Leakage-induced and disorder-activated modes from the folded acoustic branches in GaAs-AlAs superlattices

Low-frequency Raman modes in the acoustic region (5–40 ${\mathrm{cm}}^{\mathrm{-}1}$) are investigated in (001)-oriented GaAs-AlAs superlattices grown by molecular-beam epitaxy and metal-organic chemical-vapor deposition. Besides the well-known folded LA (FLA) modes, we carefully studied the folded TA (FTA) modes and also modes corresponding to the zone edge of the folded acoustic (LA and TA) branches. The former modes fulfill the selection rule of wave-vector conservation and are due to a leakage of the light polarization. The latter are activated by the disorder and are influenced by the interface defects. These disorder-activated (DA) modes, called DAFLA and DAFTA, are propagative modes which are sensitive to the period of the superlattice, contrary to the previously reported DALA and DATA which are localized in the GaAs and AlAs layers and whose frequency is fixed.