Recent Progress in Long‐Wavelength InGaN Light‐Emitting Diodes from the Perspective of Epitaxial Structure

Over the last decades, continuous technological advancements have been made in III‐nitride light‐emitting diodes (LEDs), so that they are considered as a promising replacement of traditional light sources. With the emission wavelength covering the entire visible spectrum, InGaN LEDs find various applications such as solid‐state lightings, full‐color displays, and visible light communication. However, the quantum efficiency of InGaN LEDs suffers from a dramatic decline as the emission wavelength extends from blue to green–red region. This issue restrains the lighting and display applications based on the color‐mixing monolithic lighting source system. In this review, the recent breakthroughs in long‐wavelength InGaN LEDs, together with the challenges and approaches to realize high‐indium‐composition InGaN epilayers, are introduced. These cover the different epitaxial substrates, nucleation layers, and epitaxial structures, especially multiple quantum wells active region. The related studies are also discussed to improve the long‐wavelength LEDs performance from the aspect of crystal quality, growth orientation, carrier‐injection, and 3D nanostructures. Finally, current status and perspectives for future long‐wavelength LEDs development are proposed briefly.


Introduction
InGaN alloys have a direct and tunable bandgap spanning from 0.7 to 3.4 eV, which makes them promising candidates for wide applications including visible light-emitting diodes (LEDs), lasers, detectors, and solar cells. [1] Notably, InGaN LEDs, a revolutionary visible light source for the next generation, have gained much attention due to the advantages of high luminous efficiency, low energy consumption, long operation lifetime, etc. [2] As highly bright InGaN blue LEDs were developed more than two decades ago, [3] substantial advances in epitaxial growth, material doping, and device fabrication have boosted the efficiency of LED devices. White lighting based on LEDs are becoming a world-scale replacement of fluorescent lamps to solve the increasingly serious energy shortage and environment issue. [4] At present, commercially available white LEDs are mainly fabricated using blue LEDs with yellow YAG:Ce 3þ phosphor. Stokes energy loss inevitably occurs in phosphor-converted white LEDs and deteriorates their efficiency, which can be avoided in phosphorfree white LEDs combining red, green, and blue (RGB) LEDs. [5] In recent years, small-form-factor inorganic LED displays, known as mini-LED or micro-LED, present as a viable display technology and outperform liquid-crystal displays and organic LEDs in terms of brightness, quick response, and energy efficiency. [6] Benefiting from these advantages, RGB micro-LEDs has been highlighted for full-color displays, wearable devices, high brightness augmented reality, and virtual reality devices. [7] Thus, efficient RGB emitters are highly desirable to fulfill the practical applications. To date, state-of-the-art InGaN blue LEDs achieved a considerably high external quantum efficiency (EQE) beyond 80%, and when used as excitation source, the corresponding white LEDs realized a high luminous efficacy of 249 lm W À1 , which almost approaches the theoretical limit. [8] For the current LED-display field, InGaN blue and green LEDs have been able for mass production, while the AlGaInP LEDs exhibit excellent performance as red pixels. By integrating RGB micro-LEDs via the mass transfer method, the brightness of each pixel can be precisely modulated in a wide dynamic range. [9] However, the mass transfer method is restricted by procedure and low fabrication yield. Moreover, AlGaInP LEDs suffer from high surface recombination velocities and longer carrier diffusion lengths as well as intrinsic temperature sensitivity. [10] With the chip size decreasing, the efficiency of AlGaInP red LEDs drops severely, hindering the micro-LED display applications. Therefore, monolithic integration of RGB light-emitting pixels based on the InGaN material system is of growing interest. [11] efficiency with increasing indium composition in QWs for longer wavelength emission ( Figure 1). This phenomenon is commonly known as "green gap." The origin of "green gap" is still a matter of debate and two dominant origins have been proposed to explain this phenomenon [13] : crystal quality degradation and quantum-confined Stark effect (QCSE). The lattice mismatch between substrate and epilayers, and between epilayers, gives rise to structural defects formation in epilayers. Multiple types of structural defects have been found to exist in InGaN LEDs, such as threading dislocation, [14] stacking fault, [15] point defect, [16] V-pit, [17] trench defects, [18] and misfit dislocations. [19] Structural defects deteriorate the crystal quality and some of them act as nonradiative centers that induce the Shockley-Read-Hall nonradiative process and reduce the internal quantum efficiency (IQE) of InGaN LEDs. In addition, for long-wavelength MQWs, relatively low growth temperature is required to prevent the indium desorption effect, at the cost of poor surface morphology and increased defect density. [20] Due to the immiscibility of InGaN alloys, compositional fluctuation and phase separation usually occur, hindering the realization of high-quality InGaN layers. [13a,21] Besides the crystal quality issue, lattice mismatch causes compressive strain in the LED epitaxial structure as well. This issue correlates with QCSE and compositional pulling effect. The former reduces the carrier wavefunction overlap probability, thereby decreasing the radiative recombination rate; the latter affect the indium incorporation in InGaN layers, challenging to obtain long-wavelength emission. Thus, further improvement is required for InGaN LEDs in long-wavelength region. Of course, challenges universally encountered by LED devices, such as efficiency droop, [22] carrier injection, [23] and light extraction, [24] are also present in long-wavelength InGaN LEDs. In this review, we will discuss the recent progress made in InGaN long-wavelength LEDs from the perspective of epitaxial structure, together with the important techniques to enhance the device performance. We mainly focus on the efforts devoted to settling the challenges of crystal quality and QCSE (Figure 2), and raise the outlook on the promising future of the InGaN LEDs in long-wavelength region.

