The substrate-film interface of GaAs homoepitaxial structures prepared by molecular beam epitaxy, employing three different in situ treatments for the substrate surface preparation: (1) cleaning by hydrogen radicals (H∗), (2) exposure to trisdimethylaminoarsine (TDMAAs), and (3) the usual thermal cleaning under an arsenic flux have been studied. The concentrations of interfacial residual impurities of C and O were measured by secondary ion mass spectroscopy (SIMS). These impurities pin the Fermi level producing an interfacial electric field, whose strength was evaluated by photoreflectance spectroscopy. For semi-insulating substrates, the usual thermal cleaning process resulted in very high concentrations of C (2×1019 atoms/cm3) and O (1.3×1018 atoms/cm3) at the interface, producing a large electric field (1.3×106 V/m). The impurities were drastically diminished to below the SIMS detection limit by using the H∗ cleaning, and as a result the electric field was effectively reduced. On the other hand, we observed higher concentrations of impurities, and larger interfacial electric fields on Si-doped substrates. © 1997 American Institute of Physics.