  Making use of the tight-binding Green's function technique, we have
calculated the STM images of graphite with surface and sub-surface defects,
while taking into account the relaxation of the lattice due to defects. We have
demonstrated that two different physical mechanisms may result in the formation
of hillocks in the STM images: buckling of the graphite surface due to
interstitials between the uppermost graphite layers and the enhancement of the
electron density of states close to the Fermi energy on the carbon atoms in the
vicinity of vacancies. Our results indicate that small hillocks may originate
both from the interstitial clusters and from the vacancies. By contrast,
however, large hillocks in excess of 10 \AA~ in diameter can be caused only by
interstitial clusters.