Substrate and Buffer Layer
The crystal quality of epilayers determines the LED performance. Although bulk GaN substrate contributes to a low threading dislocation density (TDD), its application is restricted by expensive fabrication cost and small size availability. Alternatively, at present, most commercial LED wafers are grown on c-plane sapphire substrate due to the low cost and excellent optical transmittance. The development in growth techniques, especially based on metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), has greatly promoted the performance of LEDs. High-quality epilayers can be achieved by introducing low-temperature AlN or GaN as buffer layer on substrate at the initial growth stage. This effectively alleviates the lattice mismatch and thus reduces the TDD extending into the following layers. Afterward, ex situ sputtered AlN nucleation layer (NL) emerged, which outperforms low-temperature GaN in terms of growth time, thermal cycles of epitaxy, and TDD. This points to another route toward efficient long-wavelength InGaN LEDs. Besides, reduction in TDD was also found by performing isoelectronic Al doping in GaN buffer layer. [25] This was successfully implemented in UV and blue LEDs with light output power (LOP) enhancement. To further solve the lattice mismatch between AlN NL and GaN epilayers, our group first proposed a concept of hybrid NL (denoted as h-NL), which was composed of sputter AlN and midtemperature GaN buffer layer. As compared to the mere sputtered AlN NL (denoted as s-NL), the midtemperature GaN in h-NL induced the formation of stacking fault band structure at the interface, which effectively relaxed the misfit stress and reduced the dislocation density in multiple quantum wells (MQWs) (Figure 3a). [26] The LOP and IQE of green LEDs grown on hybrid NL presented a 16.2% and 19.9% increment in LOP and IQE, respectively, compared to those of LEDs grown on AlN NL (Figure 3b,c). The enhanced radiative recombination efficiency was attributed to the deeper localized states and suppressed QCSE in the sample with hybrid NL. Similar to the hybrid NL structure, Chen et al. proposed a composite buffer layer consisting of sputter AlN and low-temperature GaN to increase the surface lattice constant of GaN and enhance the indium incorporation efficiency. [27] An efficient InGaN red  www.advancedsciencenews.com www.adpr-journal.com mini-LED chip with a peak wavelength of 629 nm and an external quantum efficiency of 7.4% was consequently realized. Despite these approaches, it is still challenging to grow longwavelength LEDs with high crystal quality on sapphire substrate. To address this bottleneck, patterned sapphire substrates (PSS) have been proposed and widely used in the state-of-the-art longwavelength LEDs because it can reduce the TDD and meanwhile enhances the light extraction efficiency (LEE). [28] By employing multiple fabrication processes, PSS that features different pattern shapes and sizes could be prepared prior to the epitaxial growth of LEDs, without introducing any interruption or contamination in the growth process. Thus, it has been combined epitaxial lateral overgrowth and buffer layer, feasible to obtain long-wavelength LED with low TDD. Peng et al. investigated GaN growth behaviors on sputtered AlN/PSS template under different growth modes by MOCVD. [29] Under "tsunami" growth mode, GaN grew into the truncated pyramid shape, which enabled dislocations stem from flat area bend toward the inclined planes, and the propagation of dislocations on the pattern surface was inhibited. However, dislocations formed on the pattern surface had the possibility to propagate up under "rising" growth mode, degrading the crystal quality with higher TDD. As a result, EQE decreased from 21.1% for green LEDs grown under "tsunami" growth mode to 13% for those grown under "rising" growth mode. We compared green LEDs with the same thickness of low-temperature GaN and sputtered AlN NL (19 nm) on PSS. [30] As shown in Figure 4, TDD of green LEDs on sputtered AlN/PSS template was lower than that of the green LEDs on lowtemperature GaN/PSS. Better crystal quality notwithstanding, LEDs on sputtered AlN/PSS template experienced larger inplane strain. It was found that compressive strain could be manipulated by adjusting thickness of sputtered AlN layer. When the thickness increased to 40 nm, the fabricated green LEDs featured the lowest compressive strain and especially highest EQE of 36.4% (@20 A cm À2 ), which was approximately 6.1% larger than that of green LEDs on low-temperature-GaN (19 nm)/PSS template. Adopting multiple interruptions during the green-yellow InGaN QW growth was reported to improve crystal quality. In detail, Ga and In sources were closed while maintaining ammonia flow to increase V/III ratio, leading to decreased indium clusters and relevant defects. [31] Wang et al. developed ex situ sputtered AlNO buffer layer on PSS to enhance the green LED efficiency. [32] The lattice constant of AlNO can be tuned through adjusting oxygen flow. With suitable oxygen composition, TDD and compressive stress caused by lattice mismatch were greatly reduced, paving the way for stress manipulation combining buffer layer with PSS in long-wavelength LEDs.  Compared to InGaN alloys grown by homoepitaxial growth, those grown by heteroepitaxial growth are more susceptible to strain owing to the lattice mismatch. Strain state in LED structure is related to the substrates used. As reported, compressive strain exists in GaN layers grown on sapphire substrates; by contrast, GaN layers grown on SiC and Si substrates exhibit tensile strain. Zhu et al. compared sapphire and silicon substrates for InGaN/GaN MQWs growth, finding that tensile strain presenting in GaN grown on silicon substrate modified thermodynamic limitations and facilitated indium incorporation to obtain long wavelength. [33] ScAlMgO 4 is considered as a promising substrate for InGaN alloys growth because it is lattice-matched with In 0.17 Ga 0.83 N and 1.8% lattice-mismatched with GaN. [34] Compared with InxGa 1Àx N/GaN QW on sapphire template, significantly improved IQE, emission intensity, and spatial emission uniformity were found in In x Ga 1Àx N/In y Ga 1Ày N QW on ScAlMgO 4 . [35] Besides, ZnO also acts as an attractive choice for epitaxially growing InGaN layers as its lattice constant is close to that of InGaN. In detail, for In 0.029 Ga 0.971 N, the lattice is matched to ZnO along the c-axis with a tensile lattice mismatch of À1.54% along the a-axis, whereas for In 0.168 Ga 0.834 , the lattice is matched to ZnO along the a-axis, with a compressive lattice mismatch of 1.36% along the c-axis. [36] To date, LEDs grown on ZnO substrate have been demonstrated for only blue emission region by means of MBE [37] and MOCVD. [38] To avoid the decomposition of ZnO substrate at high temperature, which caused atomic diffusion and interfacial reactions, two-step growth process or insertion of low-temperature GaN buffer layer was used and, as a result InGaN LEDs were successfully fabricated. Studies of LEDs on ZnO are still at their infancy, encouraging more efforts devoted to optimizing epitaxial structure and growth conditions to obtain high-performance devices.
