Effect of the interface on the local structure of Ge–Si nanostructures

We first discuss the limitations of Raman scattering as applied to Ge/Si nanostructures. We further summarize our recent efforts to investigate the local structure of various Ge nanostructures, namely, Ge quantum dots grown by molecular beam epitaxy (MBE) on bare Si(100), on Si(111) with a 0.3 nm SiO2 coverage, and nanocrystals embedded in SiO2, by x-ray absorption fine structure spectroscopy. In particular, the MBE growth of Ge dots on bare Si(100) has been studied as a function of the growth conditions: in most cases strong alloying with Si takes place. Ge nanoislands on Si(111) with SiO2 coverage, on the other hand, may retain the local structure of bulk Ge and be very stable against oxidation. The Ge nanocrystals embedded in SiO2 possess the structure of relaxed bulk Ge without any Ge–Si bonding. The latter two kinds of Ge nanostructures possess visible photoluminescence.