Thin, coherently strained, films of SiGe were deposited on Si(001) in the S
transki-Krastanow (SK) growth mode to form small, faceted, dislocation-free
-three-dimensional (3D) islands. The number density of these islands was de
termined as functions of SiGe alloy composition, growth rate, and substrate
temperature during growth. From these experiments, the classical model of
3D island nucleation and growth yields an approximate activation energy for
diffusion of Ge dimers on a Ge covered Si(001) surface of 0.70 eV. The dep
endence of the 3D-island number density on growth rate cannot be understood
without modifying the classical model to account for the wetting layer pre
sent in SK systems. Heteroepitaxial strain is not included in the classical
model of island nucleation and growth. A simple linear elastic model that
fits the data is developed that predicts the island number density is propo
rtional to the inverse square of the Ge mole fraction in the alloy plus a c
onstant.