The Raman scattering from amorphous silicon has been obtained to within 7 cm-1 of the exciting laser line. For hydrogenated silicon the Raman intensity does not go to zero at zero frequency as expected for a Debye solid. Instead, the Raman spectrum is essentially flat for energy shifts below approximately 50 cm-1. Amorphous silicon without hydrogen, produced by ion implantation of arsenic into single crystal silicon, showed at least a factor of 5 less scattering at low energies than heavily hydrogenated films. The temperature dependence of the intensity of the low frequency scattering could not be described by usual one-phonon or two-phonon processes. Similar low frequency scattering has been previously observed from glasses and interpreted in terms of thermally activated relaxation of two-level systems. The data indicate that the incorporation of hydrogen in amorphous silicon produces two-level systems.
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