Tunable Visible Photoluminescence from ZnO Thin Films through Mg-Doping and Annealing

Visible photoluminescence (PL) from ZnO has been found to be tunable in a wide range from blue to green and orange through chemical doping and annealing. Mgdoped, (Al, Li)-doped, and undoped ZnO thin films were deposited on glass substrates by a metal-organic decomposition method at temperatures around 600 C. The films were annealed under different atmospheres, including air, oxygen, nitrogen, and hydrogen/nitrogen. X-ray diffraction analysis and field-emission scanning electron microscope observations revealed that the films consisted of large ZnO grains 50-100 nm in size. When the Mg-doped ZnO films were annealed in nitrogen or hydrogen/nitrogen, unusual blue or bluish-white PL, respectively, was observed in response to an ultraviolet light excitation. We confirmed the bandgap broadening (approximately 0.25 eV) of the Mg-doped ZnO films as compared to that of the undoped films through observation of the absorption edge. The bluerelated PL therefore appeared to be caused by energetic shifts of the valence band and/or the conduction band of ZnO. Films annealed in the oxidizing atmospheres, on the other hand, showed yellow/orange PL. We ascribed this PL to electronic transitions between shallow and deep defect levels. Yellow PL was also observed in the (Al, Li)-doped ZnO films, suggesting that shallow donor/acceptor levels due to extrinsic defects also contributed to the yellow PL.