The Effect of Helium Dilution on Optical and Photoelectric Properties of a-Si: H Thin Films Prepared by Plasma Enhanced Chemical Vapor Deposition Technique

The effect of Helium (He) dilution on optical and photoelectric properties of hydrogenated amorphous silicon (a-Si:H) thin film samples prepared at substrate temperature of 75°C with increased growth rate by plasma-enhanced chemical vapor deposition (PECVD) technique is studied using spectrophotometric (SP) and steady-state photocarrier grating techniques (SSPG). The values of refractive index, absorption coefficient, thickness and optical energy gap, which are found from the analysis of transmission spectra using Swanepoel method, are compared with those obtained by employing variable-angle spectroscopic ellipsometry technique. The samples exhibit a tendency towards an increase in energy gap and a decrease in refractive index with the increase in He dilution. The obtained optical absorption coefficients of a-Si:H samples are compared with those of hydrogenated microcrystalline silicon (μc-Si:H) and crystalline silicon (c-Si) samples. It is also found that the change in He dilution has an influence on the ambipolar diffusion length, minority and majority mobility-lifetime products.