Two-phonon assisted free exciton recombination radiation from intrinsic silicon

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Abstract
Fine structure is reported in the luminescence spectrum associated with the two-phonon assisted decay of free excitons in intrinsic Si. The spectrum has been deconvoluted into eight components, and the major features can be identified with two-phonon processes in which a momentum-conserving phonon is emitted together with either a k=0 optical phonon OGamma or an intervalley scattering phonon connecting conduction band minima, but only by invoking 'forbidden' intervalley scattering phonons. It would appear that the complete insensitivity of the two-phonon luminescence to the symmetries of the phonons involved recently reported for Ge does not occur with Si, although there is some breakdown of the accepted symmetry selection rules.