The saturation of electron concentration and mobility at high doping concentration observed in the samples grown by low-temperature migration-enhanced epitaxy (MEE) is found to be strongly related to the morphology change in the film. The structural characteristics of the heavily Sn-doped GaAs have been further investigated by means of X-ray diffraction (XRD) measurement and transmission electron microscopy (TEM). The results indicate that the (GaAs)(1-x)(Sn-2)(x) alloy is formed in the Sn-doped GaAs with the doping concentration higher than 1 x 10(19) cm(-3). However, when the Sn concentration exceeds 1 x 10(21) cm(-3), the formation of Sn-rich clusters becomes dominant. This formation of (GaAs)(1-x)(Sn-2)(x) alloy and Sn clusters is found to be responsible for the saturation of the electron concentration and mobility in the heavily doped samples. (c) 2006 Elsevier B.V. All rights reserved.