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Abstract
High-efficiency GaN-based visible flip-chip miniaturized-light emitting diodes (FC mini-LEDs) are desirable for developing white LED-backlit liquid crystal displays. Here, we propose a full-angle Ti3O5/SiO2 distributed Bragg reflector (DBR) for blue and green FC mini-LEDs to enhance the device performance. The proposed full-angle Ti3O5/SiO2 DBR is composed of different single-DBR stacks optimized for central wavelength in blue, green, and red light wavelength regions, resulting in wider reflective bandwidth and less angular dependence. Furthermore, we demonstrate two types of GaN-based FC mini-LEDs with indium-tin oxide (ITO)/DBR and Ag/TiW p-type ohmic contacts. Experimental results exhibit that the reflectivity of full-angle DBR is higher than that of Ag/TiW in the light wavelength range of 420 to 580 nm as the incident angle of light increases from 0° to 60°. As a result, the light output powers (LOPs) of blue and green FC mini-LEDs with ITO/DBR are enhanced by 7.7% and 7.3% in comparison to blue and green FC mini-LEDs with Ag/TiW under an injection current of 10 mA. In addition, compared with FC mini-LED with Ag/TiW, light intensity of FC mini-LED with ITO/DBR is improved in side direction, which is beneficial to mix light in backlight system of liquid crystal displays (LCDs).
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Miniaturized-LEDs (Mini-LEDs) whose size is between 100 µm and 150 µm have been considered as the promising candidates for next generation displays, owing to their advantages of high brightness, high color saturation, power saving, and long lifetime [1–6]. Mini-LEDs have significant potential as local dimming backlight unit in liquid crystal displays (LCDs) to realize high dynamic range (HDR) [7–11]. The backlight white LEDs could be realized by combining the colors of red, green, and blue mini-LEDs, which enables a higher color gamut (90% BT2020) and a high contrast radio (above 10000:1) of LCDs [12–15]. However, to realize the HDR requirement of LCDs, mini-LEDs should be more efficient. Various methods applicable in conventional LEDs for improved efficiency could be applied to mini-LEDs, such as flip-chip technology [12,16–21], chip geometry shaping [22–24], and patterned sapphire substrate [25,26]. Among these methods, flip-chip technology is widely used in mini-LEDs because of its unique advantage in light extraction efficiency (LEE), heat dissipation, and current spreading.
In flip-chip mini-LEDs (FC mini-LEDs) configuration, highly reflective p-type ohmic contact, which could reflect downward photons back into sapphire substrate, plays a critical role in obtaining better LEE and further improving the efficiency of FC mini-LEDs [27–30]. It is well known that silver (Ag) and indium-tin oxide (ITO) /distributed Bragg reflector (DBR) are generally served as reflective ohmic contacts in FC LEDs due to their high reflectivity [31–33]. Owing to better heat dissipation and current spreading, the performance of FC LED with Ag/TiW is superior to that of FC LED with ITO/DBR at high injection current [20]. Nevertheless, due to higher reflectance and alleviated self-heating issue, the performance of FC LED with ITO/DBR is better than that of FC LED with Ag/TiW at low injection current [33]. Hence, ITO/DBR p-type ohmic contact is a better choice for display application of FC mini-LEDs with low operation current. However, the drawbacks of conventional single-DBR stack are narrow reflective bandwidth and strong angular dependence, which hinders the further improvement in efficiency of FC mini-LEDs [34–37]. The angular dependence was compensated by combining two single-DBR stacks with different dielectric layer thicknesses into a double-DBR stack [38]. However, as the incident angle of light increases, the blueshift of reflective bandwidth of DBR stack optimized for long central wavelength could not completely compensate the blueshift of reflective bandwidth of DBR stack optimized for short central wavelength. Consequently, the reflectivity of double-DBR stack decreases sharply in the light wavelength ranging from long central wavelength to short central wavelength as the incident angle of light increases. The strong angular dependence of double-DBR stack still restricts the improvement in performance of FC mini-LEDs. Therefore, a full-angle DBR with wider reflective bandwidth to enhance performance of blue and green FC mini-LEDs is required.
