Ge overlayers on Si(001) studied by surface-extended x-ray-absorption fine structure

The local structure of the Ge overlayers on Si(001) has been studied in situ by surface-extended x-ray absorption fine structure on the Ge K-edge, using a grazing-incidence fluorescence excitation and synchrotron radiation from a multipole wiggler. The results for 1 ML Ge on Si(001) indicated that the Ge adatoms form elongated dimers with the average Ge-Ge distance of 2.51±0.04 Å, in sharp contrast to the shortened adatom-adatom bond length for clean (2×1) Si(001). The observed adatom-substrate distance 2.40±0.08 Å indicates a contraction of the substrate Si atom by the same amount (-2.4%). The results suggest that adatoms take the ${\mathit{p}}^3$-like configuration due to a substrate-to-adatom charge transfer.