Flatband Potential and Position of Energy Bands at the Interface

Flatband Potential and Position of Energy Bands at the Interface 

Accordingly, it can be concluded that pinning of bands occurs if the potential across the Helmholtz layer remains constant A(A H = 0). 

This result has been confirmed by investigations of various other semiconductors which are available as n- as well as p-doped material, such as GaAs, InP, and SiC. As previously mentioned, the flatband potential of many systems depends on the pH of the electrolyte. An increase of pH always leads to a cathodic shift of and a corresponding change of A //. Accordingly, the energy bands of the semiconductor at the surface are always shifted upward by an increase of pH. 

It has been emphasized by Bard et al. that there may be exceptions to the model derived earlier, insofar as Fermi level pinning by surface states may occur in a similar fashion to that at semiconductor-metal junctions [33]. Such an effect would lead to an unpinning of bands at the interface. There are some examples in the literature, such as FeS2 in aqueous solutions [34, 35] and Si in methanol [36] for which an unpinning of bands has been reported. In some cases, such as TiOj, ex- 

It is interesting to note that the flatband potential may depend on the crystal faces. For instance, n-type GaAs electrodes with (111) surfaces were prepared. The electrodes where the (111) Ga face contacted the electrolyte, the flatband potential was generally found to be 0.25 V more positive than for (111) As surfaces. Such effects can be understood in terms of differences in the Helmholtz layer. The same results were obtained with corresponding p-type GaAs electrodes [63]. 

Concerning the potential distribution, comparisons of the Mott-Schottky curves and the flatband potentials as obtained with n- and p-type electrodes of the same semiconductor are of special interest. One example is GaP which has a relatively large bandgap (2,3 eV), The Mott-Schottky plots of the n- and p-type electrodes are given in Fig. 5.18 [32], Since their slopes agree very well with those predicted upon the doping of the 

17 Mott-Schottky plot of thc space charge capacity vs. electrode potential for n-GaAs in aqueous solutions under stationary conditions and after different prcpolarizations scan rate 0.2 Vs-. (After ref. [40]) 

It is further clear from Fig. 5.19 that the n-electrode has to be polarized cathodically with respect to the equilibrium potential, and the p-electrode anodically, in order to reach the corresponding flatband situation (see lower part of Fig. 5.19), provided that the positions of the energy bands at the surface are the same for the two types of electrodes. Keeping in mind that the electrode potential refers to the Fermi level of the electrode, then the difference of flatband potentials corresponds exactly to the difference of the two Fermi levels. Since the Fermi level in the bulk of a semiconductor with the usual doping ( 1() cm ) is rather close to the corresponding band, the difference in the flatband potentials approximates the bandgap of the semiconductor as found with GaP. 

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