1. Introduction

2. Size-dependent efficiency

3. Affecting factors on the EQE of µLEDs

3.1. Non-radiative recombination effect for µLEDs

µLEDs are LEDs in the dimensions less than 100 µm. Due to their small size, µLEDs have been found to have a diminishing maximum EQE as device dimensions go smaller [30,31]. Olivier et al. showed that the Shockley- Read- HaII (SRH) recombination rate increases when the µLED size is reduced [32], as shown in Figure 1a. However, the Auger recombination coefficient is almost unaffected by µLED size and remains constant, as shown in Figure 1b. The plasma dry etching technique is by far the most widely used method for defining the light-emitting mesa. Due to the large perimeter/area ratio (P/A) of µLEDs compared to large size conventional LEDs, the effect of etching processes causing high-density surface defects on the sidewalls becomes more pronounced as the chip size decreases [31].

Figure 1. Plot of the extracted coefficients A (a) and C (b) vs LED size. Reproduced from Ref. [32], with the permission of AIP Publishing.

Figure 1. Plot of the extracted coefficients A (a) and C (b) vs LED size. Reproduced from Ref. [32], with the permission of AIP Publishing.

SRH non-radiative recombination is facilitated by surface defects and dangling bonds on the surfaces of crystals, which act as traps in the bandgap. Since µLEDs have a better current spreading effect [33,34], carriers will reach the sidewalls more easily, surface compounding will be more significant and performance losses will be more severe. It is worth noting that the carrier trapping effect will be weakened by sidewall defects as a result of the shorter carrier lifespan at high currents, which will lead to peak efficiencies at larger current densities, and the peak efficiency decreases significantly with decreasing chip size, and the increase in efficiency flip current density is also an indication of enhanced surface non-radiative compounding [35], as shown in Figure 2, the blue dashed line indicates that µLED is typically driven at very low current levels, and the red dashed line indicates that higher current levels of 1 µA are necessary to generate an adequate amount of display brightness. The peak EQE and rollover current are shown for various sizes of LED chips in Figure 2c. It shows that when the size of the LED chip is reduced to below 50×50 µm², the peak EQE drops considerably.

Figure 2. (a) The relationship between IQE and current density and (b) current for LEDs of varying sizes. (c) Related current levels and peak IQEs for various LED sizes. Reproduced from Ref. [35], with the permission of John Wiley and Sons.

Figure 2. (a) The relationship between IQE and current density and (b) current for LEDs of varying sizes. (c) Related current levels and peak IQEs for various LED sizes. Reproduced from Ref. [35], with the permission of John Wiley and Sons.

Moreover, the current injection for LEDs with tiny chips must be low in order to prevent efficiency drooping when the levels of injection current density are high. These locations have more details on the correlation between current density and EQE [34,36]. Tian et al. reported a trend in the correlation of the complex coefficient with respect to temperature for different injected carrier concentrations [37]. The temperature change of the B coefficient at n= 2×10¹⁸cm^-3 and n= 1×10²⁰cm^-3 are respectively shown in Figure 3a,b. At low carrier concentrations, increasing temperature results in a considerable reduction in the B coefficient. However, when carrier concentrations are high, the B coefficient becomes less affected by temperature. As the temperature rises, the coefficient B and C decrease. Moreover, at higher carrier concentrations, the dependence of coefficient B and C to temperature is reduced, as shown in Figure 3b,d. Figure 3c investigates the trend of the coefficient C with temperature for different carrier concentrations from 300 K to 500 K. The coefficient C falls as temperature rises for all carrier concentrations. A decreased temperature dependency is acquired when the carrier concentration rises, just like in the case of coefficient B. It’s important to note that this report’s temperature dependence of the coefficient C defies the majority of theoretical and experimental trends in the literature [38,39,40,41], but it is consistent with one design’s theoretical outcome that has weak carrier confinement in the QWs [42].

Figure 3. Temperature-dependent coefficient B alterations (a) at n=2 x 10¹⁸ cm^-3 and (b) at n=1 x 10²⁰ cm^-3. (c) Calculated coefficient C in relation to n at various temperatures. (d) Alterations in coefficient C with temperature at n=1 x 10²⁰ cm^-3. Reproduced from Ref. [37], with the permission of AIP Publishing.

Figure 3. Temperature-dependent coefficient B alterations (a) at n=2 x 10¹⁸ cm^-3 and (b) at n=1 x 10²⁰ cm^-3. (c) Calculated coefficient C in relation to n at various temperatures. (d) Alterations in coefficient C with temperature at n=1 x 10²⁰ cm^-3. Reproduced from Ref. [37], with the permission of AIP Publishing.

4. Solution for increasing EQE of µLEDs

4.1. Defect density control

By reducing the density of sidewall flaws brought on by mesa etching, IQE is enhanced. In order to ascertain the potential mechanism of passivation, Kyung et al. used three passivation materials, such as SiO₂, Al₂O₃, and Si₃N₄, to investigate the chemical bonding properties at the sidewall/passivation layer interface [56]. The device structure is shown in Figure 4, they found that SiO₂ passivation was more effective than Al₂O₃ and Si₃N₄ passivation was more effective in reducing sidewall defects, and µLEDs with SiO₂ passivation exhibited high PL efficiency, high optical output power and high current density due to the fact that the Ga-O bond formation energy was lower than the Si-O bond dissociation energy, and at the interface between the GaN and the passivation layer, a great deal Ga-O bonds were formed.

Figure 4. (a) Device structure for µLEDs (left) and effect of surface passivation for various passivation layers (right). µLEDs in HAADF-STEM images with (b) SiO₂, (c) Al₂O₃, and (d) Si₃N₄ passivation layers. Reproduced from Ref. [56], with the permission of Elsevier.

Figure 4. (a) Device structure for µLEDs (left) and effect of surface passivation for various passivation layers (right). µLEDs in HAADF-STEM images with (b) SiO₂, (c) Al₂O₃, and (d) Si₃N₄ passivation layers. Reproduced from Ref. [56], with the permission of Elsevier.

