Abstract: The influence of growth temperature, Al composition x and As₄ pressure during molecular beame pitaxy on the shape of SIMS profiles, surface segregation and the diffusion of Si for the planar Si-doped Al_xGa_1-x have been investigated by using secondary-ion mass spectrometry(SIMS). The results indicate that all the Si peaks in the SIMS depth profile show asymmetric broadening, here surfajce segregation increases markedly with growth temperature and Al composition x, while the change of Si diffusion is very small and does not contribute to the shape of SIMS profile. It is shown that, the shape of the SIMS profile of δ-doped planes is almost not affected when the As₄ pressure is larger than 1.5×10^-5 mbar, but a significant increase in FWHM of this profile occurs when As₄ pressureis lower than 1.5×10^-5 mbar, because of the increased Si diffusion in this case. Among the factors, which influence the shape of SIMS profile of Si, the growth temperature is most significant, the Al composition x is the next and the influence of As₄ pressure should be taken into account only at low As₄ pressure.