ORDERED STRUCTURE AT SI GE INTERFACES

We have determined a 2 x 1 ordered Si/Ge interfacial structure on an a tomic scale, using grazing incidence x-ray diffraction and high-resolu tion transmission electron microscopy, and show that the structure dif fers from previously proposed models that use an atom pump mechanism. The observed structure indicates that atomic replacement during format ion of the ordered structure is mainly caused by Ge surface segregatio n, while atomic-scale strain due to the 2 x 1 surface reconstruction d etermines the atomic configuration of the ordered structure.