## 3 Results and Discussion
The PL intensity of LEDs with a Ag NW TCE and 45-nm-thick p-GaN layer is similar to that of the LEDs without a TCE, whereas that of LEDs with a Ag NW/NP TCE is higher than that of the LEDs without TCEs, as shown in Figure 3b. Because the p-GaN layer is 45 nm thick, QW-SP coupling is possible, and the IQE is improved. When the dipole energies in the InGaN QWs and the SP energy of the Ag are similar, the exciton dipole energies excited in the QWs is transferred to the SP modes of the Ag. Figure 2: (**a**) Transmittance of Ag NW and Ag NW/NP TCEs for different NP sizes. FE-SEM images of Ag NW/NP TCEs with NP size with (**b**) 2 \(\upmu\)m scale bar and (**c**) 500 nm scale bar, (**d**) XPS spectrum of Ag NW/NP TCE. The nanostructure and roughness of the Ag layer allows SPs with high momentum to scatter. Then, they lose their momentum, and become coupled to the radiated light. This process occurs much more rapidly than the radiative recombination of exciton dipoles in the InGaN QWs [16], and thus the PL intensity of QW can be improved. Although the PL intensity was significantly higher when only Ag NPs were used owing to increases in the IQE and LEE, Ag NPs alone cannot be used as the TCE. In addition, we found that the PL intensity did not increase when we used TCEs with only Ag NWs. Ag NWs can act as waveguides in nanoscale regions to localize the electromagnetic energy arising from the hybrid nature of SP polaritons (SPPs), which are electromagnetic (EM) waves. EM waves coupled to charge oscillations at the Ag-GaN interface and SPP waves can be optically excited at the distal ends of the NW. However, the intensity of the excited light decreases significantly with increasing propagation distance [25]. Because 20 \(\upmu\)m long NWs were used to obtain TCEs with good electrical properties, many electromagnetic waves were absorbed during propagation in the NWs. Therefore, the higher PL intensity exhibited by the LED with the Ag NW TCE cannot be attributed to photon absorption in SPPs. When Ag NPs were used with Ag NWs, the PL intensity increased by 20%. The SPPs propagate along to interface between the NW and GaN with a momentum higher than the free-space wavelength used for dipole excitation. The momentum of SPPs propagated along to interface between the NW and GaN must be removed from  
Figure 3: PL spectra of GaN LEDs with **(a)** 120 nm thick and **(b)** 45 nm thick p-GaN layer without TCE and with Ag NW, Ag NP, and Ag NW/NP TCEs and **(c)** schematic diagram of light extraction by plasmon leakage radiation and PL spectra with GaN LED (45 nm thick p-GaN layer) **(d)** without TCE and **(e)** with Ag NW/NP TCE at 10 K and 300 K.  
the surface-bound SPP wave to convert the SPPs back to photons at a desired location. NPs close to Ag NWs act as a radiative center to overcome this momentum mismatch and facilitate the momentum of SPPs to free-space photons [25; 26]. In addition, all Ag NPs can work as scattering centers because the nanostructure of the Ag layer allows the scattering of SPs with high momentum, as described above. Therefore, the increased PL intensity of the LED with a Ag NW/NP TCE can be attributed to the increase in light extraction at the NW/NP junction and the increase in scattering by Ag NPs, as shown in Figure 3c. Emission can be excited at both ends of the NW by propagation at the interface between the p-GaN layer and TCE. To assume the improvement of IQE by Ag NW/NP TCE, we tried to measure temperature-dependent photoluminescence with a 25 mW 325 nm He-Cd laser. By comparing the ratio of the integrated PL intensity between 300 K and 0 K, it was possible to find the increase in IQE [27]. The ratio of PL intensities (I300K/I10K) of LED without TEC and with Ag NW/NP TCE were 27.2% and 34.8%, respectively. Based on this result, we found that the IQE can be increased by 30% with Ag NW/NP compared to that of IQW without TCE, shown in Figure 3d,e. This result indicated that the improvement of PL intensity can be attributed to the increase in IQE by SP + QW coupling without the quenching of light extraction.