Raman scattering and x-ray absorption studies of Ge-Si nanocrystallization

We have studied the local structure of GeSi nanocrystals embedded in SiO₂ prepared by co-sputtering of Ge, Si, and SiO₂ targets onto a Si(100) substrate. From Raman scattering, we conclude that the formation of the isotropic crystalline Ge phase starts at about 800°C followed by the formation of a GeSi phase at higher temperatures. The formed nanocrystals, whose size depends on the annealing temperature, are randomly oriented. The local structure of the nanocrystals has been studied by x-ray absorption fine structure spectroscopy. They are found to consist of a relaxed Ge core with a typical diameter of ∼4 nm and the Ge-Ge bond length of 2.45 Å and of a GeSi outer shell, the Ge-Si bond length being 2.39 Å. The average composition of the grown nanocrystals is estimated to be Ge _0.75Si_0.25.