The Surface State of Semiconductor Electrodes

The interface of semiconductor electrodes iiequently contains more or less localized electron levels called either surface states or interface states. In this textbook we use the term of surface states. 

In Eqns. 5-85 and 5-86, we have distinguished the charge a of surface states from the charge of adsorbed dehydrated ions. Since these two charges are on the same semiconductor surface, it is apparently difficult to distinguish 

The interface of semiconductor electrode adsorbs both ionized basic hydroi groups -OH2 (b) and associated acidic hydroxyl groups -OH(a) in acidic solutions these hydroxyl groups introduce two-dimensional redox electron levels as identified in Eqns. 5-88 and 5-89  

These levels of interfacial redox electrons are connected with the hydrogen and oxygen electrode reactions. As noted in Sec. 5.1.2, the electron level of adsorbate particles is broadened by contact adsorption and undergoes the Franck-Condon level splitting due to a difference in adsorption energy between the oxidized particle and the reduced particle on the interface of semiconductor electrodes as shown in Fig. 5-59. 

The contact adsorption of particles other than hydroxyl groups also generates the energy levels of interfacial redox electrons in the same reason as occurs with the hydroxyl groups. 

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Simple calculation gives a comparable distribution of the electrode potential in the two layers, (64< >h/64( sc) = 1 at the surface state density of about 10cm" that is about one percent of the smface atoms of semiconductors. Figure 5—40 shows the distribution of the electrode potential in the two layers as a function of the surface state density. At a surface state density greater than one percent of the surface atom density, almost all the change of electrode potential occurs in the compact layer, (6A /5d )>l, in the same way as occurs with metal electrodes. Such a state of the semiconductor electrode is called the quasi-metallic state or quasi-metallization of the interface of semiconductor electrodes, which is described in Sec. 5.9 as Fermi level pinning at the surface state of semiconductor electrodes. 

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