Iida et al. from KAUST demonstrated that (À201) oriented β-Ga 2 O 3 substrates exhibit a relatively low lattice mismatch (%4.7%) with GaN, making them a promising substrate for nitride-based device applications. [39] The in-plane strain of GaN layers presents a very slight tensile strain and the InGaN QWs grown on (À201) β-Ga 2 O 3 substrates can increase the growth temperature by 10°C compared to that grown on sapphire substrates. Therefore, In incorporation rates into the InGaN QWs are significantly improved, leading to the production of pure red emission.

InGaN Template
Composition pulling effect, which arises from the compressive stain by lattice mismatch, limits indium incorporation in MQWs and thus hinders the development of long-wavelength devices. InGaN templates have recently attracted interest owing to their ability to tackle composition pulling effect in MQWs and to extend the emission wavelength. Abdelhamid et al. disclosed the dependence of the emission from MQWs on the indium content in underlying InGaN templates grown via semibulk approach. [40] Compared to GaN template, the relaxed In 0.1 Ga 0.9 N template induced a redshift in emission wavelength up to 62 nm, which was attributed to the reduced strain in MQWs, along with the increased indium incorporation by suppressing composition pulling effect. They further enhanced the indium composition in InGaN template, finding that the redshift of emission wavelength was about 100 nm at the expense of degraded crystal quality. [41] Bi et al reported arrays of high-crystalline quality InGaN platelets with a top c-plane area prepared by selective area metal-organic vapor phase epitaxy (MOVPE) growth with an in situ annealing that turns the pyramidal shape into a c-oriented platelet. The LED structures were grown on those InGaN platelets. [42] Such relaxed templates enabled not only the growth of QWs with high crystal quality, but also the emission shift toward green and red, depending on the indium composition in InGaN templates. To eliminate the dislocation formation due to lattice mismatch, InGaN templates were directly grown on InGaN pyramids processed by chemical mechanical polishing technique. [43] As a result, high-quality InGaN layers were obtained on strainrelaxed InGaN platelets, which offered a platform for longwavelength LEDs to develop full-color micro-LED displays. Cai et al. reported high-quality and uniform InGaN platelet arrays by selectively depositing InGaN onto submicrometer scale GaN seed arrays by MOVPE. [44] Such platelets showed a height of 120 nm, where the indium content increased from 15% at the InGaN/GaN interface to about 30% at the atomically flat surface. The obtained high-quality InGaN pseudosubstrate can serve as high-quality strain-relaxed templates for high efficiency, long wavelength optical applications.
Besides, the utilization of InGaN template was reported to allow LEDs devices grow at high temperature when maintaining high indium composition in QWs. Chan et al. developed a technique by growing InGaN buffer on a compliant decomposition stop layer, which was relaxed through the thermal decomposition of an InGaN decomposition layer underneath. [45] Using such relaxed InGaN buffer as pseudosubstrates, high-indiumcomposition QWs for red LEDs could be grown over a full 2 inch wafer at above 800°C, representing a promising approach for red LEDs with long wavelength and high crystal quality at the same time. [46] Even et al. investigated the effect of InGaN pseudosubstrate with varying a-lattice parameters (3.190, 3.200, 3.205 Å) on the emission characteristics of MQWs. [47] These pseudosubstrates were found to improve the indium incorporation rate due to the reduced compositional pulling effect caused by compressive strain. The redshift in emission wavelength for MQWs became more pronounced as a-lattice parameter of the InGaN pseudosubstrate increased. Especially for samples on InGaN pseudosubstrate with 3.205 Å, a redshift value up to 62 nm was obtained compared to samples on GaN/sapphire. Under specific growth conditions, InGaN pseudosubstrate with 3.205 Å enabled both longer wavelength and stronger emission intensity among these pseudosubstrates, which has the potential to overcome green gap issue. Red-emitting InGaN/InGaN MQWs grown on relaxed pseudosubstrate, exhibiting an IQE value above 10% at 640 nm. [48] InGaN pyramids with more than 40% indium composition were fabricated via selective area growth method with nanoimprint. [49] Despite the unwanted compositional variation, InGaN/GaN double heterostructures on nanopyramids exhibited bright red emission at 650 nm and improved internal quantum efficiency compared to those on planar structure. Researchers from UCSB developed compliant GaN on porous GaN pseudosubstrates as a universal substrate for growing strain-relaxed InGaN layers. Pasayat et al. demonstrated the growth of strain-relaxed InGaN layers on top of porous GaN. [50] InGaN-based red LEDs composed of a strain-relaxed In 0.04 Ga 0.96 N layer with a relaxation degree of 0.56 were successfully grown on InGaN pseudosubstrates, with a peak emission of 632 nm at 10 A cm À2 . [51] Notably, these devices displayed a large redshift value of 56 nm at 5 A cm À2 , which was attributed to the strain reduction achieved by the porous underlying layers. The observed enhancement in the In incorporation into the InGaN QWs was attributed to the compositional pulling effect resulting from an increase in the a-lattice constant.

