Interference-Free Determination of the Optical Absorption Coefficient and the Optical Gap of Amorphous Silicon Thin Films

A new method to determine the optical absorption coefficient (α) of thin films is presented. α of hydrogenated amorphous silicon (a-Si:H) based alloys can be accurately determined from transmittance ( T ) and reflectance ( R ) by using T /(1- R ), which almost completely eliminates disturbance from the optical interference effect. The method is applicable to any thin films, as long as the film is a single layer. Based on the interference-free α, various methods to determine the optical gap ( E OPT ) of a-Si:H, a-SiC:H, and a-SiGe:H films are discussed. The ( n α h ν) 1/3 plot and the (α h ν) 1/3 plot are most suitable for characterizing these films. The well-known (α h ν) 1/2 plot is less suited for detailed discussion of the E OPT than the cube root plot, because the plot includes a large ambiguity in the E OPT . The effect of the optical interference effect on the determination of the E OPT is also discussed.