The occurrence of a high‐resistivity region (ilayer) at the interface ofn‐type GaAs films onn+GaAs substrates has been a recurring problem in the halide synthesis transport growth of these films for microwave devices. Consistent success in the elimination of anilayer in the doping profile has been achieved by deposition of ann+epitaxial film substrate prior to growth of the activenfilm, however the nature and cause of theilayer has remained unknown. It has been proposed by others that theilayer is caused by the presence of an impurity in that region. Under this assumption, a new analytical tool, the Direct Image Mass Analyzer (DIMA, Bell & Howell/CEC) has been used to obtain depth impurity profiles of known impurities through five homoepitaxialn‐on‐n+GaAs film samples. These samples differed in the type of dopants used in the film and substrate, and in the magnitude of theilayers. The data were compared with doping profiles, and an analysis of this comparison has led to the following conclusions: (1) The impurities that are present in these samples include Si, K, Li, C, Na, Fe, F, Cl, and Mn; (2) the presence of anilayer is correlated with the presence of a high concentration (∼1×1021/cm3) of silicon in theilayer region; and (3) the origin of the silicon is probably external to the sample. The hardware of the reactor is a likely source of silicon, and a mechanism is proposed for the transport of the silicon into the growing film.