Si delta-doped GaAs layers were studied by micro-Raman spectroscopy. The spectra were recorded along the bevelled structure using the light of Ar+-ion laser (514.5 nm line) with high power density. The observed changes in the Raman spectra are discussed in the sense of coupling present between LO phonon and photoexcited electron-hole plasma and plasma of electrons arising from ionised Si atoms. Plasmon-LO-Phonon modes of the coupling of photoexcited plasma in delta-doped GaAs layers were observed for the first time. The minimal thickness of cap layer was estimated in the range of 10-19 nm depending on the doping concentration. (C) 2004 Elsevier B.V. All rights reserved.