We report on visible-light sensitivity in N-doped ZnO (ZnO:N) films that were deposited on ITO/quartz substrates by reactive magnetron sputtering. Colored ZnO:N samples showed enhanced polycrystallization and a significant decrease in optical band gap from 3.1 to 2.3 eV with increasing N doping concentration, as determined by x-ray diffraction and optical absorption measurements. Deep-level optical spectroscopy measurements revealed three characteristic deep levels located at ∼0.98, ∼1.20, and ∼2.21 eV below the conduction band. In particular, the pronounced 2.21 eV band is newly introduced by the N doping and behaves as part of the valence band, resulting in the band-gap narrowing of ZnO. Therefore, this deep level is probably one origin of visible-light sensitivity in ZnO:N.
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