Strain-Induced Atomic Rearrangements in Ge Overlayers on Si(001)

Novel features of strain-induced atomic rearrangements in germanium overlayers on Si(001).(2xl) probed by surface-sensitive XAFS are reported. It is found that the Ge adatoms on Si(001) form elongated dimers with the average adatom-adatom distance La = 2.51 ± 0.01 A, in sharp contrast to total energy calculations for an asymmetric dimer configuration. For two ML's, Ge atoms in the second layer are replaced with Si atoms in the third layer. Upon silicon overgrowth on Ge layers, the Ge atoms in the second layer are replaced with the first layer Si atoms and form Ge dimers on top. These rearrangements of surface atoms are interpreted as the strain-induced site-selective atomic migrations.