Secondary illumination effects in the photoemission spectra of GaAs and CdS

The photoemission spectra of GaAs and CdS are dramatically affected by secondary illumination of the sample. Using synchrotron radiation and a computer lineshape analysis we demonstrate that valence band and core-level spectral features are all rigidly shiftedin energy by a few tenths of an electronvolt under secondary illumination. This provides conclusive evidence that photovoltaic band-bending changes are the only cause of these effects and rules out other possible mechanisms. It is therefore right to use these phenomena for quantitative studies of band bending effects and of ultralow densities of extrinsic or intrinsic surface states.