# Improvement of Light Extraction in Deep Ultraviolet GaN Light Emitting Diodes with Mesh P-Contacts
## 1 Introduction  
Light-emitting diodes (LEDs) have rapidly developed into the main light source in the world because of the high emission efficiency of the blue band from InGaN/GaN multiple quantum wells (MQWs) grown on sapphire substrates [1; 2; 3; 4; 5; 6]. Although there is some controversy about the decline in the efficiency of blue GaN LEDs, under certain controlled operating conditions, the state-of-the-art external quantum efficiency (EQE) has reached approximately 75% [7]. The GaN material system exhibits fascinating characteristics. The AlInGaN alloy composition can be adjusted to control its emission wavelength. In principle, the emission wavelength corresponding to the bandgap of the alloy can be tuned from the infrared (IR), visible to ultraviolet (UV) bands [8; 9]. However, when the emission wavelength is adjusted to the green and ultraviolet bands, the emission efficiency of GaN-based LEDs rapidly decreases. There are many reasons for the rapid decline, including immature epitaxial growth and process technology [10; 11; 12; 13; 14; 15]. The reason why AlGaN-based UV LEDs' technology becomes important is that people see that the application of UV LEDs is of great help to our daily lives. We can simply subdivide UV into UVA, UVB, and UVC. UVA (Near-UV light source) is the wavelength range of 365 nm to 410 nm. UVC is the wavelength range of 200 nm to 280 nm, also called deep ultraviolet (DUV) light, with the advantages of short wavelength and high energy. It can destroy the molecular structure and function of nucleic acids in the cells of microorganisms (bacteria, viruses, spores, and other pathogens) through radiation in a short time and the microbial cells are lethal and cannot be regenerated to achieve the purpose of disinfection and sterilization. Therefore, the DUV LED is widely used in medical phototherapies, such as air sterilization and purification, water sterilization (including static water and flowing water), and surface disinfection [16; 17; 18].  
The efficiencies and power levels of AlGaN-based UV LEDs today are still low compared to visible wavelengths, such as blue, green, and red LEDs [19]. The main reasons for low EQE is the low light-extraction efficiency (LEE). Many LED structures have improved the LEEs by using pattern sapphire substrates [20; 21], different designs of chip structures [22; 23; 24], and different reflective materials of p-side mirrors [25; 26]. Especially in DUV LEDs, light propagating outside the escaping cone formed by the refractive index contrast between the chip and the environment, and that is reflected back to the semiconductor and substrate layer, will be absorbed by active layers, p-type layers with smaller bandgaps and electrodes that have low reflectivity to the wavelength of the DUV. Different designs of p-side structures and mirror systems have been shown to enhance light output from near-UV LEDs [27; 28; 29; 30]. In this study, we propose a DUV LED structure with mesh p-GaN/indium-tin-oxide (ITO) contacts and a Ti/Al/Ni/Au layer as a reflective layer to improve LEE. By carefully arranging the mesh configuration, we can significantly improve the emission efficiency and increase the output power of the DUV LED with an emission wavelength at 280 nm.