The hydrogen local order

While IR experiments focus on the local Si—H bonding, NMR of protons gives more information about the local environment in which the hydrogen atoms reside. NMR arises from transitions between the different spin states of the nucleus, which are split by an applied magnetic field. An isolated proton has a precisely defined resonance frequency, but the interactions between atoms in a solid modify the resonance by a variety of mechanisms. The NMR experiment is fairly complex and no attempt is made here at a detailed treatment. 

Examples of the frequency response of the resonance spectra for various a-Si H films are shown in Fig. 2.18 (Reimer, Vaughan and Knights, 1980). The line is broadened by the short lifetime of proton spin orientation. There are two distinct components to the spectrum, corresponding to fast and slowly relaxing states, with line widths of 

22-27 kHz, and 4-S kHz (compared to the resonance frequency of 56 MHz). The magnetization decay is due to dipolar interactions of the observed proton with neighboring hydrogen atoms. An immediate deduction of the two component spectrum is that there are two distinct 

The interesting changes in the concentration of hydrogen in the broad and narrow lines at different deposition conditions are shown in Fig. 2.18. In all cases the narrow line contains 2-3 at% of hydrogen and the broad line contains the reminder. This is true for hydrogen concentrations from less than 10 at% up to 50 at%. One can also compare the NMR and the IR data on the same material. For a-Si H 

Si—H bonds whereas the NMR find that more than half the hydrogen is in the broad line. 

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