Abstract
Enhancement of the external quantum efficiency of a GaN-based vertical-type light emitting diode (VLED) through the coupling of localized surface plasmon (LSP) resonance with the wave-guided mode light is studied. To achieve this experimentally, Ag nanoparticles (NPs), as the LSP resonant source, are drop-casted on the most top layer of waveguide channel, which is composed of hydrothermally synthesized ZnO nanorods capped on the top of GaN-based VLED. Enhanced light-output power and external quantum efficiency are observed, and the amount of enhancement remains steady with the increase of the injected currents. To understand the observations theoretically, the absorption spectra and the electric field distributions of the VLED with and without Ag NPs decorated on ZnO NRs are determined using the finite-difference time-domain (FDTD) method. The results prove that the observation of enhancement of the external quantum efficiency can be attributed to the creation of an extra escape channel for trapped light due to the coupling of the LSP with wave-guided mode light, by which the energy of wave-guided mode light can be transferred to the efficient light scattering center of the LSP.
Highlights
Neglected, and the porous ZnO NRs can be considered as an optically homogeneous film with an effective refractive index ranging between 1 < nZnO < nZnO
Transverse A1(TO) and E1 longitudinal E1 (LO) optical modes are observed at 379 cm−1 and 580 cm−1, respectively, which again suggests the synthesized ZnO NRs are of high crystalline quality
To validate the localized surface plasmons (LSPs)-photon resonance coupling effect to observe the electric field profile of LSPs in the vicinity of Ag NPs decorated on synthesized ZnO NRs, we performed a finite-difference time-domain (FDTD) simulation
Summary
Transverse A1(TO) and E1 longitudinal E1 (LO) optical modes are observed at 379 cm−1 and 580 cm−1, respectively, which again suggests the synthesized ZnO NRs are of high crystalline quality. The above-described observation on PL spectra indicates relatively low defect and impurity concentrations in the synthesized ZnO NRs, in good agreement with the measured micro-Raman spectra of the ZnO NRs. Most importantly, the result suggests that the use of ZnO NRs on the top surfaces of LED I (with emission wavelength of λ = 450 nm) induces a minimal amount of undesired optical absorption associated with the defect transitions of ZnO NRs; such defects would inevitably decrease the light-output power of the LED if there were many of them in the ZnO NRs
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