Formation and electronic structure of germanium nanocrystals formed by ion beam synthesis

Germanium nanocrystals embedded in SiO2 have been formed by ion implantation and subsequent thermal annealing. Ge74 ions were implanted utilising ion energy/dose combinations to yield a uniform Ge excess of ∼5 at.% over the implanted layer. Subsequent thermal annealing at 1000 °C in a 95% N2 + 5% H2 ambient for as little as 10 s was sufficient to form nanocrystals approximately 2 nm in diameter. Size distributions corresponding to differing annealing times were determined by TEM, with longer annealing times leading to the formation of larger nanocrystals. The electronic structure of the nanocrystal was studied by XAS at the Ge L3 edge, and shifts of the absorption edge position to higher energies with increasing nanocrystal size were observed. We discuss the latter in relation to quantum confinement effects.