Local structures of Si/Gen/Si(001) hetero-structure films studied by grazing incidence fluorescence X-ray absorption fine structure

The local structures around Ge atoms in the Si/Gen/Si(001) hetero-structure films (consisting of 1, 2, 4 and 8 monolayers) prepared by molecular beam epitaxy at 400℃ have been investigated by grazing incidence fluorescence X-ray absorption fine structure (XAFS). The results show that for the Si/Ge1/Si(001) or Si/Ge2/Si(001) hetero-structure film, the Ge atoms are dominantly surrounded by Si atoms as the nearest neighbor. For the Si/Ge4/Si(001) hetero-structure film, the coordination environment around Ge atoms is close to that of Si0.70Ge0.30 alloy. Even for the Si/Ge8/Si(001) hetero-structure film, the fraction of Ge-Ge coordination pair in the first shell is only 55%. This suggests that under the growth temperature of 400℃, the Ge atoms have a strong ability to migrate into the Si capping layer. With the thickness of Ge layer increasing from 1 to 2, 4 and 8ML, the amount of migrated Ge atoms increases from about 0.5 to 1.5, 2.0 and 3.0 nominal ML. We consider that the migration of Ge atoms during the growth of the Si cap is mainly attributed to the surface segregation of Ge atoms, which leads to the decrease in surface energy as well as strain energy in the Ge layer.