The properties of the photoexcited plasma in n-GaAs and its influence on micro-Raman spectra were studied on bevelled structures. The modes of the photoexcited free-carriers plasmon-LO-phonon (PLP) coupling have been detected in the surface depletion layer at room temperature in continuum wave (CW) mode. The strong modifications of the intensities of TO-phonon and LO-phonon modes were observed along the bevel. They were discussed in terms of scattering by PLP coupling. The dependence of the ratio of the intensities of TO-phonons and LO-phonons was linear along the bevel in the region of the surface depletion layer. The first derivative of this dependence is a characteristic value for the corresponding doping concentration. Five Si-doped GaAs layers with different doping levels were analyzed in this way to obtain a calibration function for determining the doping concentration profile in the very thin GaAs layers. (c) 2004 Elsevier B.V. All rights reserved.