2D Materials for Long-Wavelength LEDs
Recently, van der Waals epitaxy (vdWe) of III-nitrides on 2D materials has attracted much interests. [52] On the one hand, 2D materials as buffer layer can significantly relieve the strain in III-nitrides epilayers due to the formation of weak covalent bonds at the interfaces. This benefits the indium incorporation in the active region, and thus leads to long-wavelength emission. On the other hand, epilayers can be easily released and transferred to targeted substrates, enabling the fabrication of novel flexible devices. Developing vdWe technique offers a facile solution to overcome the lattice mismatch that hinders the performance of InGaN LEDs grown on foreign substrates. Liu et al. achieved high-quality N-polarity GaN films on graphene/sapphire by MBE. [53] Large redshift of emission wavelength from 462 to 537 nm was observed because more indium is incorporated during the growth of InGaN QWs for the N-polarity than that for Ga-polarity. Yu et al. realized the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by MOCVD. They found that graphene could effectively reduce the biaxial stress in epilayers and further improve the indium incorporation in InGaN/GaN MQWs, resulting in the redshift of emission wavelength about 21 nm. [54] Liu et al. employed a chemical activation method to generate N-O bonds that facilitated the nucleation of GaN on the h-BN surface and modify the lattice polarity of GaN to be uniform Ga-polarity one. The h-BN layer was also reported to relax the compressive strain in GaN film and improve indium incorporation, enabling the realization of pure green LEDs. [55] With help of 2D material-based layer transfer technique, vertical full-color micro-LEDs with record small size and high array density have been demonstrated. [56] This transfer technique allows the epitaxial growth of InGaN RGB LEDs with near-submicron thickness on 2D material-covered substrates, mechanical release and stacking of LEDs, and top-down fabrication to yield vertical RGB micro-LEDs. Moreover, integration of micro-LEDs with emerging 2D materials makes high-resolution and large-scale display possible, broadening their potential application prospects. [7b,c]

Prestrained Structure
To alleviate the negative impacts induced by strain, an effective and facile approach has been utilized through adopting a straincompensating structure, also known as "prestrained structure." Such structure typically consists of a low-indium-composition InGaN layer or InGaN/GaN superlattice (SL), which is prior to the growth of MQWs active region. [57] Note that indium composition in prestrained structure requires rational adjustment.
Minimal strain compensation is produced for too low indium composition, whereas spinodal decomposition and light absorption may occur for too high indium composition. [13b,58] Researchers from National Taiwan University conducted a series of experimental studies on the prestrained structures on longwavelength LED performance. [58,59] They found that prestrained structures contributed to the raised growth temperature of MQWs, capable of improving crystal quality. Besides, more indium composition atoms were incorporated in MQWs under the same growth condition. Long-wavelength LEDs with prestrained layers showed smaller wavelength shift compared to the LEDs without prestrained layers, implying that prestrained layers reduced QCSE in MQWs through reducing the in-plane strain. This further favored the fabrication of phosphor-free white LEDs featuring excellent spectrum stability. A lattice evolution model of epitaxial growth process was proposed to explain the stress-blocking effect of prelayers. [60] The first atomic layer of InGaN is strained to match GaN, and lattice constant of InGaN shows a linear increment with the InGaN film thickness before the epilayer restores to the full relaxation state at the critical thickness. Strain in each QW can be relaxed with the prelayer and the degree of stress-blocking effect varies at different cases. Our group found that prestrained structure favored the formation of deep localized states in MQWs and suppressed strain-induced QCSE, thus leading to increased IQE and LOP of yellow LEDs. [61] Lai et al. compared the crystal quality and luminescence properties of MQWs with different prestrained structures, and green mini-LEDs displayed improved optoelectronic properties utilizing appropriate prestrained structure. [62] Zhao et al. inserted SL to semipolar green LEDs and found an improvement in IQE due to the reduced point defect density. [63] The IQE improvement for semipolar LED with SL was not significant as its c-pane counterparts because extra misfit dislocations were introduced as nonradiative recombination centers. The influence of thickness ratio of InGaN to GaN in prestrained structure was investigated by Qi et al. Degraded crystal quality was observed in green LED with low thickness ratio, whereas LED with high thickness ratio showed high series resistance, indicating the importance of suitable design of prestrained structure. [64] Besides, doping effects of SL structures on device characteristics have also been investigated. Si-doped graded InGaN/GaN SL enabled reduction in the dislocation density in comparison with undoped graded SL due to the strain relaxation by Si doping. [65] Impressive advantages for Si-doped graded SL are the reduced turn-on voltage and resistance forward voltage due to its low resistivity and improved crystal quality.
