The stabilization of planar In0.4Ga0.6As on GaAs, above thickness typically reported to undergo spontaneous island formation, has been observed. The effect of growth temperature, AsH3 partial pressure, and In0.4Ga0.6As thickness on island size and density was evaluated. Planar films are stabilized by low growth temperatures and high AsH3 partial pressures. Spontaneous island formation is enhanced by high growth temperatures and low AsH3 partial pressures. The results presented are interpreted within the phenomenology of In segregation to the surface, which has been identified previously for the InGaAs/GaAs system. Based on equilibrium models for the critical thickness for island formation, the critical segregation layer thickness for island formation is predicted to be 0.6 nm. Published by Elsevier B.V.