Local structure of Ge quantum dots self-assembled on Si(100) probed by x-ray absorption fine-structure spectroscopy

The local structure of Ge quantum dots (QD's) self-assembled on Si(100) has been probed by extended x-ray absorption fine-structure and x-ray absorption near-edge structure spectroscopies. We found that in the uncapped QD's, Ge is partially oxidized (∼35%) while the other part (∼40%) alloys with Si leaving only ∼25% as a pure Ge phase. In the Si-capped dots the structure strongly depends on the growth temperature. For QD's grown at a rather high temperature of 745°C, Ge is strongly intermixed with silicon forming a Ge-Si solid solution. The fraction of Ge atoms existing as a pure Ge phase does not exceed 10%. In the QD's grown at a lower temperature (510-550°C), on the other hand, the Ge-rich phase clearly exists. The Ge-Ge bond length in the uncapped dots is close to the bulk Value of Ge, indicating elastic relaxation of the misfit strain. The Ge-Si bond length in the capped QD's grown at 745°C approaches the bulk value of Si, revealing compressive strain in the buried Si/Ge dot structures. In QD's grown at lower temperatures the Ge-Ge bond length equals 2.42 Å indicating a small compressive strain. We also found that the structural disorder is higher in the uncapped samples.