AIGaN-based UV LEDs are widely used in sterilization, disinfection, purification, phototherapy and other fields, but their performance still needs to be further improved. Due to the advantages of reduced LED size, better current spreading, lower self-heating effect, and higher light extraction efficiency, UV MicroLEDs are expected to improve quantum efficiency, thereby expanding more potential applications. This article will review the performance-enhancing technologies and applications of UV microLEDs, providing a prospect for the further development of ALGAN-based UV-LEDs. The MicroLED format offers the possibility to solve fundamental problems commonly faced by UVLEDs, but also exhibits specific features and performance advantages, opening up new application areas including high-speed optical communications, time-resolved fluorescence lifetime measurements, optical pumping, direct writing input and charge management. Index Terms - AlGaN UV MicroLED, Optical Communications, Maskless Lithography, Time Resolved Fluorescence Detection, Charge Management.
I. Introduction Ultraviolet (UV) light-emitting diodes (LEDs) play an important role in many applications such as sterilization, disinfection, purification, phototherapy, etc. Therefore, UVleds have attracted widespread attention. The global spread of COVID-19 infection has furthered the commitment to UV LED research, as deep UV light is a very effective way to destroy viruses and bacteria. UV LEDs are generally divided into UVA (320-400nm), UVB (280-320nm) and UVC (200-280nm) devices according to their emission wavelengths [1]. Alternatively, they can be designated as deep ultraviolet (DUV, 200-300nm) and near ultraviolet (NUV, 300-400nm) LEDs [2]. Ultraviolet light with wavelengths between 100nm and 200nm is classified as vacuum ultraviolet (VUV), which is currently not possible with LED technology. However, algan-based materials are very suitable for the preparation of 200-400nm, because the bandgap of gallium nitride is a national key research and development plan project (Project No.2021YFC2202500, task No. National Natural Science Foundation of China Funded project (2021YFC2202503). Shanghai Science and Technology Commission. (No. 21511101303), Frontier Technology Program Project of Jiangsu Natural Science Foundation (BE2021008-2), Fudan University-CIOMP Joint Fund Project (No. FC2020-001) and Fudan Biomedical Engineering Fund; funded by EPSRC in the UK EP/T00097X /1, EP/P02744X/1, EP/M01326X/1, EP/K00042X/1 and EP/D078555/1. (Tian Pengfei, Shan Xinyi and Zhu Shijie contributed the same to this work.) (Corresponding authors: Tian Pengfei, Gu Weidan) Tian Pengfei, Shan Xinyi and Zhu Shijie are working at the School of Information Technology and the School of Engineering Technology, Fudan University, Shanghai, China, respectively (email: Shanghai@fudan.edu.cn). Xie Enyuan, Jonathan J. D. McKendry, Gu Ai Erdan and Martin Dawson at the Institute of Photonics, Department of Physics, University of Strathclyde, Glasgow, UK (email: erdan.gu@strath.ac.uk). (3.4eV) and AlN (6.2eV) respectively Corresponds to the photon wavelengths in NUV and DUV. By adjusting the Al composition in the algan-based alloy material, the emission wavelength can be continuously changed in the ultraviolet band, and by adding the In composition, the wavelength can be further extended to 400nm, thereby achieving complete ultraviolet Wavelength coverage. Other materials suitable in principle for UVLEDs include zinc oxide [3], Ga2O3 [4], and perovskites [5], although these have not yet shown comparable performance to algan-based UV LEDs. Among them, Ga2O3 is due to Its direct wide band gap, high breakdown electric field, high electron saturation velocity, but poor n-type Ga quality have been widely used in DUV photodetectors, electroluminescent devices, and chemical sensors. 2O3 is caused by poor oxygen vacancies and defects. Thin films limit the preparation of high-performance Ga2O3-based DUV-LEDs [4]. The improvement of epitaxial quality of iii-nitride compound semiconductor materials has laid a good foundation for the development of short-emission wavelength AlGaNled. Following Nakamura et al. to achieve high-efficiency blue Following the international development of InGaNled[6] and the resulting technology, UV LEDs based on algan have also developed rapidly. However, even after a long period of development, UV LEDs, especially UVCled, still suffer from large thread dislocation densities (TDDs) , low p-doping and high transverse magnetic (TM) polarized emission, resulting in much lower quantum efficiency than InGaN blue LEDs [7]. Several reviews have been published to improve, expand the application of this technique, and explore the basic properties and underlying mechanisms of [8]. In recent years, so-called MicroLEDs between 1100um have attracted increasing attention, especially due to their application in new forms of electronic visual display technology. Relevant fundamental research shows that MicroLEDs also have superior performance characteristics compared to large-area devices, including better current spreading, lower self-heating effects, higher modulation bandwidth, higher optical power density, and higher optical power density. light extraction efficiency. These developments have prompted consideration of the potential advantages and applications of MicroLEDs operating under UV light. The microLED format offers the possibility to solve fundamental problems commonly faced with UVLEDs, but also offers specific features and performance advantages that open up new areas of application. In this review, we will focus on those in UV MicroLEDs that are expected to improve low quantum efficiency.