Apart from the merits above, prestrained structure is of great interest because it triggers the formation of V-pits, which could affect the LED performance, such as the leakage current, efficiency droop and radiative recombination efficiency. [66] V-pits tend to form at the dislocation surface termination during the growth of InGaN/GaN SL and feature inverted pyramids with (10-11) faceted sidewalls. [67] Deliberately introducing V-pits in LED epilayers has been reported to improve not only the blue LED performance but also long-wavelength LED performance. [66f,68] Compared to c-plane QWs, narrower QWs and lower indium composition were found on the V-pit sidewall, creating high potential barriers and screening TDs (Figure 5a,b). The V-pits density and size depend on the SL period number and its growth temperature, indicating that the V-pits density and size could be manipulated by adjusting SL parameter and growth condition. Our group have pioneered early experimental work on engineering V-pits for efficient green LEDs. [69] We revealed that increasing SL period and decreasing growth temperature enables the increase in density and size of V-pits. Larger V-pits exhibited higher potential barrier height, which was proved to more effectively suppress the carrier lateral diffusion into TDs. The IQE of green LEDs was found to increase with the V-pit size increasing from 99 to 207 nm, above which IQE decreased (Figure 5c). This was ascribed to the integrated effects of potential barrier and effective MQWs area, correlating with the V-pit size. Besides, we reported that low-temperature GaN layer could act as an alternative for promoting the V-pits formation to achieve high emission efficiency. [69c] Experimental results also indicated that more efficient hole injection into MQWs could be achieved with larger V-pits, leading to lower forward voltage (Figure 5d) and efficiency droop for green LEDs operated at high current regime (Figure 5e). Yao et al. reported that V-pits were formed due to the strain relaxation, and this enhanced indium incorporation in MQWs for red light emission. [57a] Jiang et al. reported that utilizing silicon substrate, prestrained structure and 3D pn junction of V-pits contributed to the improved crystal quality, reduced compressive strain, and enhanced hole injection. [70] As a result, 565 nm-yellow LEDs achieved a high wall-plug efficiency (WPE) of 24.3% at 20 A cm À2 .

Bandgap-Engineered MQWs
Active region plays a critical role in determining the carrier generation, transport, and recombination, thus directly affecting the optoelectronic properties of LEDs. Long-wavelength LEDs based on conventional square QWs suffer from the QCSE, thus causing a dramatic decrease in IQE and a quicker efficiency droop at high operating current. Fortunately, bandgap tunability of InGaN QW helps overcome these challenges by adjusting the alloy composition, which is known as bandgap-engineered technique. By taking advantage of this technique, various QW structures have been achieved, such as staggered, triangular-shaped, and W-shaped QW www.advancedsciencenews.com www.adpr-journal.com structures. [71] As a primary advantage of such bandgap-engineered QW, separation of carrier wavefunction caused by QCSE can be effectively alleviated, leading to increased carrier wavefunction overlap and radiative recombination rate in LEDs. [66e,72] Wavelength shift with increasing injection current is also suppressed in bandgap-engineered QWs, which positively enhances the device stability. Besides, such QWs contribute to the improved crystal quality and thus improved LED performance. Yellow LEDs based on staggered QWs exhibited greatly higher LOP with smaller blueshift than those based on square QWs. [71b] These were ascribed to both improved carrier wavefunction overlap (Figure 6a, b) and crystal quality. Experimental results revealed that compared to square QWs, stagger QWs featured uniform elemental distribution and abrupt interfaces (Figure 6c,d). Interestingly, the simultaneous increase in LOP and emission wavelength could be realized in LEDs based on staggered QWs. [73] By modulating the low-indium-composition layer of staggered QWs, carrier wavefunction, indium incorporation, and electric field in the following high-indium-composition layer can be simultaneously modulated. Less indium composition in low-indium-composition not only enhances the carrier wavefunction overlap, but also enhances the electric filed and alleviates composition-pulling effect. The former favors the enhanced radiative recombination rate to obtain higher LOP, and the latter enables the tilted bandgap and indium incorporation to obtain longer wavelength. Chang et al. found that asymmetric triangular QWs helped uniform carrier distribution, reduced the influence of polarization fields, and yielded low Auger recombination rate. [74] As a result, LEDs based on asymmetric triangular QWs showed higher EQE and lower efficiency droop compared to that based on square QWs. As mentioned above, LED performance degrades with the increasing indium composition in MQWs, hindering the realization of efficient longwavelength LEDs. Hybrid MQWs structure that simultaneously includes shortwavelength and long-wavelength QWs acts as another effective strategy to improve LED performance. Specifically, prior to the growth of high-indium-composition MQWs, single or several QWs are grown to reduce the in-plane strain. Researchers from Nanchang University introduced blue QWs underneath yellow QWs and found due to the more gentle evolution of lattice constants, more strain relaxation and indium incorporation could be achieved in such hybrid MQWs. Meanwhile, crystal quality in MQWs has been improved as well, leading to high EQE of yellow LEDs competitive to commercial AlGaInP yellow LEDs. [75] They replaced several orange QWs with yellow QWs to address phase separation problem and improve the spectral purity in longwavelength InGaN LEDs (Figure 6e,f ). [76] Besides, this strategy reduced the active recombination volume, which helped to shift Schematic epilayers of orange LED samples e) with nine orange QWs (denoted as Sample A) and f ) with two orange QWs and seven yellow QWs (denoted as Sample B). g) Dependence of WPE on the current density of orange LED samples. Reproduced with permission. [76] Copyright 2020, Chinese Laser Press.
