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

An ordered alkaline-earth submonolayer on a clean 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 Ge(001) could play a similar role as on structurally identical Si(001), overcoming known limitations of the 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 Ge(001) surface as a function of temperature (270−770K) for coverage 1.0 monolayer (ML) and 0.15−0.4ML, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered 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 Ge(001) with increasing coverage