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Abstract
In this work, we hybridize an air cavity reflector and a nanopatterned sapphire substrate (NPSS) for making an inclined-sidewall-shaped deep ultraviolet micro light-emitting diode (DUV micro-LED) array to enhance the light extraction efficiency (LEE). A cost-effective hybrid photolithography process involving positive and negative photoresist (PR) is explored to fabricate air-cavity reflectors. The experimental results demonstrate a 9.88% increase in the optical power for the DUV micro-LED array with a bottom air-cavity reflector when compared with the conventional DUV micro-LED array with only a sidewall metal reflector. The bottom air-cavity reflector significantly contributes to the reduction of the light absorption and provides more escape paths for light, which in turn increases the LEE. Our investigations also report that such a designed air-cavity reflector exhibits a more pronounced impact on small-size micro-LED arrays, because more photons can propagate into escape cones by experiencing fewer scattering events from the air-cavity structure. Furthermore, the NPSS can enlarge the escape cone and serve as scattering centers to eliminate the waveguiding effect, which further enables the improved LEE for the DUV micro-LED array with an air-cavity reflector.
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1. Introduction
AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) have a wide range of applications in optical communication, sterilization, water purification and medical diagnosis [1–5]. However, DUV LEDs still encounter the challenge of small external quantum efficiency (EQE) and low optical power output, which limits massive applications in the community. The primary reason for the low EQE lies in a low light extraction efficiency (LEE) that is normally below 10% [6]. The low LEE of AlGaN-based DUV LEDs primarily stems from the strong absorption of DUV photons by p-GaN layer and low reflectivity for metal mirror [7]. Furthermore, there is a large refractive index contrast between AlGaN materials and air, resulting in very remarkable total internal reflection (TIR) of photons in devices. Consequently, majority of photons encounter difficulties in entering escape cones [8,9]. Moreover, for high-Al component AlGaN-based active region, the emission of transverse magnetically (TM) polarized light propagates perpendicular to [0001] orientation, which reduces the probability of entering escape cones for photons [10]. In order to improve the LEE, various approaches have been proposed including using nanopatterned sapphire substrates (NPSS) [11,12], advanced metal reflectors [13,14], inclined sidewall structures [15–17], mesa geometry [18] or meshed p-GaN layer [19,20], etc. Among those technologies, epitaxial growth based on NPSS also helps to reduce thread dislocation density (TDD) and improve the crystalline quality, thereby increasing the internal quantum efficiency (IQE) [21]. In addition, the inclined sidewall structure for DUV micro-LED array exhibits significant potential in enhancing the LEE for both transverse electric (TE)-polarized light and TM-polarized light because the sidewall enables effective scattering effect for the light to enter escape cones [22,23]. The LEE enhancement by the sidewall can be more significant when the size for DUV micro-LED is further reduced. Integrating omnidirectional reflector (ODR) with inclined sidewalls also provide more room for increasing LEE [24]. However, the presence of total internal reflection shall induce surface plasmon polaritons (SPPs) of Al that result in a low reflectivity for the Al-based ODR [25]. To avoid the resonance absorption by SPPs, our previous study has proposed replacing metal reflector on the inclined sidewall with an air-cavity reflector. The air-cavity comprises an air cavity and a bottom metal mirror [26]. Another advantage is that the air cavity can provide new light escape paths. Nevertheless, the air-cavity-based structure requires very careful device fabrication. Therefore, developing a cost-effective process to fabricate a bottom metal mirror on the air-cavity for DUV LEDs is essentially important. Furthermore, the impact of air-cavity reflector on the optical properties for DUV micro-LED array on NPSS need exploring.
In this work, the air-cavity reflector and NPSS are utilized in an inclined-sidewall-shaped DUV micro-LED array, and a cost-effective fabrication process for making air-cavity reflector is proposed. The experimental results show the air cavity reflector can be fabricated by using hybrid photolithograph process of positive and negative photoresist (PR). The light output power (LOP) of the DUV micro-LED array on NPSS is improved by using the bottom air-cavity reflector. Moreover, two-dimensional finite-difference time-domain (2D FDTD) simulations reveal that the NPSS and the size of the micro-LED have great influence on the LEE. The reduction in device size enables a greater number of photons to be scattered by the air cavity, leading to a significant increase in LEE. Moreover, the air-cavity reflector can obviously promote the scattering effect of NPSS due to the high reflectivity and the offer for additional escape cones. Therefore, the air-cavity reflector shows great potential in enhancing the performance of DUV micro-LED array on NPSS.
