We report on Raman scattering experiments performed on GaN-AlGaN quantum well structures, using various ultraviolet excitations. Under near resonant conditions we observe an important enhancement of the multi-LO phonon scattering in the wells or in the barriers. We find that the intensity of the phonon of the quantum wells increases with the barrier width. The latter observation may be attributed to effects related to the internal electrostatic field in the GaN wells.