Initial growth of Ba on Ge(001): An STM and DFT study

An ordered alkaline-earth submonolayer on a clean $\mathrm{Si}(001)$ surface provides a template for growth of the atomically sharp, crystalline Si-oxide interface that is ubiquitous in the semiconductor device industry. It has been suggested that submonolayers of Sr or Ba on $\mathrm{Ge}(001)$ could play a similar role as on structurally identical $\mathrm{Si}(001)$, overcoming known limitations of the $\mathrm{Ge}(001)$ substrate such as amorphization of its oxidation layers. In this paper the initial stage of the Ba oxidation process, i.e., adsorption and organization of Ba atoms on the $\mathrm{Ge}(001)$ surface as a function of temperature $(270\ensuremath{-}770\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ for coverage 1.0 monolayer (ML) and $0.15\ensuremath{-}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered $\mathrm{Ge}(001)$ surface. They originate from isolated Ba adatoms, isolated Ba ad-dimers, and the Ba ad-dimers assembled into short-range, randomly distributed chains that run across the Ge dimer rows. We find from both STM measurements and DFT calculations that the latter is the dominant structure on $\mathrm{Ge}(001)$ with increasing coverage.