Semiconductors, thermodynamic instability

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

The charge carriers may reduce or oxidize the semiconductor itself leading to decomposition. This poses a serious problem for practical photoelectrochemical devices. Absolute thermodynamic stability can be achieved if the redox potential of oxidative decomposition reaction lies below the valence band and the redox potential of the reductive decomposition reaction lies above the conduction band. In most cases, usually one or both redox potentials lie within the bandgap. Then the stability depends on the competition between thermodynamically possible reactions. When the redox potentials of electrode decomposition reactions are thermodynamically more favored than electrolyte redox reactions, the result is electrode instability, for example, ZnO, Cu20, and CdS in an aqueous electrolyte. 

The comprehensive characterization of uitrathin resist (UTR) (<100 nm) processes in terms of defectivity, manufacturahility, and physical properties (structure, dynamics, stability, thermodynamic behavior, etc.) have been a central point of interest in semiconductor microlithography for quite some time. Despite many years of experimental and theoretical efforts along these lines, a number of basic questions still remain to be answered. One of these issues is the fundamental lower physical limit of the resist thickness, below which lithographic patterning is not viable. For resists based on the polyhydroxy styrene platform, this lower limit has been determined to be around 60 nm, with the onset of film instability occurring at around 55 nm. For a host of other resist platforms, this lower limit is yet to be determined. 

In Eq. 28, pEdec and Jzdec denote the anodic and cathodic decomposition energy, respectively. Two concepts are used to describe the stability of semiconductors at the electrolyte contact First, the general thermodynamic stability against corrosion is determined form the position of the semiconductor band edges with respect to the decomposition levels, as shown in Fig. 8. Situations shown encompass complete stability, complete instability, and stability against anodic or cathodic corrosion. 

In cases where the redox potentials of the electrode decomposition reactions are more thermodynamically favoured than the electrolyte redox reactions (oxidative decomposition potential more negative, reductive decomposition potential more positive, than the corresponding electrolyte redox reactions), the products of the electrolyte redox reactions have sufficient potential to drive the electrode decomposition reactions. Hence, this situation usually results in electrode instability, assuming that the electrode decomposition reaction is not kinetically inhibited. This is the case with ZnO, Cu20, and CdS in simple aqueous electrolytes, and these semiconductors are indeed unstable under these conditions. 

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