Recently developed semiempirical potential energy functions for the Ga–As–Si and Ga–As–Al systems have been applied here to determine the excess formation energy for GaAs clusters on GaAs(001̄), AlAs(001̄), Si(001), and one atomic layer As-covered Si(001) substrates as a function of cluster size and cluster shape by the Monte Carlo technique. Pyramidal type ledges on the GaAs clusters are found to be the favored ledge for the first three layers while an inverted-pyramidal type ledge is also favored in certain cases for the As1/Si(001) substrate. Cluster formation at ledges is compared with cluster formation on a flat terrace for the Si(001) and the As1/Si(001) substrates.