New Advances in the Development of High-Efficiency AlGaN Deep-Ultraviolet Light-Emitting Diodes and in the Study of Photon Transport Behavior

Recently, the research groups led by Associate Professor Chu Chunshuang, Professor Zhang Yonghui, and Professor Zhang Zihui from Guangdong University of Technology and Hebei University of Technology, in collaboration with the research group led by Associate Researcher Liu Naixin and Researcher Yan Jianchang from the Institute of Semiconductors, Chinese Academy of Sciences, have achieved significant breakthroughs in enhancing the light extraction efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs), improving hole injection efficiency, and reducing the barrier at the metal/p-type semiconductor interface.

The research team innovatively proposed: (1) simultaneously introducing scattering nanostructures on both the scribe line and the inclined sidewall, effectively overcoming the total reflection limitations at multilayer interfaces and significantly enhancing the device’s optical output power and electro-optical conversion efficiency; (2) incorporating a “photon-cycling-induced carrier regeneration” structure into the p-type hole-injection layer, which not only directly boosts the hole-injection efficiency but also effectively reduces the barrier height at the metal/semiconductor interface. The related research findings, titled “Enhancing light extraction for AlGaN-based deep-ultraviolet LEDs using scattering nanostructures on both scribe line and inclined sidewall” and “Diffusing the photon-assisted regenerated holes to increase the WPE for 259 nm AlGaN-based DUV LEDs with three-dimensional n-ZnO/p-AlGaN photon-sensitive micro-structure arrays,” have been published in the renowned journals *Optics and Laser Technology* and *Applied Physics Letters*, respectively, in the fields of optics and applied physics.

Through simulation analysis, the research team found that in deep-ultraviolet LEDs, whether it’s TE-polarized light or TM-polarized light, the proportion of light confined within each structural layer differs significantly. Most of the light is confined within the n-AlGaN layer, whereas conventional patterned substrates struggle to efficiently extract this portion of light.

Figure 1: Light confinement ratios of each layer of the DUV LED obtained via FDTD simulation

To address this challenge, the research team employed a self-assembled SiO₂ nanoparticle template technique to successfully fabricate n-AlGaN scattering nanostructures in the chip dicing lane region and the adjacent inclined sidewall areas. Experimental results show that, without compromising other optoelectronic properties, this structure boosted the optical output power of deep-ultraviolet LEDs by 13.01% and improved the electro-optical conversion efficiency by 12.92% at an injection current of 100 mA.

Figure 2 (a) Top-view SEM image of the DUV LED; (b) Magnified view of the nanostructures, with the inset showing the nanostructures on the scribe line at higher magnification; (c) Side-view SEM image of the scribe-line edge; (d) Schematic diagram of the structure of Device R versus Device A, along with histograms illustrating the statistical distributions of (d) diameter, (e) height, and (f) period of the nanostructures.

Figure 3 (a) Current-voltage characteristics of device R and device A; the inset shows the electroluminescence spectra of the two DUV-LEDs at an injection current of 100 mA; (b) Comparison of optical output power and electro-optical conversion efficiency between the two DUV-LEDs; (c) Luminescence photographs of the two DUV-LEDs at an injection current of 20 mA.

More notably, the study revealed a synergistic relationship between light extraction efficiency and the size of the cleavage grooves. FDTD simulation results indicate that in deep-ultraviolet LEDs fabricated using conventional processes, increasing the width of the cleavage grooves can indeed enhance light extraction efficiency to some extent; however, this enhancement effect is limited and eventually levels off. In contrast, in devices incorporating scattering nanostructures, the light extraction efficiency shows a continuous upward trend as the width of the cleavage grooves increases. This finding provides an important basis for the optimized design of high-power deep-ultraviolet LEDs.

Figure 4 (a) The relationship between the extraction efficiency of TE-polarized light for devices R, A, and B as a function of the ratio of cutting groove width; (b)-(d) show the XY cross-sectional electric field distributions of devices R, B, and A, respectively, under TE-polarized light; (e) shows the variation of extracted light intensity with position.

This technology boasts advantages such as simple processing, low cost, and compatibility with existing production lines. It is particularly well-suited for power-type deep-ultraviolet LEDs and Micro-LEDs, where the proportion of the chip area dedicated to the current path is relatively high, and thus holds great potential for industrial-scale application. This research provides a novel and viable technological approach for the fabrication of high-performance deep-ultraviolet LEDs.

