## 4 Semipolar LED-Based VLC Applications  
As mentioned in the above section, conventional c-plane LEDs suffer from strong spontaneous and piezoelectric polarization, leading to performance issues, and their modulation bandwidth is limited by the carrier lifetime and the parasitic capacitance. These polarization-related problems may be prevented in nonpolar and semipolar LEDs, but due to the planar anisotropic nature of the growth mode, it is difficult to produce high-quality nonpolar material [55]. In addition, nonpolar planes often suffer from poor surface morphology that prevailed over a wide range of growth conditions compared to semipolar planes that offer better crystallinity, resulting in faster transit response of the device [56]. Several papers have stated that the polarization-related electric fields for semipolar InGaN QWs can be substantially lowered, resulting in a reduced carrier lifetime. However, few studies  
Figure 2: (a) Modulation bandwidth and (b) differential carrier lifetime as a function of current density and (c) calculated log (n) vs. log (J) for LEDs with polar (green squares), semipolar (blue circles), and nonpolar (black triangles) orientations. [51] Figures reproduced with permission from American Institute of Physics Publishing. have been conducted regarding its potential applications in terms of visible light communications. As mentioned in the above section, the modulation bandwidth is limited by QCSE, and to overcome this, researchers need to grow nonpolar structures on bulk GaN. As a part of this, [PERSON] et al. demonstrated blue emitting semipolar (11-22) InGaN/GaN LEDs with the measured frequency response at -3 dB reaching 1 GHz and a back-to-back data transmission rate of over 2.4 Gbps [57]. The LED structure is composed of three- or five-period InGaN/GaN QW active layers, Si-doped n-type GaN of 1.5 \(\upmu\)m thickness, n-type In\({}_{0.01}\)Ga\({}_{0.99}\)N of 100 nm thickness, 120 nm thick Mg-doped p-type GaN, and a heavily doped p-type GaN contact layer with a thickness of 15 nm. For this study, three LED configurations were made, namely, non-mesa-etched \(\upmu\)-LED with Pd-contacts and a 50 \(\upmu\)m diameter, and 200 \(\times\) 200 and 300 \(\times\) 300 \(\upmu\)m\({}^{2}\) macro-LEDs with Ni/Au contacts. The output power of the semipolar LED is represented by the red curve in Figure 3a and is estimated at a current density of 1020 A/cm\({}^{2}\) to be about 0.81 mW with an optical power density of 415 mW/mm\({}^{2}\). The external quantum efficiency (EQE) of the semipolar LED is shown by the blue dotted line in Figure 3a, reaching a maximum value of 1.4% corresponding to an injection current density of 510 A/cm\({}^{2}\). The EQE decreases slowly up to 1.1% when the injection current density increases to 2500 A/cm\({}^{2}\). This efficiency droop in this (11-22) LED can be explained by various factors related to heat generation and current crowding [58]. This small efficiency droop in semipolar LEDs can prevent problems related to data transmission for VLC systems such as reduction in the transmission signal, which can be significantly found in (0001) LED-based systems. The frequency response of the semipolar (11-22) LEDs with different sizes, such as a non-mesa-etched 50 \(\upmu\)m diameter, 200 \(\times\) 200 \(\upmu\)m\({}^{2}\), and 300 \(\times\) 300 \(\upmu\)m\({}^{2}\), at different injection current density are shown in Figure 3b. The frequency response at -3 dB is found to increase from 42 MHz to 1030 MHz, with injection current density increasing from 4 A/cm\({}^{2}\) to 204 A/cm\({}^{2}\). The 50 \(\upmu\)m diameter LED has the highest frequency response of 1030 MHz at the injection current density of 204 A/cm\({}^{2}\). These values of frequency response are higher than that of conventional (0001) LEDs, which can be attributed to the shorter recombination lifetime due to the reduced polarization-dependent electric fields in the semipolar (11-22) QW structures.