  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.
