An longitudinal-optic-phonon-plasmon coupled mode (LOPC) has been clearly observed by Raman scattering in n-type Al(x)Ga(1-x)N films with x similar to 0.67 and different carrier densities n - 1 x 10(17)-9 x 10(17) cm(-3). The A(1)-LO-phonon mode showed a systematic frequency shift and broadening with increasing n. This is a characteristic behavior of LOPC as previously observed in n-type binary semiconductors. A theoretical line-shape fitting analysis was conducted for the LOPC profile using n and plasmon-damping rate as adjustable parameter. Assuming m*/m(0)-0.28 for the longitudinal effective mass of electron, the analysis well reproduced carrier density and mobility deduced by Hall measurement. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3670338]