The Effect of Power Density on Diffusion Length and Energy Gap of a-Si:H and nc-Si:H Thin Films Prepared by PECVD Technique

The increase in power density of 0.3, 0.5, 0.6, and 0.7 W $cm^{-2}$ for hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H) thin film samples prepared by plasma enhanced chemical vapor deposition technique causes an increase in crystalline volume fraction when the silane concentration is fixed. This increase in crystalline volume fraction is correlated to the absorption coefficient and refractive index which are determined from ellipsometric measurements. The crystallinity of samples is studied by both Raman and X-ray diffraction techniques. A mild change in the optical energy gap around an average value of 1.8 eV is noticed due to the observed change in the degree of crystallinity of the samples when power density increases. Moreover, the ambipolar diffusion length measured by the steady-state photocarrier grating technique is found to change with the increase in power density. The values of some obtained optical parameters are compared to a standard crystalline sample.