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Band bending on an -type semiconductor (after ionosorption of oxygen on levels). For semiconducting metal oxides the relative changes in work function accompanying adsorption processes contain qualitative and quantitative information (See Ref. 3), i.e., [where is the electrochemical potential, — band bending, — electron affinity]. The work function follows the change in band bending . denotes the depth of the depletion region; — elementary charge, and and are the valence and the conductance band edges in the bulk and at the surface, respectively; — the Fermi level; — the donor level at the surface; , and — an oxygen molecule in the ambient atmosphere, a physisorbed, and a chemisorbed oxygen species (, or ), respectively.
Changes of the contact potential difference (, see Fig. 1) (1) and conductance (2) of the precipitated (a) and commercial (b) (at 280 °C) under exposure (a) to dry oxygen in dry nitrogen and (b) to dry CO-synthetic air mixture in dry synthetic air. (3) Conductance of the reference sample which is exposed to the same gas atmosphere and known to show only -type behavior under the conditions of the measurements (see Ref. 19). Upper parts of panels (a) and (b) show the profiles of the oxygen partial pressure (a) and CO concentration (b).
Conductance as a function of the surface barrier : (a) simulation for intrinsic conductance and (line 1), (line 2), (line 3), (line 4), and (line 5) performed according to the model developed by Bârsan et al. (see Ref. 5): , where , , where , and are the intrinsic, initial, and actual potential barriers in their respective test conditions; (b) experimental data points and fitting curves based on the model; [see Fig. 2(a)], fitting parameters: , ; from the fit results: ; mixture ( is the predominant phase), fitting parameters: , , results: .
Schematic energy-level diagram of an -type semiconductor. We start with pure dry nitrogen atmosphere, which corresponds to a flatband situation (a). Oxygen adsorption leads to the surface states which are occupied by the electrons, this results either in a depletion layer and reduced -type surface conductivity (b) or in an inversion layer (the Fermi level lies below the intrinsic level ) and -type surface conductivity (c). For simplicity, the donor levels are not shown. In a nonstoichiometric -type the donors lie at 0.078 eV below the bottom of the conduction band (see Ref. 18).
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