Effects of the substrate thickness and chip area on the light extraction efficiency of chip-scale liquid cup encapsulated AlGaN-based deep-ultraviolet light-emitting diodes

While the demand for deep ultraviolet (DUV) light sources is rapidly growing, the efficiency of current AlGaN-based DUV light-emitting diodes (LEDs) remains very low due to their fundamentally limited light-extraction efficiency (LEE), calling for a novel LEE-enhancing approach to deliver a real breakthrough. Here, we propose sidewall emission-enhanced (SEE) DUV LEDs having multiple light-emitting mesa stripes to utilize inherently strong transverse-magnetic polarized light from the AlGaN active region and three-dimensional reflectors between the stripes. The SEE DUV LEDs show much enhanced light output power with a strongly upward-directed emission due to the exposed sidewall of the active region and Al-coated selective-area-grown n-type GaN micro-reflectors. The devices also show reduced operating voltage due to better n-type ohmic contact formed on the regrown n-GaN stripes when compared with conventional LEDs. Accordingly, the proposed approach simultaneously improves optical and electrical properties. In addition, strategies to further enhance the LEE up to the theoretical optimum value and control emission directionality are discussed. A way to overcome the low light extraction efficiencies of AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) has been shown. AlGaN-based DUV LEDs are promising DUV sources, but their light-extraction efficiency drops with decreasing wavelength because of their highly anisotropic light emission. Researchers in Korea and the United States demonstrate a strategy that exploits this characteristic. Specifically, they fabricated DUV LEDs that have light-emitting mesa stripes to efficiently extract DUV light from the AlGaN active region and reflectors between the stripes to reflect this light upward. This strategy simultaneously improves the optical and electrical properties of the DUV LEDs: they have both enhanced light output due a higher light-extraction efficiency and considerably lower operating voltages than conventional DUV LEDs. The researchers expect that the DUV LEDs can be further improved by optimizing their geometry.

AlGaN-based deep ultraviolet light-emitting diodes (DUV-LEDs), as a novel solid-state ultraviolet light source, compared with the traditional ones, have competitive advantages over traditional UV sources such as mercury (Hg) pollution free, low energy consumption, small size, and tunable wavelengths. They hold broad prospects for development in critical fields including air purification, water disinfection, biosensing, and communications. Enhancing the electro-optical conversion efficiency of DUV-LEDs is essential for achieving large-scale commercial applications in disinfection and sterilization. Improving the device’s relatively low light extraction efficiency (LEE) has proven to be an effective strategy for boosting electro-optical efficiency and overcoming technical barriers. In this review, the fundamental configurations of the AlGaN DUV-LEDs and the regulatory logic of optical polarization characteristics on LEE are summarized. The detailed discussions include the recent research advances in improving the LEE of the DUV-LEDs via optical polarization—specifically by adjusting quantum well structures, optimizing polarization-dependent light propagation paths, and incorporating additional reflection/diffraction structures. Furthermore, it outlines the challenges and development prospects for improving LEE at the optical polarization level.

AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) are widely regarded for their potential in critical applications such as sterilization, water purification, and photolithography, operating in the 200-280 nm wavelength range. However, their performance remains limited by high defect densities, inefficient p-type doping, and low light extraction efficiency, which collectively reduce the external quantum efficiency (EQE). This paper provides a comprehensive review of current doping strategies, with an emphasis on optimizing both n-type and p-type conductivity, particularly in high-aluminum-content AlGaN. Moreover, the impact of threading dislocations and point defects on material properties is analyzed, alongside recent advancements in epitaxial growth methods like metal-organic chemical vapor deposition (MOCVD) aimed at reducing defect densities. Although significant progress has been made in defect management and doping efficiency, challenges remain, particularly in enhancing p-type doping activation. The paper concludes by suggesting future directions, including co-doping strategies and stress compensation layers, to further improve DUV LED performance and enable more efficient, commercially viable devices.

In this work, we numerically investigate the N-polar AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) over the Ga-polar DUV LEDs. The light output power has increased from 7.1 mW of Ga-polar DUV LED to 18.8 mW of N-polar DUV LED at 60 mA, and the wall-plug efficiency (WPE) of N-polar DUV LED is boosted by 104% at 60 mA with the same structure of Ga-polar conventional DUV LED. Furthermore, the higher operation voltage of N-polar DUV LED induced by the large energy difference between the p-type interlayer and the p-GaN hole supplier is noted. To reduce the operation voltage of N-polar DUV LED, the structure with a staircase-like p-type interlayer is proposed. The incorporation of staircase-like aluminum composition p-type interlayer mitigates the large potential barrier for holes injection, and the operation voltage is comparable to the Ga-polar DUV LED. The reduced operation voltage further promotes the WPE of N-polar DUV LEDs. Thus, combined with the promoted electron blocking ability and the mitigated potential barrier for holes of N-polar DUV LED, the greatly enhanced WPE is as high as 4.9% at 60 mA, which is 2.88 times higher than the Ga-polar DUV LED.

