Nmr Study of Ortho-Molecular Hydrogen in Hydrogenated Amorphous Silicon

A Jeener-Broekaert three-pulse sequence is used to investigate ortho-molecular hydrogen (o-H2) in device quality amorphous silicon films prepared by plasma enhanced chemical vapor deposition (PECVD) and hot wire CVD (HWCVD). For the PECVD sample, the concentration of hydrogen molecules is ~ 11% of the total hydrogen concentration, one order of magnitude larger than that inferred from spin-lattice relaxation time measurements (~1%). Hence, most of the hydrogen molecules do not serve as effective relaxation centers. For HWCVD samples with ~3 to 4% hydrogen and very low void densities, the concentrations of hydrogen molecules are ~1% of the total hydrogen concentration. In these samples, spin-lattice relaxation measurements for bonded hydrogen indicate that the concentration of hydrogen molecules that contribute to spin-lattice relaxation is at most 0.1% of the total hydrogen concentration. Spin-lattice relaxation time (T1) measurements of ortho-molecular hydrogen indicate two very different T1's. The longer T1 is ~ 0.6 s, possibly due to an electric quadrupole-quadrupole (EQQ) interaction between o-H2 molecules and, the shorter T1 is ~ 3 ms, very close to that calculated for a two-phonon Raman process for rotating o-H2.