On the hydrogen in hydrogen-containing amorphous silicon

The present contribution reports on refined and constrained analyses of the complex proton-NMR free-induction-decay (FID) signal of hydrogen-containing amorphous silicon (a-Si:H), produced by thermal decomposition of pressurized monosilane. The spectra comprise clustered and diluted protons and the mole fractions of these are described by an algebraic sum of two exponential terms of the type I · exp(−kP) where I and k are constants and P the initial monosilane pressure in the decomposition reactor. The NMR spin-spin relaxation time of the clustered protons is found to be approximately constant (T2G ≈ 15 μs) and independent of P. For the diluted protons, the spin-spin relaxation time are found to increase monotonically with increasing P (T2L ranging from ∼85 to ∼140 μs) and could be approximated by a second-order polynomial in P. The number of diluted protons and their average nearest-neighbor separation are found to increase with increasing P from ∼7 Å at 0.28 MPa to ∼8.5 Å at 6.61 MPa. These findings show that the amorphous raw-silicon exhibits an inhomogeneous distribution of diluted protons. The distribution becomes more homogeneous with increasing P. The simplest description of the clustered configurations is that the number of protons and their average nearest-neighbor distance remain constant and independent of P, and that only the number of the clusters increases with increasing P. The amount of diluted and clustered protons, complies with the total amount of hydrogen found by chemical analysis and by FT-IR spectroscopy.