Raman spectroscopy was used to study the band bending at the interface of ZnSe/GaAs hetero-structures. A series of samples, which contained a ZnSe buffer layer, 0-35 nm thick, grown at a lower temperature than the much thicker ZnSe epilayer, by metal-organic chemical vapor phase deposition, were investigated. Compared with that of the GaAs substrate, an enhancement of the intensity of the LO GaAs phonon was found in samples grown without and with a thick (≥28 nm) buffer layer, but not in a sample grown with a 4 nm thick buffer layer. The enhancement is attributed to the electric field induced Raman scattering, resulted from a strong band bending on the GaAs side of the hetero-structure. The results suggest that the direction of the interfacial electric field on the GaAs side will reverse with increasing buffer layer thickness. Between this reversal, a near flat band condition can be achieved, as was found in a sample grown with a 4 nm buffer layer. This suggestion is consistent with the concomitant improvement of the structure of the epilayer and of the interfacial quality of the hetero-junction, which unpins the Fermi level and affects the band bending.