This paper describes a mechanism of the growth mode transition from layer-by-layer to three dimensional island formation due to the large lattice mismatch. The island formation and strain relaxation of GaAs heteroepitaxial growth onto GaP (001) by molecular beam epitaxy is studied by means of an analysis of the reflection high-energy electron diffraction (RHEED) pattern. The anisotropic strain relaxation of formed islands is in agreement with the prediction of the model which considers an elastic strain relaxation within coherent islands. The model is also supported by the oscillation of the lattice parameter of the growing film during the layer-by-layer growth. The change of the RHEED intensity profile reveals the transformation of the film structure from layer to islands and the relaxation of the misfit strain even after the growth has stopped. This finding shows the kinetically limited pseudomorphic structure of the GaAs film and suggests the presence of kinetically controlled critical thickness of the growth mode transition.