Passivation layers are deposited on µLEDs by plasma enhanced chemical vapour deposition (PECVD) and atomic layer deposition (ALD). The PECVD system provides fast deposition rates. According to the results of experiments, the electrical and optical characteristics of the devices can be somewhat improved by the passivation layers grown by the PECVD systems, for example, by improving light extraction efficiency and reducing leakage currents [57,58]. However, because PECVD methods are unable to produce passivation layers with extremely compact atom arrays, some surface defects may not be effectively passivated [59]. Therefore, due to the extremely compact atomic arrays and the accurately controlled atomic-level layer deposition technology, the ALD system is another more satisfactory solution. PECVD sidewall passivation was shown to be superior in lowering the leakage current of big devices in study by Wong et al. on the performance enhancement of µLEDs utilizing ALD systems [60], but ineffective in reducing leakage current of µLEDs smaller than 60 x 60 µm² .As shown in Figure 5a, LED-1 is a µLED without sidewall passivation, LED-2 a µLED with ICP etching and sidewall passivation applied using the ALD process, LED-3 a µLED with HF etching and sidewall passivation applied using the PECVD process, and LED-4 a µLED with HF etching and sidewall passivation applied using the ALD process. Leakage current densities in LED-2 and LED-3 are an order of magnitude greater between -2 V and 0 V than in LED-1 and LED-4 when the device size is 20 x 20 um². LED-2 and LED-3 with high leakage current densities indicate that ICP etching and PECVD create more leakage channels in the µLEDs. As shown in Figure 5b, PECVD sidewall passivation produces significantly worse results in µLEDs smaller than 60 x 60µm² compared to ALD. Figure 5c compared to Figure 5d shows that PECVD sidewall passivation produces better results in larger size LEDs due to the relatively small perimeter/area and the lack of impact of sidewall damage on the performance of larger size devices. Figure 5d reveals that for 20 x 20µm² µLEDs, the EQE of LED-4 based on ALD passivation and HF etching is higher than that of LED-3 based on HF etching and PECVD passivation, while the comparison of LED-1 and LED-2 further highlights the significance of the ALD technology for sidewall passivation.

Figure 5. (a) Properties for current density-voltage of 20 x 20 um² µLEDs with various sidewall passivation approaches. (b) The relationship between the sizes of LEDs with various sidewall passivation techniques and leakage current density at -4V. The reliance of EQE on current injection for devices with various sidewall passivation techniques (c) 100 x 100 µm² and (d) 20 x 20 um². Reproduced from Ref. [60], with the permission of Optical Society of America.

Figure 5. (a) Properties for current density-voltage of 20 x 20 um² µLEDs with various sidewall passivation approaches. (b) The relationship between the sizes of LEDs with various sidewall passivation techniques and leakage current density at -4V. The reliance of EQE on current injection for devices with various sidewall passivation techniques (c) 100 x 100 µm² and (d) 20 x 20 um². Reproduced from Ref. [60], with the permission of Optical Society of America.

Alternatively, chemical treatment is an effective method of reducing the defect density of sidewall treatments, and device performance is expected to be further enhanced by the incorporation of chemical treatment and ALD sidewall passivation. The fundamental theory of chemical processing is to passivate oroxygenate the semiconductor using wet chemicals in order to create a larger band gap material at the surface interface and produce a surface with reduced sidewall damage, surface compounding, and dangling bonds. Plasma damage can be eliminated by chemical treatments employing hydrochloric acid, ammonium sulfide, and potassium hydroxide (KOH), according to reports [61,62,63]. Forward and reverse leakage current have been shown to be reduced by chemical treatment. According to Wong et al. [64], a sidewall passivation study incorporating ALD and chemical treatment was conducted. SEM images of the LED sidewall profile before and after the KOH chemical treatment are respectively shown in Figure 6a,b. The etched nature produced a rough sidewall surface for the non-KOH treated devices. The KOH-treated sidewall developed m-plane faceted features with dimensions ranging from 50 to 200 nm. Previous investigations utilizing tetramethylammonium hydroxide (TMAH) or KOH have revealed the creation of m-plane facets on the device walls [65,66]. Figure 6c,d show the EQEs regarding current density injection in the absence and in combination with KOH and ALD sidewall treatments, respectively, with the EQEs of the larger device sizes being almost unaffected by KOH and ALD sidewall treatments. The peak EQE of 100 × 100 µm² µLED appears at the current density of 5 A cm⁻², while the peak EQE is displayed at 15 A cm⁻² when the device size is reduced to 10 × 10 µm². Besides, although the efficiency of these devices is improved by the sidewall treatment, the effect of non-radiative compounding is not completely eliminated, which is reflected in the reduced peak efficiency of the small size devices.

Figure 6. Images taken using scanning electron microscopy to compare the sidewall profile of µLEDs (a) before and (b) after KOH chemical treatment. Size-dependent properties of EQE in relation to current density for μLEDs (c) without and (d) with sidewall treatment. Reproduced from Ref. [64], with the permission of The Japan Society of Applied Physics.

Figure 6. Images taken using scanning electron microscopy to compare the sidewall profile of µLEDs (a) before and (b) after KOH chemical treatment. Size-dependent properties of EQE in relation to current density for μLEDs (c) without and (d) with sidewall treatment. Reproduced from Ref. [64], with the permission of The Japan Society of Applied Physics.

In addition, by lengthening the thermal annealing time, sidewall defects can be repaired in part, enhancing efficiency at low injection current densities [31,62]. The quantum efficiency of small-size LEDs is considerably lower than that of large-size LEDs as a result of dry etching process, which becomes a considerable problem when the LED size is ≤10 µm. A direct epitaxy method for µLEDs in which the dry etching technique used to form the µLED mesa is no longer required was conceived and developed by the Sheffield team and is known as the CSE approach [67]. Steady emission color, extremely high external quantum efficiency, reduced leakage current, improved indium integration to generate red emissions, and low parasitic capacitance are its many main benefits. This approach could be a promising candidate for the manufacture of high quality µLEDs for micro-size display and VLC applications.

4.2. Managing the spread of current

Using buried tunneling junctions (TJ) is another effective way to improve devices. Metal organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE), two regenerative growth processes that both have advantages and disadvantages for device performance, have been used to achieve tunnel junctions. When tunnel junctions are created via MBE, the hydrogen level stays low while they are growing and the conductivity of the p-GaN layer stays constant after they have grown. However, the utilization of MBE-formed tunnel junctions for large-scale manufacturing is severely hampered because of scalability [68,69,70]. The resistivity of p-GaN dramatically rises with hydrogen passivation at high hydrogen concentrations during progression, although MOCVD-formed tunnel junctions are another option due to its great reactor scalability [71,72]. TJ allow better control of the current injected into the µLED diode active layer, thereby reducing carrier arrival at the table edge, which effectively suppresses surface non-radiative recombination. This approach was applied to the tunneling connection method for GaN-based µLEDs [73]. Figure 7a shows the structure of a µLED with TJ contacts grown by MOCVD. The standard LEDs in Figure 7b,c represent LEDs without TJ structures with an epitaxial layer of ITO, while LED-1, LED2 and LED-3 have TJ structures that replace the ITO layer and they have different silicon concentrations. We can observe that the poor efficiency of p-GaN doping causes the forward voltage of the LED to increase [74], which makes the TJ structure less probable for carrier tunneling from the valence band of p⁺⁺-GaN to the conduction band of n⁺⁺-GaN under reverse bias. Figure 7c shows that the three TJ µLEDs have a higher peak EQE, which can be due to more uniform current distribution on the p-side and n-side, and that n-GaN is more optically transparent than ITO compared to standard LED, which enhance the light extraction efficiency.

