We have quantitatively investigated the suppression effect for the oxygen contamination into the InN layers by simultaneous irradiation of Ga beam during RF-MOMBE growth using the combination of the TMIn and the RF-plasma nitrogen sources. The O/C ratio and residual carrier concentrations decreased with increase in the Ga beam flux, and a linear relation was observed between O/C ratio and residual carrier concentrations. Present results strongly indicate that the origin of residual carrier concentrations is the incorporated oxygen atoms into the InN layers during growth. (c) 2006 Elsevier B.V. All rights reserved.