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 electrochemical reactions? What new processes can occur at semiconductors that are not known from metals? We begin by recapitulating a few facts about semiconductors. Electronic states in a perfect semiconductor are delocalized just as in metals, and there are bands of allowed electronic energies. According to a well-known theorem, bands that are either completely filled or completely empty do not contribute to the conductivity. In semiconductors the current-carrying bands do not overlap as they do in metals; they are separated by the band gap, and the Fermi level lies right in this gap. The band below the Fermi level, which at T = 0 is completely filled, is known as the valence band; the band above, which is empty at T = 0, is the conduction band. In a pure or intrinsic semiconductor, the Fermi level is close to the center of the band gap. At room temperature a few electrons are excited from the valence into the conduction band, leaving behind electron vacancies or holes (denoted by h+). The electric current is carried by electrons in the conduction band and holes in the valence band. The concentrations nc of the conduction electrons and pv of the holes are determined from Fermi statistics.
Direct observation of spin-resolved valence band electronic states from a buried magnetic layer with hard X-ray photoemission