  Small numbers of nitrogen dopants dramatically modify the electronic
properties of GaAs, generating very large shifts in the conduction-band
energies with nonlinear concentration dependence, and impurity-associated
spatially-localized resonant states within the conduction band. Cross-sectional
scanning tunneling microscopy provides the local electronic structure of single
nitrogen dopants at the (110) GaAs surface, yielding highly anisotropic spatial
shapes when the empty states are imaged. Measurements of the resonant states
relative to the GaAs surface states and their spatial extent allow an
unambiguous assignment of specific features to nitrogen atoms at different
depths below the cleaved (110) surface. Multiband tight binding calculations
around the resonance energy of nitrogen in the conduction band match the imaged
features. The spatial anisotropy is attributed to the tetrahedral symmetry of
the bulk lattice. Additionally, the voltage dependence of the electronic
contrast for two features in the filled state imaging suggest these features
could be related to a locally modified surface state.
