Nucleation and growth of self-assembled Ge/Si(001) quantum dots

In situ reflection high-energy electron diffraction along with atomic-force microscopy and photoluminescence spectroscopy have been used to investigate the Ge/Si(001) growth process in an ultrahigh-vacuum chemical-vapor-deposition system at temperatures varying from 550 to 700 °C. The existence of an intermediate phase between entirely pseudomorphic two-dimensional (2D) layers and 3D macroscopic islands is established. This phase which consists of pyramidal clusters with a squared base and {105} facets is found to be metastable with regard to the formation of 3D macroscopic islands. Two kinetic pathways for the growth of 3D macroscopic islands are identified versus growth temperature. At 550 °C the growth proceeds near equilibrium configuration and islands of monosize distribution can be formed. At 700 °C, coalescence is found to take place even at the early stage of growth, which results in the formation of highly inhomogeneous islands.