Confinement effect of optical phonons in Si-Ge alloy nanocrystals

Si1-xGex nanocrystals (nc-Si1-xGex, x=(43+/-1)%) with sizes of 7∼16 nm were prepared using thermal crystallization of amorphous Si1-xGex alloy. Nanocrystal shape, composition, size, and phonon property were obtained in terms of high-resolution transmission electron microscope observations and X-ray diffraction and Raman spectral analyses. It was revealed that each optical vibration mode (Ge-Ge, Ge-Si, or Si-Si mode) is of evident phonon confinement effect. Using dispersion relationship in whole Brillouin zone, we numerically simulate the Raman spectra (peak position and linewidth) of nc-Ge and nc-Si and some evident characters in phonon confinement effect are presented for comparison with those of nc-Si1-xGex