Molecular hydrogen in amorphous silicon revisited

We measured the concentration of molecular hydrogen (Hz) in hydrogenated am orphous silicon (a-Si:H) by nuclear magnetic resonance (NMR) using a Jeener -Broekaert three-pulse sequence. For samples prepared by both plasma enhanc ed chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HWCVD), the concentrations measured by this method are one order of magni tude 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 t wo different environments for molecular hydrogen, where the relaxation mech anisms differ. Molecular hydrogen in the samples prepared by HWCVD is motio nally narrowed even at 8 K, while in the sample prepared by PECVD the molec ular hydrogen is almost completely 'frozen in' at 8 K. This difference is a ttributed to the effect of the host silicon structure. (C) 2000 Elsevier Sc ience B.V. All rights reserved.