Molecular hydrogen in amorphous silicon revisited

We measured the concentration of molecular hydrogen (Hz) in hydrogenated amorphous silicon (a-Si:H) by nuclear magnetic resonance (NMR) using a Jeener-Broekaert three-pulse sequence. For samples prepared by both plasma enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HWCVD), the concentrations measured by this method are one order of magnitude larger than that inferred from the spin-lattice relaxation time (T-1) of hydrogen bonded to silicon. For the sample prepared by PECVD, we found two different environments for molecular hydrogen, where the relaxation mechanisms differ. Molecular hydrogen in the samples prepared by HWCVD is motionally narrowed even at 8 K, while in the sample prepared by PECVD the molecular hydrogen is almost completely 'frozen in' at 8 K. This difference is attributed to the effect of the host silicon structure. (C) 2000 Elsevier Science B.V. All rights reserved.