Figure 7. (a) Cross-section diagram of the treated µLEDs and layers produced using MOCVD. (b) Voltage for various µLEDs with the 20 A/cm² injection current density. (c) EQE in relation to current density for different µLEDs with sizes of 100 × 100 µm² and 40 × 40 µm². Reproduced from Ref. [73], with the permission of The Japan Society of Applied Physics.

Figure 7. (a) Cross-section diagram of the treated µLEDs and layers produced using MOCVD. (b) Voltage for various µLEDs with the 20 A/cm² injection current density. (c) EQE in relation to current density for different µLEDs with sizes of 100 × 100 µm² and 40 × 40 µm². Reproduced from Ref. [73], with the permission of The Japan Society of Applied Physics.

4.3. Mitigate the QCSE effect

As mentioned above, GaN-based LEDs, particularly In-rich red and green µLEDs, suffer greatly from the extremely strong QCSE present in InGaN/GaN based MQWs. This seriously worsens the IQE of GaN-based µLEDs. The QCSE has been reported to be mitigated by the optimizations of epitaxial structures. For example, thin quantum barriers (QBs) in multiple quantum wells (MQWs) are crucial for reducing the carrier lifetimes [75,76], which can decrease the polarization field inside the QW, enhance hole injection, and result in greater uniformity in carrier distribution across the active region. The schematic architectures of the LED I, LED II, and LED III samples are shown in Figure 8a, and their corresponding GaN QB thicknesses are 12, 9, and 6 nm. Figure 8b demonstrates the forward current-voltage properties of three manufactured LEDs. As the thickness of the QB shrinks, the forward voltage reduces. This may be explained by the fact that series resistance decreases as the thickness of QBs reduces. The series resistances for LEDs I, II, and III were measured to be 7.79, 7.28, and 6.93 Ω, respectively. The EQE of three manufactured LEDs is depicted in Figure 8c as a function of forward current. It is evident that LED II, which had a 9 nm QB thickness, had the highest performance. At an injection current of 60 mA, the efficiencies of LEDs I, II, and III are decreased by 32.7%, 27.9%, and 29.2%, respectively, from their maximum efficiencies. It has further been shown that the luminous intensity of the samples decreases significantly as the thickness is further reduced to 5 nm. This may be caused by quantum tunneling-assisted leakage of carriers and the deterioration in crystal quality, which lead to increased non-radiative losses [77]. Therefore, designing extremely thin QB samples requires fine interface quality control to meet theoretical expectations. A report on the improved performance of µLEDs by reducing QB thickness can be found here [78], and the improved EQE of µLEDs is obtained on both numerical calculations and experimental results.

Figure 8. (a) The structure diagrams of different QB-thickness LEDs. (b) The current–voltage properties for three fabricated LEDs. (c) The EQE as a function of injection currents for various LEDs. Reproduced from Ref. [76], with the permission of Elsevier.

Figure 8. (a) The structure diagrams of different QB-thickness LEDs. (b) The current–voltage properties for three fabricated LEDs. (c) The EQE as a function of injection currents for various LEDs. Reproduced from Ref. [76], with the permission of Elsevier.

By altering the growth plane to a non-polar or semi-polar plane, the impact of polarisation can be diminished or perhaps completely eliminated, offering a fundamentall solution to this issue [79,80,81,82,83]. Stabilization of wavelengths have demonstrated by MOCVD epitaxial growth of multiple quantum wells on non-polar or semi-polar planes [83], as shown in Figure 9a. Peak wavelength shifts for the m-plane and c-plane LED devices are 8.12 nm and 11.81 nm, respectively. Figure 9b shows that the m-plane devices have a slower decreasing trend, which indicates some stability and do not shift any longer under high current density injection. It may be attributed to the decrease in the m-plane polarization field, which improves the energy band tilt phenomenon. However, it can be attributed to high density stacking mistakes in the epitaxy method that the c-plane device has a considerably narrower full wavelength at half maximum than the m-plane device. Figure 9c illustrates that as the current density increases, the efficiency of c-plane decays severely compared to m-plane and that the EQE of m-plane is higher than that of c-plane at high current densities. However, high quality non-polar and semi-polar GaN-based µLEDs are grown on substrates with special crystal orientations due to cost and crystal quality issues. They are therefore not suitable for large-scale production. µLEDs are currently more suitable for growth on [0001] oriented Si [84,85] and sapphire substrates [86], as they are easier to fabricate. In addition, by progressively decreasing the dimension from the micron scale to the nanoscale, strain can be released. The fabrication of nano-LEDs, such as nanoring, nanorod, anti-nanoring, and nanohole LEDs, using a new colloidal photolithography technique based on the Talbot effect has been reported. These LEDs have lifetimes of 4, 6, 8, and 12 ns in the typical time-resolved photoluminescence test, as opposed to 15 ns for big-area LEDs [87]. However, the manufacturing process for nano-LEDs is more complex and requires special lithography systems and processes.

Figure 9. The comparisons of c-plane and m-plane µLEDs. (a) Peak wavelength, (b) FWHM, and (c) efficiency attenuation as a function of current density. Reproduced from Ref. [83], with the permission of John Wiley and Sons.

Figure 9. The comparisons of c-plane and m-plane µLEDs. (a) Peak wavelength, (b) FWHM, and (c) efficiency attenuation as a function of current density. Reproduced from Ref. [83], with the permission of John Wiley and Sons.

