Optical Bond Gap and Tauc Gap in a-SiO_x:H and a-SiN_x:H Films

Changes in the joint density of state (JDOS) of a-SiO_x:H and a-SiN_x:H films with x were investigated by optical measurements. The optical bond gap E ₀ which corresponds to the bonding-antibonding splitting energy was deduced from the dispersion of the refractive indices based on a single-oscillator model. The energy of the Si-Si bond, E ₀ ^Si-Si, was estimated from E ₀. The Tauc gap energy E _t and the energy width of the linear tail were determined from optical transmission spectra. The correlation among these characteristic energies was investigated to estimate the JDOS in these alloys. It was found that E ₀ ^Si-Si increases with x, while the bandwidth of JDOS derived from the Si-Si bond is kept constant for both a-SiO_x:H and a-SiN_x:H systems. This suggests that the increase in the Tauc gap energy is due to the increase in Si-Si bond energy rather than the change in the energy width of the Si-Si-bond-derived JDOS.