THE IMPEDANCE OF SURFACE RECOMBINATION AT ILLUMINATED SEMICONDUCTOR ELECTRODES - A NONEQUILIBRIUM APPROACH

The impedance of surface recombination at illuninated n-type semiconductor electrodes is calculated regarding non-equilibrium charge distributions. The occupancy of the surface states, as well as the hole distribution in the valence band, are calculated from charge exchange processes between the surface states and the conduction and valence bands of the semiconductor. It appears that deep lying surface states can be emptied completely at large irradiation intensities. When saturation occurs, the impedance is found to deviate strongly from its equilibrium value. However, in general saturation does not occur and the surface states are occupied almost in accordance with Fermi-Dirac statistics, even at high illumination intensities. Apparently, unless saturation occurs, the impedance for recombination at semiconductor/ electrolyte interfaces deviates only to a very small extent from its equilibrium value, and can be interpreted using simple expressions for the equivalent circuit elements. Consequently, the surface state density, the energetic position in the bandgap and kinetic parameters of dark or illuminated semiconductor surfaces can be obtained from impedance data. Hence, impedance spectroscopy is a valuable tool in the study of surface properties of semiconductor/electrolyte interfaces. © 1990.