# Improvement of Light Extraction in Deep Ultraviolet GaN Light Emitting Diodes with Mesh P-Contacts
## 4 Discussion  
In order to study the effect and design of p-mesh contacts for DUV LEDs, simplified 2D simulations were executed with Advanced Physical Models of Semiconductor Devices (APSYS) simulation program from Crosslight Software, Inc, which can deal with electrical and optical properties of LEDs. There are four structures to be compared: conventional ones (unmeshed) and three meshed ones with the same remained p-GaN area but meshed into 2, 3, and 4 segments, respectively [31; 32]. The detailed structural parameters are shown in Figure 5a. The model only deals with the region between two n-contacts similar to the schematics shown in Figure 1. In the simulation, the Shockley-Read-Hall recombination lifetime, zBN, and Auger recombination coefficient were set to be 3 ns, \(8\times 10^{-13}\) cm\({}^{2}\)/s, \(1\times 10^{-30}\) cm\({}^{6}\)/s, respectively [28; 29]. The polarization screening coefficient and carrier mobility are tuned to fit the I-V curve of conventional one obtained from the experiment, which has a turn-on voltage of 5.5 V and a serial resistance of about 85 \(\Omega\). For different absorption behavior of the four structures, ray-tracing simulations are conducted to acquire the light extraction efficiency. The light sources in the ray-tracing simulations are set according to the distribution of radiative recombination, and the absorption coefficients of p-GaN and ITO are set to be \(1.7\times 10^{5}\) cm\({}^{-1}\) and \(1\times 10^{5}\) cm\({}^{-1}\) while the other regions are assumed to be transparent [33; 34].  
Figure 5b shows the simulated L-I-V characteristics and the extraction efficiencies of the four structures, which are 37.1%, 43.6%, 41.2%, and 42.6%, respectively. Indeed, the light extraction efficiency improved by 15-17% when the p-GaN and ITO layers were meshed; however, no obvious variations of extraction efficiencies were observed when further dividing those layers into more segments. On the other hand, a monotonic increase of operating voltage was observed with the increase of segments number, and the large operating voltage caused an unbalanced MQW band diagram leading to much severe electron current leakage. Figure 5c shows the normalized electron current flowing in the growth direction, and the electron current flowing through the active region was considered to be a leakage current. It is noteworthy that, the current was not uniform over lateral direction, so the electron current in Figure 5c was the integrated result to reflect the overall condition. In Figure 5c, nearly 100% of the electron flies over the active region, which explains the unusual low output power of the four-segment structure. The leakage current effect was less severe in the three-segment structure and it showed much better light output power. Although the leakage current effect in the two-segments structure was a little bit higher than that in the unmeshed structure, the fewer light absorption in p-GaN and ITO, as well as the light scattering effect, make the light output power of the two-segment structure higher than that of the unmeshed structure, and the results were consistent with the experimental ones.  
Figure 4: Near field light emission patterns of (**a**) the conventional DUV LED and (**b**) the mesh p-GaN/ITO DUV LED.