Semiconductors oxidation processing

In Chapter 1 we consider the physical and diemical basis of the method of semiconductor chemical sensors. The items dealing with mechanisms of interaction of gaseous phase with the surface of solids are considered in substantial detail. We also consider in this part the various forms of adsorption and adsorption kinetics processes as well as adsorption equilibria existing in real gas-semiconductor oxide adsorbent systems. We analyze the role of electron theory of chemisorption on... 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

When a molecule in a solution absorbs light, an electron in the highest occupied molecular orbital (HOMO) is excited to the lowest unoccupied molecular orbital (LUMO) creating an electron vacancy, that is, a hole in the HOMO. The electron may be provided to a molecule in the solution to reduce it, whereas the hole in the HOMO may be provided to a molecule in the solution to oxidize it. This is similar to the reduction-oxidation process in the bulk semiconductor/electrolyte photoelectrolytic cell described earlier [13,17]. 

Electrons and holes generated by energetic illumination in a small semiconductor particle can easily reach the surface and can carry out reduction and oxidation processes of adsorbed species, respectively, by providing and accepting electrons before they have a chance to recombine. Therefore, the quantum yield can be high. 

Details of the chemical oxidation process are discussed in Section 5.2. The stringent requirements concerning metal contamination and the trend to more environmentally friendly processing are a constant force to improve cleaning procedures in today s semiconductor manufacturing [Me4, Sal, Ohl]. 

Mills, A Lee, S-K. Semiconductor photocatalysis. In Parsons SA, editor. Advanced oxidation processes for water and wastewater treatment. London IWA Publishing 2004 137-166. 

Photoexcitation of TiOz with wavelengths <380 nm generates an electron-hole pair [Eq. (1)], creating the potential for reduction and oxidation processes to occur at the surface of the semiconductor. 

An example where all four areas are utilized in combination with production processes is found in ozone applications in the semiconductor industry (Section B 6.1). Part of ozone s effectiveness in these four areas is derived from its production of OH-radicals. Combined processes, i. e. advanced oxidation processes, represent alternative techniques for catalyzing the production of these radicals and expands the range of compounds treatable with ozone (Section B 6.2). 

The chemical reactions of organic molecules at excited semiconductor electrodes are of course reduction and oxidation processes, but these depend on the solvents and other reactants such as water, electrolytes and molecular oxygen. Figure 4.69 gives a few examples of many such reactions which are finding applications in chemical synthesis. 

An electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) when a molecule in solution absorbs light. The excited electron in the LUMO may transfer to a neighboring molecule (oxidant) in solution, leading to the reduction of the oxidant, whereas the electronic hole (electron vacancy) in the HOMO may transfer to another neighboring molecule (reductant) in solution, resulting in the oxidation of the reductant. Quite similar photoinduced reduction-oxidation processes can occur at the semiconductor/solution (semiconductor/liquid) interface when a semiconductor in solution absorbs light. Fig. 4.1 schematically illustrates the... 

Except for the cavity interior, the possible location for oxidation of these substrates by direct or sensitized photolysis, flash photolysis, ultraviolet/ peroxide, or irradiated semiconductor (SC) particulates suggests that the sonochemical oxidation process physically mirrors the heterogeneous pho-tocatalytic process such as UV/Ti02. 

Table 1 of a paper by Murr (2) lists problems and/or concerns related to specific interface materials and specific components of SECS. In Table 2 of the same work, he related topical study areas and/or research problems to S/S, S/L, S/G, L/L, and L/G interfaces. It is also useful to divide interface science into specific topical areas of study and consider how these will apply to interfaces in solar materials. These study areas are thin films grain, phase, and interfacial boundaries oxidation and corrosion adhesion semiconductors surface processes, chemisorption, and catalysis abrasion and erosion photon-assisted surface reactions and photoelectrochemistry and interface characterization methods. The actual or potential solar applications, research issues and/or concerns, and needs and opportunities are presented in the proceedings of a recent Workshop (4) and summarized in a recent review (3). 

The sensitization of semiconductors is a special example of electron transfer quenching and may prove to be very important. A photoexcited electron may, for example, be injected with high quantum yield into the semiconductor conduction band, to produce a photovoltaic device. The hole that is left behind may then perform some useful oxidation process. 

Molecular self-assembly is a technique to form highly ordered, closely packed mono-layers on various substrates via a spontaneous chemisorption process at the interface.11,12 Earlier research done in this field includes the self-assembly of fatty acids monolayers on metal oxides,14,15 SAMs of organosilicon derivatives on metal and semiconductor oxides,16,17 and organosulfur SAMs on metal and semiconductor surfaces.18,19 Among the organosulfur SAMs, the most thoroughly investigated and characterized one is alkanethiol SAM formed on Au(l 11) surfaces.12... 

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