Interpretation and use of Mott-Schottky plots at the semiconductor/electrolyte interface

The Poisson Boltzmann equation is solved for a semiconductor without making the usual assumption of no majority carriers in the depletion layer. The capacative behaviour of the semiconductor is combined with that of the Helmholtz layer to provide a complete description of the potential distribution in the electrolyte and in the semiconductor for all applied potentials including accumulation, the flat band transition and depletion. Calculations show that good linear Mott-Schottky plots are obtained in the depletion region even when 90% of the potential is across the Helmholtz layer. A procedure is described which uses the curvature of the Mott-Schottky plot in the region of the flat band potential to determine the ratio of the semiconductor capacitance to that of the Helmholtz layer. This allows one to determine experimentally the amount of band bending in any system. The procedure is illustrated with data for tin-doped indium oxide. Reasonable values are obtained for the doping density, the Helmholtz capacitance and the true hat band potential.