Raman characterization of germanium nanocrystals in amorphous silicon oxide films synthesized by rapid thermal annealing

Raman characterization of germanium (Ge) nanocrystals embedded in amorphous silicon oxide (a-SiO2) films synthesized by rapid thermal annealing (RTA) has been carried out. The samples were prepared by cosputtering Ge and SiO2 targets using a rf magnetron sputtering machine. Ge nanocrystals can only be obtained from samples sputtered with six pieces of Ge attached to the SiO2 target. For samples annealed at different RTA temperatures, the Raman spectra indicated a transition from amorphous to nanocrystalline Ge when annealed between 600 and 750 degrees C. The spectra were analyzed in terms of phonon confinement model and the estimated nanocrystal size was between 20 and 66 Angstrom. A minimum annealing time of 160 s at 750 degrees C was necessary for Ge nanocrystal formation. Strong visible broadband photoluminescence was observed from the nanocrystals and the photoluminescence showed a blueshift with decrease in the nanocrystal size. The effect of compressive stress on nanocrystal growth was examined by varying the rampup and rampdown rates of the RTA process. The compressive stress was shown to affect the growth of the nanocrystals. (C) 1999 American Institute of Physics. [S0021-8979(99)03115-1].