Cuprous oxide (Cu2O) is a direct-gap semiconductor with band gap energy of 2.0 eV and is regarded as one of the most promising materials for application in photovoltaic cells. The attractiveness of cuprous oxide as a photovoltaic material is due to the fact that the constituent materials are nontoxic and abundantly available on Earth, and that the cuprous oxide has a high absorption coefficient in visible regions and is cheap to produce. Practical application has not been achieved to date because of the difficulty of controlling its electrical properties. In particular, it is necessary to investigate the controllability of its electrical properties and to reduce its resistivity. From previous studies, it is known that a pure phase of cuprous oxide can be obtained by adjusting the sputtering parameters. It is also known that nitrogen doping is effective for reducing the resistivity of cuprous oxide films. N-doped cuprous oxide films have been deposited onto Corning 1737 at a constant substrate temperature of 350 C using a magnetron cosputtering process in a mixture of oxygen and argon gases. The flow rate of nitrogen was kept at 12 ml/min, while other sputtering parameters were kept constant. It was found that the hole carrier concentration of nitrogen-doped cuprous oxide films increased from 9.0 x 1017 to 4.0x 1018 cm-3 resulting in a lower resistivity of 9.1 W-cm after 60 s of hydrogen plasma treatment. X-ray diffraction (XRD) analysis revealed no obvious structural change, but the roughness measured by AFM is obviously reduced after hydrogen plasma treatment. Because the mobility of the carrier is almost constant, it is believed that the hydrogen plasma treatment induces point defects in the cuprous oxide, which is effective in decreasing the resistivity of the films. 2004 Elsevier B.V. All rights reserved.