Visible-Light Sensitivity in N-Doped ZnO Films Prepared by Reactive Magnetron Sputtering
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Visible-Light Sensitivity in N-Doped ZnO Films Prepared by Reactive Magnetron Sputtering Yoshitaka Nakano, Takeshi Morikawa, and Takeshi Ohwaki TOYOTA Central Research and Development Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan
ABSTRACT 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 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. INTRODUCTION Semiconductor photocatalysis is becoming more and more attractive and important since it has a great potential to contribute to environmental problems extensively. ZnO is one of promising materials in solar energy conversion and photocatalytic field due to its photochemical properties similar to TiO2. However, ZnO, as well as anatase crystalline TiO2, becomes active under irradiation with ultraviolet light whose energy exceeds the band gap of 3.2 eV. Therefore, ZnO-based materials capable of visible-light photocatalysis are strongly required with respect to solar energy and interior lighting applications. From this point of view, visible-light-driven photocatalytic technology has attracted much attention. The modification of ZnO with the goal of improving the optical absorption and photocatalytic performances, e.g., extending spectra response into the visible-light region and enhancing photocatalytic activity, seems to be the most important interest. As for TiO2-based materials, our group has recently demonstrated that N doping into TiO2 is effective for band-gap narrowing and visible-light photocatalysis, as determined by first-principles calculations and deep-level optical spectroscopy (DLOS) [1,2]. Thus, similar results can be also expected for ZnO-based materials, based on the same concepts in accord with our research.
EXPERIMENTAL 1-µm-thick ZnO:N films were deposited on ITO/quartz substrates by reactive rf magnetron sputtering using a ceramic ZnO target in a mixture of Ar and N2. In order to control N-doping concentration in the ZnO:N films, N2 concentration in the mixture gas varied from 0 to 40 %. The ZnO reference sample without N-doping was colorless, whereas the color of the ZnO:N samples changed from brownish to umber brown with increasing N2 gas concentration. Electrical characterization of the ZnO:N samples was performed using quasi-vertical Schottky-barrier diodes [2]. All the samples showed n-type electrical behavior. Capacitancefrequency (C-f) and capacitance-voltage (C-V
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