# Improved Performance of High-Voltage Vertical GaN LEDs via Modification of Micro-Cell Geometry
## 3 Results and Discussion
The scanning electron microscopy (SEM) image of the roughened n-type GaN surface is shown in the inset of (**b**). respectively. The result agrees well with the effect of the charge diffusion length between the electrode and edge on the current spreading behavior of these samples that we have described. The heat dissipation characteristics of the HV-LEDs can be confirmed through thermal infrared imaging. The surface temperature distributions of Samples A-F are displayed in Figure 4a-f, respectively, at an injection current of 80 mA. For Samples A-F, the surface temperatures were 37.63-40.32, 38.12-39.08, 38.22-39.42, 37.12-38.36, 36.44-37.42, and 35.11-36.02 \({}^{\circ}\)C, whereas the temperature differences were 2.69, 0.96, 1.20, 1.24, 0.98, and 0.91 \({}^{\circ}\)C, respectively. Sample F possessed a lower surface temperature and a more uniform temperature distribution. Moreover, the temperature differences of Samples B and E were similar to that of Sample F. The temperature differences of Samples C and D (1.20 and 1.24 \({}^{\circ}\)C, respectively) were close to each other. However, in Sample A, a more nonuniform temperature distribution (2.69 \({}^{\circ}\)C) was observed. Compared with the electrodes of Samples B-F, the electrodes of Sample A exhibited the greater heat accumulation; the heat accumulation resulted in electrode deterioration, which reduced the reliability and lifetime of the device. In addition, the measured results of surface temperatures for these six samples were in close agreement with those of the spatial distribution images of the light output (Figure 3). According to the aforementioned results, Sample E, fabricated using four triangular cells, had the higher light extraction than that of the other samples, which can be attributed to the lower current crowding near the electrodes and a more uniform light emission in this device. On the contrary, Sample A, fabricated using four square cells, had the lowest light extraction because of the severe current-crowding phenomenon near the electrodes. Although Sample F, fabricated using four L-shaped cells, had the lower current crowding near the electrodes, its light extraction performance was worse than that of Sample E. As mentioned, in Sample F, the two-line electrode was prepared on a single cell to improve the current spreading. However, the electrode design caused another problem (i.e., the electrode-shading loss effect). Therefore, the light extraction of Sample F was reduced. Figure 5 shows the wall-plug efficiencies (WPEs) of epoxy-packaged Samples A-F as a function of injection current of 20-350 mA. The WPEs of packaged Samples A-F at 20 mA were 33.8%, 36.0%, 38.6%, 38.4%, 41.1%, and 37.2%, respectively. The WPEs of these six HV-LEDs at 80 mA were 24.2%, 27.0%, 28.1%, 28.3%, 29.7%, and 26.9%, respectively. The measured results of the I-V characteristics, the light output powers, the spatial distributions of light extraction, and the surface temperatures confirm that the design of Sample E is helpful for improving the optoelectronic performance of vertical  
Figure 3: Spatial distribution images of light output for Samples A–F (**a–f**, respectively) at an injection current of 20 mA. HV-LEDs. In addition, this study reveals that the fabrication processes of vertical LED structure are highly feasible for HV-LED applications. At present, the current mass-produced HV-LEDs are almost lateral-type fabricated on sapphire substrates. Due to severe current-crowding and heat-conduction problems, the light output power of the lateral HV-LEDs saturate at \(\sim\)200 mA. To our knowledge, at an injection current of 80 mA, the highest output power and wall-plug efficiency of current lateral HV-LEDs with epoxy packaging (connected with 2 \(\times\) 2 micro-cells, micro-cell size: 1016 \(\upmu\)m \(\times\) 1016 \(\upmu\)m) are approximately 310.0 mW and 24.8%, respectively.