Two kinds of n-GaAs layers doped with Si to a level of 10(17) cm(-3) are grown by molecular beam epitaxy on GaAs substrates : those grown on substrates cleaned using low temperature electron cyclotron resonance (ECR) hydrogen plasma and those grown on substrates cleaned by conventional thermal treatment. Comparisons between the electrical properties of both layers are made by capacitance-voltage (C-V) measurement; optical deep-level transient spectroscopy (ODLTS); Van der Pauw and Hall effect measurements and secondary ion mass spectroscopy (SIMS). It is found from SIMS analysis that for thermally cleaned wafers, C, O, and Si accumulate in the vicinity of the interface between the epilayer and the substrate, whereas only oxygen accumulates at the same interface in ECR-cleaned wafers. The C-V and Hall measurements reveal that the carrier concentration, n, as well as the electron mobility, mu, decrease in the vicinity of the interface for thermally cleaned wafers, These results can be explained theoretically by using a physical model in which Si donors are compensated by unintentionally doped C accepters. On the contrary, no degradation of n or mu is observed for ECR-cleaned wafers. In addition. ODLTS measurement reveals the presence of deep hole traps due to the transition metals Fe and Cu, and a continuously distributed interface defect state for thermally cleaned wafers, whereas such defects are not observed for ECR-cleaned wafers.