Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

As demonstrated during the COVID-19 pandemic, advanced deep ultraviolet (DUV) light sources (200–280 nm), such as AlGaN-based light-emitting diodes (LEDs) show excellence in preventing virus transmission, which further reveals their wide applications from biological, environmental, industrial to medical. However, the relatively low external quantum efficiencies (mostly lower than 10%) strongly restrict their wider or even potential applications, which have been known related to the intrinsic properties of high Al-content AlGaN semiconductor materials and especially their quantum structures. Here, we review recent progress in the development of novel concepts and techniques in AlGaN-based LEDs and summarize the multiple physical fields as a toolkit for effectively controlling and tailoring the crucial properties of nitride quantum structures. In addition, we describe the key challenges for further increasing the efficiency of DUV LEDs and provide an outlook for future developments.


Introduction
Recently, the COVID-19 pandemic has caused the outbreak of a global public health emergency. Until November 2020, more than 57 million cases, with more than 1.3 million deaths, have been confirmed. Furthermore, this ongoing disaster has led to a social and economic disruption globally, which widely raises awareness about public health and stimulated further discussion on the control means of disease transmission [1][2][3][4] . As we know, COVID-19 spreads from person to person mainly via the respiratory route with the exhalation of viruscontaining particles, respiratory droplets, or aerosols, from an infected person 5 . Indirect contact via a contaminated surface or object could also largely enhance the spread of the virus [6][7][8][9] . Strategies for preventing infection include inoculating vaccines and blocking the route of disease transmission. Until the widespread availability of highly effective vaccines, preventing virus transmission is crucial. The recommended preventive measures include social distancing, wearing masks, washing hands, and disinfecting fomites [10][11][12] . Surfaces can be decontaminated by chemical solutions, such as 70% ethanol, 0.1% sodium hypochlorite, or 0.5% hydrogen peroxide 13 , or by germicidal irradiation with deep ultraviolet (DUV) light (200-280 nm) 14,15 .
DUV radiation with high energy is known to be able to damage a microorganism's DNA or RNA, including bacteria, spores, and viruses, by changing its nucleic acids, thereby its ability to reproduce can be partially or fully impaired [16][17][18] . The germicidal effectiveness curve peak is 265 nm 19 . However, the natural solar ultraviolet light is largely blocked by the atmosphere (by 77%), and only a small fraction of DUV reaches the ground. Hence, the available DUV light derives mainly from artificial sources, such as mercury lamps, excimer lamps, and light-emitting diodes (LEDs). Mercury and excimer lamps, which are traditional sources, are large, toxic, unstable, and short lifetimes; in contrast, DUV LED has proved its remarkable advantages as well as potential applications in many fields, especially in disinfection and sterilization [20][21][22][23][24][25] . Recent researches have revealed that DUV light at 207-222 nm has significant potential to kill pathogens without damaging exposed human tissues and can be a sterilization light source that is harmless to human skin and eyes 26,27 . After decade's efforts, the level of the external quantum efficiency (EQE) of most commercial and laboratorial DUV devices still remains below 10% 28,29 (see also Fig. 14). Furthermore, the EQE dramatically decreases to approximately 1% and 0.1% when the emission wavelength is below 260 and 230 nm, respectively 30,31 . Such a low efficiency strongly restricts the range of the potential applications of DUV LEDs. Originally, the challenges to improve their performances could be attributed to systematic and interrelated difficulties in the whole structure of LED devices from the substrate, AlN basal layer, nand p-type layers, active layers, up to contacting electrodes ( Fig. 1) 32 . The further increase of the injection, radiative, extraction, and electrical efficiencies ( Fig. 1) becomes necessary to enhance the performances with high EQE of AlGaN-based DUV LEDs.