In this paper, we introduce a full-angle Ti3O5/SiO2 DBR for blue and green FC mini-LEDs. The full-angle DBR is constructed by combining DBR stacks with different thicknesses optimized for discrete central wavelengths in blue, green, and red light wavelength regions, which significantly alleviates the angular dependence and increases the reflective bandwidth. The ITO/DBR and Ag/TiW are constructed as highly reflective ohmic contacts for FC mini-LEDs. Compared with Ag/TiW, the ITO/DBR demonstrates higher reflectivity in the light wavelength range of 420 to 580 nm as the incident angle of light increases from 0° to 60°. As a result, improvements of ∼7.7% and ∼7.3% in LOPs of blue and green FC mini-LEDs with ITO/DBR are attained with respect to that of blue and green FC mini-LEDs with Ag/TiW. Our study exhibits that the full-angle DBR provides a promising strategy for the development of high-efficiency blue and green FC mini-LEDs for display application.
2. Experimental
The GaN epitaxial layers of blue and green mini-LEDs are grown on c-plane PSS using metal-organic chemical vapor deposition (MOCVD) method. The GaN-based blue mini-LED consists of a 25 nm-thick low temperature GaN nucleation layer, a 3.0 µm-thick undoped GaN buffer layer, a 2.5 µm-thick Si-doped n-GaN layer, a 12 pair In0.16Ga0.84N (3 nm) /GaN (12 nm) multiple quantum wells (MQWs), a 40 nm-thick p-Al0.2Ga0.8N electron blocking layer, and a 112 nm-thick Mg-doped p-GaN layer. The epitaxial layers of green mini-LED are identical to that of blue mini-LED except for MQWs. The MQWs of green mini-LED consist of 12-pair In0.25Ga0.75N (3 nm) /GaN (12 nm). The LED wafer is subsequently annealed at 750 °C to activate Mg in the p-GaN layer.
Figure 1 shows the schematic of fabrication process for FC mini-LED with ITO/DBR. The detailed fabrication process steps are described as follows: (a) n-via hole is defined by using inductively coupled plasma (ICP) etching based on BCl3/Cl2 mixture gas. (b) electron beam evaporation is employed to deposit 90-nm-thick ITO transparent conductive layer, followed by thermal anneal in N2 atmosphere at 540 °C for 20 min to strengthen p-ohmic contact. (c) Cr/Al/Ti/Pt/Au metal layer is deposited onto the n-GaN layer and ITO to serve as n- and p-electrodes by electron beam evaporation. (d) DBR consisting of 14-pair alternating Ti3O5/SiO2 with different thicknesses is sputtered by ion beam deposition, followed by the formation of p-electrode hole through DBR using ICP etching based on CHF3/Ar/O2 mixture gas. (e) Cr/Al/Ti/Pt/Ti/Pt/Au metallization is deposited onto n-via hole and p-electrode hole as n- and p-contact pads.
Figure 2 shows the schematic of fabrication process for FC mini-LED with Ag/TiW. The process consists of following steps: (a) ICP etching based on BCl3/Cl2 mixture gas is employed to form n-via hole. (b) Ag (100 nm) /TiW (50 nm) /Pt (10 nm) /TiW (50 nm) /Pt (25 nm) /Ti (30 nm) /Pt (25 nm) /Ti (30 nm) /Pt (25 nm) /Ti (30 nm) /Pt (60 nm) /Ti (30 nm) stacks are deposited onto p-GaN as p-electrode. (c) Cr/Al/Ti/Pt/Au metallization layer is deposited into n-via hole as n-electrode. (d) DBR insulating layer consisting of 14-pair Ti3O5/SiO2 is deposited by ion beam deposition. The ICP etching based on CHF3/Ar/O2 mixture gas is used to form p-electrode hole through Ti3O5/SiO2 DBR; (e) Cr/Al/Ti/Pt/Ti/Pt/Au metal layer is deposited onto n-via hole and p-electrode hole as n- and p-contact pads.
