Semiconductor Electrodes: XLIX . Evidence for Fermi Level Pinning and Surface‐State Distributions from Impedance Measurements in Acetonitrile Solutions with Various Redox Couples

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Abstract
Capacitance‐voltage (C‐V) measurements were made for the single crystal semiconductors ; , , p‐Si, , , and in acetonitrile containing a number of redox couples whose potentials spanned a potential regime much wider than the bandgaps. The flatband potential evaluated from capacitance‐potential (C‐V) measurements (Mott‐Schottky plots) exhibited three types of behavior with varying solution redox potentials: (i) varied monotonically with for p‐Si, , and ; (ii) for and , did not shift for couples located negative of the midgap potential, but varied monotonically for couples positive of this value; (iii) for the layer‐type, compounds ( , ), was almost independent of . These differences were ascribed to differences in surface‐state densities. For crystals, (001) face etched with molten and reduced, evidence for surface states at two different potentials was obtained from the in‐phase component of the total admittance. Tentative assignment of these states is to lattice defects. The states closer to the conduction band are assigned to oxygen vacancies and the deeper states to Ti (III), The densities of surface states evaluated from vs. ω plots for and p‐Si are around 1010 and 1013 cm−2, respectively. These two values represent different situations, i.e., while the former value of is not sufficient for pinning the Fermi level, the latter value is sufficiently high for the occurrence of Fermi level pinning.

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