4.4. Improving the crystalline quality

Pattern substrate technology, buffer layer technology, and other technologies are more widely used in the heteroepitaxial growth of GaN on sapphire or silicon substrates to reduce dislocation density and improve crystal quality [88]. The homogeneous epitaxial technology of high-quality GaN substrates can obtain high-quality LED epitaxial wafers, but this substrate is costly, and it is tough to obtain large-sized epitaxial wafers. Buffer insertion layers such as GaN and AlN are often used in the epitaxial growth of sapphire-based LEDs [89], and the buffer layers in the preparation of silicon substrate LEDs usually include AlGaN/GaN, AlN/GaN superlattice and so on [90,91]. Nucleation centers can be provided by the insertion of buffer layers for GaN growth, and this can reduce the dislocation density of GaN. Yang et al. filter dislocations by growing single-layer AlN buffer layer and optimized superlattice insertion layer on silicon substrate [90]. Figure 10a and Figure 10b respectively show the epitaxial structure and EQE of the device as a function of current density. At the current density of 4 A cm^-2, the green and yellow LED EQE with a luminescent wavelength of 551 nm is increased to 37.7%. Chen et al. obtain Nanopatterned Sapphire (NPSS) by nanoimprinting, and grow GaN on NPSS substrate [92]. The stress in the grown GaN thin film can be effectively relaxed. The dislocation density decreases to 1.8 x 10⁸ cm^-2, and surface roughness reduced to below 0.1 nm.

Figure 10. (a) Schematic epitaxial structure with preparation layer. (b) Room-temperature EQE as a function of current density. Reproduced from Ref. [90], with the permission of Elsevier.

Figure 10. (a) Schematic epitaxial structure with preparation layer. (b) Room-temperature EQE as a function of current density. Reproduced from Ref. [90], with the permission of Elsevier.

4.5. Increasing the LEE for µLEDs

For conventional flip-chip µLEDs, a highly reflective Ag metal contact fabricated by using electron beam irradiation is proposed for better light extraction from the substrate [93]. Figure 11 demonstrates that the LOP of the EBI-based µLED is greater than that of the unbased device. Moreover, at the current density of 83 A/cm², EBI-based µLED has a greater EQE (37.2%). As can be seen in Table 1, the IQE at drive current of the two types of LEDs is almost equal, and the rise in EQE is caused by a rise in LEE.

Table 1. Optoelectronic performances at the 83 A/cm² current density with and without EBI-based μLEDs. Reproduced from Ref. [93], with the permission of Elsevier.

Table 1. Optoelectronic performances at the 83 A/cm² current density with and without EBI-based μLEDs. Reproduced from Ref. [93], with the permission of Elsevier.

µ-LEDs type	EQE (%)	LEE (%)	IQE (%)	LOP (mW)
With EBI	37.2	81.2	54.5	9.44
Without EBI	32.6	70.6	53.1	8.27

Figure 11. The comparisons with and without EBI on Ag thin film-based µLEDs. (a) LOP properties, (b) EQE properties. Reproduced from Ref. [93], with the permission of Elsevier.

Figure 11. The comparisons with and without EBI on Ag thin film-based µLEDs. (a) LOP properties, (b) EQE properties. Reproduced from Ref. [93], with the permission of Elsevier.

Due to the increase in critical angle, sidewall passivation can improve light extraction [57,94]. Hwang et al. deposited an omnidirectional reflector (ODR) structure on the sidewall of a µLED by ion beam deposition [95], which can reduce light loss and enhance the light extraction efficiency for µLEDs. ODR structures are mainly composed of silicon dioxide (Si0₂), silicon pentoxide (Ta₂O₅) SiO₂ and Ta₂O₅ are alternately overlapped, with Ta₂O₅ being deposited in the outermost layer. The carefully designed reflective structure reflects the light into the µLED, ultimately improving the performance of the device.

In addition, An investigation is numerically done on the function of a superlattice distributed Bragg reflector (SL DBR) as the p-type electron blocking layer in the GaN-based µLED [96]. The 2, 4, and 6 pairs of DBRs in LED devices are represented by SD A, SD B, and SD C in Figure 12a, respectively. Figure 12b shows that the EQE of SD C is 18.5% greater than that of Ref, and LEE enhances with the number of DBR pairs.

Figure 12. (a) Schematic diagram of µLEDs. (b) The output power and EQE as a function of current density. Reproduced from Ref. [96], with the permission of Optical Society of America.

Figure 12. (a) Schematic diagram of µLEDs. (b) The output power and EQE as a function of current density. Reproduced from Ref. [96], with the permission of Optical Society of America.

An inclined mesa favors the optical reflection towards the substrate and can be an easier approach to increase the LEE [97,98,99]. Hang at al have numerically investigated and demonstrated the impact of different inclination mesa angles on the optical and electrical properties for GaN-based µLEDs [99]. The simulation study has shown that enhanced LEE can be obtained by appropriately reducing the mesa angle of µLEDs. However, there is a trade-off between LEE and IQE in that when the inclined mesa angle is reduced, despite the enhanced LEE, the increased electric field in the sidewall region enhances the QCSE therein and the non-radiative recombination rate induced by surface defects will increase. Therefore, the inclined mesa angle should therefore be carefully designed and optimized. Due to the excellent scattering ability of the tilted countertop sidewall of µLEDs, and the certain thickness of the air layer between the n-GaN layer and the sapphire substrate that can be utilized. Jia at al propose the air-cavity patterned sapphire substrate (AC-PSS) as the light filter [100]. By combining the filtering effect of AC-PSS and the scattering effect of the tilted countertop sidewall, LEE can be enhanced while also improving optical crosstalk. Although manufacturing the above air layer is challenging in the reality, its structure provides important reference value for enhancing LEE.