2. Experiment
We firstly fabricate NPSS on which the hole pattern has a diameter of ∼700 nm with a depth of ∼300 nm. The period for hole pattern is approximately 1 µm. Next, a 3 µm-thick AlN buffer layer is grown on the NPSS. Then, AlGaN-based DUV LED heterostructures are grown on NPSS by using metal-organic chemical vapor deposition (MOCVD) system. A 1 µm-thick Si-doped n-Al0.60Ga0.40N electron injection layer, five periods of 3-nm-thick Al0.45Ga0.55N/12-nm-thick Al0.56Ga0.44N multiple quantum wells (MQWs), 20 nm-thick Mg-doped p-Al0.60Ga0.40N electron barrier layer (p-EBL), a 50 nm-thick p-Al0.40Ga0.60N layer and a 50 nm-thick p-GaN layer are grown to make LED structures.
Two different DUV micro-LED arrays are fabricated according to the process flow chart depicted in Fig. 1. The DUV micro-LED with an ODR on the inclined sidewall is named as Device 1, while the DUV micro-LED with an air-cavity consisting of an integrated bottom metal mirror and the inclined sidewall is deemed as Device 2. Both devices have 10 × 10 inclined-sidewall-shaped micro-LED arrays that are obtained by conducting photolithography and dry etching processes. The structure of each micro-LED pixel is a truncated cone. The top diameter of a single truncated cone is 30 µm, and the spacing among truncated cones is 5 µm. After dry etching process, a Ti/Al/Ti/Au (20/30/60/100 nm) multilayer film is deposited on n-AlGaN as the n-contact, which is then annealed in an N2 environment at 650°C for 1 minute. A Ni/Au (10/10 nm) layer is deposited on p-GaN using serving as p-electrode, which followed by annealing process in O2 at 450°C for 3 min. Subsequently, a layer of 100 nm-thick SiO2 is deposited. The deposited SiO2 is patterned and etched using buffer oxide etchant (BOE) to expose both the n-electrode and p-electrode. For Device 1, Al/Ti/Au (800/20/100 nm) multilayers are deposited to connect all Ni/Au p-contact electrodes on the truncated cone arrays, as illustrated in process A in Fig. 1. Device 2 is manufactured according to process B depicted in Fig. 1, and the processes of making n-electrode, p-electrode and SiO2 deposition is the same as that of Device 1. The most critical process step for Device 2 is the fabrication of the bottom metal mirror. Initially, a thin layer of positive PR (AZ3120) is spin-coated on the epitaxial wafer at a speed of 6000 r min-1 for 30 s. After the sample is exposed for 5 s under 365 nm ultraviolet light, the entire sample is immersed in the developing solution for 10 s. The exposure and development time are optimized carefully to ensure effective removal of the positive PR on the Ni/Au. However, the positive PR among truncated cones has to be kept for subsequent negative PR coating. Meanwhile, the remaining positive PR is ∼ 200 nm lower than the mesa. Next, a layer of negative PR (n-lof2020) is spin-coated on the epitaxial wafer at a speed of 3000 r min-1 for 30 s. Subsequently, the entire sample is exposed and developed to obtain the pattern of bottom metal mirror. The unexposed negative PR is removed by the developer while the unexposed positive PR remains intact. Therefore, during negative PR lithograph process, the positive PR under the unexposed negative PR pattern will not be exposed. Only the unexposed negative PR is removed during development, and the positive PR remains among the truncated cone arrays. Hence, a layer of Al/Ti/Au multilayers can be deposited on top of both Ni/Au and the remained positive PR. When the positive PR is removed by the developer, the air cavity can be formed under the Al/Ti/Au multilayer. Finally, the air cavity with bottom metal mirror and inclined-sidewall can be fabricated for the Device 2. In addition, by using the conventional manufacturing process [27], a conventional planar DUV LED is fabricated which is entitled as Reference device. All devices have dimensions of 350 × 350 µm2. These fabricated devices are not packaged. The electroluminescence (EL) spectra and optical power of all devices are measured by standard integrating spheres from the sapphire side at room temperature.
