We have analyzed the incorporation of Si &dgr; doping during the molecular‐beam epitaxial (MBE) growth of GaAs and Al0.25Ga0.75As using secondary‐ion mass spectroscopy. At high substrate temperatures (≥580 °C) a significant and asymmetric broadening is observed in both GaAs and Al0.25Ga0.75As. This is due to the combined effect of thermal diffusion and migration towards the surface during MBE growth. This study points out that a low substrate temperature (≤540 °C) and a short time are required during MBE crystal growth to achieve confined &dgr; doping.