## 3 Results and Discussion
It is further proved that the ICP etching process of patterned sidewalls is damage-free process. The results are in accordance with the comparison of forward voltage shown in Figure 3.  
In order to reveal the effect of the patterned sidewalls structure on the optical properties of the devices, the relationship between light output power (LOP) and current density are studied as shown in Figure 6. And Table 1 shows the saturation LOP of all studied devices. With the increase of the current density, the LOP of all studied devices increases rapidly and then gradually becomes saturated. At the same current density, the LOP of the micro-LEDs with concave circular sidewalls are greater than those of the micro-LEDs with flat sidewalls, and the LOP of the micro-LEDs with concave-convex circular composite structure sidewalls are larger than those of the micro-LEDs with concave circular sidewalls. Meanwhile, the LOP of the micro-LEDs using the concave-convex circular composite structure sidewalls are increased along with the decrease of the radius of the concave- and convex-circle. The saturation current density of Devices A-F are 2420.3, 2433.1, 2458.5, 2458.5, 2458.5, and 2458.5 A/cm\({}^{2}\), respectively. As well, the corresponding saturation LOP of Devices A-F are respectively 37.08, 37.46, 39.75, 39.09, 38.67, and 38.11 mW. Compared to Device A with flat sidewalls, the light output characteristics of Devices B-F are improved to varying degrees. In particular, the LOP of Device C using the concave-convex circular composite structure sidewalls with a radius of 2 \(\upmu\)m is increased by 7.2%. This is because the patterned sidewalls structure reduces the TIR of the micro-LEDs, increasing the emitting probability of photons at the sidewalls and thereby increasing the LEE by permitting photons to find the escape cones of the horizontal direction [32,33,36]. However, the LOP of Device B using concave circular sidewalls is only increased by 1%, which is lower than those of Devices C-F using concave-convex circular composite structure sidewalls. This is because the patterned sidewalls structure can increase LOP, but the Device B loses nearly 5.15% of the active region area due to the use of concave circular sidewalls, which offsets part of the increased LOP, thus resulting in  
Figure 5: \(I(dV/dI)\) versus current density of all studied devices. an LOP enhancement of only 1%. With the decrease of the radius of the concave- and convex-circle, the LOP of the micro-LEDs with concave-convex circular composite structure sidewalls is increased. This is because for the same size of the device, the smaller the radius of the concave- and convex-circle, the more the light emitting sidewall surface that the photon can escape, the easier it is to escape, thus achieving the improvement of LOP. In order to further verify the influence of the concave-convex circular composite structure sidewalls on light output characteristics, the relationship between the calculated wall plug efficiency (WPE) and current density is studied as shown in Figure 6, the EL intensity as a function of wavelength is investigated as shown in Figure 7. At a current density of 1006.3 A/cm\({}^{2}\), the WPE of Devices A-C are 7.81%, 7.83%, and 8.36%, respectively. Similarly, Devices C and B have an enhancement of 7.04% and 0.3% compared with Device A, respectively. The EL intensities of Device C are much higher than that of Devices A and B. Furthermore, compared to Device A, the EL intensities of Device B are slightly greater. The results are consistent with the comparison of LOP shown in Figure 2:0 It is further proved that the concave-convex circular composite structure sidewalls achieve an improvement in micro-LED light efficiency without losing a large amount area of active region, on account of the TIR reduction.