5. Conclusions

References

1. Lin, J.Y.; Jiang, H.X. Development of microLED. Appl. Phys. Lett. 2020, 116, 100502. [Google Scholar] [CrossRef]
2. Wierer, J.J.; Tansu, N. III-Nitride Micro-LEDs for Efficient Emissive Displays. Laser Photonics Rev. 2019, 13, 1900141. [Google Scholar] [CrossRef]
3. Lee, H.E.; Shin, J.H.; Park, J.H.; et al. Micro light-emitting diodes for display and flexible biomedical applications. Adv. Funct. Mater. 2019, 29, 1808075. [Google Scholar] [CrossRef]
4. Liu, Z.; Zhang, K.; Liu, Y.; Yan, S.; Sun, X. Fully multi-functional GaN-based micro-LEDs for 2500 PPI micro-displays, temperature sensing, light energy harvesting, and light detection; IEEE International Electron Devices Meeting (IEDM): San Francisco, USA, 2018; pp. 38.1.1–38.1.4. [Google Scholar]
5. Lin, C.C.; Fang, Y.H.; Kao, M.J.; Huang, P.K.; Chang, F.P.; Yang, L.C. Ultra-Fine Pitch Thin-Film Micro LED Display for Indoor Applications. In SID Symposium Digest of Technical Papers; 2018; 49, pp. 782-785.
6. Santos, J.M.; Jones, B.E.; Schlosser, P.J.; Watson, S.; Herrnsdorf, J.; Guilhabert, B.; ... & Dawson, M.D. Hybrid GaN LED with capillary-bonded II–VI MQW color-converting membrane for visible light communications. Semicond. Sci. Technol. 2015, 30, 035012.
7. Hori, A.; Yasunaga, D.; Satake, A.; Fujiwara, K. Temperature and injection current dependence of electroluminescence intensity in green and blue InGaN single-quantum-well light-emitting diodes. J. Appl. Phys. 2003, 93, 3152–3157. [Google Scholar] [CrossRef]
8. Trindade, A.J.; Guilhabert, B.; Xie, E.Y.; Ferreira, R.; McKendry, J.J.D.; Zhu, D.; ... &Dawson, M.D. Heterogeneous integration of gallium nitride light-emitting diodes on diamond and silica by transfer printing. Opt. Express 2015, 23, 9329–9338.
9. Han, H.V.; Lin, H.Y.; Lin, C.C.; Chong, W.C.; Li, J.R.; Chen, K.J.; ... & Kuo, H.C. Resonant-enhanced full-color emission of quantum-dot-based micro LED display technology. Opt. Express. 2015, 23, 32504–32515.
10. Chung, K.; Sui, J.; Demory, B.; Ku, P.C. Color mixing from monolithically integrated InGaN-based light-emitting diodes by local strain engineering. Appl. Phys. Lett. 2017, 111, 041101. [Google Scholar] [CrossRef]
11. Gong, Z.; Jin, S.; Chen, Y.; et al. Size-Dependent Light Output, Spectral Shift, and Self-Heating of 400 nm InGaN Light-Emitting Diodes. J. Appl. Phys. 2010, 107, 013103. [Google Scholar] [CrossRef]
12. Yin, Y.F.; Lan, W.Y.; Lin, T.C.; et al. High-Speed Visible Light Communication Using GaN-Based Light-emitting Diodes With Photonic Crystals. J. Lightwave Technol. 2017, 35, 258–264. [Google Scholar] [CrossRef]
13. Rashidi, A.; Monavarian, M.; Aragon, A.; et al. Nonpolar M-Plane InGaN/GaN Micro-Scale Light-Emitting Diode with 1.5 GHz Modulation Bandwidth. IEEE Electron Device Lett. 2018, 39, 520–523. [Google Scholar] [CrossRef]
14. Lan, H.Y.; Tseng, I.C.; Kao, H.Y.; et al. 752-MHz Modulation Bandwidth of High-Speed Blue Micro Light-Emitting Diodes. IEEE J. Quantum Electron. 2018, 54, 1–6. [Google Scholar] [CrossRef]
15. Windisch, R.; Knobloch, A.; Kuijk, M.; et al. Large-Signal-Modulation of High-Efficiency Light-Emitting Diodes for Optical Communication. IEEE J. Quantum Electron. 2001, 36, 1445–1453. [Google Scholar] [CrossRef]
16. Akselrod, M.S.; Bruni, F.J. Modern Trends in Crystal Growth and New Applications of Sapphire. J. Crystal Growth 2012, 360, 134–1451. [Google Scholar] [CrossRef]
17. Komine, T.; Nakagawa, M. Fundamental Analysis for Visible Light Communication System Using LED Light. IEEE T. Consum. Electr. 2004, 50, 100–107. [Google Scholar] [CrossRef]
18. Zhao, S.; Ma, X. A Spectral-Efficient Transmission Scheme for Dimmable Visible Light Communication Systems. J. Lightwave Technol. 2017, 35, 3801–3809. [Google Scholar] [CrossRef]
19. Komine, T.; Nakagawa, M. Integrated System of White LED Visible-Light Communication and Power-Line Communication. IEEE T. Consum. Electr. 2003, 49, 71–79. [Google Scholar] [CrossRef]
20. Pang, G.; Kwan, T.; Liu, H.; et al. LED Wireless. IEEE Ind. Appl. Mag. 2002, 8, 21–28. [Google Scholar] [CrossRef]
21. Wun, J.M.; Lin, C.W.; Chen, W.; et al. GaN-Based Miniaturized Cyan Light-Emitting Diodes on a Patterned Sapphire Substrate With Improved Fiber Coupling for Very High-Speed Plastic Optical Fiber Communication. IEEE Photonics J. 2012, 4, 1520–1529. [Google Scholar] [CrossRef]
22. Mckendry, J.J.D.; Massoubre, D.; Zhang, S.; et al. Visible-Light Communications Using a CMOS-Controlled Micro-Light-Emitting-Diode Array. J. Lightwave Technol. 2012, 30, 61–67. [Google Scholar] [CrossRef]
23. Zhu, S.; Yu, Z.; Liu, L.; et al. Enhancing the Spontaneous Emission Rate by Modulating Carrier Distribution in GaN-Based Surface Plasmon Light-Emitting Diodes. Opt. Express 2017, 25, 9617. [Google Scholar] [CrossRef] [PubMed]
24. Liu, B.; Chen, D.; Lu, H.; et al. Hybrid Light Emitters and UV Solar-Blind Avalanche Photodiodes based on III-Nitride Semiconductors. Adv. Mater. 2020, 32, 1904354. [Google Scholar] [CrossRef] [PubMed]
25. Wang, Q.; Yu, J.C.; Tao, T.; et al. Fabrication and characterization of GaN-based micro-LEDs on silicon substrate. Chin. Phys. Lett. 2019, 36, 088501. [Google Scholar] [CrossRef]
