Strain relaxation of Ge films grown on a Si(001)-2×1 surface by molecular beam epitaxy

The crystalline quality of Ge films grown on a Si(100)-2×1 substrate by molecular beam epitaxy has been studied by means of in situ reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering spectroscopy. The Ge layers of thickness up to 4000 Å were deposited at 400 °C with and without thin (200 Å) Ge buffer layers predeposited at 200 °C. It was found that the buffer layers were effective in improving the quality of the thin Ge overlayers, while they had no noticeable effect on the thick layers whose x-ray and Raman spectra were comparable to that of bulk Ge. It was also found that, even after the Ge films became relaxed with a generation of dislocations, they were not free from strain due to the lattice mismatch. This finding is supported by the quantitative estimate of the Raman frequency shift in terms of the uniaxial strain model.