## 4 Semipolar LED-Based VLC Applications
This wavelength shift is attributed to strong QCSE in the semipolar \(\upmu\)-LEDs resulting from the reduced polarization-related electric field, leading to an increase in the QW band gap. The FWHM for the emission peaks shows a slight change of only 3 nm from 5 to 50 mA, suggesting a low defect density and good uniformity in the structure of the device. Figure 4c shows the relationship between the current density (J) and the voltage (V) in the forward voltage, while the inset gives the Current density-Voltage (J-V) relation at reverse voltage used to evaluate the leakage current. The device has a 2.2 V turn-on voltage that can be considered good performance for the green \(\upmu\)-LEDs, but the device shows a high resistance that could be due to reverse polarization or weak ohmic contact in the p-type region. However, this resistance will not affect the 3-dB bandwidth for VLC device application because \(\upmu\)-LEDs have extremely small capacitance. The inset curve shows that the device has a low leakage current density of about \(10^{-6}\) A/cm\({}^{2}\), which may be due the passivation layer formed by the ALD process and low defect density in the semipolar GaN. As the chip size gets smaller, the sidewall effects and surface recombination can lead to leakage, and, from this result, the passivation effect is believed to be important to avoid such problems. Figure 3: (**a**) P\({}_{\rm opt}\) and corresponding EQE of the LED plotted as a function of \(I(J)_{\rm inject}\); (**b**) normalized frequency response curves of the non-mesa-etched 50 \(\upmu\)m diameter, 200 \(\times\) 200 \(\upmu\)m\({}^{2}\), and 300 \(\times\) 300 \(\upmu\)m\({}^{2}\) (11-22) LEDs measured at different \(J_{\rm inject}\) (\(I_{\rm inject}\) = 4 mA); (c) eye diagrams at 2 Gbps and 2.4 Gbps of an LED with a non-mesa-etched 50 \(\upmu\)m diameter (11-22) measured at \(I_{\rm inject}\) = 10 mA. [57] Figures reproduced with permission from Optical Society of America. The EL spectra of the semipolar \(\upmu\)-LED using a polarizer are measured as shown in Figure 5a. The two EL spectra correspond to the polarizer directions along (1-210) and (10-1-4) defined as x\({}^{\prime}\)-and y\({}^{\prime}\)-directions, respectively. The x\({}^{\prime}\)- and y\({}^{\prime}\)-direction correspond to the maximum and the minimum intensity of the emitted light through the polarizer. Using these values, the polarization ratio of the semipolar \(\upmu\)-LED is calculated and found to be 0.3, corresponding to injection current of 50 mA. The polarization ratio as a function of the injection current is shown in the inset in Figure 5a, indicating that the polarization is independent of the injection current. The characteristics of polarization can be useful when it comes to data transmission more efficiently, as polarization-multiplexing technology can be used to exploit light polarization and help to improve the rate of transmission [61]. Figure 5b displays the TRPL measurement for semipolar (20-21) \(\upmu\)-LED and the commercial c-plane \(\upmu\)-LED performed at low power of 20 \(\upmu\)W. Some important parameters have been extracted from the fitting curves, such as decay lifetime, \(\uptau\) and fractions of electron-hole pairs, \(\upalpha\). The lifetime of the minority carrier for semipolar and c-plane \(\upmu\)-LEDs has been estimated using these parameters, and found to be 2.65 and 9.00 ns, respectively. Due to low polarization-related electrical fields and greater overlap of the electron-hole wave function, the semipolar \(\upmu\)-LED has a shorter lifetime than that of c-plane \(\upmu\)-LED. Hence, it would result in faster carrier recombination for semipolar LEDs and, correspondingly, a higher frequency bandwidth. Figure 4: (a) Schematic diagram of (20-21) \(\upmu\)-LED structures; (b) electroluminescence (EL) spectra at different currents and an image of lighting from the device; (c) Current density-Voltage (J–V) curve of the semipolar \(\upmu\)-LEDs. [60] Figures reproduced with permission from ACS Publications.