Infrared-ellipsometry evidence of disorder-induced vibrational frequency shifts in hydrogenated-amorphous-silicon thin films

In situ infrared ellipsometry is used systematically to study the thickness-dependent vibrational frequency shift of the Si-H stretching mode in plasma-deposited hydrogenated-amorphous-silicon thin films. The shift effect is shown to be dependent on the deposition conditions. After a detailed discussion of the possible physical origins of the frequency shift it is established that the latter is a result of the increased structural disorder at the film-substrate interface. A physical picture in terms of film-growth mechanisms is proposed and the role of hydrogen discussed. An analytical model based on the polarized medium theory enabled us to obtain the effective dynamical charges of the Si-H bond vibration in different bonding configurations. The values of the effective charges thus obtained are (0.41 +/- 0.01)e and (0.24 +/- 0.03)e (where e denotes the elemental charge) for the SiH and SiH2 group vibrations, respectively. Highly sensitive infrared techniques are shown to be capable of probing the local structural order of the material, as in the case of Raman scattering spectroscopy. The results and conclusions presented in the following are expected to be helpful in our batter understanding of the intimate Link between vibrational and structural properties of alpha-Si:H.