Formation and decay of metastable Ge clusters on Ge(001)

The initial stages of room temperature homoepitaxial growth of Ge(001) have been studied with scanning tunneling microscopy. The smallest entities observed after deposition are dimers. Several different adsorption sites for the dimers have been identified. We have observed three different diffusion pathways for these adsorbed dimers at room temperature: two along and one across the substrate dimer rows. As well as the isolated dimers, metastable dimer clusters and epitaxial islands are observed. The metastable dimer clusters always consist of chains of trough dimers oriented along a 〈130〉 direction and have a profound influence on the buckling registry of the Ge(001) substrate. At room temperature the metastable dimer clusters show limited mobility. By switching a terminating (tail) or middle (body) dimer between equivalent positions the cluster can change its shape. The metastable dimer clusters eventually convert to epitaxial islands. Based on the experimental observations, a pathway for this conversion process is proposed.