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

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Abstract
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 (nαhν)1/3 plot and the (αhν)1/3 plot are most suitable for characterizing these films. The well-known (αhν)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.