Optical dielectric function of hydrogenated amorphous silicon: Tetrahedron model and experimental results

Abstract

A Si-centered tetrahedron model is developed for the determination of the effect of hydrogen on the visible–near-ultraviolet dielectric function ε of a-Si:H. Infrared-absorption and film-density measurements on the a-Si:H films studied here have been utilized to obtain the concentrations and relative fractions of the four tetrahedra, Si-${\mathrm{Si}}_{4\mathrm{-}\nu }$ ${\mathrm{H}}_{\nu }$ (ν=0–3), considered in the model and present in the films. These results have indicated that random bonding of hydrogen, with no excess of the dihydride Si-${\mathrm{Si}}_2$ ${\mathrm{H}}_2$ tetrahedron, occurs for a-Si:H films deposited at ${\mathit{T}}_{\mathit{s}}$=250 and 450 °C, but that nonrandom bonding with an excess of dihydride occurs for the ${\mathit{T}}_{\mathit{s}}$=110 °C film and for the annealed 250 °C film. The results of the Si-centered tetrahedron model have been used in the effective-medium approximation to obtain predictions for ε=${\epsilon }_1$+i${\epsilon }_2$ that are in good agreement with experimental results presented here for the a-Si:H films studied. This model has thus been shown to provide a very useful framework for predicting ${\epsilon }_1$ and ${\epsilon }_2$ in the visible–near-uv region and for interpreting the nature of the bonding of hydrogen in these a-Si:H films.