Owing to the fast scale-down of the structural size of advanced materials and the rapid development of epitaxial instruments and techniques, quantum structures gradually exhibit their unique advantages over the traditional device structure of semiconductors and have been widely implanted into DUV LEDs (Fig. 1). The internal quantum efficiency (IQE) is mainly related to the quality of the active layers with quantum structure, such as single quantum wells (SQWs) and multi-quantum wells (MQWs). In principles, the scale of semiconductor quantum structure is only a few nanometers. Its growth can usually be accomplished under non-equilibrium conditions, where the growth kinetics appears very complicated and dependent on the field of chemical potentials of molecules. The pre-reaction of precursors, the adsorption, diffusion, and desorption on the substrate are subjected to extremely complicated parameters. As we know, the cohesion of Al atoms and the difficulty of their migration on the substrate surface strongly restrict the improvement of the quality of MQWs 33 . On the other hand, because the III-nitrides possess large spontaneous and piezoelectric polarization, the polarization electric fields in MQWs separate carriers for effective radiative recombination [34][35][36] . Meanwhile, the heteroepitaxy and heterostructure inevitably subject AlGaN layers to large and complicated strain fields 37 , this strongly affects the crystal quality and causes the piezoelectric fields. Therefore, the carrier confinement in quantum structures plays a key role in overlapping carriers against polarization field and in the operation of optoelectronic devices 38,39 . When the quantum structure is reduced to the atomic scale, lattice discontinuities must be taken into account. The carrier injection efficiency is closely related to the quality of the conductive layer, e.g., the net carrier concentration in the nand p-type conductive layers. For III-nitrides, p-type doping is much more difficult than n-type doping. In the case of GaN, the activation energy of p-type-doped Mg acceptor is as high as~160 meV, thereby resulting in a hole concentration lower than the electron concentration by 1-2 orders of magnitude 40 . This problem becomes much serious as the Al content in AlGaN increases. It has been proved that a low Mg doping concentration in AlGaN materials is highly relative to the higher formation energy of Mg impurities 40 and the activation energy of Mg acceptor increases linearly (465-758 meV in AlN) 41 . The light extraction efficiency (LEE) is closely related to the refractive index of the material and the optical fields. Generally, photons emitted from the active layer of a DUV LED must propagate out of the device to form effective lighting. However, light will be reflected at the interface between media and will be absorbed by por n-type layers and electrodes. For AlGaN, the total reflection angle is only 26°4 2 , thereby resulting in extremely low LEE. On the other hand, AlGaN materials with high Al content have significant optical anisotropy. The emitted light from the active layer has a much larger transverse magnetic (TM) polarized portion 43,44 , which propagates laterally towards the sidewall of c-plane AlGaN epilayers. This means that most of the light emission cannot be extracted out of the top face of the device. From the aforementioned facts, one can realize that in various parts of the DUV device, within critical quantum structures, and on crucial problems, multiple physical fields have been proved important in affecting, controlling, and even adjusting the properties of nitride quantum structures, the performance of devices, and the behaviors of various particles, as illustrated in Fig. 2. Hence, to overcome the efficiency bottleneck of the AlGaN-based DUV light sources includes not only simple technical issues but also deep scientific problems. After decades of efforts by worldwide researchers in this community, the features of these physical fields have been revealed and could be summarized into a toolkit for intentional tuning of the properties of nitride quantum structures. Once the expected performance of AlGaN-based DUV LEDs is achieved, the relative application market could explode rapidly.

Manipulation of fields of chemical potentials
One of the most fundamental and crucial issues is to improve the crystal quality of AlN basal layers. Starting with the substrate, systematic works have addressed the buffer techniques beneath the AlN epilayer, including reactive plasma deposited AlN nucleation layers 45,46 , low/ high-temperature AlN buffer layers 47 , double AlN buffer layers 48 , superlattice (SL) buffer layers 49 , microtrenches 50,51 , nanopatterned sapphire substrates 52 , and nanopatterned AlN buffer layers 48 . In the growth process, epitaxial strategies have been proposed as migrationenhanced metal-organic chemical vapor deposition (MOCVD) 53 , migration-enhanced lateral epitaxial overgrowth of AlN 50 , and multilayered AlN [54][55][56] . However, the realization of AlGaN with atomically abrupt surfaces and/ or interfaces is still challenging in MOCVD technique. From the viewpoint of the microscopic growth mechanisms with basic constituent units, including the Al/N atoms, Al-N molecule, and Al-N 3 cluster, the different migration behaviors strongly depend on the field of their chemical potentials (Fig. 3), which allows for using hierarchical growth units via appropriate control and choice of precursors in the growth process. In this process, the AlN epilayers could be grown with more compact and smoother surface morphologies as well as optimized crystal qualities 57 . To shift the DUV emission towards shorter wavelength with efficient light extraction from the top face of the device, the construction of GaN/AlN quantum structures has become a widely concerned issue for the replacement of high-Al-content AlGaN alloys.
Aiming at the precise tailoring of critical parameters of the AlN and GaN heterostructures, the digitally stacked GaN/AlN structure, i.e., short-period GaN/AlN SLs, has been proposed. The short period indicates the extremely abrupt and ultrathin well and barrier layers with a thickness of just a few atomic layers. For such an advanced structure, the coherent lattice, abrupt interface, and rapid alternation are of great significance in the growth technique.
