Space semiconductor surface

Upon immersion of the CdSe semiconductor into the electrolyte, electron exchange at the interface occurs until equilibrium is attained. At equilibrium, the Fermi level of the semiconductor is adjusted by the presence of a space charge layer at the semiconductor surface. This layer is due to the difference between the Fermi level of the semiconductor and the Fermi level of the electrolyte which is measured at the redox couple (X) The potential drop at the space charge layer and the amount of band bending also depend on the degree of Fermi level mismatch at the semiconductor-... 

It will be seen that the values of the space-charge capacities are low (-0.01-1 fiF cm 2) compared with the capacities (-17 (J.F cm 2) of the region between the semiconductor surface and the OHP plane, the Helmholtz-Perrin parallel-plate region. That is why the space-charge capacities (the inverted parabolas) are noticed, for the observed capacity is given by two capacitors in series, the space charge, Csc, and Helmholtz-Perrin HP capacitors. Thus,... 

Let us note one vital point, which is of methodological importance. It has been traditionally accepted in electrochemistry to choose the positive direction of the electrode potential

positive electrode charge. Here the zero potential is assumed to be that of the reference electrode, which coincides, within a constant, with the potential in the solution bulk (— oo). On the other hand, in physics of semiconductor surface the potential is usually reckoned from the value in the semiconductor bulk ( ) the enrichment of the surface with electrons, i.e., the formation of a negative space charge, corresponding to the positive potential of the surface. In particular, this statement directly follows from the Boltzmann distribution for electrons and holes in the space-charge region in a semiconductor ... 

The potential distribution ( ) in the space-charge region is described, with due account for Eq. (13), by the self-consistent Poisson-Boltzmann equation. Its first integral can be calculated analytically, so that the electric field at the semiconductor surface Ssc = — /dx is expressed as (Garrett and Brattain, 1955 see also Frankl, 1967)... 

The potential distribution in the space chaige layer can be obtained by solving the Poisson equation for a given charge distribution. For a semiconductor/elecirolyte interface such as that shown in Fig.4.2, the potential,0(x), at a distance, x, from the semiconductor surface is given as follows ... 

If the lateral distances of the atoms are changed, this is called surface reconstruction. This is, for instance, observed with the (100) faces of Au, Ir, Pt, and W. Figure 8.4 shows two types of surface reconstruction that lead to a doubling of the lattice spacing in one direction. Semiconductor surfaces tend to exhibit surface reconstruction due to the directional character of the dangling bonds at the surface. 

The sheaf of the C 2 vs. E straight lines generally evidences some slow process proceeding either on the semiconductor surface (e.g. adsorption, charging of surface states) or in the space charge region (e.g. the deep donor or acceptor ionization mentioned above). 

A fifth type of space charge layer, the deep depletion layer, may be formed under non-equilibrium conditions at the semiconductor surface when a high voltage is applied such that an inversion layer should form, but either (a) minority carriers are not available to accumulate at the surface in the time allotted or (b) the minority carriers are consumed in an electrochemical reaction as soon as they reach the surface. Such a space charge layer is unlikely to form within semiconductor electrodes at open circuit and is included here solely for completeness. 

The potentials of the capacity minima are strongly dependent on the pH of the electrolyte. The current potential curves show the same dependence. These potential differences are not caused by changes in the space charge. It must be assumed that the source of these potential differences lies between the semiconductor surface and the Helmholtz plane in the electrolyte. Adsorption of OH ions may be an explanation of this effect. 

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