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

Abstract

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. Mg-doped, (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 band-gap 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 blue-related 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.

Reversible change in electrical and optical properties in epitaxially grown Al-doped ZnO thin films

“…Though the origin of the orange emission is controversial, the overall consensus of previous studies is that a singly and negatively charged interstitial oxygen, O i Ј, plays an important role in emitting the orange light in ZnO. [20][21][22][23] The orange emission of the air-annealed AZO films proves that air annealing of the AZO films incorporates oxygen into AZO; and produces a great amount of O i Ј in AZO. O i Ј acts as an acceptor in ZnO and occupies 1 electron/ interstitial.…”

Optical characterization of europium-doped indium hydroxide nanocubes obtained by Microwave-Assisted Hydrothermal method

“…Much effort has been dedicated to the study of PL in nanostructural materials since the first visible PL at room temperature was observed in porous silicon for the first time 37 . Rare ions are used to dope oxides not only as a probe to investigate local centers and energy [38][39][40] , but also to provoke changes in the optical behavior.…”

Preparation and photocatalytic activity of multi-walled carbon nanotubes/Mg-doped ZnO nanohybrids

“…Furthermore, the substitution of Zn by Mg makes the polarization of its neighboring O 2− ion decrease, causing the defect band to move towards the ultraviolet [33]. Meanwhile, the Mg-doping can shift the conduction-band and valence-band levels to the same degree in relation to the V O level [34].…”

“…Among various dopants, Mg 2+ has a ionic radius of 0.57Å, * E-mail: jxccs1934@gmail.com which is very close to the ionic radius of Zn 2+ (0.60Å), therefore, Mg ions have a large solubility in ZnO and ability to replace Zn, but Mg does not give rise to significant changes in the lattice constants [4]. Mg doping can create localized impurity levels by replacing the Zn site of ZnO lattice, or inserting into the crystal lattice of ZnO, resulting in creating localized impurity levels and creating many intermediate energy gaps in ZnO crystal [5][6][7][8][9]. Therefore, Mg-doping can improve the photocatalytic activity of ZnO nanostructures significantly [10].…”