Researches on the growth of GaN/AlN short-period SLs was pioneered by Khan et al. in 1990s. The switched atomic layer epitaxy yields a sharp absorption edge and clear interfaces 58 . Following works on this issue further revealed that high-quality GaN/AlN short-period SLs possess properties that are significantly different from traditional continuous AlGaN epilayer [59][60][61][62] . In 2011, Rodaka et al. worked on AlN/GaN SLs and explored the relationship of the binary alloy growth rates with the interfacial quality 63 . Taniyasua et al. employed the GaN/AlN short-period SLs on SiC substrates as the active layer for DUV LEDs. By decreasing the GaN well thickness from 2.5 monolayers (MLs) down to 0.9 ML via the MOCVD method, they achieved a short-wavelength emission at 236.9 nm from the c-plane surface 64 . In 2014, the GaN/AlN short-period SLs with sharp interfaces grown by plasma-assisted molecular beam epitaxy (PA-MBE) was demonstrated by Kuchuk et al., however, these SLs showed compositional fluctuation and non-uniform random distribution 65 .
Although MBE method provides high controllability of short-period SLs with atomic layer-by-layer epitaxy, the slow growth rate makes it unsuitable for industrial productions. The medium growth rate enables MOCVD an ideal method for typical nitrides growth. However, the epitaxial growth in MOCVD actually is under nonequilibrium conditions, which include extremely complicated kinetics and dynamic processes. Especially for the AlN/GaN heterostructural epitaxy, challenges lie in the control of the decomposition and pre-reaction of MO precursors as well as the kinetic processes of deposition, such as adsorption, diffusion, and desorption on the substrate surface [66][67][68] . On the other hand, the Al atom has a high adhesion coefficient and slow migration velocity due to the limit of meddling reaction temperature 69 . How to overcome these problems attracted broad interests of researchers in this community.
Based on the hierarchical growth of AlN epilayers, the atomically tunable well and barrier layers in the shortperiod (AlN) m /(GaN) n SLs were grown on AlN/sapphire template by Gao et al. in 2014 70 . By switching the growth sequence instantaneously, the short-period AlN/GaN SLs wells achieve coherent growth. Clear and atomically abrupt interfaces, as well as single atomic layers of GaN, were recognized. In 2019, Gao et al. further demonstrated the underlying growth mechanism of constituent elements during the formation of the digital alloyed integral MLs revealed that the extreme circumstance of the nitrogen-rich condition could effectively stabilize the nitrogen adatoms with higher smoothness on the Gaterminated interface. Al-rich condition favors the formation of Al layer while the deposition of Ga adatomic layer appears insensitive to the atmosphere. Based on these principles, the manipulation of the fields of chemical potentials was proposed to grow constituent elements of AlN and GaN layer-by-layer (Fig. 4a). The precisely integral MLs with atomic flatness and abrupt interfaces have been achieved without observable compositional fluctuations, as shown in Fig. 4b. The concept of chemical potential manipulation strongly indicates a practical scheme for the precise controlling of the extreme quantum structures under non-equilibrium growth conditions, e.g., in the MOCVD system.

Manipulation of strain fields
On account of the heteroepitaxial growth and heterostructural construction, AlGaN epilayers and quantum structures are inevitably subjected to large misfit strains 77 . This has been well known as a fundamental situation of AlGaN materials and related devices. Recently, research works have been conducted to minimize the influence of misfit strain by releasing it through various techniques. Furthermore, the stress field within the AlGaN quantum systems has gradually been considered and utilized as an operable tool to manipulate the structural and optoelectronic properties of their functional structures and advanced devices.
For misfit strain release, there have been three important research branches: the gradient stress field methods through epitaxial lateral overgrowth (ELOG) techniques, the multi-period SLs inserting layers, and the van-der-Waals epitaxial growth with buffering by 2D materials.
The ELOG concept and related technique were first proposed in 1997 and applied in GaN epitaxial growth, thereby effectively proving the crystal quality by lowering the density of threading dislocations (TDs) (Fig. 5c) 78,79 . In ELOG method, the crucial technique is to pattern the template or substrate with dielectric mask or etched trenches, which could allow the selected area overgrowth of epilayer above. Afterward, a lateral overgrowth could be achieved by enhancing the growth laterally and coalescing over the mask or void 80 . In 2005, Cai et al. established a novel scheme based on Auger electron spectroscopy (AES) system for high-spatial-resolution strain measurement (in nanometer scale) and investigated the strain field distribution on ELOG area (Fig. 5a, b) 81 . It has been found that, together with the bending of TDs, a crucial stage for strain release could occur within a distance range above the mask, thereby leading to the turning of the propagation direction of TDs laterally 82 . This is regarded as the main reason for the release of misfit strain and the improvement of GaN epilayer crystal quality. Thereafter, the ELOG technique has been applied to the AlN epitaxy and extended to nano-patterned substrates. In 2007, Asif Khan et al. showed that micro-stripe-patterned sapphires or AlN/FSS templates could effectively enhance the light output power of DUV LEDs by reducing the TDs 83 85 . The full width at half-maximum (FWHM) of the X-ray rocking curve was 165/185 arcsec for (002)/(102) planes, respectively. A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. In 2020, Hagedorn et al. reported an 800 nm-thick, fully coalesced, and crack-free AlN grown on two-inch holetype nanopatterned sapphire wafers by high-temperature annealing (1680°C) method 86 .
