First-principles investigation of tip-surface interaction on a GaAs„110... surface: Implications for atomic force and scanning tunneling microscopies

Ab initio pseudopotential total energy techniques are used to investigate the tip-surface interaction in atomic force microscopy on a GaAs~110! surface with a Si tip. Our simulations show significant surface relaxation effects in the near contact region, which lead to a complicated behavior of the total energy and force curves. In particular, the tip-induced displacement of the Ga atoms can exceed 1 Å even in the attractive force region, leading to hysteresis in the energy and force curves. These large tip-induced relaxations of the surface Ga atoms provide a natural explanation to the simultaneous imaging of both anions and cations in recent nearcontact scanning tunneling microscopy experiments on this surface. We show that, for tip-surface distances where the surface topography remains unchanged and for a charge neutral Si tip, only the anion sublattice can be resolved in noncontact atomic force microscopy. Close to contact, our simulations prove that, even for atomically sharp tips ~1! there is a significant contribution to the total interaction from tip atoms different from the apex atom; ~2! large lateral ~bonding! forces on the tip apex may develop and change the tip structure well before significant normal repulsive forces appear. @S0163-1829~99!09939-7#