Figure 1. (a) Image from CCD of UV MicroLED and conventional LED [12]. (b) (Left) EQE comparison of current densities for UVCleds of different sizes. (Right) Peak EQE and peak current density for different sizes [13]. (c) (Left) Current densities of LOP and EQE versus nanorods and planar LEDs. (Right) Normalized angle-resolved electroluminescence spectra of nanorods and planar LEDs [14]. These figures are reproduced with permission by reference.
II. UV Particles Since the wide bandgap of AlGaN-based materials from 3.4 to 6.2 eV, as mentioned above, covers almost the entire UV spectral range (200-400 nm), AlGaN-based materials are ideal and suitable materials for UV LEDs. In addition, compared with traditional UV light sources, ALGAN-based LEDs have the advantages of controllable emission wavelength, low power consumption, long lifespan, environmental friendliness, and easy integration. However, the external quantum efficiency (EQE) of AlGaN-based UV LEDs is still relatively low, especially at shorter wavelengths (<365 nm). In recent years, many methods have been proposed to improve the efficiency of AlGaN-based UV LEDs. Among them, some studies have shown that reducing the chip size to microscale can improve the efficiency of LEDs [11-15]. Adiwarahan et al. reported an interconnected micro-LED geometry to improve current spreading in LEDs, reduce device series resistance and thermal impedance, and thus increase maximum light output power [11]. as the picture shows. 1(a), the UV-continuously connected MicroLED geometry has more uniform current spreading in CW operation [12] compared to the conventional one with severe lateral current crowding effect. Generally, when the overall size of the device and the light-emitting area are the same, the smaller the device size and the larger the spacing, the better the heat dissipation [16]. However, some applications may impose practical constraints on minimum size or pitch, such as to match the geometry of the optics. Yu et al. The size-dependent optical and electrical properties of UVCAlGaN were investigated.
Based on LEDs as shown. 1(b) [13]. It is well known that TM polarized emission increases with increasing Al content in the active region. The increase in TM polarized emission means that photons cannot be easily extracted from the light escape cone, resulting in low light extraction efficiency (LEE) and EQE values. For smaller chip size LEDs, the enhanced LEE is attributed to the increased outcoupling of tm polarized photons. In addition, Zhang et al. Further downscaling to the nanoscale, the 274nm nanorod LEDs demonstrate 2.5 times the light output power (LOP) and EQE values of large planar LEDs, as shown. 1(c) [14]. Figure 1(c) also shows the beam divergence angles of planar LEDs and nanorod LEDs. The larger the divergence angle of the nanorod LED is, the greater the vertical photon extraction amount of the nanorod LED is. It is also worth noting that there have also been reports of MicroLEDs composed of nanowires. Liu et al. The relatively high EQE of green nanowires is 5.5% due to the effective surface strain relaxation of the nanowires [17], which may provide a feasible route to improve the efficiency of UV emitters. Therefore, these studies all show that reducing the scale of UV LEDs can improve the current spreading and suppress the lateral transport of carriers in mqw, which is a promising way to realize efficient UV emitters. In the next section, we will review the current status and challenges of UV MicroLEDs, as well as the latest technologies to improve the efficiency of UV MicroLEDs. A. Technical Status and Challenges of Ultraviolet MicroLEDs EQE, defined as the ratio of the number of photons emitted from the device to the number of carriers injected into the device, is a key performance parameter of LEDs. This paper summarizes the EQE values of AlGaN micro-LEDs and conventional LEDs in the ultraviolet spectral range (200-400 nm), see Figures [13, 14, 18-27]. .2 So far, most of the research has focused on the different applications of UV MicroLEDs, but relatively few studies on the improved EQE of UV MicroLEDs. In the UV band, researchers from the University of Science and