Based on atomic force microscopy, high-angle annular dark-field scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy, Ge out-diffusion effects on SiGe quantum dots to form nano-rings were studied using the ultrahigh vacuum chemical vapour deposition. The epitaxial Si layer grown on quantum dots with SiH4 and H2 precursors could not cover the whole quantum dots, and the central area was passivated by the hydrogen. After annealing longer than 1h at 500C in vacuum, the Ge atoms at the central area of quantum dots diffused outwards to form nano-rings. However, the epitaxial Si layer grown with SiH4 and He precursors could cap all over the quantum dots, and the Si cap layer retards the Ge out-diffusion at the central area of quantum dots, resulting in 4h annealing time. Moreover, for the uncapped quantum dots, the annealing in H2 also prevents the Ge out-diffusion at the central area and no nano-rings were observed up to 5h annealing at 500C.

Ge Out Diffusion Effect on SiGe Nanoring Formation. W.H.Tu, S.H.Huang, C.W.Liu: Journal of Applied Physics, 2012, 111[7], 076103