The dopant effect of In0.5Ga0.5P/GaAs epitaxial layers has been studied using Raman spectroscopy. Our study of the LO phonon-plasmon interaction shows one mode behavior even though In1−xGaxP alloys are known to have partial two model behavior. We attribute this apparent one mode behavior to enhanced charge transfer from In–P bonds to Ga–P bonds which is induced by the local lattice distortion originating from bond length mismatch. Our work also shows a TOmmode between the InP-like LO mode and the GaP-like LO mode. In terms of this TOmmode, we discuss the valley depth ratio,b/a, which was formerly attributed in the literature to CuPt-type ordering without clear explanation. ©1997 American Institute of Physics.