Submonolayer island-size distributions are obtained with scanning tunneling microscopy and used to infer the nucleation and growth kinetics of islands on the three low-index surfaces of GaAs. Comparison with Monte Carlo simulations reveals that on the (110) and $\left(111\right)A$ surfaces, random nucleation is followed by the attachment and detachment of single atoms at island edges. But on the (001) surface (using ${\mathrm{As}}_4$), nucleation is initiated in the trenches of the $2\times 4$ reconstruction by pairs of Ga atoms. Growth then proceeds over locally filled trenches, also by the capture of pairs of Ga atoms.

• Received 20 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.3938