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

We investigated the optical characteristics of thin (<22 nm) hydrogenated a morphous silicon (a-Si:H) films by spectroscopic ellipsometry (SE). For com parison, we prepared H-2 diluted as well as undiluted a-Si:H samples on c-S i substrates. As the thickness decreases, the peak positions of dielectric functions (epsilon(r), epsilon(i)) Shifted to the higher-energy sides both in the H-2 diluted and the undiluted a-Si:H films. In addition, a noticeabl e difference of growth behavior between the H-2 diluted and the undiluted a -Si:H films was observed. To preclude effects caused by the incorporated hy drogens, the samples were annealed for sufficient dehydrogenation, which wa s verified by Fourier transform infrared spectroscopy (FTIR). Even after th is dehydrogenation, there still remained a difference between the H-2 dilut ed and the undiluted samples, also, the evolution of the (epsilon(r), epsil on(i)) peak positions remained in the same relation as before the annealing . A simple chemical-alloy effect between silicon and hydrogen cannot explai n 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 struc ture difference is enhanced by H-2 addition. (C) 2000 Elsevier Science B.V. All rights reserved.