The 3D islands of the Stranski-Krastanow system Ge/Si(001) that form either
during the annealing of previously hat and nearly strain-relieved Ge films
at 1020 K or directly at the Ge deposition at 1020 K are found to be compo
sed of a mixture of Ge and Si, thus pointing to considerable interdiffusion
at 1020 K. Direct measurement of the elastic energy unambiguously reveals
that neither the 3D islanding nor the Si in-diffusion are driven by the red
uction of misfit strain; this strain being the result of increasing configu
rational entropy.