Nonuniform alloying in Ge(Si)/Si(001) quantum dots

The composition profile of pyramid shaped Ge(Si)/Si(001) quantum dots has been modeled using a combination of atomistic total energy calculations and a Metropolis Monte Carlo process. The analysis of the non-uniform composition profile has revealed the important, separate roles of the strain energy, the surface energy, and the mixing energy as driving forces of the alloying and segregation process. The surface energy was found to drive a redistribution of Ge into the surface layer of the quantum dot, which was followed by two Si-rich layers. In the vertical direction Si is found to redistribute to the bottom resulting in a Ge-rich apex of the quantum dot. This result is compared to a phenomenological description of the composition profile by Tersoff [Phys. Rev. Lett. 81, 3183 (1998)]. The possibility of a misinterpretation of experimental measurements of the composition profile as a result of the Ge-rich surface layer is discussed.