26. Kou, J.; Shen, C.C.; Shao, H.; Che, J.; Hou, X.; Chu, C.; Tian, K.; Zhang, Y.; Zhang, Z.H.; Kuo, H.C. Impact of the surface recombination on InGaN/GaN-based blue micro-light emitting diodes. Opt. Express 2019, 27, 643–653. [Google Scholar] [CrossRef] [PubMed]
27. Sato, H.; Sugahara, T.; Hao, M.; Naoi, Y.; Kurai, S.; Tottori, S.; Yamashita, K.; Nishino, K.; Sakai, S.; Romano, L.T. Surface pretreatment of bulk GaN for homoepitaxial growth by metalorganic chemical vapor deposition. Jpn. J. Appl. Phys. 1998, 37, 626. [Google Scholar] [CrossRef]
28. Sugahara, T.; Sato, H.; Hao, M.; et al. Direct evidence that dislocations are non-radiative recombination centers in GaN. Jpn. J. Appl. Phys. 1998, 37, L398. [Google Scholar] [CrossRef]
29. Cho, J.; Schubert, E.F.; Kim, J.K. ; Efficiency droop in light-emitting diodes: Challenges and countermeasures. Laser Photonics Rev. 2013, 7, 408–421. [Google Scholar] [CrossRef]
30. Hwang, D.; Mughal, A.; Pynn, C.D.; Nakamura, S.; DenBaars, S.P. Sustained high external quantum efficiency in ultrasmall blue III-nitride micro-LEDs. Appl. Phys. Express 2017, 10, 032101. [Google Scholar] [CrossRef]
31. Tian, P.; McKendry, J.J.D.; Gong, Z.; Guilhabert, B.; Watson, I.M.; Gu, E.; Chen, Z.; Zhang, G.; Dawson, M.D. Size-dependent efficiency and efficiency droop of blue InGaN micro-light emitting diodes. Appl. Phys. Lett. 2012, 101, 231110. [Google Scholar] [CrossRef]
32. Olivier, F.; Daami, A.; Licitra, C.; et al. Shockley-Read-Hall and Auger non-radiative recombination in GaN based LEDs: A size effect study. Appl. Phys. Lett. 2017, 111, 022104. [Google Scholar] [CrossRef]
33. Herrnsdorf, J.; McKendry, J.J.D.; Zhang, S.; et al. Active-matrix GaN micro light-emitting diode display with unprecedented brightness. IEEE Trans. Electron Devices 2015, 62, 1918–1925. [Google Scholar] [CrossRef]
34. Konoplev, S.S.; Bulashevich, K.A.; Karpov, S.Y. From large-size to micro-LEDs: scaling trends revealed by modeling. Phys. Status Solidi (a). 2018, 215, 1700508. [Google Scholar] [CrossRef]
35. Huang, Y.; Tan, G.; Gou, F.; et al. Prospects and challenges of mini-LED and micro-LED displays. J. Soc. Inf. Display. 2019, 27, 387–401. [Google Scholar] [CrossRef]
36. Olivier, F.; Tirano, S.; Dupré, L.; et al. Influence of size-reduction on the performances of GaN-based micro-LEDs for display application. J. Lumin. 2017, 191, 112–116. [Google Scholar] [CrossRef]
37. Tian, P.; McKendry, J.J.D.; Herrnsdorf, J.; et al. Temperature-dependent efficiency droop of blue InGaN micro-light emitting diodes. Appl. Phys. Lett. 2014, 105, 171107. [Google Scholar] [CrossRef]
38. Galler, B.; Drechsel, P.; Monnard, R.; Rode, P.; Stauss, P.; Froehlich, S.; Bergbauer, W.; Binder, M.; Sabathil, M.; Hahn, B.; Wagner, J. Influence of indium content and temperature on Auger-like recombination in InGaN quantum wells grown on (111) silicon substrates. Appl. Phys. Lett. 2012, 101, 131111. [Google Scholar] [CrossRef]
39. Kioupakis, E.; Yan, Q.; Steiauf, D.; et al. Temperature and carrier-density dependence of Auger and radiative recombination in nitride optoelectronic devices. New J. Phys. 2013, 15, 125006. [Google Scholar] [CrossRef]
40. Kioupakis, E.; Rinke, P.; Delaney, K.T.; et al. Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes. Appl. Phys. Lett. 2011, 98, 161107. [Google Scholar] [CrossRef]
41. Bertazzi, F.; Goano, M.; Bellotti, E. Numerical analysis of indirect Auger transitions in InGaN. Appl. Phys. Lett. 2012, 101, 011111. [Google Scholar] [CrossRef]
42. Hader, J.; Moloney, J.V.; Koch, S.W. Temperature Dependence of Radiative and Auger Losses in Quantum Wells. IEEE J. Quantum Electron. 2008, 44, 185–191. [Google Scholar] [CrossRef]
43. Dadgar, A.; Bläsing, J.; Diez, A.; et al. Metalorganic chemical vapor phase epitaxy of crack-free GaN on Si (111) exceeding 1 µm in thickness. Jpn. J. Appl. Phys. 2000, 39, L1183. [Google Scholar] [CrossRef]
44. Kikuchi, A.; Kawai, M.; Tada, M.; et al. InGaN/GaN multiple quantum disk nanocolumn light-emitting diodes grown on (111) Si substrate. Jpn. J. Appl. Phys. 2004, 43, L1524. [Google Scholar] [CrossRef]
45. Zhao, D.G.; Xu, S.J.; Xie, M.H.; et al. Stress and its effect on optical properties of GaN epilayers grown on Si (111), 6H-SiC (0001), and c-plane sapphire. Appl. Phys. Lett. 2003, 83, 677–679. [Google Scholar] [CrossRef]
46. De, S.C.; Meneghini, M.; La, G.M.; et al. Role of defects in the thermal droop of InGaN-based light emitting diodes. J. Appl. Phys. 2016, 119, 094501. [Google Scholar]
47. Gong, Z.; Jin, S.; Chen, Y.; et al. Size-dependent light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes. J. Appl. Phys. 2010, 107, 013103. [Google Scholar] [CrossRef]
48. Lu, S.; Liu, W.; Zhang, Z.H.; et al. Low thermal-mass LEDs: size effect and limits. Opt. Express 2014, 22, 32200–32207. [Google Scholar] [CrossRef]
49. Ploch, N.L.; Rodriguez, H.; Stolmacker, C.; et al. Effective thermal management in ultraviolet light-emitting diodes with micro-LED arrays. IEEE T. Electron Dev. 2013, 60, 782–786. [Google Scholar] [CrossRef]
50. Choi, H.W.; Jeon, C.W.; Dawson, M.D.; et al. Mechanism of enhanced light output efficiency in InGaN-based microlight emitting diodes. J. Appl. Phys. 2003, 93, 5978–5982. [Google Scholar] [CrossRef]
51. Lin, H.Y.; Sher, C.W.; Hsieh, D.H.; et al. Optical cross-talk reduction in a quantum-dot-based full-color micro-light-emitting-diode display by a lithographic-fabricated photoresist mold. Photon. Res. 2017, 5, 411–416. [Google Scholar] [CrossRef]
