Observation of folded acoustic phonons in nanocrystalline silicon/ amorphous silicon multilayers

We report in this letter the observation of folded acoustic phonons in hydrogenated nanocrystalline silicon/amorphous silicon multilayers with visible emission, which are prepared in a plasma enhanced chemical vapor deposition system. In the low-frequency range of 10–100 cm, the obtained Raman spectra clearly show some folded doublets from longitudinal acoustic phonons. Using the elastic continuum model, we calculated their frequencies and the obtained results were in agreement with the experimental ones. In addition, some broad folded doublets and additional peaks were clearly observed in the sample with thin nancorystalline sublayers. We attributed them to the mixing of longitudinal and transverse acoustic phonons due to the layered structure. A confined acoustic mode was also proposed to be responsible for the strongly folded longitudinal acoustic phonon peak at 61 cm. © 1996 American Institute of Physics. @S0003-6951~96!00939-4#