Fig. 1. Process flow charts for DUV micro-LED array with ODR on inclined sidewall (Process A) and with the air-cavity reflector (Process B).
3. Results and discussion
The cross-sectional profile for Devices 1 and 2 is tested by scanning electron microscope (SEM), as shown in Fig. 2. The inset in Figs. 2(a) and (b) show SEM images from a bird’s-eye-view for Devices 1 and 2, respectively. It can be clearly seen that the sidewall of Device 1 in Fig. 2(a) is coated with metal mirror. For Device 2, an air cavity is formed below the metal mirror, confirming the fabrication of the floating metal mirror on the air cavity. The angle of the inclined sidewall is measured to be approximately 63°. The top diameter of micro-LED is about 30 µm, and the spacing among the micro-LED array is 5 µm. The ohmic contact electrode is positioned at the center of the truncated cone. Furthermore, it can also be observed that both Devices 1 and 2 are grown on NPSS, resulting in numerous air holes due to the coalescence of AlN epitaxial layer on NPSS [28].
Fig. 2. SEM images of the cross section for (a) Device 1 and (b) Device 2. Inset: SEM images from a bird’s-eye-view for Device 1 and Device 2.
Figure 3 shows the electroluminescence (EL) spectra for the three devices at the injection current of 40 mA. The peak wavelength of the Reference device is 273 nm, while Devices 1 and 2 exhibit peak wavelengths of 275 nm. The longer peak wavelength observed for Devices 1 and 2 at the same current can be attributed to the larger current density due to their smaller active region area [29]. Furthermore, the EL intensities for Devices 1 and 2 are higher than that for Reference device. Notably, Device 2 shows the highest EL intensity owing to the reduced Al-caused SPP resonance absorption. The luminescence photographs for the three devices that are taken from the p-GaN side are shown in the inset of Fig. 3. Because our fabricated devices are flip-chip ones, thus the few photons can be emitted from the top side of the mesa. In addition, the micro-LED array pattern beneath the metal reflector of Device 1 can be observed from the luminescence photograph of Device 1, but it cannot be observed from that of Device 2. The luminescence photograph for metal reflector of Device 2 is the same as that of Reference device. This further suggests that the metal reflector of Device 2 exhibits a relatively flat surface due to its air-cavity structure.
Fig. 3. EL spectra for Reference, Devices 1 and 2 at an injection current of 40 mA. Inset: the luminescence photographs for the three devices under the microscope.
Figure 4(a) illustrates the current-voltage (I-V) curves for all fabricated DUV LEDs. The rectifying effect for Device 2 proves that the air cavity Al reflector has interconnected each p-electrode without any short-circuiting effect for Mirco LED array. The forward voltages at the current of 40 mA for Reference, Devices 1 and 2 are 13.11 V, 14.42 V, 14.37 V, respectively. When compared to Reference device, forward voltage for Devices 1 and 2 are higher due to the decrease of the effective p-type ohmic contact area. The effective ohmic contact areas for Devices 1 and 2 are same, and thus their electrical characteristics are not impacted. This also indicates that the air cavity reflector has minimal impact on the current transport. Figure 4(b) shows the current-dependent optical power curves for the three devices. When compared with Reference device, the optical powers for Devices 1 and 2 are enhanced by 15.74% and 27.19% at an injection current of 40 mA, respectively. The enhanced optical power is attributed to the inclined sidewall structure for Devices 1 and 2, which is more favorable for light extraction. In addition, the optical power for Device 2 is even 9.88% higher than that for Device 1 at an injection current of 40 mA. This can be attributed to the fact that the air-cavity-shaped inclined sidewall reduces the light absorption from the metal. In addition, the air-cavity structure provides additional light escape paths between the mesas, leading to the further increase in LEE.
