Defect-induced band gap states and the contact charging effect in wide band gap insulators

Abstract

Color centers, created by electron bombardment, have been studied as model defects on the (100) surfaces of the wide band gap insulators NaCl and KCl with electron energy loss spectroscopy (EELS) and UV photoelectron spectroscopy (UPS). Both salts were grown as thin epitaxial films on the same Ge(100) surface so that they could be directly compared in situ. At substrate temperatures below 200 K, characteristic losses of F and M centers could be identified on both materials. Close to room temperature, however, high electron exposures resulted in additional losses in the band gap due to surface and bulk plasmons of Na and K clusters. Only the defects turn out to be chemically reactive. Color centers dissociate water, but leave salicylic acid (SA) intact as a molecule, which, however, is more strongly bound compared with the undistorted surfaces. Both the color centers and the adsorbed molecules provide unoccupied electronic states in the band gap to which thermal electronic excitation is possible. We propose that these states are essential for charge exchange during contact charging and discuss this phenomenon in context with our experiments.