  Spatial distribution of local tunneling conductivity was investigated for
deep and shallow impurities on semiconductor surfaces. Non-equilibrium Coulomb
interaction and interference effects were taken into account and analyzed
theoretically with the help of Keldysh formalism. Two models were investigated:
mean field self-consistent approach for shallow impurity state and Hubbard-{I}
model for deep impurity state. We have found that not only above the impurity
but also at the distances comparable to the lattice period both effects
interference between direct and resonant tunneling channels and on-site Coulomb
repulsion of localized electrons strongly modifies form of tunneling
conductivity measured by the scanning tunneling microscopy/spectroscopy
(STM/STS).
