  The core-level and valence band electronic structure of the well-defined
near-surface layer of n-GaAs (100) has been studied by synchrotron-based
high-resolution photoelectron spectroscopy before and after modification of the
layer by an Ar+ ion beam in the 1.5 - 2.5 keV energy range. Conversion of the
conductivity type from n into p has been revealed in the irradiated layer
several nm thick. The effect manifests itself in shifts of the core-levels and
valence band edge by the value comparable to the bandgap width. Transformation
on the conductivity type has been assumed to be caused by Ga-antisite point
defects generated by ion bombardment. The possibility of local formation of a
p-n nanojunction within the ion-beam spot has been shown.