Figures 3(a) and 3(b) show the schematic illustrations of FC mini-LEDs with ITO/DBR and Ag/TiW ohmic contacts, respectively. The dimension of FC mini-LEDs is 120 × 350 µm2. The plan-view images of FC mini-LEDs were taken by a field emission scanning electron microscope (SEM, TESCAN MIRA 3, UK). The transmission electron microscope (TEM) samples were prepared using focused ion beam (FIB, TESCAN GAIA3 XMH, Czech Republic) technique. The cross-sectional structures of FC mini-LEDs were analyzed using FIB combined with SEM. The analysis of structural characteristics for FC mini-LEDs was completed using TEM (JEM-F200, Japan) in combination with energy-dispersive x-ray (EDX) mapping spectroscopy. The reflectance spectra of ITO/DBR and Ag/TiW were measured using ultraviolet/visible/near infrared spectrophotometer with universal reflectance accessory (LAMDA 950, USA). The current versus voltage (I-V) characteristics were measured by using a semiconductor parameter analyzer (Keysight B2901A, USA). The light output power versus current (L-I) characteristics were measured by using a high-precision photometric colorimetric and electric test system (HAAS-2000, China).
Fig. 3. Schematic illustrations of (a) FC mini-LED with ITO/DBR ohmic contact and (b) FC mini-LED with Ag/TiW ohmic contact.
3. Results and discussions
The DBR is made by stacking high refractive dielectric layers (H) and low refractive dielectric layers (L) with quarter-wavelength thickness ($\lambda /4n$, where $\lambda $ is the central wavelength of reflectivity spectrum, n is the refractive index of the material) based on thin-film interference effect [39]. The layer thickness is determined by the following equation [40]:
where ${n_H}$ and ${n_L}$ are the index of high index layer and low index layer, respectively, while ${t_H}$ and ${t_L}$ are the thickness of high index layer and low index layer, respectively.We investigated angular dependence of single-DBR stack using the commercial software TFCalc. The refractive indices of Ti3O5 and SiO2 in the simulation are fixed at 2.37 and 1.46, respectively. Figure 4(a) shows the reflectance spectra of 14-pair Ti3O5 (47.75 nm) /SiO2 (79.07 nm) single-DBR stack optimized for central wavelength at 450 nm. The reflectance decreases in blue light wavelength region when incident angle of light exceeds 50°, as shown in Fig. 4(a). Figure 4(b) shows the reflectance spectra of 14-pair Ti3O5 (54.76 nm) /SiO2 (89.10 nm) single-DBR stack optimized for central wavelength at 520 nm. It is clearly seen in Fig. 4(b) that the reflectance decreases sharply in green light wavelength region when incident angle of light exceeds 40°. Figure 4(c) shows the reflectance spectra of 14-pair Ti3O5 (65.73 nm) /SiO2 (107.56 nm) single-DBR stack optimized for central wavelength at 620 nm. It is obvious that the 14-pair Ti3O5 (65.73 nm) /SiO2 (107.56 nm) single-DBR stack exhibits high reflectivity in green light wavelength region when incident angle exceeds 40°. However, when the incident angle is less than 40°, the reflectivity is low in green light wavelength region.
Fig. 4. Reflectance spectra of (a) 14-pair Ti3O5 (47.75 nm) /SiO2 (79.07 nm) single-DBR stack, (b) 14-pair Ti3O5 (54.76 nm) /SiO2 (89.10 nm) single-DBR stack, (c) 14-pair Ti3O5 (65.73 nm) /SiO2 (107.56 nm) single-DBR stack, (d) double-DBR I, (e) double-DBR II, and (f) full-angle DBR versus incident angle of light. The blue and green rectangle regions show blue and green light wavelength regions, respectively.
To alleviate the angular dependence, we combine 7-pair Ti3O5 (65.73 nm) /SiO2 (107.56 nm) single-DBR stack optimized for a central wavelength at 620 nm and 7-pair Ti3O5 (54.76 nm) /SiO2 (89.10 nm) single-DBR stack optimized for a central wavelength at 520 nm into double-DBR I. Figure 4(d) shows the reflectance spectra of double-DBR I versus incident angle of light. As the incident angle increases, the reflectivity of double-DBR I is high in green light wavelength region but low in blue light wavelength region. To further broaden the reflective bandwidth, we combine 7-pair Ti3O5 (65.73 nm) /SiO2 (107.56 nm) single-DBR stack optimized for central wavelength at 620 nm and 7-pair Ti3O5 (47.75 nm) /SiO2 (79.07 nm) single-DBR stack optimized for central wavelength at 450 nm into double-DBR II. Figure 4(e) shows the reflectance spectra of double-DBR II versus incident angle of light. The double-DBR II exhibits high reflectivity in both blue and green light wavelength regions when the incident angle is less than 40°. However, it was seen in Fig. 4(e) that the reflectivity of double-DBR II decreases sharply in light wavelength region II as the incident angle of light increases. This indicates that as the incident angle of light increases, the blueshift of reflective bandwidth of DBR stack optimized for 620 nm could not completely compensate the blueshift of reflective bandwidth of DBR stack optimized for 450 nm due to the large gap between the short central wavelength (450 nm) and the long central wavelength (620 nm). In addition, the narrow reflective bandwidth of double-DBR stack leads to the decrease of reflectivity in light wavelength region I and region III, as shown in Fig. 4(e).