52. Monavarian, M.; Rashidi, A.; Feezell, D. A Decade of Nonpolar and Semipolar III-Nitrides: A Review of Successes and Challenges. Phys. Status Solidi (a). 2019, 216, 1800628. [Google Scholar] [CrossRef]
53. McKendry, J.J.D.; Massoubre, D.; Zhang, S.; et al. Visible-light communications using a CMOS-controlled micro-light-emitting-diode array. J. Lightwave Technol. 2011, 30, 61–67. [Google Scholar] [CrossRef]
54. Shen, Y.C.; Mueller, G.O.; Watanabe, S.; et al. Auger recombination in InGaN measured by photoluminescence. Appl. Phys. Lett. 2007, 91, 141101. [Google Scholar] [CrossRef]
55. Zhu, S.; Lin, S.; Li, J.; et al. Influence of quantum confined Stark effect and carrier localization effect on modulation bandwidth for GaN-based LEDs. Appl. Phys. Lett. 2017, 111, 171105. [Google Scholar] [CrossRef]
56. Son, K.R.; Murugadoss, V.; Kim, K.H.; et al. Investigation of sidewall passivation mechanism of InGaN-based blue microscale light-emitting diodes. Appl. Surf. Sci. 2022, 584, 152612. [Google Scholar] [CrossRef]
57. Chen, W.; Hu, G.; Lin, J.; et al. High-performance, single-pyramid micro light-emitting diode with leakage current confinement layer. Appl. Phys. Express 2015, 8, 032102. [Google Scholar] [CrossRef]
58. Ley, R.T.; Smith, J.M.; Wong, M.S.; et al. Revealing the importance of light extraction efficiency in InGaN/GaN microLEDs via chemical treatment and dielectric passivation. Appl. Phys. Lett., 2020, 116, 251104. [Google Scholar] [CrossRef]
59. Dingemans, G.; Van de Sanden, M.C.M.; Kessels, W.M.M. Influence of the deposition temperature on the c-Si surface passivation by Al₂O₃ films synthesized by ALD and PECVD. Electrochem. Solid-state Lett. 2009, 13, 76. [Google Scholar] [CrossRef]
60. Wong, M.S.; Hwang, D.; Alhassan, A.I.; et al. High efficiency of III-nitride micro-light-emitting diodes by sidewall passivation using atomic layer deposition. Opt. express 2018, 26, 21324–21331. [Google Scholar] [CrossRef]
61. Choi, W.H.; You, G.; Abraham, M.; et al. Sidewall passivation for InGaN/GaN nanopillar light emitting diodes. J. Appl. Phys. 2014, 116, 013103. [Google Scholar] [CrossRef]
62. Yang, Y.; Cao, X.A. Removing plasma-induced sidewall damage in GaN-based light-emitting diodes by annealing and wet chemical treatments. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.--Process., Meas., Phenom. 2009, 27, 2337–2341. [Google Scholar] [CrossRef]
63. Le Maoult, C.; Vaufrey, D.; Martin, F.; Martinez, E.; Nolot, E.; Cadot, S.; Gheeraert, E. Analysis of InGaN surfaces after chemical treatments and atomic layer deposition of Al₂O₃ for µLED applications. In Proc. SPIE 11280, Gallium Nitride Materials and Devices XV, 16 February 2020. 16 February.
64. Wong, M.S.; Lee, C.; Myers, D.J.; et al. Size-independent peak efficiency of III-nitride micro-light-emitting-diodes using chemical treatment and sidewall passivation. Appl. Phys. Express 2019, 12, 097004. [Google Scholar] [CrossRef]
65. Zhuang, D.; Edgar, J.H. Wet etching of GaN, AlN, and SiC: a review. Mater. Sci. Eng. R Reports 2005, 48, 1–46. [Google Scholar] [CrossRef]
66. Tang, B.; Miao, J.; Liu, Y.; et al. Enhanced light extraction of flip-chip mini-LEDs with prism-structured sidewall. Nanomaterials-Basel 2019, 9, 319. [Google Scholar] [CrossRef] [PubMed]
67. Cai, Y.; Bai, J.; Wang, T. Review of a direct epitaxial approach to achieving micro–LEDs. Chinese Phys. B 2023, 32, 018508. [Google Scholar] [CrossRef]
68. Hamdy, S.W.; Young, E.C.; Alhassan, A.I.; et al. Efficient tunnel junction contacts for high-power semipolar III-nitride edge-emitting laser diodes. Opt. Express 2019, 27, 8327–8334. [Google Scholar] [CrossRef] [PubMed]
69. Malinverni, M.; Martin, D.; Grandjean, N. InGaN based micro light emitting diodes featuring a buried GaN tunnel junction. Appl. Phys. Lett 2015, 107, 051107. [Google Scholar] [CrossRef]
70. Young, E.C.; Yonkee, B.P.; Wu, F.; et al. Hybrid tunnel junction contacts to III–nitride light-emitting diodes. Appl. Phys. Express 2016, 9, 022102. [Google Scholar] [CrossRef]
71. Kuwano, Y.; Kaga, M.; Morita, T.; et al. Lateral hydrogen diffusion at p-GaN layers in nitride-based light emitting diodes with tunnel junctions. Jpn. J. Appl. Phys. 2013, 52, 08JK12. [Google Scholar] [CrossRef]
72. Lee, S.G.; Forman, C.A.; Kearns, J.; et al. Demonstration of GaN-based vertical-cavity surface-emitting lasers with buried tunnel junction contacts. Opt. Express 2019, 27, 31621–31628. [Google Scholar] [CrossRef]
73. Hwang, D.; Mughal, A.J.; Wong, M.S.; et al. Micro-light-emitting diodes with III–nitride tunnel junction contacts grown by metalorganic chemical vapor deposition. Appl. Phys. Express 2017, 11, 012102. [Google Scholar] [CrossRef]
74. Smorchkova, I.P.; Haus, E.; Heying, B.; et al. Mg doping of GaN layers grown by plasma-assisted molecular-beam epitaxy. Appl. Phys. Lett. 2000, 76, 718–720. [Google Scholar] [CrossRef]
75. Lin, G.B.; Kim, D.Y.; Shan, Q.; et al. Effect of quantum barrier thickness in the multiple-quantum-well active region of GaInN/GaN light-emitting diodes. IEEE Photonics J. 2013, 5, 1600207–1600207. [Google Scholar]
76. Wang, C.K.; Chiou, Y.Z.; Chang, S.J.; et al. On the effect of quantum barrier thickness in the active region of nitride-based light emitting diodes. Solid State Electron. 2014, 99, 11–15. [Google Scholar] [CrossRef]