Fig. 4. (a) Current-voltage characteristics and (b) optical power as a function of the injection current for Reference, Devices 1 and 2.
To further reveal the underlying physics mechanism, we analyze the LEEs for the three devices by using two-dimensional finite-difference time-domain (2D FDTD) simulations. Figure 5 shows the model for Reference device, Devices 1 and 2. Simulation parameters such as the layer thickness, micro-mesa size, inclination angle, etc. are set according to the experimental structure. According to the SEM picture in Fig. 2, a prismatic cavity array with 1 µm period and 1.5 µm height can be observed between sapphire and AlGaN. To represent the effect of NPSS, the same prismatic cavity array is embodied between sapphire and AlGaN as shown in Fig. 5. Due to the limitation of computer memory, the horizontal size of the entire simulation region is 159 µm, and the two lateral boundary conditions are set to 100% reflectivity of the metal, thus treating the finite horizontal size as infinite [25]. The top and bottom boundary conditions are set as perfectly matched layer (PML) to completely absorb the electromagnetic energy [19]. The AlGaN truncated cones for Device 1 are completely covered by Al metal. The sidewalls of the truncated cones for Device 2 are not covered with Al metal, but there is a flat floating Al metal reflector on the p-electrode and the air cavity is also set. A single dipole that is placed in the middle of the MQWs layer is used as the radiant source for DUV LED [26]. When emitting TM-mode (TE-mode) light, the dipole source polarization direction is parallel to the Y-axis (X-axis) direction. The power monitor is placed 500 nm away from the bottom of the sapphire to collect the light emitted from the bottom of the device. LEE is defined as the ratio of the total extraction power collected by the power monitor to the total power emitted by the dipole source [9]. For more accurately monitoring the LEE, the distance between the power monitor and the sapphire bottom is larger than the peak wavelength. Therefore, the LEE is not affected by electromagnetic waves at the sapphire/air interface.
To investigate the impact of NPSS on LEE, various devices without NPSS are also simulated. Device A features a flat sapphire substrate and an inclined sidewall covered with metal reflector. Device B on flat sapphire substrate is designed with air cavity reflector. Figures 6(a) and (b) show that the LEE curves of TE- and TM-polarized light for micro-LED arrays in terms of different micro-mesa sizes. It can be observed that the increased LEE ratio of TE-polarized light of Device 2 compared to Device 1 in Fig. 6(a) is smaller than that of TM-polarized light. It suggests that the air cavity reflector has a more pronounced effect on enhancing the LEE of TM-polarized light. This is because compared to the TE-polarized light, a higher ratio of TM-polarized light can reach the inclined sidewall. It should be noted that the overall trend observed in Figs. 6(a) and (b) indicates a gradual increase in LEE as the size decreases. This is because when the micro-mesa size of micro-LEDs becomes smaller, the lateral propagation distance for the photons towards the inclined sidewall is shorter. Consequently, more photons are scattered directly into escape cones without experiencing too many scattering events from the inclined sidewall and air-cavity structure. The reduced extracted length also leads to a lower absorption for photons, thereby effectively improving the LEE [30]. In addition, the LEE of Device 2 (Device B) is higher than that of Device 1 (Device A), which verifies the advantage of utilizing an air cavity reflector. Moreover, it can be observed that the LEE of Device 1 (Device 2) is higher than that of Device A (Device B) due to the scattering effect by the NPSS.
Fig. 6. (a) TE- and (b) TM- polarized LEEs for Devices 1 and 2 with NPSS and without NPSS as a function of micro-mesa size.
