Dielectric function ofa-Si:H based on local network structures

Abstract

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 $\ensuremath{\varepsilon}$${}_{2}$ spectra obtained from various $a$-Si:H layers is expressed completely by the SiH${}_{2}$ bond density in the $a$-Si:H and reduces strongly with increasing the SiH${}_{2}$ content, indicating that microvoids present in the $a$-Si:H network are surrounded by the SiH${}_{2}$ bonding state. On the other hand, no significant void formation occurs by the generation of the SiH local bonding due to rather dense surrounding structures. Depending on the SiH${}_{n}$ bonding states, the whole $a$-Si:H dielectric function shifts toward higher energies, as the SiH${}_{n}$ hydrogen contents in the $a$-Si:H increase. Based on these findings, we have established a new $a$-Si:H dielectric function model that incorporates the void structure terminated with SiH${}_{2}$ and $\ensuremath{\varepsilon}$${}_{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.