The study of p-type doping in low-Be content Zn1-xBexSe layers deposited on GaAs is transferred from the layer to the substrate side where a built-in dense hole gas is evidenced. The idea is to use the extreme sensibility of the associated LO-P mode to give evidence for large hole transfer from Zn1-xBexSe to GaAs in case of even moderate p-doping of Zn1-xBexSe. As a matter of fact the built-in hole accumulation in GaAs appears to be systematically reinforced. Moreover, in some N-doped layers hole densities as high as 10(17)Cm(-1) in accordance with C-V results could be directly inferred from a clear weakening of the ZnSe-like longitudinal optical (LO) mode. Quantitative treatment of the LO phonon-plasmon lineshapes in ternary Zn1-xBexSe alloy is developed within the dielectric approach of Hon and Faust, by using equations of motion and polarization derived from the MREI model.