Optical characterization of hydrogenated amorphous silicon thin films deposited at high rate

The aim of this paper is to provide a better understanding of hydrogenated amorphous silicon thin films (a-Si:H) in relation to their optical properti es: refractive index, optical gap, absorption coefficient, thickness and su rface roughness. The transmission spectrum of the Rims, deposited with vari ous rf discharge power densities by an optimized plasma enhanced chemical v apor deposition (PECVD) method, at a high rate (> 10 Angstrom/sec), was mea sured over a range in wavelength from 500 to 1100 nm. An approximate model is utilized to describe the surface roughness. Ln this model, the surface r oughness is modeled as a mixed layer of 50 percent of a-Si:H and 50 percent of air and the optical constant of the rough layer is derived using the Br uggemann effective medium approximation (EMA). The gradient iteration metho d of numerical analysis is used to solve the nonlinear equations in the stu dy. Our results show that the potential underestimation of refractive index and resulting overestimation of film thickness can be overcome by consider ing the reflection of the rough surface. The method is carried out on the t ransmission data and the influence of rf discharge power density on the pro perties of the film is discussed in detail.