Abstract
We have carried out barrier-height imaging of oxygen-adsorbed Si(001) and Ge(001) surfaces and investigated the adsorption-induced change in the barrier height at various oxygen-reacted sites. On Ge(001)2×1, we find that the most highly reactve site shows a large decrease (-1.6 eV in the occupied state) in barrier height. A similar but smaller decrease in barrier height is also observed at oxidized type-C defects on Si(001) 2×1. The observed low barrier height suggests that the oxygen-induced surface dipole at these reaction sites makes a negative contribution to the barrier height. It is also shown that the barrier-height difference between oxidized and unoxidized type-C defects can be used to distinguish them which show little difference in scanning tunneling microscopy (STM) contours.