GE OVERLAYERS ON SI(001) STUDIED BY SURFACE-EXTENDED X-RAY-ABSORPTIONFINE-STRUCTURE

The local structure of the Ge overlayers on Si(001) has been studied i n situ by surface-extended x-ray absorption fine structure on the Ge K -edge, using a grazing-incidence fluorescence excitation and synchrotr on radiation from a multipole wiggler. The results for 1 ML Ge on Si(0 01) indicated that the Ge adatoms form elongated dimers with the avera ge Ge-Ge distance of 2.51+/-0.04 Angstrom, in sharp contrast to the sh ortened adatom-adatom bond length for clean (2 x 1) Si(001). The obser ved adatom-substrate distance 2.40+/-0.08 Angstrom indicates a contrac tion of the substrate Si atom by the same amount (-2.4%). The results suggest that adatoms take the p(3)-like configuration due to a substra te-to-adatom charge transfer.