Band bending surface dipoles

Fig. 4.2 Schematic illustrations of (a) the charge distribution, (b) the charge-density distribution, (c) the potential distribution, and (d) the band bending at the semiconductor/redox electrolyte interface, assuming that no surface charge nor surface dipole is present.

Fig. 4. (a) Band bending at semiconductor surface caused by charge—Q in surface states, (b) Effect of adsorbates (A) having a dipole moment and changing the charge of the space-... 

Among the related methods, specific experimental designs for applications are emphasized. As in-system synchrotron radiation photoelectron spectroscopy (SRPES) will be applied below for chemical analysis of electrochemically conditioned surfaces, this method will be presented first, followed by high-resolution electron energy loss spectroscopy (HREELS), photoelectron emission microscopy (PEEM), and X-ray emission spectroscopy (XES). The latter three methods are rather briefly presented due to the more singular results, discussed in Sections 2.4-2.6, that have been obtained with them. Although ultraviolet photoelectron spectroscopy (UPS) is an important method to determine band bendings and surface dipoles of semiconductors, the reader is referred to a rather recent article where all basic features of the method have been elaborated for the analysis of semiconductors [150]. 

A water molecule from the atmosphere (HjO ) reacts with pre-adsorbed oxygen ions (O d)) and two Cu sites (2Cuq) on the surface under the formation of two terminal hydroxyl groups (2(Cuj - OH )). Hie appearance of the two terminal hydroxyl groups is responsible for the increase in the electron affinity (formation of local surface dipoles) and the cancellation of a hole (h+) determines the decrease in the band bending. Sa is the freed adsorption site for chemisorbed oxygen. 

Ionized donors inside the material are indicated by , and the electrons they have released to the conduction band, by —. If a metal were in electrical contact with the semiconductor, electrons would be redistributed until the Fermi level, Ei-, separating the occupied from the unoccupied levels in the metal, were near the conduction-band edge of the semiconductor. Then, if the electrons near the surface were eaten up by surface reconstruction, as shown in part (b), an electric dipole layer would arise, giving the potential hill, or Schottky barrier, shown in part (c). The hill or htirrier would bend the bands, as indicated in part (d). One can say that the Fermi level is pinned midgap at the surface, though it is near the conduction-band edge in the interior. 

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