Formation and evolution of a self-organized hierarchy of Ge nanostructures onSi(111)−(7×7): STM observations and first-principles calculations

Abstract

We report scanning-tunneling-microscopy observations and first-principles calculations for the formation and evolution of self-organized Ge nanostructures on $\mathrm{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces for Ge coverages up to 0.5 ML. We show that individual Ge atoms initially form a triangular lattice. At higher coverages, Ge nanoparticles $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter gradually form in both the faulted and unfaulted half unit cells with an initial preference in the faulted halves, ultimately driving ordered hexagonal arrays. The underlying $7\ifmmode\times\else\texttimes\fi{}7$ surface periodicity, the triangular single-Ge lattice, and the nanoparticle hexagonal superstructures coexist. Charge transfer from Si adatoms to Ge nanoparticles is shown to play a key role in the self-organization.