Optical dielectric function and infrared absorption of hydrogenated amorphous silicon nitride films: Experimental results and effective-medium-approximation analysis

Abstract

The optical dielectric function ε (1.5–6.5 eV), ir absorption (400–4000 ${\mathrm{cm}}^{\mathrm{-}1}$), and film density have been measured for a series of hydrogenated amorphous silicon nitride (a-${\mathrm{Si}}_{\mathit{x}}$ ${\mathrm{N}}_{\mathit{y}}$ ${\mathrm{H}}_{\mathit{z}}$) films deposited at 400 °C via rf plasma-enhanced chemical vapor deposition for varying [${\mathrm{NH}}_3$]/[${\mathrm{SiH}}_4$] ratios R. From a detailed analysis of the ir and density results, the concentrations of Si-N, N-H, Si-H, and Si-Si bonds and of Si, N, and H atoms have been obtained for the films studied. An effective-medium-approximation analysis of the measured ε based on the Si-centered tetrahedron model presented in the preceding paper has provided a more sensitive means of determining the concentration of Si-Si bonds in the films and has demonstrated that careful measurements of ${\mathrm{\epsilon }}_1$ and ${\mathrm{\epsilon }}_2$ can serve as a useful probe of the bonding in these alloys. Approximately 9×${10}^{20}$ Si-Si bonds/${\mathrm{cm}}^3$ have been found in N-rich films which are close in composition to silicon diimide, Si(NH${)}_2$, and these Si-Si bonds have been found to have a significant influence on both the optical energy gap ${\mathit{E}}_{\mathrm{opt}}$ and the refractive index n of the films. New results obtained from the ir absorption measurements include the identification of (1) a shoulder near 1030 ${\mathrm{cm}}^{\mathrm{-}1}$ on the main 880-${\mathrm{cm}}^{\mathrm{-}1}$ Si-N(s) band and (2) a weak absorption feature near 640 ${\mathrm{cm}}^{\mathrm{-}1}$ which is not hydrogen related. It has been found that the N-rich diimidelike films prepared here have very low porosities and are thermally stable up to 700 °C, properties which will be important for their future applications.