Dielectric function of a-Si:H based on local network structures

For hydrogenated amorphous silicon (a-Si:H) layers prepared by plasma-enhanced chemical vapor deposition, we have found clear relationships between the dielectric function in the ultraviolet/visible region and SiH(n) (n = 1-2) local bonding states by applying real-time spectroscopic ellipsometry and infrared attenuated total reflection spectroscopy. In particular, the amplitude of the epsilon(2) spectra obtained from various a-Si:H layers is expressed completely by the SiH(2) bond density in the a-Si:H and reduces strongly with increasing the SiH(2) content, indicating that microvoids present in the a-Si:H network are surrounded by the SiH(2) bonding state. On the other hand, no significant void formation occurs by the generation of the SiH local bonding due to rather dense surrounding structures. Depending on the SiH n bonding states, the whole a-Si:H dielectric function shifts toward higher energies, as the SiH(n) hydrogen contents in the a-Si:H increase. Based on these findings, we have established a new a-Si:H dielectric function model that incorporates the void structure terminated with SiH(2) and epsilon(2) spectral shift induced by the SiH n local structures. This model is appropriate for a wide variety of a-Si:H layers deposited at different substrate temperatures and plasma conditions and, conversely, allows the characterization of the SiH n contents from the a-Si:H dielectric functions in the ultraviolet/visible region. From results obtained in this study, the local network structures and electronic states of a-Si:H are discussed.