SL is another quantum structure with very short-period QWs. In 2002, Asif Khan et al. revealed that the insertion of a set of AlN/AlGaN SLs (Fig. 6a) could significantly reduce the biaxial tensile strain, thereby resulting in 3-mm-thick, crack-free Al 0.2 Ga 0.8 N layers 87 . It was also observed that the TDs could merge in the SLs region and consequently, the density of TDs is reduced greatly.  88 . The strain of the grown layer could be controlled by the structure of the inserted (AlN/GaN). It was found that the crystal quality of the grown layer could be improved by increasing the tensile strain in a-axis (compressive strain in c-axis). The FWHM of (0002) and (1012) were decreased to 79 and 853 arcsec, respectively. In recent years, 2D materials, such as graphene and hexagonal boron nitride (h-BN) are emerging advanced materials with various breakthroughs, which exhibit unique properties and functionalities [89][90][91] . Owing to their week outof-plane van der Waals interaction, several pioneering works have been conducted on misfit strain release and quality improvement of AlN and AlGaN epilayers and quantum structures. In 2012, Kobayashi et al. first demonstrated that the h-BN can form a release layer that enables the mechanical transfer of GaN-based device structures onto foreign substrates (Fig. 6c) 92 . In 2016, Cai et al. achieved a large-roll synthesis of monolayer h-BN film by CVD method and presented the overgrowth of thick GaN wafer over 200 μm through the van der Waals epitaxy with h-BN buffering, free of residual strain (Fig. 6d) 93 . In 2018, Qi et al. utilized graphene as a buffer layer for the growth of an AlN film on a sapphire substrate and revealed the relaxation of compressive strain as well as the reduction TDs in AlN epilayer ( Fig. 6e) 94,95 . In 2020, Wei et al. further showed that the AlN grown on graphene will prefer the lateral growth and quick coalescence on the nano-patterned substrate, resulting in low strain and low dislocation density (Fig. 6f) 96 .
Based on these achievements, it has been realized that the strain field could be intentionally managed for AlGaN quantum structures, aiming at energy band engineering, transition controlling, and emission tuning. In 2012, J. E. Northrup et al. reported that the polarization of the light emitted from DUV-LEDs can be controlled by engineering the strain state in the active region (Fig. 7a) 97 . The compressive strains lead to a reordering of the valence sub-band of AlGaN quantum structures and consequently the enhancement of the degree of light polarization (Fig. 7b) 98 .
To modulate the strain of the AlGaN quantum structures, numerous attempts have been proposed. Highly compressively strained QWs were realized by using AlN bulk or patterned AlN/sapphire as substrates owing to the differences of thermal expansion coefficients and the coalescence process 99 underlying n-AlGaN layer, thereby enhancing the PL intensity 101 . In 2020, Zhang et al. adopted multiple alternation cycles of low-and high-temperature growth to modulate the strain state of the AlN template, and the polarization degree of AlGaN QWs effectively increased from 41.5% to 61.9% 102 . In addition to these growth conditions, Kang et al. reported that the strain state of QWs has also been affected by the electrical injection due to the electron accumulation in active regions 103 . A direct measurement technique was developed to study the stress variations of AlGaN MQWs under electrical injection. A tensile stress was found to be enhanced when the injecting current increases (Fig. 7c), thereby causing CH band to lift upward and the degree of polarization to decrease. It was revealed that the relaxation of tensile strain or the increase of compressive strain pulls the discrete quantum states of heavy holes and light holes back to the valence band maximum (VBM), thereby dramatically improving the total spontaneous emission rate (Fig. 7d). In 2021, Kang et al.
proposed compressively strained (AlN) 8 /(GaN) 2 nanorods by strain engineering digitally alloyed GaN well, thereby enabling the emission wavelength to reach 220 nm in the far-UVC with a higher transition probability from the heavy-and light hole bands. Moreover, they pushed the limits of QW structures based on AlGaN materials 104 . This recent progress suggests that the control of the strain fields of high Al-content AlGaN MQWs is a promising way to improve the transverse electric (TE) polarized emission and increase the quantum efficiency in DUV optoelectronic devices.