www.advancedsciencenews.com www.adpr-journal.com the WPE peak toward lower current density (Figure 6g). Based on the proposed structure, a series of efficient InGaN orange and red LEDs with high WPE were successfully developed. Iida et al. demonstrated orange LEDs with hybrid MQWs that consisted of a blue-green single QW and orange InGaN QWs. [77] The hybrid MQW LEDs showed narrower full widths at half maximum (FWHM) of 51 nm and 2.2-fold enhancement in LOP compared to the control LEDs (at 20 mA). Using similar method, compressive strain could be further decreased with thick underlying GaN layers. [78] Thus, LOP of 0.64 mW and EQE of 1.6% were realized for red LEDs with 8 μm-thick underlying GaN layers at 20 mA. It was reported that hybrid MQWs benefited the generation of stronger localization effect and longer emission wavelength compared to the conventional MQWs, realizing a high luminous efficiency of 19.58 lm W À1 for LEDs in the green gap range. [13a] In addition to the studies of QWs, quantum barrier has also been widely investigated as it can increase the hole injection capability and reduce the electric field in the active region. Li et al. inserted InGaN layer with varying thickness into GaN quantum barrier and found that holes got accelerated by the polarization-induced electric field in the properly designed GaN/InGaN/GaN quantum barriers (QBs), so hole injection was improved. [23b] Besides, QCSE was reduced if appropriate thick InGaN insertion layer was used, leading to increased radiative recombination rate of green LEDs. Our group proposed a stacked GaN/AlN last quantum barrier to improve green LED performance. [79] At 15 mA, LED device with such QB achieved a 35% increase in LOP and a reduction of forward voltage compared to the LED with conventional GaN last quantum barrier.
Embedding Al(Ga)N interlayer in InGaN/GaN MQWs was demonstrated to improve the performance of long-wavelength LEDs. AlGaN has the ability to introduce tensile strain for QWs, leading to improved crystal quality. [80] This effect was enhanced with increasing Al composition in AlGaN interlayer, together with improvement of EQE and LOP for red LEDs. [81] Annealing AlGaN interlayer was essential to form sharp heterointerfaces and remove nonradiative recombination centers in MQWs. [82] Alhassan et al. reported that green LEDs with AlGaN interlayer achieved the peak EQE and luminous efficacies of 44.3% and 239 lm W À1 , respectively. [83] They further optimized the AlGaN layer thickness and GaN barrier growth temperature to develop high-performance green LEDs. [84] AlInN layer inserted in InGaN/GaN MQWs plays a similar role as AlGaN insertion layer to some extent. Apart from introducing tensile strain, AlInN was found to prevent the outdiffusion of indium into GaN QB and improve the quantum confinement within the QW. The latter resulted in enhanced optical transitions and higher emission efficiency. With AlInN insertion layer, MQWs showed %4-7 times enhancement in radiative efficiency over the conventional MQWs in the green spectral region. [85] It was found that LED with AlN insertion layer between InGaN QW and GaN QB showed shorter emission compared to that without AlN layer, arising from the large polarization difference between InGaN and AlN. [86] Moreover, they investigated the effect of thickness and composition of Al(Ga)N layer on the emission wavelength, correlating with the dominance of the quantum confinement and polarization effect. By inserting AlN layer, polarization electric field and energy band were modulated, which enabled carrier transport and eventually reduced the operation voltage of red LEDs. Table 1 gives a summary for the reported studies of long-wavelength InGaN LEDs with bandgap-engineered MQWs. www.advancedsciencenews.com www.adpr-journal.com

Semi/Nonpolar MQWs
Commercial InGaN LEDs grown on (0001) c-plane sapphire suffer from severe electric field in MQWs, causing decreased LED efficiency. To tackle this problem, semipolar and nonpolar crystal orientations are used instead of c-plane orientation for epitaxial growth of LEDs. Li et al. reported efficient semipolar (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) 550 nm InGaN LEDs with low defect densities of basal stacking faults (BSFs) and TDs. With the current increasing, small blueshift with low efficiency droop was realized in semipolar LEDs. [87] In addition, semipolar orientations were investigated to realize monolithic phosphor-free white LEDs with high LOP and polarization ratio, promising for backlighting sources in liquid crystal displays. [88] The and 590 nm via changing the InGaN QW thickness. [89] Bai et al. demonstrated (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) semipolar green to amber LEDs on overgrown