The angular dependence of double-DBR stack results from narrow reflective bandwidth as well as large gap between long central wavelength and short central wavelength. The reflective bandwidth of DBR stack could be calculated by the following equation [41]:
Table 1. The detailed thickness of each dielectric layer and central wavelength of each stack
Figure 4(f) shows the reflectance spectra of the full-angle DBR at different incident angle of light. In light wavelength region II, the reflectivity decrease of full-angle DBR is remarkably less than that of double-DBR II as incident angle of light increases. Moreover, in light wavelength region I and region III, the reflectivity of full-angle DBR is higher than that of double-DBR II. The results demonstrate that full-angle DBR has less angular dependence and wider reflective bandwidth compared with double-DBR stack.
We measured the reflectance spectra of full-angle DBR and Ag/TiW at various incident angles by using ultraviolet/visible/near infrared spectrophotometer. Figure 5(a) shows the measured reflectance spectra of Ag/TiW and full-angle DBR at incident angles of 0°, 10°, 20°, 30°, 40°, 50°, and 60°. The electroluminescent (EL) spectra of blue and green mini-LEDs are also shown in Fig. 5(a). The peak wavelength of blue and green FC mini-LEDs is 465 nm and 520 nm, respectively. The reflectivity of full-angle DBR is much higher than that of Ag/TiW in the light wavelength range of 420 to 580 nm as the incident angle of light increase from 0° to 60°, as shown in Fig. 5(a), revealing that FC mini-LEDs with full-angle DBR can obtain higher LEE. Figures 5(b) and 5(c) show cross-sectional TEM images and EDX mapping spectroscopy of full-angle Ti3O5/SiO2 DBR and Ag/TiW/Pt/TiW/Pt/Ti/Pt/Ti/Pt/Ti/Pt/Ti, respectively.
Fig. 5. (a) Measured reflectance spectra of Ag/TiW and full-angle DBR at incident angles of 0°, 10°, 20°, 30°, 40°, 50°, and 60° as well as the EL spectra of blue and green LEDs. (b) Cross-sectional TEM images of full-angle Ti3O5/SiO2 DBR and corresponding EDX mapping spectroscopy. (c) Cross-sectional TEM images of Ag/TiW/Pt/TiW/Pt/Ti/Pt/Ti/Pt/Ti/Pt/Ti and corresponding EDX mapping spectroscopy.
Figure 6 shows the top-view and cross-sectional SEM images of the fabricated FC mini-LEDs (named as FC mini-LED I and FC mini-LED II). In FC mini-LED I, Ag-based metallic reflector (Ag/TiW/Pt/TiW/Pt/Ti/Pt/Ti/Pt/Ti/Pt/Ti) is employed as highly reflective p-ohmic contact. In FC mini-LED II, transparent ITO combined with full-angle Ti3O5/SiO2 DBR is used as highly reflective p-ohmic contact. Figures 6(b) and 6(c) show the cross-sectional SEM images of FC mini-LED I with Ag/TiW along A-A and B-B directions, as marked in Fig. 6(a). Figures 6(e) and 6(f) show the cross-sectional SEM images of FC mini-LED II with ITO/DBR along C-C and D-D directions, as marked in Fig. 6(d).
Fig. 6. (a) Top-view SEM image of FC mini-LED I with Ag/TiW. Cross-sectional SEM images of FC mini-LED I with Ag/TiW milled by FIB along (b) A-A and (c) B-B directions. (d) Top-view SEM image of FC mini-LED II with ITO/DBR. Cross-sectional SEM images of FC mini-LED II with ITO/DBR milled by FIB along (e) C-C and (f) D-D directions.
