Valence state manipulation of single Fe impurities in GaAs by STM
  The incorporation of Fe in GaAs was studied by cross-sectional scanning tunneling microscopy (X-STM). The observed local electronic contrast of a single Fe atom is found to depend strongly on its charge state. We demonstrate that an applied tip voltage can be used to manipulate the valence and spin state of single Fe impurities in GaAs. In particular we can induce a transition from the Fe{3+)- 3d5 - isoelectronic state to the Fe{2+} - 3d6 - ionized acceptor state with an associated change of the spin moment. Fe atoms sometimes produce dark anisotropic features in topographic maps, which is consistent with an interference between different tunneling paths. 
J. Bocquel, V. R. Kortan, R. P. Campion, B. L. Gallagher, M. E.   Flatt\'e, and P. M. Koenraad
cond-mat.mes-hall cond-mat.mtrl-sci