Surface atom ionization of covalent semiconductor electrodes

1 Surface atom ionization of covalent semiconductor electrodes 

We discuss the dissolution of surface atoms from elemental semiconductor electrodes, which are covalent, such as silicon and germanium in aqueous solution. Generally, in covalent semiconductors, the bonding orbitals constitute the valence band and the antibonbing orbitals constitute the conduction band. The accumulation of holes in the valence band or the accumulation of electrons in the conduction band at the electrode interface, hence, partially breaks the covalent bonding of the surface atom, S, (subscript s denotes the surface site). 

As shown in Fig. 9-7, removing a pair of electrons from the covalent bonding orbital proceeds via an intermediate surface radical, S, as indicated in Eqn. 9-24 [Gerischer-Mindt, 1968]  

The activation energies of the respective elementary steps are denoted as g, Agl, 4gc, and dgj the corresponding anodic reaction rates are denoted as u., iJb, Vc, and Ud. Then, we obtain the rate equations shown in Eqns. 9-25a through 9-25d  

In the two different steps, Eqns. 9-24a and 9-24b, for the formation of intermediate radicals, the initial state differs in energy by an amount equivalent to the band gap as shown in Fig. 9-8. Consequently, the activation energy differs in the two steps as shown in Eqn. 9-26  

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