Equilibrium shape diagram for strained Ge nanocrystals on Si(001)

We introduce a chemical-thermodynamic model to explain the formation and an nealing behavior of Ge nanocrystalline islands grown on Si(001). Assuming t he nanocrystals are essentially large adsorbed molecules, we propose a simp le free energy expression for islands of different shapes interacting with each other via the substrate on which they reside. Nanocrystal growth, disa ppearance, and shape transitions are all consonant with a near-equilibrium system constrained by mass conservation and characterized by interisland re pulsions. We construct an equilibrium shape diagram from experimentally det ermined free energy differences between island shapes and use it to resolve several anomalies that have been noted for the Ge on Si(001) system.