We study long wave polar optical modes at semiconductor surfaces (GaAs
) and interfaces (GaAs/ALAs). We have considered also the cases in whi
ch the surface or interface is kept at a fixed electrostatic potential
. The spectrum of excitations then shows significant differences. The
existence of localized and resonant modes has been obtained by studyin
g the spectral strength through the surface Green function matching (S
GFM) method.