Corrosion process semiconductors

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

Electrochemical properties of silicon single crystals, usually cuts of semiconductor wafers, have to be considered under two distinct respects (1) As an electrode, silicon is a source of charge carriers, electrons or positive holes, involved in electrochemical reactions, and whose surface concentration is a determining parameter for the rate of charge transfer. (2) As a chemical element, silicon material is also involved in redox transformations such as electroless deposition, oxide generation, and anodic etching, or corrosion processes. 

Specific features of corrosion processes at semiconductors (as against to metals) are caused by the fact that charge carriers of both signs, namely conduction band electrons and valence band holes, take part in charge exchange between a solid and a solution. Therefore, the condition of Eq. (43) is insufficient, so account should be made of charge balance for each type of the carriers because equilibrium between the bands, which is established via generation-recombination processes, may not be reached. 

It is quite natural that the thermodynamic approach does not allow photocorrosion processes to be described comprehensively. In a number of cases, kinetic peculiarities of reactions play an important role (see, for example, Bard and Wrighton, 1977) these peculiarities are caused by the effect of crystalline structure, state of the semiconductor surface, etc. A detailed description of a complicated reaction with several particles in the solution and crystal lattice involved usually encounters considerable difficulties. Therefore, at this stage the kinetic approach is used to reveal purely qualitative regularities of corrosion processes. 

Ion sputtering mass spectrometry has been applied to several problems in the analysis of solids with various types of instruments. These include studies of semiconductor devices as shown in Fig. 3. oxygen concentrations and concentration gradients and of processes of oxidation in a variety of metals, some catalytic and corrosion processes on metals, and the chemistry of trace elements in geologic specimens. The distribution of trace elements in lunar rocks has also been studied. 

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). 

Miller, B. 1984. Charge transfer and corrosion processes at III-V semiconductor/electrolyte interfaces. J. Electroanal. Chem.. 168. 91-100. 

In the next sections we discuss some selected examples of the use of ATR spectroscopy to study semiconductors, polymers, and corrosion processes. 

Much is known about the growth, crystallography and reconstruction of semiconductor (SC) surfaces from STM ultrahigh vacuum experiments. In this environment the first atomic resolution image was obtained for a semiconductor subslralc" before it was done for metal substrate. In the case of air and in particular in liquid enviromnent the situation is different. This is because of the presence of the native oxides formed in air on SC surface, and corrosion processes taking place easily in solution. 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Illumination of a semiconductor under open-circuit conditions in an etching (oxidizing) solution gives rise to corrosion even in darkness. In the simplest case where the cathodic partial reaction of a corrosion process proceeds exclusively through the conduction band and the anodic one through the valence band, the corrosion rate for specimens of any conductivity type is limited by the minority-carrier supply to the surface and is therefore low in darkness. Illumination accelerates corrosion processes. Comparison with the case considered above shows that here the chemical polarization of the semiconductor by an oxidizer introduced into the solution acts as anodic polarization. 

Our comprehensive understanding of materials corrosion fundamentals has advanced considerably over the decades. Modern corrosion science has made it clear that the corrosion process on metals and semiconductors consists of an anodic oxidation and a cathodic reduction both occurring across the material-aqua-solution interface. These reduction-oxidation reactions depend on the interfacial potential and hence on the electrode potential of materials. 

Photosensitivity Many corrosion products possess semiconducting properties. When interacting with photons (see Chapter 1, Volume 6) of higher energy than the band gap of the semiconductor, electron-hole pairs are generated, which change the conditions for chemical reactions in different ways. Yet, there is not ample evidence of photosensitivity in atmospheric corrosion processes. One... 

On a short timescale (seconds to minutes), illuminated OCP analysis is a nondestructive technique for most materials. However, extended periods of illumination at open-circuit conditions may lead to corrosion of the photoelectrode surface [2]. Therefore, it is best to minimize the time of exposure to high-intensity illumination. Intense illumination can heat the solution (especially if the infrared radiation is not pre-filtered) at the electrode surface, which can cause a slow drift of the measured potential over the course of seconds or hours, depending on the rate of heating. In addition, drifts in the potential response may also be the result of corrosion processes or slow adsorption of cations or anions in solution to the semiconductor surface. A more rapid photovoltage response is often desirable and can be indicative of a better material. 

To improve the solar response of a photoelectrode, a proper match between the solar spectrum and the band gap of the semiconductor should be maintained. When a single band gap semiconductor is used, a band gap in the vicinity of 1.4 eV is most desirable from the standpoint of optimum solar-conversion efficiency. An important criterion is that the minority carrier that is driven toward the semiconductor-electrolyte interface should not participate in a photocorrosion reaction that is detrimental to the long-term stability of the photoelectrode. Photocorrosion can be viewed in terms of either kinetic or thermodynamic considerations and the real cause may be a mixture of both. From thermodynamic perspective, a photoanode is susceptible to corrosion if the fermi level for holes is at a positive potential with respective to the semiconductor corrosion potential [21]. The corrosion can be prevented or at least inhibited by choosing a redox couple that has its /ijedox more negative than that for the corrosion process [22,... 

Corrosion Studies In order to fully understand the phenomena underpinning corrosion processes, a detailed structural and chemical understanding of the dissolution activity of metals, al loys, and semiconductors is required [24] Volume 4 of the Encyclopedia is con cemed with corrosion and oxide films The ability of SPMs to provide high level resolution structural information and to map shape evolution in real time... 

These limitations can be overcome by SECM. One of the important advantages of SECM is that the tip does not need to be a saniconductor. Therefore, any saniconductor electrode can be probed using a Pt UME tip, for example. Since very small currents are measured at the UME tip, the IR drop is minimal. Additionally, the UME tip quickly reaches a steady state due to the small area of the electrode so that effects due to double layer charging are avoided. Because the UME tip is poised at a potential specific to the redox couple used, corrosion processes taking place at semiconductor electrode do not contribute to the tip current. 

At different beam energies and wavelengths, specific characteristic bonds will interact with the beam. The whole process must be performed in an ultrahigh vacuum environment (UHV) and can reveal detailed information about the molecular composition of a surface. The technique is often used in industry to study catalysis, polymer surface modification, corrosion, adhesion, semiconductor and dielectric materials, electronics packaging, magnetic media, and thin film coatings [36,37]. 

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