77. Liu, W.; Liang, F.; Zhao, D.; et al. Reduction in the photoluminescence intensity caused by ultrathin gan quantum barriers in InGaN/GaN multiple quantum wells. Crystals 2022, 12, 339. [Google Scholar] [CrossRef]
78. Chang, L.; Yeh, Y.W.; Hang, S.; et al. Alternative strategy to reduce surface recombination for InGaN/GaN Micro-light-emitting diodes—thinning the Quantum barriers to manage the current spreading. Nanoscale Res. Lett. 2020, 15, 1–9. [Google Scholar] [CrossRef] [PubMed]
79. Rashidi, A.; Monavarian, M.; Aragon, A.; et al. Nonpolar ${m} $-plane InGaN/GaN micro-scale light-emitting diode with 1.5 GHz modulation bandwidth. IEEE Electr. Device L. 2018, 39, 520–523. [Google Scholar] [CrossRef]
80. Dinh, D.V.; Quan, Z.; Roycroft, B.; et al. GHz bandwidth semipolar (112 2) InGaN/GaN light-emitting diodes. Opt. Lett. 2016, 41, 5752–5755. [Google Scholar] [CrossRef]
81. Chakraborty, A.; Haskell, B.A.; Keller, S.; et al. Nonpolar InGaN∕GaN emitters on reduced-defect lateral epitaxially overgrown a-plane GaN with drive-current-independent electroluminescence emission peak. Appl. Phys. Lett. 2004, 85, 5143–5145. [Google Scholar] [CrossRef]
82. Sharma, R.; Pattison, P.M.; Masui, H.; et al. Demonstration of a semipolar (10 1¯ 3¯) In Ga N∕ Ga N green light emitting diode. Appl. Phys. Lett. 2005, 87, 231110. [Google Scholar] [CrossRef]
83. Liu, Y.; Feng, F.; Zhang, K.; et al. 4.4: Low Efficiency Attenuation and Stable Monochromaticity for Non-polar M-plane Micro-light-emitting-diodes (Micro-LEDs). SID Symp. Dig. Tech. Pap. 2022, 53, 62–65. [Google Scholar] [CrossRef]
84. Ma, J.; Zhu, X.; Wong, K.M.; et al. Improved GaN-based LED grown on silicon (111) substrates using stress/dislocation-engineered interlayers. J. Cryst. Growth 2013, 370, 265–268. [Google Scholar] [CrossRef]
85. Kim, T.; Jung, Y.H.; Song, J.; et al. High-efficiency, microscale GaN light-emitting diodes and their thermal properties on unusual substrates. Small 2012, 8, 1643–1649. [Google Scholar] [CrossRef]
86. Dingemans, G.; Van de Sanden, M.C.M.; Kessels, W.M.M. Influence of the deposition temperature on the c-Si surface passivation by Al2O3 films synthesized by ALD and PECVD. Electrochem. Solid-state Lett. 2009, 13, H76. [Google Scholar] [CrossRef]
87. Wang, L.; Liu, Z.; Li, Z.; et al. Nanostructure nitride light emitting diodes via the Talbot effect using improved colloidal photolithography. Nanoscale 2017, 9, 7021–7026. [Google Scholar] [CrossRef] [PubMed]
88. Liu, B.; Chen, D.; Lu, H.; et al. Hybrid Light Emitters and UV Solar-Blind Avalanche Photodiodes based on III-Nitride Semiconductors. Adv. Mater. 2020, 32, 1904354. [Google Scholar] [CrossRef] [PubMed]
89. Amano, H.; Sawaki, N.; Akasaki, I.; et al. Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer. Appl. Phys. Lett. 1986, 48, 353–355. [Google Scholar] [CrossRef]
90. Yang, X.; Zhang, J.; Wang, X.; et al. Enhance the efficiency of green-yellow LED by optimizing the growth condition of preparation layer. Micro and Nanostructures 2020, 141, 106459. [Google Scholar] [CrossRef]
91. Zhang, J.; Wang, X.; Liu, J.; et al. Study on Carrier transportation in InGaN based green LEDs with V-pits structure in the active region. Opt. Mater. 2018, 86, 46–50. [Google Scholar] [CrossRef]
92. Chen, Y.; Chen, Z.; Li, J.; et al. A study of GaN nucleation and coalescence in the initial growth stages on nanoscale patterned sapphire substrates via MOCVD. CrystEngComm 2018, 20, 6811–6820. [Google Scholar] [CrossRef]
93. Hong, I.Y.; Islam, A.B.M.H.; Kim, T.K.; et al. Impact of grain growth of silver reflective electrode by electron bombardment on external quantum efficiency of III-nitride micro-light-emitting diode arrays. Appl. Surf. Sci. 2020, 512, 145698. [Google Scholar] [CrossRef]
94. Yang, C.M.; Kim, D.S.; Park, Y.S.; et al. Enhancement in light extraction efficiency of GaN-based light-emitting diodes using double dielectric surface passivation. Opt. Photonics J. 2012, 2, 185–192. [Google Scholar] [CrossRef]
95. Hwang, D.; Mughal, A.; Pynn, C.D.; et al. Sustained high external quantum efficiency in ultrasmall blue III–nitride micro-LEDs. Appl. Phys. Express 2017, 10, 032101. [Google Scholar] [CrossRef]
96. Yan, G.; Hyun, B.R.; Jiang, F.; et al. Exploring superlattice DBR effect on a micro-LED as an electron blocking layer. Opt. Express 2021, 29, 26255–26264. [Google Scholar] [CrossRef]
97. Zhang, Y.; Meng, R.; Zhang, Z.H.; 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. 2017, 9, 1–9. [Google Scholar] [CrossRef]
98. Tian, M.; Yu, H.; Memon, M.H.; et al. Enhanced light extraction of the deep-ultraviolet micro-LED via rational design of chip sidewall. Opt. Lett. 2021, 46, 4809–4812. [Google Scholar] [CrossRef] [PubMed]
99. Hang, S.; Zhang, G.; Chu, C.; et al. On the impact of the beveled mesa for GaN-based micro-light emitting diodes: electrical and optical properties. Opt. Express 2022, 30, 37675–37685. [Google Scholar] [CrossRef] [PubMed]
100. Jia, T.; Zhang, M.; Zhang, G.; et al. Suppressing optical crosstalk for GaN/InGaN based flip-chip micro light-emitting diodes by using an air-cavity patterned sapphire substrate as a light filter. Opti. Express 2023, 31, 2931–2941. [Google Scholar] [CrossRef]