Ge Epitaxial Overlayers on Si(001) Studied by Surface-Sensitive X-Ray Absorption Fine Structure: Evidence for Strain-Induced Surface Rearrangement

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Abstract
The structure of Ge epitaxial overlayers on well-oriented Si(001) ( Ge n /Si(001), n 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, ∼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 ( ∼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.