Manipulation of atomic orbital coupling
In current modern optoelectronic devices, the IQE is closely related to the energy band structure in the active Aside from the challenges at the technical level, the orbital configuration of quantum levels in MQWs should be the most fundamental, which could directly influence the probability of particle transition between the band-edge states and thus the photon behaviors. In 2013, Lin et al. studied the optical anisotropy in AlGaN and recognized that the light emission polarized perpendicular to the c-axis is closely related to the near band-edge transitions occurring between the conduction bottom and the top of the valence bands 108 . The conduction band minimum (CBM) at Γ point is solely composed of s-orbitals with even symmetry along any axis going through its center. In previous study on the optical anisotropy in AlGaN, it was recognized that, in contrast to the Ga-rich AlGaN, the VBM is dominated by the CH band in Al-rich AlGaN instead of HH/LH bands (Fig. 8b) 108,109 . With respect to c-axis, the CH band at VBM is composed of p zorbitals with odd symmetry. On account of parity selection rules, the interband transitions at the bandgap are readily accessible for TM polarized light (E⊥c) propagating laterally in the c plane 110 . With higher transition energy, the available TE polarized light emitting outward from the c plane is much less, and this limits the LEE. pointed out that the conventional confinement was based on the continuous potential derived from the heterostructure band offset 112 , which did not consider the atomic orbital role in the quantum confinement. As the quantum structures go down to the atomic scale, the lattice discontinuity become increasingly unavoidable in a more micro perspective, where the pivotal role of the orbital intercoupling is at the forefront. The orientational sensitivity of the active valence p-states becomes strong along the confining direction in the quantum structures 113 .
The induced energy gain has magnitudes matched to the band offset with changes in sign depending on the microscopic details in the orbital inter-coupling. Therefore, the barrier potential for the confinement is determined by the joint effect of orbital inter-coupling and the band offset. Recent studies of the orbital-state coupling revealed that the head-on coupling between p-orbitals yielding the ppσ coupling is favorable for the band offset compensation, while the sideways coupling of parallel p-orbitals causing the ppπ coupling is favorable for the barrier enhancement, as shown in Fig. 8c. The energy gain with changes in sign contributing to the compensation or enhancement of the band offset crucially depends on the orbital coupling orientation with respect to the quantum confinement. By varying the confining directions, the orbital engineering has been proposed to customized quantum confinement to tailor luminance intensity. The interaction between the charge confinement of the hole band and the orbital coupling modulating is demonstrated by inclining the well plane via constructing the well on the semipolar and nonpolar planes implemented in the microrods. The higher emission intensity from the QW on the nonpolar plane is confirmed by localized cathodoluminescence. The concept of orbital engineering provides a fertile base for designing new materials through the combination of numerous orbital configurations, as well as size-dependent electrical and optical properties of quantum structures caused by quantum confinement effects.

Manipulation of photonic fields
In optoelectronic devices, light generation, emission, absorption, and propagation are all highly correlated to the photonic field. For DUV devices, it has been widely concerned that the low LEE strongly hinders the rapid improvement of the light output power. It is well recognized that the emission light in Al-rich AlGaN QWs is primarily dominated by TM polarization, which propagates mainly towards the sidewall. The LED fabrication for sidewall collection is very difficult. In the past decade, researches have been widely conducted on these related issues, e.g., the enhancement of light extraction, the switching of the light propagation modes, the emission enhancement by electromagnetic coupling, etc. Therefore, it is demonstrated that the photonic field plays a crucial role in effectively operating the photon behavior and enhancing the photon extraction.
To increase the LEE, many efforts have been made to operate the light propagations, including the development of novel transparent electrode materials, the introduction of high reflective electrodes, and the fabrication of photonic nanostructures. As we know, the DUV light could be easily absorbed by most matters due to the high energy and short wavelength. Generally, materials that are transparent to DUV light have a wide bandgap and possess insulating properties. Such materials are rare, and the pursuit of novel materials and advanced techniques is difficult.
