Visible-light sensitivity in N-doped ZnO films prepared by reactive magnetron sputtering

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 similar to 0.98, similar to 1.20, and similar to 2.21 eV below the conduction band. In particular, the pronounced 2.21 eV band is newly introduced by the N doping and makes the dominant contribution to the band-gap narrowing of ZnO. Therefore, this deep level is probably one origin of visible-light sensitivity in ZnO:N.