GaN on microrod template, showing reduced blueshift compared to c-plane LEDs. [90] Christophe et al. grew InGaN MQWs on m-plane nonpolar GaN at low temperature to enhance In incorporation and achieved light emission of 500-550 nm. [91] Further, they demonstrated a dual-color LED that achieved wavelength band emission at 400-450 and 450-550 nm with three sets of blue MQWs and seven sets of In-rich green MQWs. [92] Shojiki et al. reported N-polar LEDs with blue to red emission wavelength, which were realized by optimizing growth conditions MOVPE. [93] Due to the intrinsic nonpolar property of zincblende crystal structure, polarization electric field is absent in InGaN epilayers grown along the zincblende growth direction. Moreover, zincblende InGaN has smaller bandgap compared to the wurtzite counterpart, that is, less indium is required for longer wavelength emission, making it especially attractive for LED devices. Emission wavelength at 540 nm was reported for 10 nm-thick zincblende InGaN QWs with indium composition of 11%. [94] 6. Electron-Blocking Layer Typically, AlGaN electron-blocking layer (EBL) is used in InGaN LEDs to suppress electrons overflow from the MQWs active region. However, increased barrier height for holes is accompanied by using AlGaN EBL, which decreases the hole injection efficiency and encourages the optimal design of EBL structure. Tzou et al. reported quaternary InAlGaN/GaN SL EBL that reduced the electric fields in MQWs, leading to a smoother band diagram and more uniform carrier distribution. [95] As a result, efficiency droop was reduced from 52% for green LED AlGaN EBL to 30% for LED with SL EBL. Usman et al. theoretically demonstrated that replacing AlGaN EBL and p-GaN with p-InGaN/p-GaN SL EBL showed improved efficiency and LOP due to reduced electron leakage and increased hole injection. [96] In addition, it has to be considered that the growth temperature for AlGaN EBL is relatively higher than that for MQWs growth. This induces a thermal damage for the crystal quality of MQWs, especially for long-wavelength LEDs consisting of high-indium-composition MQWs. AlInN layers were proposed as an EBL alternative due to low growth temperature, high conduction band offset, and lattice-matching capability with GaN. [97] Quantum efficiency of green LEDs were thus promoted with alleviated thermal damage for MQWs during EBL growth, more electron confinement as well as reduced defect generation. Besides, the impact of AlGaN, graded AlGaN, AlInGaN, and AlInN EBL on green LED efficiency was studied and compared numerically. [98] Varying degrees of improvement in IQE and LOP were achieved by optimizing EBL structure. Ren et al. proposed a hybrid EBL by partially incorporating a small amount of indium in the AlGaN EBL, which promoted the green LED performance due to the abovementioned advantages. [99] Zhang et al. found that graded AlGaN EBL reduced compressive strain in the last GaN QB, leading to obviously weakened QCSE and enhanced EQE. [100] P-type InGaN conduction layer with a graded indium composition was designed to replace AlGaN EBL and p-GaN layer for InGaN green LEDs, which achieved an enhancement in LOP. [101] In addition to optimizing EBL structure, another approach was reported to improve hole injection by inserting AlGaN/GaN graded SL between last QB and EBL. [102] With such insertion structure, the effective barrier height of holes was reduced by 57 meV, which helped holes easily inject into MQWs. Besides, the insertion structure reduced polarization field near the last QB, and thus the efficiency droop radio for green LED was significantly reduced. A low-temperature GaN layer with varying Mg-doping depth was inserted between MQWs and EBL to improve the InGaN green LEDs performance. [103] It was found that the low-temperature-GaN interlayer prevented MQWs from thermal damage by high temperature of EBL growth. This contributed to the ameliorated indium diffusion and compositional variation, realizing increased emission wavelength with reduced FWHM. With the increasing Mg doping depth in the low-temperature-GaN interlayer, the hole injection was improved and electron leakage current was suppressed at the expense of increased nonradiative recombination. Green LEDs with an optimized Mg-doping depth in low-temperature-GaN interlayer achieved a substantial enhancement in EQE and LOP as well as suppression of efficiency droop and compared with the LEDs without interlayer. Huang et al. proposed a modulararchitected p-type region including a 60 nm-thick polarizationinduced graded AlGaN and a 130 nm-thick Al 0.2 Ga 0.8 N/GaN SLs to enhance p-type conductivity. [104] The fabricated green micro-LEDs arrays showed improvement in optical power, modulation bandwidth, and data rate.

