SURFACE-STATES AND FERMI-LEVEL PINNING AT SEMICONDUCTOR ELECTROLYTE JUNCTIONS

The influence of surface modification procedures on the electronic interface properties of p-InP, p-Si, p- and n-type GaAs and p-CuInS2 is investigated at the semiconductor redox electrolyte contact. Plots of photovoltage versus redox potential allow the determination of surface state densities using a simple model. The calculated surface-state densities D(s) range from 10(11) cm-2 eV-1 in an unpinned situation to about 2 X 10(14) cm-2 eV-1 for pronounced Fermi level pinning evidenced by independence of the photovoltage on changes of the redox potential. On InP and GaAs, deviations from the linear relationship of photovoltage and redox potential are observed. The deviations are attributed to an energetically non-uniform distribution of surface states. The model considerations are extended to include variations of D(s) with electron energy. In some cases, a dependence of the cell voltage in the dark on redox potential is observed which is attributed to electrochemical reactions which lead to a dynamic pinning of the Fermi level.