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 (alpha) of thin films is presented. alpha 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 alpha, 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-alpha-h-nu)1/3 plot and the (alpha-h-nu)1/3 plot are most suitable for characterizing these films. The well-known (alpha-h-nu)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.