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.