Phonons in Si/GaAs superlattices

Phonon dispersion in strained heterovalent (Si)n/(GaAs)m (001) superlattices has been calculated using first-principles force constants. Raman scattering experiments have been conducted on fifteen-period (Si)2/(GaAs)28 and (Si)3/(GaAs)50 superlattices synthesized by molecular-beam epitaxy. In addition to folded acoustic modes, confined Si-like and quasiconfined GaAs-like optical modes appear in the superlattice spectra, in spite of the extremely small thickness of the Si layers. Quantitative agreement is found between measured and calculated phonon frequencies, confirming that a description in terms of strain- and confinement-induced shifts of the optical phonons is appropriate for these structures.