Direct imaging of dopants in GaAs with cross-sectional scanning tunneling microscopy

We report the direct imaging of individual electrically active dopants on cross-sectionally cleaved GaAs using scanning tunneling microscopy and compare these results to theory. The observation of these dopants is due to an enhancement in the tunneling current in the neighborhood of an ionized dopant atom in the top several surface layers. In highly p-doped GaAs, for tunneling out of the valence band, the dopants in the top several surface layers appear as individual circular hillocks about 2 nm in diameter, superimposed on the As sublattice, as expected. From the size of the hillock and the symmetry of the As sublattice enhanced by the hillock one can infer whether the dopant lies in the top, second or deeper layers.