Charge surface states

Meissner D, Lauermann I, Memming R, Kastening B (1988) Photoelectrochemistry of cadmium sulfide. 2. Influence of surface-state charging. J Phys Chem 92 3484-3488... 

If the surface state charge a is relatively great, the potential of the compact layer, is given by Eqn. 5-67 ... 

The potential i sc of the space charge layer can also be derived as a fixnction of the surface state charge Ou (the surface state density multiplied by the Fermi function). The relationship between of a. and M>sc thus derived can be compared with the relationship between and R (Eqn. 5-67) to obtain, to a first approximation, Eqn. 5-68 for the distribution of the electrode potential in the space charge layer and in the compact layer [Myamlin-Pleskov, 1967 Sato, 1993] ... 

As the Fermi level of the electrode approaches the surface state level of high state density, the surface state is charged or discharged as a capacitor. For convenience sake, we express the sum of a. and in Eqn. 5-86 as the surface state charge Qu and the capacity due to the surface state charge as the surface state capacity C.. Then, the interfadal capadty C is represented by the capadly of an equivalent drcuit shown in Fig. 5-60. 

The surface state capacity, Ch, is apparently zero in the range of potential where the Fermi level is located away from the surface state level (the state of band edge level pinning). As the Fermi level is pinned at the surface state, the capacity Ch increases to its maximum which is equivalent to the capacity Ch of the compact layer, because the surface state charging is equivalent to the compact layer charging in the state of Fermi level pinning. 

Meissner, D., Lauermann, I., Memming, R., Kastening, B. 1987c. Photoelectrochemistry of cadmium sulfide, part II influence of surface state charging. J. Phys. Chem., to be published. 

The surface concentration of electrons depends on the potential drop (band bending) in the semiconductor, and in the absence of complications due to surface state charging (Fermi level pinning), it is given by (cf. equation (8.5))... 

The causes of this anomalous behavior are still not fully understood. It appears likely that many factors are involved surface film formation, varying potential drop across the Helmholtz region caused, for example, by surface state charging, etc. Even crystallographic orientations appear to be important [163]. These aspects have been discussed by other authors [14, 159, 164]. 

FIGURE 1.9. Surface charge in an n-type semiconductor space charge, at various doping levels at AFs of -0.3 and -l.OV and surface state charge, i2s as a function of surface state density, assuming half-occupancy. Potential drop across the Helmholtz layer is AVh = 0.085 (pC/cm ) assuming 8n = 4 and d = 3 A. (Reprinted with permission from Bard et al. 1980 American Chemical Society.)... 

Finally, the surface state charge or the fixed oxide charge illustrated in Fig. 3.26d is a result of an oxide growth process that has the following characteiislics ... 

The surface state charge is due to the excess ionic silicon present in the oxide during the oxidation, waiting to react with the oxidizing species that has diffused across the oxide/silicon interface as illustrated in Fig. 3.27." ... 

The change of potential due to the surface state charge can be calculated from... 

Eo energy at which there is zero surface state charge 1.2... 

Fig. 6.1 Band diagrams of a n-type semiconductor (a) prior to contact with the electrolyte solution (assuming no defects or surface state charges), (b) in contact with the solution in absence of illumination, (c) in contact with the solution in the presence of moderate illumination, and (d) in contact with the solution in the presence of intense illumination and at the Ef. Illustrated are the conduction band (Ec), Fermi level ( p), and valence band ( v) of the semiconductor. Also shown are the Gaussian distribution of the redox states in the solution, shown as the density of states of oxidized (Doxidized) and reduced (Dreduced) species along with the corresponding Fermi level (fipsoiution), as described in more detail elsewhere [1]...

D) RECONTACT. FILLS THE SURFACE STATES. CHARGE BUILDS UP. 

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