Low-frequency Raman scattering of Ge and Si nanocrystals in silica matrix

Ge and Si nanocrystals (nc- Ge and nc- Si) with average sizes in the range of 2- 7 nm, embedded in silica matrix, were fabricated for investigating their acoustic-phonon vibrational properties. The freely elastic sphere theory was found to be unsuitable for explaining the low-frequency phonon vibration character of both nc- Ge and nc- Si in our current experiments. We have assigned the observed low-frequency Raman peaks to "LA-like mode" and "TA-like mode" in terms of their polarization and depolarization behaviors. In addition, it is revealed that the lattice contraction phenomenon exists in nc- Ge and nc- Si with sizes smaller than 4 nm, which leads to a contrary effect against matrix traction on the phonon vibrational frequencies. © 2005 American Institute of Physics.