The semiconductor-electrolyte interface

Abstract

Many naturally occurring substances, in particular the oxide films that form spontaneously on some metals, are semiconductors. Also, electrochemical reactions are used in the production of semiconductor chips, and recently semiconductors have been used in the construction of electrochemical photocells. So there are good technological reasons to study the interface between a semiconductor and an electrolyte. Our main interest, however, lies in more fundamental questions: How does the electronic structure of the electrode influence the properties of the electrochemical interface, and how does it affect electro-chemical reactions? What new processes can occur at semiconductors that are not known from metals? 11.2 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. 11.1). 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 φ(x) in the surface region entails a bending of the bands, since the potential contributes a term −e 0 φ(x) to the electronic energy. Consider the case of an n-type semiconductor. We set φ = 0 in the bulk of the semiconductor. If the value φ s of the potential at the surface is positive, the bands band downwards, and the concentration of electrons in the conduction band is enhanced (see Fig. 11.2). This is called an enrichment layer. If φ s < 0, the bands bend upward, and the concentration of electrons at the surface is reduced; we speak of a depletion layer. On the other