STRAIN-INDUCED ATOMIC REARRANGEMENTS IN GE OVERLAYERS ON SI(001)

Novel features of strain-induced atomic rearrangements in germanium ov erlayers on Si(001)-(2x1) probed by surface-sensitive XAFS are reporte d. It is found that the Ge adatoms on Si(001) form ''elongated'' dimer s with the average adatom-adatom distance L-a = 2.51 +/- 0.01 Angstrom , in sharp contrast to total energy calculations for an asymmetric dim er configuration. For two ML's, Ge atoms in the second layer are repla ced with Si atoms in the third layer. Upon silicon overgrowth on Ge la yers, the Ge atoms in the second layer are replaced with the first lay er Si atoms and form Ge dimers on top. These rearrangements of surface atoms are interpreted as the strain-induced site-selective atomic mig rations.