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Abstract — In this paper, we present and analyse Secondary Ion Mass Spectrometry (SIMS) measurements of oxygen concentration in 3C SiC epitaxial layers on Si. The concentration of oxygen determined by SIMS was as high as 1019 to 1020 atom/cm3. Unlike silicon, oxygen can act as donor atoms in SiC with calculated ionization levels of 200 meV [1]. It is generally believed that the main contribution of dopant concentration in the unintentionally doped SiC film is related to background nitrogen. Because of the high ionisation level, oxygen is not electrically active at room temperature. By measuring the conductivity of the films at higher temperatures, we extracted three donor energy levels: EA1 =79 meV, EA2 = 180 meV, and EA3 = 350 meV. The activation energy of 180 meV could be associated with the calculated ionization level for oxygen. Further analysis of the conductivity measurements at elevated temperatures will be performed to determine the electrically active donor concentration that is associated with the activation energy of 180 meV