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.