  Cross sectional scanning tunneling microscopy (XSTM) and density functional
theory have been used to characterize the structure of GaSb digitally doped
with Mn. The Mn dopants are found in both isolated substitutional form as well
as in large clusters of zinc-blende MnSb commensurate with the surrounding GaSb
matrix. Theoretical calculations predict that these two forms of Mn in the
digitally doped layers will have a very different appearance in XSTM images.
Substitutional Mn enhances the local density of states near the surface, thus
appearing higher in a filled-state image. In contrast, MnSb clusters induce
substantial structural relaxation at the {110} surface, and therefore appear as
localized depressed regions with negligible perturbation of the surrounding
GaSb.
