Thickness dependence of optical properties of hydrogenated amorphous silicon films

Abstract

A detailed study of the thickness dependence of optical constants of glow discharge prepared hydrogenated amorphous silicon thin films (a-Si:H) is reported in this paper. The thicknesses of the films range from 0.32 to 1.77 μm. The optical constants, refractive index n( λ), absorption coefficient α( λ), extinction coefficient ( k( λ) and consequently the real and imaginary parts of the dielectric constant and the optical bandgap, are determined from the transmission spectrum alone, obtained over the wavelength region of 550 nm to 900 nm by means of a spectrophotometer. The absorption coefficient α, refractive index n and the extinction coefficient k are found to increase for thinner films (0.32 to 0.9 μm) and showed apparently little change for thicker films (above 0.9 μm). The bandgap E g, calculated from both (ahν) 1 2 versus hv and (ε ″) 1 2 versus hv, is found to increase with increasing film thickness. Activated electrical conduction also showed an increase in the fermi level for increasing film thickness. These results are discussed in terms of the quality of the films.