We studied the effect of carbon incorporation on the material and electrical properties of Ta2O5 thin film. We doped the Ta2O5 films with carbon using pulsed-dc reactive and rfmagnetron sputtering of Ta2O5 performed in an Ar/O2/CO2 plasma. In thick (70 nm) films, an optimal amount (0.8 - 1.4 at.%) of carbon doping reduced the leakage current to 10−8 A/cm2 at +3 MV/cm, a four orders of magnitude reduction compared to that in a pure Ta2O5 film grown in similar conditions without CO2 in the plasma. This finding suggests that carbon doping can significantly improve the dielectric leakage property at an optimal concentration. X-ray Photoemission Spectroscopy (XPS) analysis showed the presence of carbonate in these electrically improved carbon-doped films. Analysis by high-resolution transmission electron microscopy (HRTEM) exhibited no morphological or structural changes in these carbon doped films. Carbon doping showed no improvement in the leakage current in thin (10 nm) Ta2O5 films. This phenomenon is explained by a defect compensation mechanism, in which the carbon-related defects remove carriers at low concentrations but form a hopping conduction path at high concentrations.