Island growth from a surface instability in the GeSi system

We have used in situ electron microscopy to observe the process of elastic relaxation during lattice mismatched epitaxial growth of Ge_xSi_1-x alloys on Si(001). In the range of composition approximately 0.2<x<0.6, relaxation occurs by the formation of a surface instability, as predicted theoretically. On further growth, the amplitude of the ripple increases until the {105} facet angle is reached and the surface becomes covered in a densely packed array of pyramidal islands. These pyramids later coarsen and transform into domes. We compare this growth mode with the more familiar nucleation and coarsening process which has been seen at higher strain.