  We study long-wavelength polar optical modes at semiconductor interfaces of
GaAs|Al_{(x)}Ga_{(1-x)}As and take into account influence of the Al
concentration. We have considered two cases in which the interface is kept at
unfixed and fixed electrostatic potential. The spectrum of excitation then
shows existence of localized and resonant modes to depend on the concentration
of Al. For the case of a fixed electrostatic potential, a splitting of the
resonant mode near the transverse threshold is found for certain ranges of the
concentration x. The existence of these modes has been obtained by computing
the spectral strength through the Surface Green Function Matching (SGFM)
method.
