Dissolution of covalent semiconductors

In the anodic dissolution of covalent semiconductors, the transfer of surface ions across the compact layer (Helmholtz la r) occurs following the ionization of surface atoms S, illustrated in Eqn. 9-33, as described in Sec. 9.2.1  

As shown in Fig. 9-9, the interfacial double layer of semiconductor electrode consists of a space charge layer with the potential of in the semiconductor and a compact layer with the potential of at the electrode interface. The potential 4+sc across the space charge layer controls the process of ionization of smface atoms (Eqn. 9-24) whereas, the potential across the compact layer controls the process of transfer of surface ions (Eqn. 9-33). The overvoltage iiac across the space charge layer and the overvoltage t b across the compact layer are eiq)ressed, respectively, in Eqn. 9-34  

When electronic equilibrium is established in the space charge layer, the concentration of interfacial electrons is given by n, = n exp (- e A /k T) and the concentration of interfacial holes is given by Pt = p exp(e A lk T) n and p are the concentrations of electrons and holes, respectively, in the semiconductor interior. In general, the ionization of surface atoms (Eqn. 9-24) is in quasiequilibrium so that the concentration of surface ions depends on the overvoltage 

The anodic current i of the transfer of surface ions, Eqn. 9-33, across the compact layer is given by Eqn. 9-36  

Equation 9-36 3delds the anodic ion transfer current, i, in the state of band edge level pinning (ti = isc, t h = 0) as shown in Eqn. 9-37  

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Fig. 9-7. Ionization of surface at oms followed by ion tnnsfer across an electrode interface in anodic dissolution of covalent semiconductor S = covalently bonded atom in semiconductor S. = surface atom of semiconductor s = surface radical = surfisce ion 825 = hydrated ion OHP = outer Helmholtz plane.

For covalent semiconductors described in Sec. 9.2.2, holes in the valence band or electrons in the conduction band are required for initially breaking the covalent bonding of surface atoms to form surface ions thus, the dissolution of covalent... 

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). 

Fig. 9-10. Polarization curves of anodic dissolution and cathodic deposition of n-type and p-type covalent semiconductor electrodes n-SC (p-SC) = n-type (p-type) semiconductor electrode i (i ) = anodic dissolution (cathodic deposition) current Cp = Fermi level.

The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Dissolution of Semiconductors. The mechanism of dissolution of semiconductors is essentially similar to that of metals, described above. The basic difference in the dissolution behavior of metals and semiconductors lies in the concentration and type of charges responsible for surface reactions. In semiconductors, the concentration of charge carriers is much smaller than in metals because of the predominantly covalent nature of bonding. The electron transfer process may involve either valence band or conduction band electrons at the semiconductor electrode while only conduction band electrons take part at metal electrodes. Furthermore, the kinetics of dissolution of metals is determined by electrochemical reactions occurring in the soiution or at the solution-metal interface, whereas the rate-determining process in the dissolution of semiconductors may also involve phenomena taking place inside the surface. 

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