The number of In atoms in a critical cluster, i*, in Stranski-Krastanow (S-K) mode of InAs islands was determined to be 1-10. The i* was determined using an activation energy E-A of 2.0 eV determined from an Arrhenius plot of the saturated density of InAs islands formed on a GaAs (001) surface by S-K mode of molecular beam epitaxy [K. Shiramine et al., J. Cryst. Growth 242, 332 (2002)], and an activation energy of 1.6 eV for migration (surface diffusion) of In adatoms, inferred from other references. The common value similar to2.0 eV of E-A in S-K mode was ascribed to the small i*. (C) 2004 American Vacuum Society.