It is demonstrated that the measurement of a bulk phonon property, i.e., the Raman intensity of the resonantly excited symmetry forbidden LO-phonon via its dependence on the surface electric field, is a sensitive method to measure surface band bending in polar semiconductors. The observation of coupled phonon-plasmon modes shows the existence of flat band conditions on clean, UHV-cleaved (110)GaAs. This condition can also be reached on pinned surfaces by photoexcitation with laser power densities in excess of 500 W/cm2. A curve fitting of the observed L+/L−-spectrum with the Lindhard–Mermin dielectric function yields information on the wave-vector smearing due to strong light absorption and impurity scattering.