Optical evidence of amorphous-network change in the initial-growth stage a-Si:H

We investigated the optical characteristics of thin (<22 nm) hydrogenated amorphous silicon (a-Si:H) films by spectroscopic ellipsometry (SE). For comparison, we prepared H2 diluted as well as undiluted a-Si:H samples on c-Si substrates. As the thickness decreases, the peak positions of dielectric functions (er,ei) shifted to the higher-energy sides both in the H2 diluted and the undiluted a-Si:H films. In addition, a noticeable difference of growth behavior between the H2 diluted and the undiluted a-Si:H films was observed. To preclude effects caused by the incorporated hydrogens, the samples were annealed for sufficient dehydrogenation, which was verified by Fourier transform infrared spectroscopy (FTIR). Even after this dehydrogenation, there still remained a difference between the H2 diluted and the undiluted samples, also, the evolution of the (er,ei) peak positions remained in the same relation as before the annealing. A simple chemical-alloy effect between silicon and hydrogen cannot explain these phenomena which may be a result of changes in the amorphous network. It is presumed that the structure of the amorphous network in the initial-growth stage is different from that of the amorphous network and the structure difference is enhanced by H2 addition.