Valence state manipulation of single Fe impurities in GaAs

Cross-sectional STM was used to characterize Fe doped GaAs. We show that, by controlling the tip-induced band bending, Fe atoms can be brought from their isoelectronic state (Fe3+)-3d5 state into their (Fe2+)-3d6 ionized acceptor state. This STM-induced valence manipulation that involves the (de)population of the d-shell of the Fe atom differs from our previous experiments on Mn-acceptors and Si-donors where we changed its charge state by adding or removing a valence band hole or a conduction band electron respectively that is bound by the impurity. In addition for specific tunneling conditions a peculiar contrast is observed under which Fe atoms appear as dark anisotropic features.