Potential profile and band bending

When a semiconducting electrode is brought into contact with an electrolyte solution, a potential difference is established at the interface. The conductivity even of doped semiconductors is usually well below that of an electrolyte solution so practically all of the potential drop occurs in the boundary layer of the electrode, and very little on the solution side of the interface (see Fig. 7.3). The situation is opposite to that on metal electrodes, but very similar to that at the interface between a semiconductor and a metal. 

The variation of the electrostatic potential p(x) in the surface region entails a bending of the bands, since the potential contributes a term —eo4 (x) to the electronic energy. Consider the case of an n-type semiconductor if the value f s of the potential at the surface is positive, the bands band downwards. We set 4 = 0 in the bulk of the semiconductor and the concentration of electrons in the conduction band is enhanced (see Fig. 7.4). This is called an enrichment layer. If cj)s 0, the bands bend upward, and the concentration of electrons at the sur- 

Mutatis mutandis the same terminology is applied to the surface of p-type semiconductors. So if the bands bend upward, we speak of an enrichment layer if they bend downward, of a depletion layer. 

The total interfacial capacity C is a series combination of the space-charge capacities C c of the semiconductor and C oi of the solution side of the interface. However, generally Csoi Csc, and the contribution of 

Semiconductors that are used in electrochemical systems often do not meet the ideal conditions on which the Mott-Schottky equation is based. This is particularly true if the semiconductor is an oxide film formed in situ by oxidizing a metal such as Fe or Ti. Such semiconducting films are often amorphous, and contain localized states in the band gap that are spread over a whole range of energies. This may give rise 

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