In 2013, Cai et al. successfully synthesized via solution method the ultrafine and super long Cu nanowires (NWs) as transparent electrodes and revealed the unique full and high transparency (higher than 90%) from DUV to nearinfrared region (200-3000 nm) (Fig. 9a) 114 . The light transmission mechanism on NWs network electrode has been regarded as a photon penetration and diffraction through the empty space between NWs, which is in the In 2016, core-shell structured Cu NWs with various metal shells were achieved by one-pot method 115 , and together with broad work-function tunability, Cu@Pt NWs transparent electrodes led to the efficient ohmic contact to AlGaN-based DUV LEDs (275 nm) with enhanced light output power (wall plug efficiency of 3%) by 103% (Fig. 9b) 116 . Ohmic type contact with the high transparency (higher than 90%) to DUV light has been obtained. Reflective electrodes have been introduced to the top surface and sidewall of DUV LED mesas to enhance the light extraction. In 2015, Kim et al. proposed the sidewall emission-enhanced DUV LEDs with three-dimensional Al reflectors between the narrow mesa stripes 117 , which could effectively enhance the light extraction of TM light (Fig. 9d). In 2017, Takano et al. proposed the combination of the AlGaN:Mg p-type contact layer and the Rh mirror electrode, which has significantly increased the output power and the EQE (more than 20%) of DUV-LEDs 28 . On the other hand, in 2018, Chen et al. introduced a motheye microstructure on the backside of a sapphire substrate and demonstrated an optical polarization of high degree (more than 80%) as well as the enhanced TE mode light intensity, thereby resulting in a doubled LEE (Fig. 9e) 42 .
Recent developments of surface plasmon polaritons (SPPs) have opened the new way to improve the efficiency and performance of solid-state light sources because of their capability of controlling light propagation at subwavelength scale 118 . However, most efforts have been devoted to surface-plasmon-(SP-) enhanced light emission at visible wavelengths for LEDs since 1990 (Fig. 10a,  b) [118][119][120][121][122] . In 2010, Lin et al. reported an efficient enhancement of UV-light emission from AlGaN/GaN SQW by depositing various metallic thin films onto the epitaxial layers 123 . In the case of AlGaN/GaN SQW excited from the top, the emission was enhanced via SP-QW coupling in the presence of both Ag and Al thin films. However, they only predicted that Al film could be extended to enhancement in DUV region.
Usually, in a DUV LED structure with high Al-content Al x Ga 1−x N alloys as the active region, the emitted photons in the active layers can only partially escape from the top and bottom surfaces when inside an escape cone, with the emission polarized perpendicular to the c-axis (TE waves). The dominant emissions would be polarized parallel to the c-axis (TM waves), thus the rest part of TE waves outside the escape cone, and the entire TM waves would transmit along the direction perpendicular to the caxis, thereby implying that DUV emission can no longer be extracted easily. In light of the fact that only the  (Fig. 10c) 125 . For ultrathin Al layer deposited on the top of the DUV LEDs, parallel to the TM waves dominate in high Al-content Al x Ga 1−x N alloys, while the top surface of the Al layer is not perfectly flat. Associated with the same frequency of TM waves and SPPs bridged by Al/AlGaN interface, the SPPs will propagate through the Al layer and thereupon recombine and emit light efficiently. Through these steps, the light extraction towards the top surface of DUV LEDs is enhanced by the SP-TM wave coupling. The light extraction was increased by 217% and 136% in peak photoluminescence intensity with a wavelength of 294 and 282 nm, respectively. Furthermore, the cathodoluminescence measurements provided evidence that the IQE of the DUV LEDs coated with Al layer was not enhanced by SP-QW coupling, thus the extraction of DUV light towards the top should be significantly enhanced. Thereafter, versatile Al metallic structures embedded into the DUV range were proposed for efficient SP-based enhancement. Huang et al. optimized the metallic Al thin layer with polygonal geometry Al nanoparticles by localized surface plasmon (LSP) resonance (Fig. 10d), both the top-and bottom-emission EL at a wavelength of 279 nm were effectively enhanced 126  The resultant IQE for the DUV LED was increased by 57.7% (Fig. 10f) 130 .
In addition, extensive studies have been dedicated to extracting the DUV light from the devices, such as surface texturing [131][132][133] , substrate patterning 84 , anti-reflective coatings 134 , highly reflective mirrors on top of the p-(Al) GaN 135 and on the inclined sidewalls along the edge of the square-shape active mesa 136,137 . In an attempt to couple lateral emission to the outward emission of the top surface, the photonic crystal based on nanostructure is designed with air voids, nanopillar, nanorod, and NW structures [138][139][140][141] . From the band engineering perspective, recent results by Lin et al. addressed the compensation operation of asymmetry implemented by introducing some additional asymmetric periodicities into the matrix material to balance the intrinsic optical anisotropy in Alrich AlGaN 108 . Specifically, the compensation of the Δ cr was successfully achieved by the superimposition of variable asymmetrical ultrathin SLs into the anisotropic AlGaN host with high Al content. The optical isotropy supporting the transmission of TE and TM light with the same energy is accessible in ultrathin GaN/AlN SLs allowing for higher light emission and extraction.

