Electronic structure of Co islands grown on the √3 × √3-Ag/Ge(111) surface

By means of room temperature scanning tunneling spectroscopy (RT STS), we have studied the electronic structure of two different Ag/Ge(111) phases as well as Co islands grown on the √3 × √3-Ag/Ge (111) forming either √13 × √13 or 2 × 2 patterns. The spectrum obtained from 4 × 4-Ag/Ge(111) structure shows the existence of a shoulder at 0.7 V which is also present in the electronic structure of the Ge(111)-c2 × 8 and indicates donation of Ge electrons to electronic states of the Ag-driven phase. However, this fact is not supported by the electronic spectrum taken from the √3 × √3-Ag/Ge (111). The complexity of the Co-√13 × √13 islands bonding with the substrate is mirrored by a large number of peaks in their electronic spectra. The spectra obtained from the Co-2 × 2 islands which had grown on the step differ from those taken from Co-2 × 2 islands located along the edge of the terrace by a number of peaks at negative sample bias. This discrepancy is elucidated in terms of dissimilarities of Co-substrate interaction accompanying Co islands growth on different areas of the stepped surface.