Resonant Raman scattering in hydrogenated amorphous silicon films

Micro-Raman measurements were carried out to investigate the microstructure of a-Si:H samples prepared by plasma enhanced chemical vapor deposition (PECVD) using 647.1 nm and 514.5 nm laser lines as excitation sources. It is found that the frequency of TO mode downshifts with increasing excited photon energy without significantly changing its width, while LO mode expands to a large extent. The above results suggest that the variation of LO and TO mode is caused by resonant Raman effect. With the increasing of excitation energy, smaller a-Si clusters are excited thus results in large redshift of TO band according to the quantum confinement effect, another possible explanation may be related with the existing of a highly disordered layer near the free surface in a-Si:H film. In conjunction with observation of LO band variation, we further deduce that the highly disordered layer may have lower H content.