Adatom Densities on GaAs: Evidence for Near-Equilibrium Growth

We examine the equilibrium of a compound semiconductor surface under molecular beam epitaxy (MBE) conditions, both theoretically and experimentally. For GaAs, the Ga chemical potential and adatom density depend sensitively on As pressure as well as temperature. Our results suggest that MBE growth may take place under conditions much closer to equilibrium than has been believed. We also show that standard one-component models cannot, even in principle, reproduce both the adatom density and its temperature dependence.