Si-doped Al0.4Ga0.6N (Si-Al0.4Ga0.6GN) epilayers grown on an AlGaN window layer (WL) with different Al contents are prepared using a high-quality A1N buffer layer by metal-organic chemical vapor deposition.Surface morphology, crystalline quality and electric properties of these epilayers are investigated by using atomic force microscopy, x-ray diffraction, Raman scattering spectrum and Hall techniques.Results show that the surface morphology of these epilayers are mainly determined by the Si-doping level which, together with the effect of Al content of WL, also has an obvious impact on the electron concentrations.On the other hand, the insertion of AlGaN WL is helpful to the increase of Si doping level and conductivity of subsequently grown Si-Al0.4Ga0.6N epilayers.However, the insertion as well as the increase of Al content of WL result in increase of dislocation densities, compressive strain in Si-Al0.4Ga0.6N epilayers, and tilt of A1N subgrains at the top interface of the buffer layer.Further, the degradation of crystalline quality with the Al content of WL exerts a decisive influence on the conductivity of the Si-Al0.4Ga0.6N epilayers grown on WL with Al content of 0.6 through a dramatic decrease in electron mobility.