Manipulation of polarization fields
Because they are non-centro-symmetric and have highdegree ionicity, wurtzite III-nitrides exhibit strong spontaneous and piezoelectric polarization effects, which induces a strong built-in internal polarization field along [0001] direction 142 . The polarization field causes the band bending in (QWs, which results in a redshift of the emission and an overlap reduction of the electron and hole wave-functions, commonly known as "Quantum Confined Stark Effect (QCSE)". Finally, the QCSE limits the radiative efficiency of III-nitride light emitters [143][144][145] . Great efforts have been made to reduce or eliminate the polarization field of the QWs in active region through various techniques. Furthermore, the polarization field within the III-nitride quantum structures has also been manipulated to achieve a high free-hole concentration in p-type AlGaN.
For the reduction or elimination of polarization field, doping in the active region, polarization-matched AlGaInN barriers, and varying QWs thickness have been proposed. The Si-doping QWs are most widely used to screen the polarization field for InGaN-based LEDs 42,146,147  Reducing the QW width is another method for suppressing the effects of the polarization field in QW. Hirayama et al. in 2008 exhibited that the utilization of a thin QW in the active region was beneficial to increase the IQE of AlGaN DUV-LEDs 150 . As another approach, there has been an effort to substitute the conventional GaN barriers with quaternary AlGaInN barriers 151 . The use of quaternary alloys enables the interface polarization charge to be tuned over a range of values while keeping the bandgap constant. Therefore, the polarization-matched quaternary barriers can be realized with appropriately designed, which leads to less polarization electric field and improvement of the device performance.
In contrast to QWs in active region where the polarization field decreases the radiative efficiency, the polarization field is beneficial for p-type doping. Mg is the only known viable p-type dopant of III-nitride semiconductors 40 . However, it shows large activation energy (465-758 meV in AlN) in III-nitride semiconductors 40 , thereby only a small fraction of the dopant are ionized at room temperature. A large number of approaches, including SLs structure of p-type AlGaN and polarizationinduced hole doping, have been proposed to assist the ionization of Mg acceptors by leveraging the polarization engineering 152,153 .
Generally, the p-type AlGaN SLs consist of several thin p-AlGaN layers with alternating Al compositions, in which the periodic oscillation of the valence band edge induced by the polarization field can make the Mgacceptor level close to the Fermi-level (Fig. 11a, b) 152,153 . The effective acceptor activation energy is thus reduced and high hole concentration can be achieved in SLs. In 1996, Schubert et al. first revealed in their theoretical work that the SLs doping can increase the acceptor activation efficiency by more than one order of magnitude 152 155 . Because of the charge transferring from the Si-doped interface to Mgdoped interface, the internal electric fields in SLs were significantly intensified (Fig. 11c, d). Thus, the increased band bending caused the Mg acceptor level to be much closer to the Fermi-level. The Hall effect measurement results revealed that a hole concentration as high as 5.77 × 10 18 cm −3 was achieved, which was twice that in modulation-doped SLs.
Although 2D hole gases of high density can be formed in these SLs structures, they suffer from low conductivity along the c-axis. To enhance vertical hole conductivity, Zheng et al. proposed a novel three-dimensional (3D) Mgdoped SL in 2016 156 . The first-principle simulations indicated that the hole potential barrier along the c-axis significantly deceased in the 3D SL, thereby attributing to the stronger p z -hybridization between Mg and N. Therefore, the hole in the 3D SLs were more delocalized rather than concentrated in the well, compared to those in the conventional SLs (Fig. 12a). Further analysis of the site-decomposed density of states (DOS) of Mg and N atoms showed that the higher value in p z -DOS nearby the Fermi level of N atoms bonded with Mg was much higher (Fig. 12b), thereby producing higher concentration of vertical hole. Based on the theoretical results, p-type 3D Al 0.63 Ga 0.37 N/Al 0.51 Ga 0.49 N SLs were realized by adjusting the nitridation time at the initial growth stage of MOCVD. The hole concentration reached a value of 3.5 × 10 18 cm −3 , while the corresponding resistivity was as low as 0.7 Ω cm at room temperature, thereby exhibiting a conductivity improvement by 10 times in compared to that of conventional SLs.
