Germanium diffusion and strain relaxation in Si/Si1-xGex/Si structures

The thermal stability of strained Si_1-xGe_x layers grown by molecular beam epitaxy on Si(100) was measured using Rutherford backscattering spectrometry, secondary ion mass spectroscopy and high resolution X-ray diffractometry (HRXRD). Diffusion experiments were carried out on Si_1-xGe_x layers 50 nm thick (x = 0.07, 0.16 and 0.33) annealed at temperatures between 775 and 1010 °C for different times. The diffusion of germanium was evaluated from the broadening of the RBS and SIMS germanium profiles, while the strain relaxation was deduced from the angular shift of the (400) reflection in HRXRD. The diffusion coefficient thus measured proved to be strongly dependent on the local germanium concentration in the film. In the tails of the profile, the diffusion coefficient was comparable with the value for germanium in bulk silicon while in the centre of the film an enhanced diffusion was found. Both the initial germanium fraction x in the as-grown film and the presence of misfit dislocations hah only minor influence on the diffusion behaviour. It is concluded that safe thermal processing of these structures is possible up to 850 °C for several hours.