In order to better understand the optical scattering process by the air cavity reflector and the NPSS, Figs. 7(a)-(c) show the electric field distributions and propagation paths of TM-polarized light for Device A, Device B, Device 2 on XY cross section. The top diameter for the micro-mesa is 30 µm, respectively. The intensity distribution of extracted light in terms of the position are shown in Fig. 7(d). It should be noted that Fig. 7(d) only shows the light intensity in the right half for the simulated structures due to the symmetric optical profiles in Figs. 7(a)-(c). The three main regions corresponding to the main escape path for the extracted light from the three devices are labeled by the dashed line in Fig. 7(d). Region (1) originates from the direct emission of light by the light source, which then propagates into the escape cone as indicated by light path ① in Figs. 7(a)-(c). The light intensity of region (1) is identical for Devices A and B due to the same light path ①. However, for Device 2, there is a NPSS in light path ①. The NPSS can disrupt the TIR and enlarge the escape cone. Therefore, Device 2 shows the strongest intensity in region (1). For Device A, the light intensity in regions (2) and (3) originates from the light scattered into the escape cones by the inclined sidewall as indicated by light paths ② and ③ [see Figs. 7(a) and 7(b)]. It is worth noting that the light following the light path ③ experiences multiple TIRs before reaching the inclined sidewall. Therefore, the reflectivity of reflector between each micro-LED pixel will significantly affect the light intensity in region (3). For Devices B and 2, the light intensity in regions (2) and (3) not only comes from the reflection of the inclined sidewall, but also results from the air cavity reflector that provides additional light escape path ④ as depicted in Figs. 7(b) and (c). Namely, the light entering the air cavity can be reflected by the bottom metal mirror, and then shall propagate towards the AlGaN parallel interface between each micro-LED pixel and will be extracted into air space. Therefore, the light intensity of Devices B and 2 in regions (2) and (3) is higher than that of Device A. In addition, in order to make more light reflected into the escape cones, the light incident angle for inclined sidewall should fall within the range of 37.38°∼82.62° which is defined as the reflection escape cone. The critical angle of the AlGaN/SiO2 interface is calculated to be 35° according to the Snell's law. It can be attained that the incident angles within the reflection escape cone are larger than the critical angle of 35°. Therefore, all light extracted through the reflection escape cone are totally internally reflected by the inclined sidewall. For the incident angle within the reflection escape cone, the reflectivity of the AlGaN/SiO2/Air is higher than that of the AlGaN/SiO2/Al because the TIR causes the surface plasmon resonance absorption by the Al reflector [26]. Therefore, the LEEs of Devices B and 2 are higher than that of Device A. In addition, we carefully observe the region (4) in Fig. 7(d), and we find that the light intensity in region (4) for Device 2 is higher than that of Device B. We believe it can be attributed to light path ⑤ as depicted in Fig. 7(c). In the case of Devices A and B, some light is guided between AlGaN and the sapphire interface due to TIR as indicated by light path ⑤ in Fig. 7(b). However, in Device 2, the NPSS can serve as scattering centers to break the waveguiding effect and finally scatter the light into the air cavity. Subsequently, these scatted photons experience the reflection from the bottom reflector and are extracted into air space. The stronger light intensity in the air cavity of Device 2 in Fig. 7(c) confirms this phenomenon. Therefore, the NPSS further improves the LEE of the DUV micro-LED array with air cavity reflector.
Fig. 7. Light intensity distributions in the XY cross section and schematic diagrams for the propagation paths of the TM-polarized light for (a) Device A, (b) Device B and (c) Device 2. The top diameter of Devices A, B, and 2 are 30 µm for the micro-mesa. Light paths of ①, ② and ③ denote different propagations paths in all Devices. (d) Light intensity of extracted light versus the position.
4. Conclusion
In summary, the combination of an air cavity reflector and NPSS is employed to enhance the LEE of the inclined-sidewall-shaped DUV micro-LED arrays. Additionally, a cost-effective hybrid fabrication process by using positive and negative photoresist is developed to make such air cavity reflector. The experimental results demonstrate that the DUV micro-LED arrays with the air cavity reflector and NPSS are superior to the DUV micro-LED arrays with inclined sidewall metal reflector and NPSS. This is due to the fact that the air cavity reflector reduced the optical absorption by the sidewall metal. Moreover, this design provides additional light escape paths for light to escape into the air space. In addition, the air cavity reflector plays a more significant role in Micro-LED with smaller pixel sizes. Our finding report that NPSS can further enhance the LEE of Micro-LED array with air cavity reflector by expanding the angle range of the escape cone and serving as scattering centers.
