Thermodynamics of coherently-strained GexSi1-x nanocrystals on Si(001): alloy composition and island formation.

We determined the enthalpic and entropic contributions to the thermodynamics of coherently strained nanocrystals grown via deposition of pure Ge on Si(001) surfaces at 600 and 700 degrees C by analyzing their composition profile and local strain. We found that the free energy associated with the entropy of mixing, which drives GexSi1-x alloy formation, was significantly larger than the relaxation enthalpy that produces the islands. Thus, entropy plays a significant role in the evolution of the size and shape of the islands during growth through the strong thermodynamic drive to form an alloy.