Mesopores silicon

Gao, T. Gao, J. Sailor, M. J., Tuning the response and stability of thin film mesoporous silicon vapor sensors by surface modification, Langmuir. 2002, 18, 9953 9957... 

If the wafer is not fixed in the cell, a mechanical wafer support is advisable. The ohmic contact can be an integral part of such a sample fixture, as shown in Fig. 1.5 a. During formation of mesoporous silicon on highly doped substrates at low bias (0-1.5 V), it was found that such a contact can even be immersed into the electrolyte without a significant degradation of its electrical properties. It is remarkable that mesoporous silicon formation takes place under the contact, too, without significant degradation of the contact properties. 

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],... 

Fig. 7.8 The below-bandgap transmission of free-standing mesoporous silicon compared to the bulk transmission of the corresponding n+ and p+ substrates. The strong increase in...

Microcrystallites of direct semiconductors usually show a simple exponential decay of the PL intensity P with time, with time constants r in the ps and ns range at RT. A similar simple exponential decay (r = 20ms at 2 K) is observed for PL from mesoporous silicon of high porosity, which shows a weak confinement effect... 

Microporous and mesoporous silicon can be partly oxidized by chemical or electrochemical methods, while thermal oxidation is needed for a complete conversion of the porous layer to oxide. This complete conversion was the aim of the first studies in this field. 

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. 

The structural features of micro- and mesoporous silicon are much smaller than the wavelength of visible light, and so these materials may be treated according to the EMA. The dependence of the porosity of micro- and mesoporous silicon on formation current density and substrate doping density can be used to generate layers with a single or a periodic change in the dielectric constant... 

Combining aspects of carbon and silicon chemistry while at the same time expanding the tool box of the periodic table, recently, a first report on the templated synthesis of mesoporous silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) as analogs of the well-known mesoporous silica materials discussed in many chapters of the book has appeared (Fig. 25.5),58 opening an even wider horizon for the exploration of SiC-related nanomaterials in the fields covered in this book. 

FEATURES OF THE NICKEL ELECTROCHEMICAL DEPOSITION INTO MESOPOROUS SILICON... 

Features of the nickel electrochemical deposition into mesoporous silicon are discussed. The process was controlled by the surface potential of a sample relative to the reference Ag/Cl electrode. Complete pore filling with metal is reported. Cross-sectional SEM studies of the samples at various deposition stages allowed the deposition mechanism to be revealed. 

Nickel-mesoporous silicon structures are of considerable industrial interest for various applications. Anisotropy of magnetic properties of the nickel nanowires inside porous silicon conditioned by their high aspect ratio is applicable for the magnetic store production [1], Moreover, these structures offer much promise for the rectenna (a special type of antenna that is used to directly convert microwave energy into DC electricity) fabrication. So, it is of value to study in detail the process of the nickel electrodeposition into pores of porous silicon and elaborate control methods for pore filling with metal. 

Figure 1. The surface potential response during the nickel electrochemical deposition onto mesoporous silicon.

The nickel electrodeposition into pores of mesoporous silicon begins from the metal grain formation randomly all over the surface of the silicon skeleton. The size of grains increases up to 100 nm with the deposition time and further nickel deposition is accompanied by the increase in the number of grains, which finally coalesce to threads. The moment of complete pore filling with nickel is controlled by the surface potential of the sample. 

An electrochemical Pt deposition from the diamine nitrite solution onto monoctystalline, macro- and mesoporous silicon is presented. Pt grain size versus deposition time was determined from the SEM data. A catalytic reactivity of the Pt coated electrodes was estimated by the calculation of the effective surface area with a voltammetry technique. 

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]. 

These curves were obtained experimentally for Pt electrodeposited onto the monocrystalline, macro- and mesoporous silicon surfaces. There are two peaks corresponding to the oxidation of adsorbed hydrogen, a potential area of a double layer, and an area of the oxygen adsorption. The current increase at the potential of 1.45 V and higher is associated with the release of molecular oxygen. In the cathodic polarization, peaks concerned with the reduction of adsorbed oxygen, a feebly marked potential area of the double layer, and a maximum of the hydrogen adsorption are observed. 

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. 

V. Lehmann, A. Luigart, and V. Corbel, On the morphology and the electrochemical formation mechanism of mesoporous silicon, Electrochem. Soc. Proc. 97(7), 132 1997. 

Mesoporous silicon oxides with different channel networks, sizes, and shapes ... 

---