For the deep-ultraviolet photons that do not escape from the p-type region, the research team proposed a reverse-biased three-dimensional n-ZnO/p-AlGaN microstructure array to achieve continuous energy conversion between light and electricity, as shown in Figure 5. This design, on the one hand, suppresses the hole depletion effect at the surface of the p-AlGaN layer, thereby reducing the contact resistance of the p-type region; on the other hand, it significantly enhances the hole injection efficiency into the active region. Experimental results indicate that, at an injection current of 45 mA, the forward operating voltage of the proposed device is reduced by 25%, while the optical output power increases by 47%. As a result, the wall-plug efficiency (WPE) of the 259 nm AlGaN-based deep-ultraviolet LED is improved by 46%, as illustrated in Figure 6.

Figure 5: Schematic diagram of the fabrication process flow for Device R and Device A: (a) Epitaxial growth; (b) ICP etching to form the mesa structure; (c) Deposition of an SiO₂ layer on Device R; (d) Deposition of an n-type electrode on Device R; (e) Deposition of a p-type electrode on Device R; (f) ICP etching of Device A to form micropores; (g) Filling with a ZnO layer; (h) Deposition of an SiO₂ layer on Device A; (i) Deposition of an n-type electrode on Device A; (j) Deposition of a p-type electrode on Device A.

Figure 6: Current–voltage characteristics of Device A and Device R plotted on (a) semi-logarithmic coordinates and (b) linear coordinates; (c) normalized WPE of Device A and Device R.  

This is primarily attributable to the photonic-free-carrier energy conversion physical process realized in Figures 7(a) and (b): The photosensitive n-ZnO/p-AlGaN junction converts photons into electron-hole pairs, with the regenerated holes being reinjected into the quantum well, while the electrons are extracted from the device through the n-ZnO region. The one-dimensional energy-band diagram of n-ZnO/p-AlGaN/n-ZnO shown in Figure 7(c) indicates that the built-in electric field at the n-ZnO/p-AlGaN interface significantly enhances the lateral band bending. This not only facilitates the transport of holes toward the p-type layer but also suppresses the hole-depletion effect at the surface of the p-AlGaN layer. Figures 7(d) and 7(e) respectively illustrate the longitudinal energy-band distributions beneath the n-ZnO micropillars and between two adjacent micropillars. As shown in Figure 7(d), the electric field formed at the n-ZnO/p-AlGaN junction exerts a “hole-acceleration” effect: while promoting the transport of holes toward the p-type layer, it can also directly inject holes into the quantum well. Thus, the charge-coupling effect of the three-dimensional n-ZnO/p-AlGaN structure can directly facilitate the injection of regenerated holes into the multiple-quantum-well region and simultaneously counteract the hole-depletion effect at the surface of the p-type layer, thereby enhancing the light-emitting efficiency of deep-ultraviolet LEDs and reducing the contact resistance of the device.

Figure 7 (a) Schematic diagram of the three-dimensional structure of device A; (b) Carrier generation and transport process; (c) Cross-sectional one-dimensional band diagram of the n-ZnO/p-AlGaN/n-ZnO structure under an injection current of 45 mA; (d) Longitudinal one-dimensional band diagrams of the n-ZnO/p-AlGaN/quantum-well heterojunction region and (e) the p-GaN/p-AlGaN/quantum-well heterojunction region.  

Jia Yuedong, a master’s student at Hebei University of Technology, is the first author of the paper titled “Enhancing light extraction for AlGaN-based deep-ultraviolet LEDs using scattering nanostructures on both scribe line and inclined sidewall.” Professor Zhang Yonghui from Hebei University of Technology, Professor Zhang Zihui from Guangdong University of Technology, and Associate Researcher Liu Naixin from the Institute of Semiconductors, Chinese Academy of Sciences, are the co-corresponding authors. Other collaborating institutions include Shanxi University, the Institute of Semiconductors, Chinese Academy of Sciences, and Shanxi Zhongke Luan Ultraviolet Optoelectronics Technology Co., Ltd.

Associate Professor Chu Chunshuang from Guangdong University of Technology is the first author of the paper titled “Diffusing the photon-assisted regenerated holes to increase the WPE for 259 nm AlGaN-based DUV LEDs with three-dimensional n-ZnO/p-AlGaN photon-sensitive micro-structure arrays,” with Professor Zhang Zihui from Guangdong University of Technology serving as the corresponding author. Collaborating institutions also include Professor Zhang Yonghui from Hebei University of Technology, Associate Researcher Liu Naixin and Researcher Yan Jianchang’s teams from the Institute of Semiconductors, Chinese Academy of Sciences, and others.

This research was supported by the National Key Research and Development Program (2022YFB36051002023YFB3609703), the National Natural Science Foundation of China (62275073), the Shanxi Province Science and Technology Cooperation and Exchange Project (202404041101049), and the Hebei Province Natural Science Foundation (F2025202016).