Ortho-molecular hydrogen in hydrogenated amorphous silicon

Using a Jeener-Broekaert three-pulse sequence to measure directly the conce ntration of o-H-2 by H-1 nuclear magnetic resonance (NMR), we find that thi s concentration is one order of magnitude larger than that previously infer red from spin-lattice relaxation time (T-1) measurements. At 300 K, this co ncentration of o-H-2 contributes at most 30% to the narrow H-1 NMR line att ributed to hydrogen bonded to silicon. For a plasma-enhanced-chemical-vapor -deposition (PECVD) sample, two distinct values of T-1 for o-H-2 are found, only one of which contributes to the T-1 for bonded hydrogen. In hot-wire- chemical-vapor-deposition samples, the line shape of o-H-2 exhibits motiona l narrowing at lower temperatures, suggesting a more ordered structure than in a typical PECVD sample. (C) 2000 American Institute of Physics. [S0003- 6951(00)02105-7].