ELONGATED DIMER STRUCTURE FOR GE OVERLAYERS ON SI(001) - SYMMETRICAL OR ASYMMETRIC

The structure of Ge dimers on well-oriented Si(001) has been studied b y surface-sensitive X-ray absorption fine structure (XAFS) technique i n situ after the growth by molecular beam epitaxy (MBE). The bond leng ths values for adatom-adatom and adatom-substrate pairs were determine d from the Ge K-extended X-ray absorption fine structure (EXAFS). For 1 monolayer (ML) Ge on Si(001), the elongated Ge dimer structure with the Ge-Ge bond length of 2.51 +/- 0.01 Angstrom was observed, The obse rved local structure around Ge atoms indicates that the adatoms take t he p(3)-like configuration commonly observed for symmetric dimers. The results suggest that a substrate-to-adatom charge transfer is induced by surface strain.