Figure 7(a) shows the current-voltage (I-V) characteristics of green FC mini-LED I and FC mini-LED II. The inset in Fig. 7(a) is the optical image of green FC mini-LEDs after flip-chip bonding on a proper package. At 10 mA, the forward voltages of green FC mini-LED I and FC mini-LED II are 2.9 V and 3.0 V, respectively. Owing to the high electrical conductivity of Ag/TiW p-type ohmic contact, the forward voltage of green FC mini-LED I is lower than that of green FC mini-LED II. Figure 7(b) shows the light output power-current (L-I) characteristics of green FC mini-LED I and FC mini-LED II. At 10 mA, LOPs of green FC mini-LED I and FC mini-LED II are 4.1 and 4.4 mW, respectively. The green FC mini-LED II exhibits 7.3% improvement over green FC mini-LED I in LOP due to the higher reflectivity of full-angle Ti3O5/SiO2 DBR in green light wavelength region.
Fig. 7. (a) I-V characteristics of green FC mini-LED I and FC mini-LED II. The inset shows optical image of green FC mini-LEDs after flip-chip bonding on a proper package. (b) L-I characteristics of green FC mini-LED I and FC mini-LED II. (c) I-V characteristics of blue FC mini-LED I and FC mini-LED II. The inset shows optical image of blue FC mini-LEDs after flip-chip bonding on a proper package. (d) L-I characteristics of blue FC mini-LED I and FC mini-LED II. (e) Far-field radiation pattern of green FC mini-LED I and FC mini-LED II. (f) Far-field radiation pattern of blue FC mini-LED I and FC mini-LED II.
Figure 7(c) shows I-V characteristics of blue FC mini-LED I and FC mini-LED II. The inset in Fig. 7(c) is the optical image of blue FC mini-LEDs after flip-chip bonding on a proper package. At 10 mA, the forward voltages of blue FC mini-LED I and FC mini-LED II are 3.0 V and 3.1 V, respectively. Figure 7(d) shows the L-I characteristics of blue FC mini-LED I and FC mini-LED II. At 10 mA, the LOPs are 9.1 mW for blue FC mini-LED I and 9.8 mW for blue FC mini-LED II. The LOP of blue FC mini-LED II is increased by 7.7% in comparison with that of blue FC mini-LED I. The improved LOP for blue FC mini-LED II is due to the higher reflectivity of full-angle Ti3O5/SiO2 DBR in blue light wavelength region. Figures 7(e) and 7(f) show normalized far-field angular radiation patterns of green and blue FC mini-LEDs, respectively. It could be clearly seen that the light intensity of FC mini-LED II is significantly improved in side direction compared with FC mini-LED I. In addition, the improved light intensity in side direction of FC mini-LED II is beneficial for the mixing light in backlight system of LCDs. Therefore, when the FC mini-LED II is applied to form backlight system, fewer mini-LEDs are required to realize the same luminance uniformity in backlight system, reducing the cost and power consumption of displays.
4. Conclusion
In summary, a novel full-angle Ti3O5/SiO2 DBR was developed to enhance the performances of blue and green FC mini-LEDs. The full-angle DBR consists of different single-DBR stacks optimized for discrete central wavelengths at 689, 647, 645, 631, 619, 606, 585, 543, 502, 497, 464, 437, 433, and 390 nm, which exhibits wider reflectance bandwidth and less angular dependence compared with conventional DBR structures. In addition, ITO combined with full-angle DBR and Ag/TiW serve as p-type ohmic contacts for FC mini-LEDs. The experiment results exhibit that the ITO/DBR shows higher reflectivity in comparison with Ag/TiW in the wavelength range of 420 to 580 nm as the incident angle of light increases from 0° to 60°. As a result, the light output powers (LOPs) of blue and green FC mini-LEDs with ITO/DBR is 9.8 mW and 4.4 mW at 10 mA, which are 7.7% and 7.3% higher than that of blue and green FC mini-LEDs with Ag/TiW, respectively. The full-angle Ti3O5/SiO2 DBR could provide a promising method for the realization of high-efficiency visible FC mini-LEDs.
Funding
National Natural Science Foundation of China (51675386, 51775387, 52075394); National Youth Talent Support Program.
