  Autocompensated Si-doped GaAs is studied with cross-sectional scanning
tunneling spectroscopy (X-STS). The local electronic contrasts of
substitutional Si(Ga) donors and Si(As) acceptors under the (110) cleavage
plane are imaged with high resolution. Si(Ga) donor atoms exhibit radially
symmetric contrasts. Si(As) acceptors have anisotropic features. The
anisotropic acceptor contrasts are traced back to a tunnel process at the
valence band edge. They reflect the probability density distribution of the
localized acceptor hole state.
