The influence of H2/(H2 + Ar) ratio on microstructure and optoelectronic properties of microcrystalline silicon films deposited by plasma-enhanced CVD

Hydrogenated microcrystalline silicon films were deposited by glow discharge decomposition of SiH4 diluted in mixed gas of Ar and H2. By investigating the dependence of the film crystallinity on the flow rates of Ar and H2, we showed that the addition of Ar in diluted gas markedly improves the crystallinity due to an enhanced dissociation of SiH4. The infrared-absorption spectrum reveals that the fraction of SiH bonding increases with increasing the rate ratio of H2/(H2 + Ar). The surface roughness of the films increases with increasing the flow rate ratio of H2/(H2 + Ar), which is attributed to the decrease of massive bombardment of Ar ions in the plasma. Refractive index and absorption coefficient of the films were obtained by simulating the optical transmission spectra using a modified envelope method. Electrical measurements of the films show that the dark conductivity increases and the activation energy decreases with the ratio of H2/(H2 + Ar). A reasonable explanation is presented for the dependence of the microstructure and optoelectronic properties on the flow rate ratio of H2/(H2 + Ar).