Funding
National Key Research and Development Program of China (2022YFB3605100); National Natural Science Foundation of China (61975051, 62074050, 62275073); Guangdong Basic and Application Basic Research Foundation (2021B151520022).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. P. Tian, X. Shan, S. Zhu, et al., “AlGaN Ultraviolet Micro-LEDs,” IEEE J. Quantum Electron. 58(4), 1–14 (2022). [CrossRef]
2. Y. Chen, J. Ben, F. Xu, et al., “Review on the Progress of AlGaN-based Ultraviolet Light-Emitting Diodes,” Fundam. Res. 1(6), 717–734 (2021). [CrossRef]
3. M. H. Memon, H. Yu, H. Jia, et al., “Quantum Dots Integrated Deep-Ultraviolet Micro-LED Array Toward Solar-Blind and Visible Light Dual-Band Optical Communication,” IEEE Electron Device Lett. 44(3), 472–475 (2023). [CrossRef]
4. H. Yu, M. H. Memon, H. Jia, et al., “Deep-Ultraviolet LEDs Incorporated with SiO2-Based Microcavities Toward High-Speed Ultraviolet Light Communication,” Adv. Opt. Mater. 10(23), 1–7 (2022). [CrossRef]
5. S. Liang and W. Sun, “Recent Advances in Packaging Technologies of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes,,” Adv. Mater. Technol. (Weinheim, Ger.) 7(8), 1–17 (2022). [CrossRef]
6. H. Yu, M. H. Memon, H. Jia, et al., “A 10 × 10 deep ultraviolet light-emitting micro-LED array,” J. Semicond. 43(6), 062801 (2022). [CrossRef]
7. T. Jia, B. Wang, G. Zhang, et al., “Improving the Performance of Free-pGaN Deep-Ultraviolet Light-Emitting Diodes by Embedding Self-Assembled Ni Nanoparticles Between p-AlGaN/p-Electrode,” IEEE Trans. Electron Devices 70(12), 6410–6414 (2023). [CrossRef]
8. Q. Yue, K. Li, F. Kong, et al., “Analysis on the effect of amorphous photonic crystals on light extraction efficiency enhancement for GaN-based thin-film-flip-chip light-emitting diodes,” Opt. Commun. 367, 72–79 (2016). [CrossRef]
9. X. Hu, X. Liang, L. Tang, et al., “Enhanced Light Extraction Efficiency and Modulation Bandwidth of Deep-Ultraviolet Light-Emitting Diodes with Al Nanospheres,” Crystals 12(2), 289 (2022). [CrossRef]
10. H. Wan, S. Zhou, S. Lan, et al., “Light Extraction Efficiency Optimization of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes,” ECS J. Solid State Sci. Technol. 9(4), 046002 (2020). [CrossRef]
11. P. Dong, J. Yan, J. Wang, et al., “282-nm AlGaN-based deep ultraviolet light-emitting diodes with improved performance on nano-patterned sapphire substrates,” Appl. Phys. Lett. 102(24), 241113 (2013). [CrossRef]
12. S. Zhou, X. Zhao, P. Du, et al., “Application of patterned sapphire substrate for III-nitride light-emitting diodes,” Nanoscale 14(13), 4887–4907 (2022). [CrossRef]
13. Z. Liao, Z. Lv, K. Sun, et al., “Improved efficiency of AlGaN-based flip-chip deep-ultraviolet LEDs using a Ni/Rh/Ni/Au p-type electrode,” Opt. Lett. 48(16), 4229 (2023). [CrossRef]
14. L. Wang, F. Xu, J. Lang, et al., “Transparent p-type layer with highly reflective Rh/Al p-type electrodes for improving the performance of AlGaN-based deep-ultraviolet light-emitting diodes,” Jpn. J. Appl. Phys. 62(3), 030904 (2023). [CrossRef]
15. K. W. Peng, M. C. Tseng, S. H. Lin, et al., “Sidewall geometric effect on the performance of AlGaN-based deep-ultraviolet light-emitting diodes,” Opt. Express 30(26), 47792–47800 (2022). [CrossRef]
