Porous silicon initiation

Fig. 8.6 Schematic set-up of a DIOS-TOF-MS system. Initially, the sample is deposited on the porous silicon surface. Subsequently, a laser pulse is directed to the silicon surface, and the analytes are desorbed. Ions that are generated are transferred into a time-of-flight mass spectrometer.

Although the data shown in Fig. 3 clearly indicate that this reaction proceeds via a chain mechanism, it is not clear how this reaction is initiated. Possible mechanisms that must be considered include exciton-based schemes involving surface localized holes that facilitate the attack of the alkene nucleophile (such as proposed for the white light alkylation of porous silicon [23]). Flowever, given the low efficiency of the initiation process it is difficult to completely rule out the role of photogenerated radicals from impurities in solution, even when high purity reactants are used. 

Y. Kato, T. Ito, and A. Hiraki, Initial oxidation process of anodized porous silicon with hydrogen atoms chemisorbed on the inner surface, Jpn. J. Appl. Phys. 27, L1406, 1988. 

The porous SiC is fabricated from commercial SiC substrate (4H or 6H) by electrochemical etching. An electrolyte is placed in contact with the SiC substrate. A bias is introduced across the electrolyte and the semiconductor materials causing a current to flow between the electrolyte and the semiconductor material. The SiC partially decomposes in this electrolyte and forms high density of pores with nano-scale diameter. This decomposition initiates from the carbon-face of SiC substrate because the carbon-face is less chemically inert compared with the silicon-face. These as-etched pores have a depth of approximately 200 pm but do not reach the silicon-face of SiC. To fabricate porous silicon-face SiC (silicon-face is used as the growth plane for GaN), SiC with thickness of tens of micrometers is polished away from the silicon-face to expose the surface pores. Two surface preparation procedures, hydrogen polishing and chemical mechanical polishing, have been applied to the as-polished silicon-face porous SiC to improve its surface perfection. 

The growth of this structure at the interface between porous silicon film and silicon substrate has been tentatively explained in [17]. In the silicon dissolution mechanism, it is known that positive carriers (holes) are involved. It is assumed that initially the pore walls are depleted of holes. 

Initial reaction rates 6.5 X 10 molmin" m with the porous silicon support vs 4.5 X 10 mol min m with the powdered... 

The term biocompatibility is defined as the ability of a material to perform with an appropriate host response in a specific situation" (Williams 2008). A biocompatible material can be inert, where it would not induce a host immune response and have little or no toxic properties. A biocompatible material can also be bioactive, initiating a controlled physiological response. For porous silicon, bioactive properties were initially suggested based on the observation that hydroxyapatite (HA) crystals grow on microporous silicon films. HA has implications for bone tissue implants and bone tissue engineering (Canham 1995). An extension of this work showed that an applied cathodic current was able to further promote calcification on the surface (Canham et al. 1996). More recently, Moxon et al. showed another example of bioactive porous silicon where the material promoted neuron viability when inserted into rat brains as a potential neuronal biosensor, whereas planar silicon showed significantly fewer viable neurons surrounding the implant site (Moxon et al. 2007). 

Implants of porous silicon membranes under the rat conjunctiva demonstrated similar results (Fig. 1). A small inflammatory response was initially observed, but histological examination of the... 

The future development of porous silicon (PS)-based optoelectronic devices depends on a proper understanding of electrical transport properties of the PS material. Electrical transport in PS is influenced not only by each step of processing and fabrication methods but also by the properties of the initial base substrate. This chapter endeavors to chronologically document how the knowledge base on the nature of carrier transport in PS and the factors governing the electrical properties has evolved over the past years. The topics covered include the proposed electrical transport models including those based on effective medium theories, studies on contacts, studies on physical factors influencing electrical transport, anisotropy in electrical transport, and attempts to classify the PS material. 

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