Technology of China (USTC) fabricated UV LEDs emitting at 275 nm in different sizes of 300, 200, 100, 50 and 20 um, and they found that the EQE peak value of 20 um was 1.2%, compared with that of 300 um [13] ] compared to an increase of 20%. In addition, the University of South Carolina (USC) also reported a 5-micron MicroLED, which achieved a record brightness of 570Wcm-2 with a peak EQE of 2% [18]. Currently, for conventional sized LEDs in the UVC region, RIKEN reports the highest EQE for UVC LEDs emitting at 275 nm [20], reaching 20%. In the UVA band, researchers from Tsinghua University (THU) fabricated a high-resolution 960 540 UV MicroLED array with a single pixel size of 8um and a peak EQE of 5.5% [19]. Many commercial UV LEDs in the UVA band (320-400nm), especially in the 365-400nm region, show high performance. However, most UVB and UVCled still have poor eqe. Although reducing chip size is a powerful technique for improving UVled efficiency, the EQE of UV MicroLEDs is still relatively low, as shown in the figure. 2. And there is room for improvement. There are many factors that limit the efficiency of UV MicroLEDs. Figure 3 illustrates the technical challenges posed by each layer of a typical MicroLED structure. First, because sapphire is low-cost and transparent across the entire UV spectral region, it is often used as a substrate for UV MicroLEDs. However, the LEE on the substrate side will be limited due to the limitation of the extraction cone caused by sapphire [28]. Second, the large lattice mismatch between the sapphire substrate and AlGaN leads to the high TDD of the AlGaN layer. Single crystal AlN is used for strain management to reduce the defect density of AlGaN layers [29,30]. To obtain AlN with low defect density on sapphire substrates, various methods have been proposed, including hydride vapor phase epitaxy of AlN[31], migration-enhanced metal organic chemical vapor deposition of AlN[32], and high temperature AlN[33] Metal organic vapor phase epitaxy. However, in the AlN layer, the UV absorption of the AlN buffer layer caused by residual impurities such as oxygen, carbon, silicon, etc., is still a problem [34]. Third, it is difficult to obtain high n-type and p-type doping efficiencies due to the increased ionization energy of Si donors and Mg acceptors with high Al content, which further affects the performance of UV MicroLEDs [35]. The limitations of the n-type doping of AlGaN result in high contact resistance, which results in high voltage operation and low electrical efficiency. In addition, AlGaN has insufficient p-doping, resulting in a lower concentration of holes injected into the active region and thus poor ohmic contact. Several research groups have explored solutions to improve p-doping efficiency in AlGaN, including the use of Mg-doped superlattices [36], Mg-delta doping [37], and Mg-doped hexagonal boron nitride [38] ] instead of pAlGaN. Typically, a p-GaN layer is grown on top of p-AlGaN to provide better ohmic contact and improved hole supply. However, the strong absorption of UV light by the p-GaN layer results in poor LEE. These factors also present challenges to conventional sized UV LEDs. Compared with conventional-sized devices, UV MicroLEDs with micro-sized mesas facilitate the extraction of TM polarized emission, resulting in improved LEE. Furthermore, as mentioned above, the UV MicroLED approach improves current spreading for efficient thermal management. In contrast to the enhancement of LEE and thermal management in MicroLEDs, the reduced LED size makes it easier for carriers to reach the edge of the mesa, resulting in significant surface nonradiative recombination, which reduces efficiency at low current densities [39]. In addition, dry etching is usually employed to define the light-emitting region (active area) of MicroLEDs, which often results in many defects in the sidewalls of the mesa, resulting in more non-radiative recombination, reducing the internal quantum efficiency (IQE) [10]. Therefore, improving the efficiency of UV MicroLEDs needs a lot of research. B. Techniques to Improve the Efficiency of Ultraviolet MicroLEDs In order to achieve efficient light extraction, some research