## 4 Semipolar LED-Based VLC Applications
The data transmission rates for the (11-22) LEDs are also evaluated to demonstrate that they can clearly be used for data transmission. A digital oscilloscope (Agilent Infiniium DSO80804A) collected the large signal properties/back-to-back data transmission rates of the LEDs. It is found that the transmission rate for the non-mesa-etched 50-\(\upmu\)m diameter LED reaches over 2.4 Gbps when a 2 V peak-to-peak amplitude is applied at an injection current greater than 10 mA. Figure 3c shows the eye diagram for the non-mesa-etched 50 \(\upmu\)m diameter LED at 2.0 and 2.4 Gbps when biased at 10 mA. For 300 \(\times\) 300 \(\upmu\)m\({}^{2}\) LEDs, the transmission rate is found to surpass 600 Mbps at an injection current of 50 mA, and these high speeds and data rates of (11-22) LEDs suggest that they can be used as photodetectors in the visible range. However, data communication rates can be made even faster for semipolar (11-22) LEDs by using higher-level encoding methods that have already been demonstrated in conventional (0001) LEDs, achieving transmission rates of up to 5 Gbps. However, when it comes to white light VLC applications using blue LEDs, many challenges come into existence, such as poor efficiency, stability issues, color rendering problems, etc. Thus, the ultimate solution is to develop the RGB light emission for obtaining white LEDs, but still there are some challenges in terms of optical performance for green emitting LEDs. In this regards, [PERSON] et al. demonstrate semipolar (11-22) green InGaN/GaN LEDs by achieving a modulation bandwidth of 540 MHz at an injected current density of 92 A/cm\({}^{2}\)[59]. This give a clear picture about the suppression of the polarization-related electric field in semipolar LEDs, however, with an increase in In content, the polarization for the semipolar LEDs will increase, which will affect the device performance. It is found that the modulation bandwidth of the semipolar LED increases with the increase in injection current density, implying that the modulation bandwidth is not restricted by the junction capacitance and the 3 dB bandwidth can be further improved beyond this value. The high modulation bandwidth of this semipolar device is attributed to shorter carrier lifetime (0.31 ns), which has been extracted from the time-resolved photoluminescence (TRPL) measurement. This value is the record modulation bandwidth for GaN-based macro-LEDs. There is another recent improvement in the modulation bandwidth for green emitting LEDs from [PERSON] et al. by demonstrating green semipolar (20-21) InGaN/GaN \(\upmu\)-LEDs with a high bandwidth for visible light communication [60]. The structure of the LED comprised of three pairs of InGaN/GaN MQWs as the active region, n-type GaN having a thickness of 1.5 \(\upmu\)m and a 100 nm thick p-type layer. The LED structure process flow started with the deposition of semipolar (20-12) GaN on patterned sapphire substrate (PSS) using a low-pressure chemical vapor deposition (MOCVD). Figure 4a demonstrates the \(\upmu\)-LED device structure optimized to achieve high performance in VLC applications by means of different designs. To obtain a low RC time constant, the LED's electrode has been optimized to suppress the capacitance. The active region and the electrode have been designed in a circular manner to enhance the current spreading and increase the electrical efficiency. To remove the effect of sidewall defects from the etching process, an Al\({}_{2}\)O\({}_{3}\) layer deposited by the atomic layer deposition (ALD) is employed. The DBR at the back of the LED is used to improve the device's output power. Figure 4b represents the electroluminescence (EL) spectra of the semipolar (20-21) \(\upmu\)-LEDs with an increasing injection current from 5 mA to 50 mA, and the inset in Figure 4b displays the photograph of the device at a current of 10 mA. The EL spectra show a 14.9 nm wavelength shift, which becomes stable after an injection current of 50 mA, indicating a good performance.