We have prepared GaAs substrates prior to molecular beam epitaxial growth by the following in situ treatments: (1) The usual thermal cleaning under an arsenic flux, (2) cleaning by hydrogen radicals (H*), and (3) exposure to trisdimethylaminoarsine (TDMAAs). The effects of these treatments on the optical properties and built-in electric fields in GaAs/GaAs structures were studied. In order to investigate the effects of the substrate type on the properties of the GaAs epilayers, undoped semi-insulating (SI) GaAs (100) and Si-doped n(+)-GaAs(100) substrates were used. Reflection high-energy electron diffraction during the growth, and atomic force microscopy in air showed that the smoothest surface morphology was obtained for the layer grown on a H*-cleaned SI substrate at 570 degreesC. For Si-doped substrates the smoothest layer was obtained on a TDMAAs-treated substrate. The concentrations of interfacial residual impurities of C and O were measured by secondary ion mass spectroscopy (SIMS). For SI substrates, the usual thermal cleaning process resulted in very high concentrations of C (2X10(19) atoms/cm(3)) and O (1.3x10(18) atoms/cm(3)) at the interface. The impurities were drastically diminished to below the SIMS detection limit by using H*-cleaning. We observed higher concentrations of impurities on Si-doped substrates. Internal electric fields due to the interfacial impurities were detected by the presence of Franz-Keldysh oscillations in the room temperature photoreflectance spectra. The samples with the highest amount of interfacial impurities presented the strongest internal electric fields. Photoluminescence results showed a clear correlation between the amount of interfacial impurities and signal intensity, the lower the impurity content the stronger the photoluminescence intensity. The signal associated with carbon impurities dominates the photoluminescence spectra for GaAs layers grown on SI substrates, while for samples grown on Si-doped substrates the signal coming from the substrate is the dominant one.