Acknowledgments
The authors also acknowledge valuable support from the National Youth Talent Support Program.
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. T. Wu, C. W. Sher, Y. Lin, C. F. Lee, S. Liang, Y. Lu, S. W. H. Chen, W. Guo, H. C. Kuo, and Z. Chen, “Mini-LED and Micro-LED: Promising candidates for the next generation display technology,” Appl. Sci. 8(9), 1557 (2018). [CrossRef]
2. E. L. Hsiang, Z. Yang, Q. Yang, Y. F. Lan, and S. T. Wu, “Prospects and challenges of mini-LED, OLED, and micro-LED displays,” J. Soc. Inf. Disp. 29(6), 446–465 (2021). [CrossRef]
3. G. Tan, Y. Huang, M. C. Li, S. L. Lee, and S. T. Wu, “High dynamic range liquid crystal displays with a mini-LED backlight,” Opt. Express 26(13), 16572–16584 (2018). [CrossRef]
4. E. L. Hsiang, Q. Yang, Z. He, J. Zou, and S. T. Wu, “Halo effect in high-dynamic-range mini-LED backlit LCDs,” Opt. Express 28(24), 36822–36837 (2020). [CrossRef]
5. S. Kikuchi, Y. Shibata, T. Ishinabe, and H. Fujikake, “Thin mini-LED backlight using reflective mirror dots with high luminance uniformity for mobile LCDs,” Opt. Express 29(17), 26724–26735 (2021). [CrossRef]
6. E. Chen, J. Guo, Z. Jiang, Q. Shen, Y. Ye, S. Xu, J. Sun, Q. Yan, and T. Guo, “Edge/direct-lit hybrid mini-LED backlight with U-grooved light guiding plates for local dimming,” Opt. Express 29(8), 12179–12194 (2021). [CrossRef]
7. M. Y. Deng, E. L. Hsiang, Q. Yang, C. L. Tsai, B. S. Chen, C. E. Wu, M. H. Lee, S. T. Wu, and C. L. Lin, “Reducing Power Consumption of Active-Matrix Mini-LED Backlit LCDs by Driving Circuit,” IEEE Trans. Electron Devices 68(5), 2347–2354 (2021). [CrossRef]
8. B. Tang, J. Miao, Y. Liu, H. Wan, N. Li, S. Zhou, and C. Gui, “Enhanced light extraction of flip-chip mini-LEDs with prism-structured sidewall,” Nanomaterials 9(3), 1–8 (2019). [CrossRef]
9. Y. Huang, E. L. Hsiang, M. Y. Deng, and S. T. Wu, “Mini-LED, Micro-LED and OLED displays: present status and future perspectives,” Light Sci. Appl. 9(1), 1–16 (2020). [CrossRef]
10. Y. Sun, J. Fan, M. Liu, L. Zhang, B. Jiang, M. Zhang, and X. Zhang, “Highly transparent, ultra-thin flexible, full-color mini-LED display with indium-gallium-zinc oxide thin-film transistor substrate,” J. Soc. Inf. Disp. 28(12), 926–935 (2020). [CrossRef]
11. Y. M. Huang, T. Ahmed, A. C. Liu, S. W. H. Chen, K. L. Liang, Y. H. Liou, C. C. Ting, W. H. Kuo, Y. H. Fang, C. C. Lin, and H. C. Kuo, “High-Stability Quantum Dot-Converted 3-in-1 Full-Color Mini-Light-Emitting Diodes Passivated with Low-Temperature Atomic Layer Deposition,” IEEE Trans. Electron Devices 68(2), 597–601 (2021). [CrossRef]
12. K. P. Chang, Y. T. Tsai, C. C. Yen, R. H. Horng, and D. S. Wuu, “Structural design and performance improvement of flip-chip AlGaInP mini light-emitting diodes,” Semicond. Sci. Technol. 36(9), 095008 (2021). [CrossRef]
13. K. Masaoka and Y. Nishida, “Metric of color-space coverage for wide-gamut displays,” Opt. Express 23(6), 7802–7808 (2015). [CrossRef]
14. Y. Huang, G. Tan, F. Gou, M. C. Li, S. L. Lee, and S. T. Wu, “Prospects and challenges of mini-LED and micro-LED displays,” J. Soc. Inf. Disp. 27(7), 387–401 (2019). [CrossRef]
