GE EPITAXIAL OVERLAYERS ON SI(001) STUDIED BY SURFACE-SENSITIVE X-RAY-ABSORPTION FINE-STRUCTURE - EVIDENCE FOR STRAIN-INDUCED SURFACE REARRANGEMENT

The structure of Ge epitaxial overlayers on well-oriented Si(001) (Ge-n/Si(001), n < 7) has been studied by the surface-sensitive X-ray absorption fine structure (XAFS) technique in situ after layer-by-layer growth by molecular beam epitaxy (MBE). It is found that surface rearrangements unique to the number of Ge layers take place. For 1 monolayer (ML) Ge on Si(001), elongated Ge dimers with a local structure characteristic of s(2)p(3) configuration are observed, suggesting that surface strain induces a substrate-to-adatom charge transfer. For 2 ML Ge, similar to 1/2 of Ge atoms in the second layer are replaced with Si atoms in the third layer, relieving elastic strain in the second layer caused by a large atomic size mismatch (similar to 10%) between the adatom and substrate atom. A. possible model structure with the Ge0.5Si0.5 double layer interface is proposed. The results suggest that surface strain induces a site-specific atomic migration channel between the adjacent layers, serving as a driving force of interface mixing associated with the growth of Ge on Si or Si on Ge.