Novel Diffusion Mechanism on the GaAs(001) Surface: The Role of Adatom-Dimer Interaction

Employing first principles total energy calculations we have studied the behavior of Ga and Al adatoms on the GaAs(001)- $\ensuremath{\beta}2$ surface. Beside the adsorption site we identify two diffusion channels that are characterized by different adatom-surface dimer interaction. Both affect the adatom migration: in one channel the adatom jumps across the surface dimers and leaves the dimer bonds intact; in the other one the dimer bonds are broken. The two channels are taken into account to derive effective adatom diffusion barriers. We find a strong diffusion anisotropy for both Al and Ga adatoms and, in agreement with experiments, higher diffusion barriers for Al than for Ga.