15. X. Zhao, B. Tang, L. Gong, J. Bai, J. Ping, and S. Zhou, “Rational construction of staggered InGaN quantum wells for efficient yellow light-emitting diodes,” Appl. Phys. Lett. 118(18), 182102 (2021). [CrossRef]
16. W. Guo, N. Chen, H. Lu, C. Su, Y. Lin, G. Chen, Y. Lu, L. L. Zheng, Z. Peng, H. C. Kuo, C. H. Lin, T. Wu, and Z. Chen, “The Impact of Luminous Properties of Red, Green, and Blue Mini-LEDs on the Color Gamut,” IEEE Trans. Electron Devices 66(5), 2263–2268 (2019). [CrossRef]
17. B. Lu, Y. Wang, B. R. Hyun, H. C. Kuo, and Z. Liu, “Color Difference and Thermal Stability of Flexible Transparent InGaN/GaN Multiple Quantum Wells Mini-LED Arrays,” IEEE Electron Device Lett. 41(7), 1040–1043 (2020). [CrossRef]
18. R. H. Horng, H. Y. Chien, K. Y. Chen, W. Y. Tseng, Y. T. Tsai, and F. G. Tarntair, “Development and Fabrication of AlGaInP-Based Flip-Chip Micro-LEDs,” IEEE J. Electron Devices Soc. 6, 475–479 (2018). [CrossRef]
19. Y. C. Lee, H. C. Kuo, C. E. Lee, T. C. Lu, and S. C. Wang, “High-performance (AlxGa1-x)0.5In0.5P-based flip-chip light-emitting diode with a geometric sapphire shaping structure,” IEEE Photonics Technol. Lett. 20(23), 1950–1952 (2008). [CrossRef]
20. S. Zhou, X. Liu, H. Yan, Z. Chen, Y. Liu, and S. Liu, “Highly efficient GaN-based high-power flip-chip light-emitting diodes,” Opt. Express 27(12), A669–A692 (2019). [CrossRef]
21. L. B. Chang, C. C. Shiue, and M. J. Jeng, “High reflective p-GaN/Ni/Ag/Ti/Au Ohmic contacts for flip-chip light-emitting diode (FCLED) applications,” Appl. Surf. Sci. 255(12), 6155–6158 (2009). [CrossRef]
22. J. M. Smith, R. Ley, M. S. Wong, Y. H. Baek, J. H. Kang, C. H. Kim, M. J. Gordon, S. Nakamura, J. S. Speck, and S. P. Denbaars, “Comparison of size-dependent characteristics of blue and green InGaN microLEDs down to 1 µm in diameter,” Appl. Phys. Lett. 116(7), 071102 (2020). [CrossRef]
23. Z. Zhuang, D. Iida, and K. Ohkawa, “Effects of size on the electrical and optical properties of InGaN-based red light-emitting diodes,” Appl. Phys. Lett. 116(17), 173501 (2020). [CrossRef]
24. S. Lu, Y. Zhang, Z.-H. Zhang, B. Zhu, H. Zheng, S. T. Tan, and H. V. Demir, “High-Performance Triangular Miniaturized-LEDs for High Current and Power Density Applications,” ACS Photonics 8(8), 2304–2310 (2021). [CrossRef]
25. H. Hu, B. Tang, H. Wan, H. Sun, S. Zhou, J. Dai, C. Chen, S. Liu, and L. J. Guo, “Boosted ultraviolet electroluminescence of InGaN/AlGaN quantum structures grown on high-index contrast patterned sapphire with silica array,” Nano Energy 69, 104427 (2020). [CrossRef]
26. S. Zhou, S. Yuan, Y. Liu, L. J. Guo, S. Liu, and H. Ding, “Highly efficient and reliable high power LEDs with patterned sapphire substrate and strip-shaped distributed current blocking layer,” Appl. Surf. Sci. 355, 1013–1019 (2015). [CrossRef]
27. J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, G. Christenson, Y. C. Shen, C. Lowery, P. S. Martin, S. Subramanya, W. Götz, N. F. Gardner, R. S. Kern, and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” Appl. Phys. Lett. 78(22), 3379–3381 (2001). [CrossRef]
