silicon layer was formed by anodizing of p-type Si wafers of 12 Ohm-cm resistivity in an aqueous solution of HF acid and DMSO (10 46 by volume parts) at the current density of 8 mA-cm [1]. Pore channels distributed with the surface density of 6T0 cm look like long straight holes with inlet diameters of 1.5 pm. An uniform 1 pm thick mesoporous silicon layer was fabricated by anodizing of n" -type Si wafers of 0.01 Ohm-cm resistivity in a solution of HF acid, water and isopropanol (1 3 1 by volume parts) at the current density of 60 mA-cm . The mesoporous silicon sample formed looks like Si layer perpendicularly pierced through by pore channels with diameter of about 20 nm. The number of pores per square centimetre is up to 2-10 [2]. 

The determination of the true specific surface area of the Pt electrode is possible with the analysis of the hydrogen anodic region of the potentiodynamic curve [7]. It amounts to 80 a.u. for macroporous electrode and 840 a.u. for mesoporous one in comparison to the surface of monocrystalline silicon. 

Macroporous silicon has the surface area less then 10 m -cm, while the surface area of mesoporous silicon is about 200 m -cm" [8]. In our case, it means that the increase of the specific surface area goes in the connection with the total specific surface area of Pt grains. The total specific surface area for the mesoporous silicon sample is obvious to be higher than for macroporous silicon because of greater number of nucleation centers. 

The relatively high newly developed mesopore surface area of 120 m g is however in contrast to alkaline-treated silicalite-1, in which despite the high degree of unselective silicon dissolution hardly any mesoporosity has been observed. Due to the uncontrolled silicon extraction in the absence of framework aluminum, mostly large macropores were obtained in alkaline-treated silicalite [18]. We have previously reported a correlation between mesopore surface area development and framework Si/Al ratio of the parent... 

We carried out comparative studies of the effect of the porous structure of carbon materials on electrochemical electrode characteristics using various carbide carbons (CCs). Main structural characteristics for CCs based on silicon carbide are presented in Table 27.3 and those for titanium carbide are in Table 27.4. Specific surface areas were calculated on the basis of the nitrogen adsorption data with calculation using the DFT technique. This method is used to measure micropores and mesopores, but not macropores. 

Meso- and macroporous silicon (Bisi et al. 2000) have been compared in their ability to support cell attachment and growth. Osteoblasts were found to grow preferentially on the macroporous surface, which had pore sizes of 1 pm. This was attributed to the wide areas of flat silicon in between the pores for the cells to attach to. In contrast, the two types of mesoporous silicon, which had pore sizes between 50 nm and 15 nm, respectively, both gave low cell attachment (Sun et al. 2007). 

Kim JH, Kim KP, Lyu HK, Woo SH, Seo HS, Lee JH (2009) Three dimensional macropore arrays in p-type silieon fabrieated by electrochemical etching. J Korean Phys Soc 55(1) 5-9 Loni A, Canham LT (2013) Exothermic phenomena and hazardous gas release during thermal oxidation of mesoporous silicon powders. J Appl Phys 113 173505 Lysenko V, Vitiello J, Remaki B, Barbier D (2004) Gas permeability of porous silicon nanostructures. Phys Rev E 70 017301... 

Ouyang H, Christopherson M, Fauchet PM (2005) Enhanced control of porous silicon morphology fi"om macropore to mesopore formation. Phys Stat Solidi (a) 202(8) 1396-1401 Pacholski C (2013) Photonic crystal sensors based on porous silicon. Sensors 13 4694-4713 Roura P, Costa J (2002) Radiative thermal emission from silicon nanoparticles a reversed story from quantum to classical theory. Eur J Phys 23 191-203 Scherer WG, Smith DM, Stein D (1995) Deformation of silica aerogels during characterisation. JNon Cryst Solids 186 309-315... 

With its high surface area and reactive Si-H and Si-Si moieties, porous Si is particularly susceptible to air, water, or chemical oxidation. Thermal oxidation is employed by the microelectronic industry to produce high-quality oxides on silicon, and this approach also works with porous Si. However, the extent of oxidation of a porous Si sample can be significantly greater than with flat silicon due to the small features in the porous nanostructure. For example, 1 nm of oxide on the surface of a flat Si wafer is considered a minor degree of oxidation, whereas 1 nm of oxide on a microporous Si sample that consists of 2 nm features is essentially completely oxidized. Thus it is important to know not only time and temperature but also feature size in the micro-, meso-, or macroporous structure to understand the oxidation process in porous Si (see chapters Oxidation of Macroporous Silicon, Oxidation of Mesoporous Silicon ). 

At a temperature of 900 °C, the porous Si skeleton will completely convert to silicon oxide (Eq. 8), although the length of time needed to accomplish this transformation depends on the type of sample microporous silicon will t5q)ically convert within an hour, mesoporous silicon requires 3 h (Pacholski et al. 2005), and the conversion of macroporous silicon may not be complete even after 12 h. As mentioned above, this is directly related to the thickness of the silicon features and the rate of oxygen diffusion through the oxide layer. 

Lehmann V, Griining U (1997) The limits of macropore array fabrication. Thin Solid Films 297 13 Lehmann V, Stengl R, Luigart A (2000) On the morphology and the electrochemical formation mechanism of mesoporous silicon. Mater Sci Eng B 69-70 11 Ogata YH, Kobayashi K, Motoyama M (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10 163... 

Herino R, Bomchil G, Barla K, Bertrand C, Ginoux JL (1987) Porosity and pore size distributions of porous silicon layers. J Electrochem Soc 134(8) 1994-2000 International Organization for Standardization (2006a) Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption - Part 2 analysis of mesopores and macropores by gas adsorption. ISO 15901-2 2006(E)... 

Fig. 1 Low-to ultrahigh porosity silicon structures (a) <5 % macroporous wafer (b) buried porosity for layer transfer (Terheiden et al. 2011) (c) 35 % porosity membrane for thermoelectrics (Tang et al. 2010) (d) a 40 % macroporous photonic crystal waveguide (Muller et al. 2000) (e) microfabricated 60 % mesoporous microparticles for drag delivery (Chiappini etal. 2010) (f) a 70 % mesoporous nanowire for photocatalysis (Quet al. 2010) (g) double-walled silicon nanotubes ( 80 % porosity) for battery anodes (Wu et al. 2012) (h) photoluminescent 95 % mesoporous aerocrystal (Canham et al. 1994)...

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