Accordingly, there are two possible approaches to increasing the IQEs of green LEDs, i.e., decreasing the nonradiative recombination rate and/or increasing the radiative recombination rate. For instance, improving crystal quality via the reduction of the threading dislocation density is one method of increasing the IQE by decreasing the nonradiative recombination rate [10], and controlling the quantum confined stark effect in MQWs via strain relaxation is another possible method of increasing the IQE by increasing the radiative recombination rate [11]. Among the several methods of increasing the IQE of GaInN-based LEDs, surface plasmon (SP) coupling with excitons in MQWs has been intensively studied over the last 15 years as a way to increase the radiative recombination rate [12; 13; 14; 15; 16; 17; 18; 19]. SPs are the collective oscillations of free electrons in metals at the interfaces between metals and dielectrics. SPs excited by the interaction between light and metal surfaces can enhance the recombination rate and increase Raman scattering intensities. When the resonance frequency of the surface plasmon polaritons (SPPs) overlaps with the emission frequency of the MQWs at the metal and semiconductor interface, the MQW energy can transfer to the SPs. Consequently, this new path of recombination increases the spontaneous radiative recombination rate, and the IQE thus increases. Thus, this is an approach to improve IQEs by increasing the radiative recombination rate rather than by decreasing the nonradiative recombination rate. From this point of view, the IQEs of conventional LEDs (Equation (1)) and of SP-enhanced LEDs (Equation (2)) are given by [20; 21]  
\[\eta_{IQE}=\frac{R_{R}}{R_{R}+R_{NR}}, \tag{1}\]  
\[\eta_{IQE\_SP}=\frac{R_{R}+R_{SPC}\cdot C_{ext}}{R_{R}+R_{NR}+R_{SPC}}, \tag{2}\]  
where \(R_{R}\), \(R_{NR}\), \(R_{SPC}\), and \(C_{ext}\) are the radiative recombination rate, the nonradiative recombination rate, the SP coupling rate, and the probability of photon extraction from the SP energy, respectively. From these definitions in Equations (1) and (2), we can observe that when the \(R_{SPC}\) is much greater than \(R_{R}\) and \(R_{NR}\), the IQE of an SP-enhanced LED drastically increases. To obtain a higher \(R_{SPC}\) for SP-enhanced LEDs, it is of prime importance to match the resonance frequency of the SPPs to the emission frequency of the MQWs. In GaInN-based SP-enhanced LEDs, Al, Ag, and Au thin films are typically used as plasmonic materials. The resonance frequencies of the SPPs for Al, Ag, and Au thin films on the GaN layer, i.e., Al/GaN, Ag/GaN, and Au/GaN, correspond to \(\sim\)5.1 eV (\(\sim\)243 nm), \(\sim\)2.8 eV (\(\sim\)442 nm), and \(\sim\)2.3 eV (\(\sim\)539 nm), respectively [21]. Since the resonance frequency of the SPPs is determined by the material constant, matching the resonance frequency of the SPPs to the emission frequency of the MQWs is fundamentally impossible with a single plasmonic material. To overcome this problem, there have been efforts to tune the resonance frequency of SPPs via double-metallic Au/Ag multiple layers [22], Ag/Si\({}_{3}\)N\({}_{4}\)/Au multiple layers [23], Ag/ZnO/Au multiple layers [24], and Ag/TiO\({}_{2}\)/Au multiple layers [25]. Furthermore, double-metallic NPs are frequently employed in such devices as sensors [26], solar cells [27], and lasers [28] to improve their performance. In practice, it is difficult to realize SP-enhanced LEDs covered with a thin metal film deposited on the device surface because photons typically are emitted into the free space via the device surface. For this reason, SP-enhanced LEDs covered with self-assembled nanoparticles (NPs) on top of a p-GaN layer are often studied [29; 30; 31; 32; 33]. In this case, the collective oscillations of electrons in randomly distributed metal NPs coupled with excitons in MQWs occur via localized surface plasmon (LSP) modes. In particular, the resonant frequency is dependent on the shape and size of NPs in LSP modes.