Polarization-induced hole doping is another approach for p-type doping in AlGaN. This method utilizes the gradually stacked polarization discontinuity in the Alcomposition-graded AlGaN layer, which results in the formation of 3D bound charges. Thus, a built-in electric field is generated, which can activate the acceptor and make the valence band of AlGaN grading layer smoother to facilitate vertical hole transport, consequently, a 3D hole gas is generated (Fig. 13) 157 . This mechanism of polarization-induced hole formation was first proposed by Simon  Apart from the free-carrier density, the efficient injection of holes into the active region is also intricately dependent on carrier mobility. Although a free-hole density of the order of 10 18 cm −3 has been achieved in high Al-content AlGaN, the hole mobility is 10 times smaller than the electron mobility. In other words, a strong asymmetry between electron-and hole-transport still exists in DUV LEDs, thereby resulting in the electron overflow from the active region to the p-type layer without recombination as well as self-heating, which, in turn, leads to the efficiency droop 162,163 . Furthermore, the poor conductivity of p-type layers leads to higher contact and epilayer resistances and limits their current spreading length. This consequently causes the severe self-heating effect.
To block the electron overflow, p-AlGaN electron blocking layer (EBL) is typically employed, but it causes a penalty in operating voltage at the same time. Thus, a major effort is required to design the EBL structures. EBL and grading the alloy composition respectively, to guarantees a smooth hole injection into the active region 166,167 . In 2019, Lang et al. adopted an Alcomposition and thickness-graded multiple quantum barriers structure as polarization-modulated EBL to enhance the carrier transport in DUV LED 168 . Furthermore, hole reservoir layers with different structures, such as graded AlGaN SLs 169 , Al-composition-graded layer 112 , and inverted-V-shaped quantum barrier 170 , showed significant suppression in the efficiency droop of DUV LEDs.
To eliminate the self-heating, some attempts have been conducted for DUV LEDs. In 2002, Shatalov et al. demonstrated that the current crowding in DUV LEDs could be alleviated by using the strip-geometry p-electron 171 . In 2004, Adivarhan et al. proposed a 10 × 10 array of interconnected micropixel structure to reduce both the device series resistance and the thermal impedance 172 . In 2009, they demonstrated that the vertical current conduction geometry of a device could also effectively reduce thermal impedance 173  This progress demonstrates that the proper design of MQWs, p-type structure, EBL, hole reservoir layers, current spreading layer, and device geometry play an important role in increasing the efficiency of DUV LED.

Conclusion and outlook
In summary, we have reviewed recent progress in the development of novel concepts and techniques on AlGaN-based LEDs and summarized that multiple physical fields could build the toolkit for effectively controlling and tailoring the crucial properties of nitride  Fig. 13 Schematic illustration of polarization-induced p-type doping in graded polar heterostructures. a Sheets of charge dipoles in every unit cell of the crystal. The net unbalanced polarization charge is shown in (b), which leads to the electric field in (c), and the energyband bending in the valence band in (d) if holes are not ionized. Field ionization of holes results in a steady-state energy-band diagram shown in (e), which highlights the smooth valence-band edge without any potential barriers for hole flow. E f , is the Fermi level; E c and E v are the conduction and valence-band edges, respectively; and E g is the band gap. Figures reproduced from ref. 157 , © 2010 AAAS quantum structures. By manipulating the fields of chemical potentials, the short-period GaN/AlN SLs that are atomically flat and abrupt interfaces can be realized for the replacement of high-Al-content AlGaN alloys. To release misfit strain during heteroepitaxial growth and heterostructural construction, different approaches such as the ELOG, the multi-period SLs inserting layers, and the van-der-Waals epitaxial growth have been adopted. Furthermore, the strain fields within the AlGaN QWs can be intentionally managed to improve the TE polarized emission and increase the quantum efficiency in DUV LEDs. To improve the IQE of AlGaN MQWs, the optimization of orbital-state coupling was proved significant in enhancing the combination of numerous orbital configurations as well as size-dependent electrical and optical properties. Meanwhile, the polarization field could be reduced by methods such as doping in the active region, polarization-matched AlGaInN barriers, and varying QWs thickness for improving the radiative efficiency. In contrast, the polarization field could also be manipulated to achieve a high free-hole concentration in p-type AlGaN. The photonic field plays a crucial role in effectively operating the photon behavior and enhancing the photon extraction. Various techniques, including novel transparent electrodes, high reflective electrodes, photonic nanostructures, surface plasmon coupling, and surface texturing, have been developed to operate the light propagations. Moreover, the TE-polarized dominated emission could be enhanced by band engineering and thus lead to increased LEE.