Confinement of optical phonon modes in thin (GaAs)(n)(AlAs)(n) superlattices

We present theoretical investigations of atomic vibrations in thin (GaAs)(n )(AlAs)(n), superlattices grown along the symmetry directions [001], [110] and [111] using the adiabatic bond charge model. We find that optical phono n modes get quantized for all layer thicknesses, including the monolayer ca se. In particular, we show that the energy gap between the primary and seco ndary quantized longitudinally optic modes is dependent upon the direction of growth in addition to the period of thickness of the superlattice. We sh ow that the direction dependence is due to the polarity of the system. We a lso calculate the unfolding behaviour of the quantized superlattice frequen cies and provide a comparison with experimental Raman peaks. (C) 1999 Elsev ier Science B.V. All rights reserved.