# Improvement of Light Extraction in Deep Ultraviolet GaN Light Emitting Diodes with Mesh P-Contacts
## 2 Materials and Methods  
In this work, trimethylgallium (TMGa), trimethylaluminum (TMAI) for the group-III element, and ammonia (NH\({}_{3}\)) for the group-V element were used in the low-pressure metal-organic chemical vapor deposition (MOCVD) to grow the DUV LED structure on an AlN template grown (0001)-oriented c-plane sapphire substrate. Firstly, a 25 nm-thick AlN template was grown as a buffer layer followed by a 1.5 \(\upmu\)m-thick AlN layer, and a 2 \(\upmu\)m-thick Si-doped Al\({}_{0.45}\)Ga\({}_{0.55}\)N layer. Then, five periods of Si-doped Al\({}_{0.4}\)Ga\({}_{0.6}\)N/Al\({}_{0.6}\)Ga\({}_{0.4}\)N Multiple Quantum Well (MQW)s comprising 6 nm-thick Si-doped Al\({}_{0.4}\)Ga\({}_{0.6}\)N quantum wells and 6 nm-thick Si-doped Al\({}_{0.6}\)Ga\({}_{0.4}\)N barriers were grown on the Si-doped Al\({}_{0.45}\)Ga\({}_{0.55}\)N. Finally, 20 nm-thick p-Al\({}_{0.7}\)Ga\({}_{0.3}\)N layer electron blocking layers (p-EBLs) doped with Mg were grown, followed by a 50 nm-thick p-Al\({}_{0.3}\)Ga\({}_{0.7}\)N layer and a 50 nm-thick p-GaN for the current spreading contact layer. The carrier concentrations were estimated to be \(4\times 10^{18}\) cm\({}^{-3}\) in the n-AlGaN layer by the Hall measurement. The doping concentration of Mg in p-GaN was estimated to be \(8\times 10^{18}\) cm\({}^{-3}\). The schematic layer structure of the based-DUV LEDs is shown as Figure 1a. The chip process begins with the use of inductively coupled plasma (ICP) etching to define a 510 \(\upmu\)m \(\times\) 510 \(\upmu\)m mesa with an array of a circular recess having a diameter of 40 \(\upmu\)m for n-contacts. For n-ohmic-contact electrodes, a stack of Ti/Al/Ti/Au layers was deposited by electron beam (E-beam) evaporation and then alloyed at 800 \({}^{\circ}\)C for 180 s in an N\({}_{2}\) atmosphere to reduce the contact resistance.  
Figure 1: (**a**) The schematic epi-structure of based-DUV LEDs- Deep Ultraviolet. (**b**) The schematics of the conventional DUV LED. (**c**) The schematics of the mesh p-GaN/ITO DUV LED within four n-contacts.  
After that, two kinds of p-ohmic-contact structures were fabricated for comparison. The conventional DUV LED was deposited with an ITO layer by the sputtering system on the p-GaN layer as the p-ohmic-contact layer, and a stack of Ti/Al/Ni/Au was deposited by the E-beam evaporation on the ITO layer as a reflective layer. However, the DUV attenuation coefficients in the p-GaN and ITO are large, which will lead to low light efficiency. In order to improve this situation, the mesh p-GaN/ITO structure was proposed and formed by etching in the ITO layer and P-GaN layer of the DUV LED, and formed an array of circular holes with a diameter of 9 \(\upmu\)m. Next, an Al\({}_{2}\)O\({}_{3}\) layer (50 nm) deposited by atomic layer deposition (ALD) and a SiO\({}_{2}\) layer (50 nm) deposited by plasma-enhanced chemical vapor deposition (PECVD) were applied as the passivation layer to cover the sidewalls of these circular holes from the leakage current. Similarly, the reflective layer was deposited on the ITO layer. The novel structure reduces the volume of the p-GaN layer and ITO layer, which can decrease the light absorption in the chip. Figure 1b,c are three-dimensional schematic views of the conventional DUV LED and the mesh p-GaN/ITO DUV LED within four n-contacts.  
For more accurate measurements, we did the packaging process of the conventional DUV LED and the mesh p-GaN/ITO DUV LED. We placed packages of the two structures into an integrating sphere with Instrument Systems CAS 140 CT to measure the light output intensity-current (L-I) characteristics, the current-voltage (I-V) characteristics, and the emission spectra of the two structures. We used the Ophir SP620U beamview measurement system to measure the emission patterns of the two structures.