16. Q. Chen, H. Zhang, J. Dai, et al., “Enhanced the Optical Power of AlGaN-Based Deep Ultraviolet Light-Emitting Diode by Optimizing Mesa Sidewall Angle,” IEEE Photonics J. 10(4), 1–7 (2018). [CrossRef]
17. M. Tian, H. Yu, M. H. Memon, et al., “Enhanced light extraction of the deep-ultraviolet micro-LED via rational design of chip sidewall,” Opt. Lett. 46(19), 4809 (2021). [CrossRef]
18. S. Xiao, H. Yu, M. H. Memon, et al., “In-Depth Investigation of Deep Ultraviolet MicroLED Geometry for Enhanced Performance,” IEEE Electron Device Lett. 44(9), 1520–1523 (2023). [CrossRef]
19. Y. Zheng, Y. Zhang, J. Zhang, et al., “Effects of Meshed p-type Contact Structure on the Light Extraction Effect for Deep Ultraviolet Flip-Chip Light-Emitting Diodes,” Nanoscale Res. Lett. 14(1), 149 (2019). [CrossRef]
20. S. Y. Kuo, C. J. Chang, Z. T. Huang, et al., “Improvement of light extraction in deep ultraviolet GaN light emitting diodes with mesh P-contacts,” Appl. Sci. 10(17), 5783 (2020). [CrossRef]
21. W. Luo, S. M. Sadaf, T. Ahmed, et al., “Breaking the Transverse Magnetic-Polarized Light Extraction Bottleneck of Ultraviolet-C Light-Emitting Diodes Using Nanopatterned Substrates and an Inclined Reflector,” ACS Photonics 9(9), 3172–3179 (2022). [CrossRef]
22. Y. Zhang, R. Meng, Z. H. Zhang, et al., “Effects of Inclined Sidewall Structure with Bottom Metal Air Cavity on the Light Extraction Efficiency for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes,” IEEE Photonics J. 9(5), 1–9 (2017). [CrossRef]
23. J. W. Lee, J. H. Park, D. Y. Kim, et al., “Arrays of Truncated Cone AlGaN Deep-Ultraviolet Light-Emitting Diodes Facilitating Efficient Outcoupling of in-Plane Emission,” ACS Photonics 3(11), 2030–2034 (2016). [CrossRef]
24. S. Zhang, Y. Liu, J. Zhang, et al., “Optical polarization characteristics and light extraction behavior of deep-ultraviolet LED flip-chip with full-spatial omnidirectional reflector system,” Opt. Express 27(20), A1601 (2019). [CrossRef]
25. Y. Zhang, J. Zhang, Y. Zheng, et al., “The effect of sapphire substrates on omni-directional reflector design for flip-chip near-ultraviolet light-emitting diodes,” IEEE Photonics J. 11(1), 1–9 (2019). [CrossRef]
26. J. Zhang, L. Chang, Y. Zheng, et al., “Integrating remote reflector and air cavity into inclined sidewalls to enhance the light extraction efficiency for AlGaN-based DUV LEDs,” Opt. Express 28(11), 17035 (2020). [CrossRef]
27. Y. Chen, J. Che, C. Chu, et al., “Balanced resistivity in n-AlGaN layer to increase the current uniformity for AlGaN-based DUV LEDs,” IEEE Photon. Technol. Lett. 34(20), 1065–1068 (2022). [CrossRef]
28. F. J. Xu and B. Shen, “Progress in high crystalline quality AlN grown on sapphire for high-efficiency deep ultraviolet light-emitting diodes,” Jpn. J. Appl. Phys. 61(4), 040502 (2022). [CrossRef]
29. H. Yu, M. H. Memon, D. Wang, et al., “AlGaN-based deep ultraviolet micro-LED emitting at 275 nm,” Opt. Lett. 46(13), 3271 (2021). [CrossRef]
30. R. Floyd, M. Gaevski, K. Hussain, et al., “Enhanced light extraction efficiency of micropixel geometry AlGaN DUV light-emitting diodes,” Appl. Phys. Express 14(8), 084002 (2021). [CrossRef]
