Stable co-emission of UV, green and red light in ZnO thin films with rapid annealing treatment
Introduction
ZnO thin film is an important multifunctional material, which has potential applications in many fields [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Recently, its applications in solid light-emitting devices especially attract attention [11], [12], [13]. At room temperature, ZnO has a direct wide bandgap of ∼3.37 eV and a large exciton binding energy of 60 meV, which make it produce UV emission with a wavelength range from 370 to 390 nm originating from recombination of free excitons. In most cases, the higher the crystalline quality of ZnO thin films and the lower the defects density, the better the UV emission performance. Currently, ZnO-based UV light-emitting devices have been fabricated by some researchers [10], [11]. Besides UV emitters, ZnO thin films are also the ideal materials for fabricating solid white light-emitting devices. This is because, besides the UV light, ZnO can also produce visible emissions with different wavelengths at the same time, such as blue [14], green [15] and red light [16]. In an appropriate proportion, these emissions can be mixed into a white emission. If one wants to obtain excellent ZnO-based light-emitting devices, he should master the luminescence characteristics of ZnO materials first. Although many investigations on luminescence behavior of ZnO have been done, the visible emission mechanisms are still controversial now. Therefore, it still needs to carry out more studies on luminescence phenomenon of ZnO materials prepared by different techniques with various deposition conditions.
Although the visible emission mechanisms of ZnO are still in debate, many researchers deem that the green emission is mainly related with oxygen vacancy defects [15], [17] and the red emission is mainly associated with oxygen interstitial defects [16], [18]. Theoretically, oxygen vacancy and oxygen interstitial should not exist at the same time with large quantity; thus, the strong co-emission of green and red light should also not be easy to happen. From the reported results, it can be seen that the strong co-emission of green and red light is indeed rarely to be observed, which seems to support the above opinions on the green and red emission mechanisms. However, some groups recently reported the strong co-emission of green and red light in ZnO materials [8], [19], [20]. For example, Yang et al. [20] prepared ZnO nanocrystals by sol-gel method and found that the samples had a strong co-emission of green and red light. The strong co-emission of green and red light or green/red emissions with different wavelengths at the same time suggests that it should have more than one source producing green and red emissions. Tu et al. [19] found that there were three red emissions centered at 600, 660 and 730 nm, respectively. They deemed that the red emissions centered at 600 and 660 nm were mainly caused by oxygen interstitial defects and another centered at 730 nm was attributed to the deep levels of zinc interstitials (Zni). In this study, ZnO thin films were prepared by electron beam evaporation and rapidly annealed at different temperatures; it was found that the samples showed stable and strong co-emission of green and red light. Combined with the structural and component analysis of the films as well as the opinions of others, we tentatively gave a suggestion on the green and red emission mechanisms.
Section snippets
Experiments
ZnO thin films were deposited by electron beam evaporation. The substrate is glass slide with a thickness of 1 mm and the source material for the films is high-purity ZnO particles. When the films are deposited, the ZnO particles are put into a copper crucible from which it is ∼1.5 m to the substrates. The growth temperature is 250 °C and the working pressure is 3.0 × 10−2 Pa (Ar/O2 = 2/1). Four samples as-prepared are rapidly annealed at 300, 400, 450 and 500 °C respectively, for 10 min and accordingly
The crystal structure, surface morphology and composition of the ZnO thin films
Fig. 1 shows the XRD patterns of the as-deposited ZnO thin film and those annealed at different temperatures. All the samples show a diffraction peak at 34.4° or so which corresponds to the diffraction of the (002) plane of wurtzite ZnO, indicating that ZnO crystals are preferentially oriented along the c-axis direction. With the rise of annealing temperature, the (002) peak is gradually intensified and its full width at half maximum is gradually decreased, meaning that the crystalline quality
Conclusion
In this work, ZnO thin films were deposited by electron beam evaporation and rapidly annealed at different temperatures. The photoluminescence results showed that all the films exhibited a strong co-emission of UV, green and red light. Although the samples were rapidly annealed at different temperatures, their luminescent features like wavelength and intensity was basically stable. The XPS analysis showed that the main point defects in these samples were oxygen vacancies and the rapid annealing
Acknowledgements
This work is supported by the Open Project Program of Labs in Nanjing University of Information Science and Technology in 2015 (15KF085) and by the Innovation and Entrepreneurship Training Program for College Students in Jiangsu Province (Grant no. 201410300083X),and partially supported by the Natural Science Foundation of Jiangsu Province (Grant no. BK20141483). Dr. Junfeng Wang thanks the financial support from the National Natural Science Foundation of China (Grant no. 11504175).
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2019, Materials Chemistry and PhysicsCitation Excerpt :The introduction of copper into the ZnO lattice strongly modifies the properties of underdoped material. For example, using pulsed electron deposition (PED), ZnO thin films were prepared [33], and then rapidly annealed at different temperatures. In such films a strong joint emission of ultraviolet, green, and red colors, which occurs quite rarely, was observed.