Porous silicon applications

Aouida, S., Saadoun, M. and Boujmil, M. E Effect of UV irradiation on the structural and optical features of porous silicon Application in silicon solar cells , (2004) Appl. Surf. Sci. 238,193-8. 

Methods for electrochemical, catalytic (metal assisted), and deep reactive ion etching (DRIE) of silicon have been developed, which enable fabrication of arrays of deep cylindrical or modulated pores, walls, tubes, combinations of these, and other forms with vertical walls (Wu et al. 2010). As a rule, the regular arrays produced by electrochemical etching are characterized by constant porosity and pore depths (up to 500 pm) and form a planar front propagating into the substrate. Various devices and functional elements for micromechanics, photonics, chemical power sources, microfluidics, photovoltaics, etc. (see Porous Silicon Application Survey chapter), are commonly fabricated on the basis of these arrays by post-anodization treatment intended to modify the structure and properties of macroporous silicon to raise or reduce its porosity, change the shape of pores, transform the pore array into a column array, change the properties of the inner surface of pores, coat it with the film of a metal or insulator, open up pores, fill pores with various fillers, dope the silicon walls, etc. Some procedures can be performed locally, which requires formation of a pattern and subsequent structuring. 

Prokes, S.M. (1996) Porous silicon nanostructures, in Nanomaterials Synthesis, Properties and Applications, eds. Edelstein, A.S. and Cammarata, R.C. (Institute of Physics Publishing, Bristol and Philadelphia) p. 439. 

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

Electropolishing is well established as a simple, in situ method to separate porous silicon layers from the silicon electrode. By switching the anodic current density from values below JPS to a value above JPS, the PS film is separated at its interface to the bulk electrode. The flatness of a PS surface separated by electropolishing is sufficient for optical applications, as shown in Fig. 10.10. 

Starodub NF, Starodub VM (2002) Porous silicon some theoretical aspects and practical application as transducer for immune sensor. In Extended abstracts of the 3rd international conference porous semiconductors science and technology, Puerto de la Cruz, Tenerife, Spain, pp 155-157, 10-15 Mar 2002... 

Svechnikov SV, Sachenko AV, Sukach GA et al (1994) Light emitted layers of the porous silicon preparation, abilities and application (review). Optoelectronics and Semiconductor Technique N27 3-28... 

In addition to dense monolithic ceramics, porous silicon nitrides are gaining more importance in technological applications [24], Some porous silicon nitrides with high specific surface area have already been applied as catalysis supports, hot gas filters and biomaterials [25], There is an emerging tendency to facilitate silicon nitride as biomaterial, because of specific mechanical properties that are important for medical applications [25], Moreover, in a recent study it was shown that silicon nitride is a non-toxic, biocompatible ceramic which has the ability to propagate human bone cells in vitro [25], Bioglass and silicon nitride composites have already been realized to combine... 

In order to improve the separating performance of HPTLC pre-coated plates silica gel 60 even at larger applied volumes, as may be necessary at low sample concentrations, and with a rapid and simple technique of application, HPTLC pre-coated plates silica gel 60 with so-called "concentrating zones" were developed (10, 11, 12). This type of plate consists of two distinct layer sections, namely the separating layer proper consisting of silica gel 60 and a concentrating zone composed of an inert, porous silicon dioxide. These two sorbent materials pass into one another at a clearly defined boundary-line in such a way that the eluant is offered no resistance as it passes through. 

Recent applications of FNS include the dynamics of the electrical potential fluctuation in an electromembrane system [iv], analysis of the fluctuations of the electrical current in electrochemically deposited conducting polymers [v], and forecasting electrical breakdown in porous silicon [vi]. 

Examples of applications of silicon electrochemistry are exploiting the properties of the silicon-electrolyte contact for analytical purposes, e.g., HF tester and pinhole detector, which directly exploit the special properties of the electrochemical reactions at anodically- or cathodically-polarized silicon electrodes, and the preparation of devices based on porous silicon and silicon oxide (formed by anodic processes). 

Non-Debye dielectric relaxation was also observed in porous silicon (PS) [25,160,161], PS has attracted much attention recently, mainly due to its interesting optical and electro-optical properties that can be utilized for device applications [164,165], So far, most of the activity in this field has focused on the intense visible photoluminescence (PL) from nano-PS and the underlying physical mechanism that is responsible for the generation of light. In addition, transport and dielectric relaxation phenomena in PS have also attracted... 

In terms of biosensing applications using such layers, again cholera toxin detection on a porous silicon substrate [85] has been reported. Also biotin-avidin interaction by QCM [86], glutamate detection [87], as well as protein membrane interactions [88, 89] have been studied. 

There are essentially three different ways how to prepare nanometer sized silicon particles. The porous silicon is, as already mentioned, prepared by anodic etching of silicon wafers in an HF/ethanol/water solution [6, 7]. The microporous silicon has typically a high porosity of 60-70 vol.%, and it consists of few nm thin wires which preserve the original orientation of the wafer. The thickness of the wires varies within the PS layer and the material is very brittle. Free standing PS films can be prepared by application of a high current density after the usual etching of the desired thickness of the PS. 

On semiconductors light emission is induced by injection of electrons into the conduction band and subsequent band-to-band radiative recombination with holes (Fig. 38a). The process is reminiscent of electroluminescence or cathodolumines-cence and works with p-type substrates only (at n-type specimens no hole is available at the surface). Tunnel biases of 1.5-2 V are necessary in the case of GaAs, for instance. Figure 38b is a photon map of a GaAlAs/GaAs multiquantum well obtained by Alvarado et al. [140], The white stripes are regions where photons are emitted and correspond to the GaAs layers. The lateral resolution is about 1 nm and is limited by the diffusion distance of minority carriers. In Sec. 5.1 we have seen an example of the application of this technique in the case of porous silicon layers. 

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

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. 

Continuous Pt films at the surface of porous silicon cannot be applicable as catalytic coats for fuel cells electrodes. Quite the contrary, Pt coats should save its porosity to allow an easy penetration of gaseous iuel and to have the effective surface area as high as possible. So, the layers of electrodeposited Pt of about 100 nm in thickness, as illustrated in Fig. Ic, are optimal catalytic Pt films for micro fuel cell electrodes. 

Another problem in application of the basic theories is associated with surface geometry. Most theories are developed to describe the relationships among the area-averaged quantities such as charge density, current density, and potentials assuming a uniform electrode surface. In fact, the silicon surface may not be uniform at the micrometer, nanometer, or atomic scales. There can be great variations in the distribution of reactions from extremely uniform, for example, in electropolishing, to extremely nonuniform, for example, in the formation of porous silicon. 

T. C. Teng, An investigation of the application of porous silicon layer to the dielectric isolation of integrated circuits, J. Electrochem. Soc. 126, 870, 1979. 

R. C. Frye, The formation of porous silicon and its applications to dielectric isolation, Mater. Res. Soc. Symp. Proc. 33, 53, 1984. 

L. Koker and K. W. Kolasinski, Applications of a novel method for determining the rate of production of photochemical porous silicon. Mater. Sci. Eng. B69-70, 132, 2000. 

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