Abstract
Large reflection losses at interfaces in light-emitting semiconductor devices cause a significant reduction in their light emission and energy efficiencies. Metal nanoparticle (NP) surface coatings have been demonstrated to increase the light extraction efficiency from planar high refractive index semiconductor surfaces. This emission enhancement in Au NP-coated ZnO is widely attributed to involvement of a green (∼ 2.5 eV) deep level ZnO defect exciting localized surface plasmons in the NPs. In this work, we achieve a 6 times enhancement of the ultra-violet excitonic emission in ZnO nanorods coated with 5 nm Au NPs without the aid of ZnO defects. Cathodoluminescence (CL) and photoluminescence (PL) spectroscopy revealed that the increased UV emission is due to the formation of an additional fast excitonic relaxation pathway. Concurrent CL-PL measurements ruled out the presence of charge transfer mechanism in the emission enhancement process. While time-resolved PL confirmed the existence of a new excitonic recombination channel that is attributed to exciton relaxation via the excitation of rapid non-radiative Au interband transitions that increases the UV spontaneous emission rate. Our results establish that ZnO defect levels ∼ 2.5 eV are not required to facilitate Au NP induced enhancement of the ZnO UV emission.
Highlights
Light emitting diodes (LEDs) fabricated from nanorods have clear and significant advantages over conventional planar device structures
The hexagonal Zinc oxide (ZnO) NRs with a growth axis along the direction exhibit an average diameter of 40 ± 10 nm and length of around 700 nm, which are oriented at different angles to the normal direction of the Si substrate
Broad deep level (DL) red emission centered at ~ 1.7 eV has been assigned to native point defects,[37,38,39,40] the RL intensity is noticeably stronger in the PL spectrum compared with its CL counterpart
Summary
Light emitting diodes (LEDs) fabricated from nanorods have clear and significant advantages over conventional planar device structures. Zinc oxide (ZnO) nanorods are attractive for LED applications owing to their: (i) attractive optical, electrical and mechanical properties, (ii) large surface to volume ratio, (iii) availability in a large assortment of bespoke shapes and sizes and (iv) facile growth on a wide variety of substrates.[1,2,3] because ZnO has a direct wide band gap at room temperature (Eg = 3.37 eV) as well as a large exciton binding energy of 60 meV ZnO, it is a very promising material for the development of ultra-violet (UV) LEDs.[4,5,6,7,8] despite the high luminescence efficiency of the near band edge (NBE) in ZnO, only a small fraction of this light generated is emitted due to large internal surface reflection losses arising from the high refractive index difference at the air – ZnO interface. It has been established that this optical limitation on the light extraction efficiency can be overcome by using a nanostructured gold thin-film surface coating, which has been found to significantly enhance that ZnO NBE emission output.[9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]
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