Molecular hydrogen in hydrogenated amorphous silicon: NMR evidence

We measured the concentrations of molecular hydrogen in hydrogenated amorphous silicon $(a-\mathrm{S}\mathrm{i}:\mathrm{H})$ 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 molecular hydrogen concentrations are about one order of magnitude larger than those inferred from spin-lattice-relaxation time ${(T}_1)$ measurements. There are two distinct environments for hydrogen molecules. In one the molecules are essentially isolated and act as relaxation centers for hydrogen bonded to silicon atoms. In the other, the hydrogen molecules are clustered and do not relax the bonded hydrogen. Samples prepared by HWCVD have a lower transition temperature for freezing of the molecular hydrogen motion than those prepared by PECVD. This behavior is attributed to a more ordered silicon structure in HWCVD samples. The nonexponential behavior of the dipolar relaxation and possible relation with the defects and doping is discussed.