Blue Electroluminescence from Eu²⁺-Doped GaN@SiO₂ Nanostructures Tuned to Industrial Standards

Abstract

Significant progress in solid state lighting has been achieved since the development of nitride-based blue light-emitting diodes (LEDs) from Nichia chemical industries, 1À5 but reliable and highly efficient blue LEDs are still in high demand especially polymer-based LEDs because of their cheap, flexible, and substrate suitability for daily use. 6À9 Particularly, blue-emitting materials with CIE coordinates nearly X = 0.15 and Y = 0.15 are really important as defined by the Department of Energy (DOE) and Optoelectrical Industry Development association (OIDA). 9 Blue-emitting materials are not only a major constituent for red-green-blue full color displays but also a key emitting element for generating white light. For example, combination of the GaN-based blue LED (λ emi ∼ 465 nm) with the Y 3 Al 5 O 12 : Ce 3+ yellow phosphor (λ emi ∼ 555 nm) gives white light. 4 Blue and UV LEDs also have great prospect in data-storage capacities in compact discs (CDs) and digital video discs (DVDs), because the storage density of compact discs is inversely proportional to the square of the laser wavelength. 2 Despite the application of organic electroluminescent devices in making blue organic lightemitting diodes (OLEDs), their low efficiency and stability still remain a challenge and calls for the use of very robust materials such as GaN. 10À13 GaN has a direct wide band gap (3.4 eV) at room temperature and is a promising host electroluminescence (EL) material for multicolor emitting phosphor. Doping of GaN thin films with Tm 3+ , Er 3+ , and Eu 3+ ions results, upon excitation, in blue, green, and red emission, respectively. 14 Furthermore enhancement in the blue emission intensity was observed on doping Tm 3+ in Al 1 Ga 1-X N instead of GaN thin film. The CIE coordinates were tuned to X = 0.13, Y = 0.09, and the enhanced emission from Tm 3+ ions was proportional to the Al content in the film. 15 Recently, Yang et al. 16 fabricated GaN-based blue LEDs with an EL peak centered at 447 nm. In another report Zhao et al. 17 obtained blue EL by using Au/AlN/p-Si heterostructure and had an EL peak centered at 490 nm. The CIE coordinates are X = 0.25, Y = 0.33, as calculated by us, which show a deviation from the blue region.ZnO based heterostructures were also applied in the fabrication of EL devices. The blue EL emission from n-ZnO/nMg y Zn 1-y O/Zn 1-x Cd x O/P-SiC heterojunction diodes as reported by Nakamura et al. 18 has an EL peak centered at 480 nm. The CIE coordinates, as calculated by us, are X = 0.29, Y = 0.32, which is away from the required blue. Li et al. 6 also fabricated a blue EL device using ZnO based heterojunction diodes in the presence of active layer of CdZnO with an El peak centered at 459 nm. Although the peak centered in the blue, the CIE coordinates, as calculated by us, are X = 0.62, Y = 0.32, again away from blue. Ultraviolet-blue LEDs device fabricated by Guo et al. 19 used ZnO nanowires/polymer/p-GaN in heterojunction structure with the EL emission peaks were at 400 nm. The CIE coordinates, as calculat...

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