Electronic structure of monolayer and double-layer Ge on Si(001)

Electronic structures of the epitaxial Ge layers during the very initial growth on the $\mathrm{Si}\mathrm{}\left(001\right)2\times 1$ surface have been studied by angle-resolved photoemission using synchrotron radiation. The surface band structures are investigated in detail for a clean single-domain $\mathrm{Si}\mathrm{}\left(001\right)2\times 1$ surface, a single-domain $\mathrm{Si}\mathrm{}\left(001\right)1\times 2-\mathrm{Ge}$ surface at one monolayer of Ge, and a double-domain $\mathrm{Si}\mathrm{}\left(001\right)2\times 1-\mathrm{Ge}$ surface at two monolayers. The two different surface-state bands due to the up-dimer atom states of the buckled dimers on the surface are identified for all three above surfaces with very similar dispersions. In addition, a few surface-resonance bands due to the back bonds of the surface dimers are also identified and their detailed dispersions are determined. These results are compared with the previous experimental and theoretical reports for the band structures of the Si(001), Ge(001), and Ge/Si(001) surfaces in relation to the structure and stoichiometry of the initial Ge layers on $\mathrm{Si}\mathrm{}\left(001\right)2\times 1.$ The surface band structure is shown to be insensitive to the intermixing of Ge and Si atoms on the surface within the resolution of the present measurement.