3D Nanostructures
Due to the QCSE, conventional InGaN QW-based LEDs have been experiencing significant fluctuations in color and exhibit www.advancedsciencenews.com www.adpr-journal.com a severe blueshift in wavelength. [105] However, the advancements in augmented reality and virtual reality are driving the miniaturization of optical devices to the micron or submicron levels, which impose more critical requirements on the light output performance of LEDs. It is well known that as the size decreases, the sidewall defects caused by etching increase, leading to Shockley-Read-Hall (SRH) nonradiative recombination and thus reducing the efficiency of the device, which is called the size effect. [106] Despite these obstacles, the development of long-wavelength LEDs remains an ongoing challenge, severely hindering the widespread application of micro-LEDs. Recent studies have explored the potential for addressing the challenges in LED performance through the use of 3D nanostructures, such as nanowires, nanorods, and nanopyramids. These structures aim to relieve strain, increase the emitting surface area, and improve LEE. The use of nanowires in GaN-based devices has been proposed for over two decades and has been implemented through metal-catalyzed chemical vapor deposition on Si and sapphire substrates. [107] Subsequent research has explored the structural characteristics, photoluminescence properties, surface morphology, and defects generated during nanowire growth. [108] The high aspect ratio of nanowires leads to improved mechanical and optoelectronic properties, including good mechanical flexibility that can withstand high deformation without plastic relaxation. [109] Additionally, nanowires can significantly improve crystal growth quality by reducing dislocation density and releasing stresses induced by thermal expansion coefficient mismatches. For LED applications, core-shell nanostructures have been shown to increase the active area and further suppress the QCSE. [110] Most recently, Liu et al. successfully grew N-polar InGaN nanowires containing multiple InGaN quantum disks on an N-polar GaN template on a sapphire substrate, achieving a maximum external quantum efficiency of 11% for LEDs with lateral dimensions as small as 750 nm without any packaging, as shown in Figure 7a. [111] Pandey et al. reported an ultrahigh efficiency nanowire excitonic LED in the submicron green light spectrum. [112] By utilizing the large exciton oscillator strength of quantum-confined nanostructures, with an EQE of 25.2% and a WPE of 20.7%, which are the currently highest values known for the same size, providing an effective way to break the efficiency bottleneck of green LEDs. In addition, the insertion of AlGaN barriers in the QWs of nanowire LEDs enables the screening of the QCSE, which is realized by the transfer of free electrons into the QWs induced by polarization positive bond charges, thus effectively solving the problem of wavelength blueshift. [113] Through optimization of the Mg doping within the nanowires, the plasma damage-induced deleterious effect can be eliminated, resulting in a peak EQE and WPE of %8.3% and %4.6% for submicrometer red LEDs with an emission wavelength at %650 nm. [114] Remarkably, as the diameter of nanowires decreases, the lateral diffusion of In adatoms is significantly enhanced, resulting in an increase of In incorporation and longer emitting wavelengths. [115] This observation is crucial for the development of monolithically integrated full-color micro-LEDs, offering a new avenue toward the creation of ultrasmall and ultrahigh resolution displays, as shown in Figure 7b-d. Researchers from University of Sheffield developed a selective overgrowth method to fabricate ultrabright and ultrasmall micro-LEDs in the longwavelength region. The selective overgrowth took place within SiO 2 microhole arrays that could be controlled to possess varying structural parameters for micro-LEDs without extra passivation and mesa etching processes. By this method, green and red micro-LED array chips with EQE of 6% and 1.75% have been achieved, respectively. [116] The microhole arrays enhanced indium content InGaN QWs with at an elevated growth temperature due to the enhanced strain relaxation. Moreover, combined with nanoporous-GaN/undoped-GaN distributed Bragg reflectors (DBR), the fabricated green micro-LEDs showed a 0.5-fold increase in EQE, reduced electroluminescence spectral line width and blueshift compared to green micro-LEDs without DBR. [117] Meanwhile, several other novel nanostructures are also being developed and applied to enhance the optoelectronic properties of long-wavelength LEDs. For instance, the growth of nanopyramids with sixfold semipolar facets on SiO x or SiN x masked c-plane GaN by selective area growth is an alternative option for enhancing the optoelectronic performance of InGaN-based LEDs, as illustrated in Figure 7e,f. Such configuration can be used for improving the performance of high In-component LEDs because, in addition to the lower polarization field of these crystal planes, the 3D nanostructures provide better strain relaxation for crystal growth and reduce the defect density, thereby lowering the piezoelectric field. [118] In addition, the special hexagonal structure provides more paths for the light to escape, resulting in an improved LEE of the devices. High material homogeneity of the nanopyramid can be achieved by inserting precisely shaped GaN seeds prior to InGaN growth. [119] During the growth of nanopyramids, a slower growth rate and a higher In incorporation were obtained with a larger diameter. [120] The diffusion of precursors in the SiO 2 mask region plays a very important role in the coaxial growth of the multiple quantum shells of the nanopyramids. For the quantum dots (QDs), quantum wires (QWRs), and QWs grown on the top, edges, and sidewalls of the nanopyramids, they have different dimension-dependent optical transition behaviors. [121] Due to the strong carrier localization, they are able to provide high emission efficiency at long wavelengths. Therefore, the nanopyramid structure is a promising candidate for implementation of efficient micro-LED display technology.

Conclusion and Perspectives
In summary, we give an overview of the recent progress in InGaN long-wavelength LEDs and related techniques. Efforts devoted to epitaxial growth are expected to fundamentally address the challenges induced by lattice mismatch, and to mainly improve the IQE of LEDs. On the one hand, reducing structural defects, such as dislocations, point defects, and stacking faults, leads to reduced nonradiative recombination centers and improved crystal quality. Correspondingly, wavelength shift, FWHM, and efficiency droop for LED devices are alleviated, thus facilitating the device performance. There is an inevitable compromise between indium incorporation and crystal quality. It is urgently demanded to exploit novel strategies to incorporate more indium atoms in MQWs while minimizing the degradation of crystal quality. On the other hand, carrier transport behavior and carrier wavefunction in MQWs can be modulated by engineering the epilayer bandgap, which helps the carrier easily inject and uniformly distribute in MQWs. As different substrates, NLs and epitaxial structures used will affect the LED efficiency to a considerable degree, and how to combine and make the best use of their own advantages to obtain high-performance LED is a critical obstacle in the future development. This requires the comprehensive studies of growth methods and growth conditions, maybe together with the in-depth investigation of emerging vdWE and 3D-nanostructure techniques applied in long-wavelength LEDs.