The initial stages of homoepitaxy on GaAs(001) are studied with atomic-resolution scanning tunneling microscopy and Monte Carlo simulations that include the zinc blende structure of GaAs, the ( $2\times 4$) reconstruction of the (001) surface, and the kinetics of ${\mathrm{As}}_2$ incorporation. The reconstruction is found to favor nucleation on the top-layer arsenic dimers and to cause small islands to be unstable until they adopt the local ( $2\times 4$) structure.

• Received 9 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.633