The profile of a line defined by focused ion beam irradiation of GaAs(
100) was characterised in situ using scanning probe microscopy. Curren
t-imaging tunnelling spectroscopy was used to characterise the surface
defects induced by the high energy Ga+ ions. Spatially reproducible f
eatures, approximately one per ion and 2-3 nm in diameter, were observ
ed in the irradiated surface. Differential conductance spectra of thes
e features indicated that the image contrast was due to acceptor state
s induced in the surface bandgap. The density of these defects was use
d to form a profile of the ion beam in the semiconductor surface. The
resulting profile was in excellent agreement with the two-Gaussian fit
reported by previous work.