## 7 Improvement of Modulation Bandwidth
Metal organic chemical vapor deposition (MOCVD) is used to fabricate the GaN-based LED epilayers consisting of 18 pairs of InGaN/GaN MQWs created on the top of 3 \(\upmu\)m thick Si-doped n-type GaN. The 35 nm thick Al\({}_{2}\)O\({}_{3}\) layer is deposited on the top of p-type GaN, which was created above the MQWs, followed by the deposition of the 10 nm thick Ni layer. After rapid thermal annealing (RTA), inductively coupled plasma (ICP) etching was conducted for the Al\({}_{2}\)O\({}_{3}\), p-GaN and the active layer with a 25 nm etched depth in order to create a nanorod structure. Then, HfO\({}_{2}\) is deposited to shield the nanorod and the active layer and is later coated by an Ag layer, which will become Ag nanoparticles after RTA under N\({}_{2}\) at a temperature of 550 \({}^{\circ}\)C. After getting the Ag NPs on the top of the device, the space between the nanorods is filled with spin-on-glass (SOG) with a thickness of 500 nm, and this is baked in a furnace for 30 min at 550 \({}^{\circ}\)C, as shown in Figure 17d. Finally, ICP etching has been performed for the mesa to expose the n-type and p-type GaN layer for electrode deposition. After the metal electrode deposition using Cr/Pt/Au, the device is ready, as illustrated in Figure 17f. Another nanorod LED structure has also been created to check the influence of Ag NPs on the modulation bandwidth of the device. Figure 16: (**a**) YAG:Ce phosphor-based LED; (**b**) LED with a red luminescent microspheres (LMS) layer; (**c**) frequency response comparison; (**d**) modulation bandwidth comparison between LEDs with phosphor and LMS. [92] Figures reproduced with permission from Optical Society of America. In the following section, the electrical as well as optical characteristics of the two LEDs with and without Ag NPs were performed and analyzed. The photoluminescence (PL) spectra of the two LEDs have been recorded at two different conditions, as shown in Figure 18a, one at room temperature and another at 10 K, to evaluate the corresponding IQE of the two LEDs. The IQE of the nanorod LED with Ag NPs at room temperature is found to be 61%, which is higher than that without Ag NPs, i.e., 51%. The higher IQE of the LED with Ag NPs can be attributed to a strong QW-SP coupling, which leads to an efficient energy transfer from excitons in MQWs to SPs. Furthermore, there is a peak wavelength shift of about 4 nm, and the PL intensity of the LED with Ag NPs is reduced by 4.6 times that with Ag NPs. This reduction in PL intensity correlates to the MQWs region's high IQE and the low scattering efficiency of Ag NPs, as most of the SP energy is dissipated rather than scattered due to the small Ag NP size. However, while the device's luminous efficacy is very low, the QW-SP coupling can create a new energy transition channel for the electron-hole pairs, resulting in an improvement in the spontaneous emission rate for high-speed LED implementation. Moreover, because of its high quality factor and high Purcell factor, the small size Ag NPs can significantly increase the spontaneous emission rate [96,97]. Figure 18b shows the J-V characteristics of the nanorod LEDs with and without Ag NPs, showing a turn-on voltage of about 4V for both devices. Based on the J-V characteristics, the series resistances of nanorod LEDs with and without Ag NPs have been evaluated, and they were found to be 67 \(\Omega\) and 89 \(\Omega\), respectively. The capacitances for both the LEDs with and without Ag NPs have been calculated, and they were found to be 3 pF corresponding to a current density of 57 A/cm\({}^{2}\). Using these values of series resistances and capacitances, the calculated RC-limited bandwidths of the two LEDs with and without Ag NPs are about 790 MHz and 590 MHz, respectively. However, these calculated values of modulation bandwidths are much higher than the measured value, which is approximately 30 MHz for the LED with Ag NPs.