PHONON PROPERTIES OF (311) GAAS ALAS SUPERLATTICES/

Raman scattering spectra of GaAs/AlAs superlattices grown along the [3 11] direction are compared with results of lattice-dynamical calculati ons. Confined optical as well as folded acoustic-phonon modes are clea rly observed at frequencies in excellent agreement with those predicte d. Several of the bulk branches hybridize along the low-symmetry [311] direction, producing confined optical modes in the superlattice of mi xed longitudinal/transverse character. This hybridization complicates also the assignment of a mode order to the confined phonons, since the y encompass components deriving from several bulk branches with differ ent effective wave vectors. We demonstrate that a proper assignment ca n only be made by comparison to the calculated eigenvector envelope fu nctions. We discuss how the crystal symmetry affects the angular dispe rsion of the optic modes.