28. O. B. Shchekin, J. E. Epler, T. A. Trottier, T. Margalith, D. A. Steigerwald, M. O. Holcomb, P. S. Martin, and M. R. Krames, “High performance thin-film flip-chip InGaN-GaN light-emitting diodes,” Appl. Phys. Lett. 89(7), 071109–2007 (2006). [CrossRef]
29. B. P. Yonkee, E. C. Young, S. P. DenBaars, S. Nakamura, and J. S. Speck, “Silver free III-nitride flip chip light-emitting-diode with wall plug efficiency over 70% utilizing a GaN tunnel junction,” Appl. Phys. Lett. 109(19), 191104–6 (2016). [CrossRef]
30. K. P. Hsueh, K. C. Chiang, Y. M. Hsin, and C. J. Wang, “Investigation of Cr- and Al-based metals for the reflector and Ohmic contact on n-GaN in GaN flip-chip light-emitting diodes,” Appl. Phys. Lett. 89(19), 191122 (2006). [CrossRef]
31. J. Y. Kim, M. K. Kwon, I. K. Park, C. Y. Cho, S. J. Park, D. M. Jeon, J. W. Kim, and Y. C. Kim, “Enhanced light extraction efficiency in flip-chip GaN light-emitting diodes with diffuse Ag reflector on nanotextured indium-tin oxide,” Appl. Phys. Lett. 93(2), 021121 (2008). [CrossRef]
32. C. H. Lin, C. F. Lai, T. S. Ko, H. W. Huang, H. C. Kuo, Y. Y. Hung, K. M. Leung, C. C. Yu, R. J. Tsai, C. K. Lee, T. C. Lu, S. C. Wang, and S. Member, “Enhancement of InGaN-GaN Indium-Tin-Oxide Flip-Chip Light-Emitting Diodes With TiO2-SiO2 Multilayer Stack Omnidirectional Reflector,” IEEE Photonics Technology Lett. 18(19), 2050–2052 (2006). [CrossRef]
33. S. Zhou, X. Liu, Y. Gao, Y. Liu, M. Liu, Z. Liu, C. Gui, and S. Liu, “Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts,” Opt. Express 25(22), 26615–26627 (2017). [CrossRef]
34. T. Zhi, T. Tao, B. Liu, Y. Yan, Z. Xie, H. Zhao, and D. Chen, “High Performance Wide Angle DBR Design for Optoelectronic Devices,” IEEE Photonics J. 13(1), 1–6 (2021). [CrossRef]
35. X. Ding, C. Gui, H. Hu, M. Liu, X. Liu, J. Lv, and S. Zhou, “Reflectance bandwidth and efficiency improvement of light-emitting diodes with double-distributed Bragg reflector,” Appl. Opt. 56(15), 4375–4380 (2017). [CrossRef]
36. W. Cai, W. Wang, B. Zhu, X. Gao, G. Zhu, J. Yuan, and Y. Wang, “Suspended light-emitting diode featuring a bottom dielectric distributed Bragg reflector,” Superlattices Microstruct. 113, 228–235 (2018). [CrossRef]
37. S. Zhou, H. Xu, M. Liu, X. Liu, J. Zhao, N. Li, and S. Liu, “Effect of dielectric distributed bragg reflector on electrical and optical properties of GaN-based flip-chip light-emitting diodes,” Micromachines 9(12), 650 (2018). [CrossRef]
38. S. Zhou, B. Cao, S. Yuan, and S. Liu, “Enhanced luminous efficiency of phosphor-converted LEDs by using back reflector to increase reflectivity for yellow light,” Appl. Opt. 53(34), 8104–8110 (2014). [CrossRef]
39. J. Zou, Z. Yang, C. Mao, and S. T. Wu, “Fast-response liquid crystals for 6G optical communications,” Crystals 11(7), 797 (2021). [CrossRef]
40. M. A. Kats, R. Blanchard, P. Genevet, and F. Capasso, “Nanometre optical coatings based on strong interference effects in highly absorbing media,” Nat. Mater. 12(1), 20–24 (2013). [CrossRef]
41. H. Kim, M. Kaya, and S. Hajimirza, “Broadband solar distributed Bragg reflector design using numerical optimization,” Sol. Energy 221, 384–392 (2021). [CrossRef]
