Dense H2 and proton NMR in a-Si:H.

Abstract

A model is proposed which resolves an apparent paradox in the results and interpretation of various experiments detecting H and ${\mathrm{H}}_{2}$ in a-Si:H. A number of measurements indicate that the ${\mathrm{H}}_{2}$ in a-Si microvoids is at densities of the order of the density of solid ${\mathrm{H}}_{2}$. However, at these densities, the electric quadrupole-quadrupole (EQQ) interactions between molecules in bulk fluid or solid ${\mathrm{H}}_{2}$ dominate the nuclear spin relaxation ${T}_{1}$(${\mathrm{H}}_{2}$) and its temperature dependence in such a way that it cannot be used to explain the proton NMR ${T}_{1}$(H) relaxation results for Si-bonded H. In order to fit the a-Si:H relaxation data it is necessary that the effective EQQ interaction varies widely among ${\mathrm{H}}_{2}$ in a-Si:H. The ${T}_{1}$(H) relaxation in a-Si:H arises from interaction with that part of the adsorbed ${\mathrm{H}}_{2}$ fraction which has a small effective EQQ.