Ni/Au is employed to ensure a high-quality p-type ohmic contact for most AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs). However, the substantial absorptivity of ultraviolet light by Ni/Au significantly impacts the light extraction efficiency (LEE). In this work, to reduce the absorptivity of a Ni/Au electrode, we conduct wet-etching to the Ni/Au electrode after the formation of ohmic contact between Ni/Au and p-AlGaN. Experimental results show that the optical transmittance of the Ni/Au electrode can be improved from 35.2% to 42.68% after the Ni/Au is wet-etched for 5 min. When compared with the reference device, the optical power and the wall plug efficiency (WPE) of the proposed device are enhanced by 10.24% and 9.89% at an injection current of 100 mA, respectively. Although the proposed DUV LED exhibits a 0.30 V increase in forward voltage, this does not affect the device’s stability after making a 1000-h lifetime test. This proposed method not only can improve the LEE but also is fully compatible with the mass production of DUV LEDs.

Despite a rapidly-growing demand for efficient man-made DUV light sources, widespread adoption of AlGaN-based DUV LEDs is currently obstructed by extremely poor extraction of DUV photons due to the intrinsic material properties of AlGaN including low hole concentration and poor light extraction efficiency (LEE). Conventional LEE-enhancing techniques used for GaInN-based visible LEDs turned out to be ineffective for DUV LEDs due to a strong absorption of DUV light by p-GaN contact layer, and predominant TM polarized anisotropic emission from Al-rich AlGaN multi-quantum well (MQW) active region grown on c-plane sapphire substrate. Therefore, a new LEE-enhancing approach addressing the unique intrinsic property of AlGaN DUV LEDs is strongly desired. In this study, we present DUV LEDs having arrays of TC shaped active mesas coated with MgF2/Al reflectors on the inclined sidewalls to extract strong TM-polarized in-plane emission trough the sapphire substrate. Ray tracing simulations reveal that the TC DUV LEDs show an isotropic emission pattern and much enhanced light-output power in comparison with stripe-type DUV LEDs with the same MgF2/Al reflectors. Consistent with the ray tracing simulation results, the TC DUV LEDs show an isotropic emission pattern with 37.1% higher light-output power as well as lower operating voltage than the stripe-type DUV LEDs. Based on our results, we suggest strategies to design an optimized DUV LEDs for further enhancing the optical and electrical performances simultaneously. In addition, we propose a next generation DUV LED with an array of Al nanoparticles capable of enhancing IQE and LEE simultaneously by surface plasmon resonance coupling.

An inclined sidewall scattering structure with air cavity characterized by a metal bottom and flat parallel top (Bottom_metal) is proposed to enhance the light extraction efficiency (LEE) for AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). Compared to the reported sidewall metal inclined sidewall (Sidewall_metal) structure, the Bottom_metal structure can greatly enhance the LEE of DUV LEDs based on three-dimensional finite difference time domain simulations. Further analysis indicates that the existence of the air cavity promotes the Bottom_metal DUV LEDs to mainly utilize the total internal reflection and the Fresnel scattering to scatter the light into the escape cone, which avoids the light absorption from the sidewall metal mirror in the Sidewall_metal structure. Moreover, the unique air cavity having a bottom metal also enhances the scattering ability of the Bottom_metal DUV LEDs because any light within the cavity directing downward will be reflected back, and the parallel top interface of air cavity/AlGaN functions as additional out-light planes not limited by total internal reflection.

AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) are being developed for their numerous applications such as purification of air and water, sterilization in food processing, UV curing, medical-, and defense-related light sources. However, external quantum efficiency (EQE) of AlGaN-based DUV LEDs is very poor (<5% for 250nm) particularly due to low hole concentration and light extraction efficiency (LEE). Conventional LEE-enhancing techniques used for GaInN-based visible LEDs turned out to be ineffective for DUV LEDs due to difference in intrinsic material property between GaInN and AlGaN (Al<~30%). Unlike GaInN visible LEDs, DUV light from a high Al-content AlGaN active region is strongly transverse-magnetic (TM) polarized, that is, the electric field vector is parallel to the (0001) c-axis and shows strong sidewall emission through m- or a-plane due to crystal-field split-off hole band being top most valence band. Therefore, a new LEE-enhancing approach addressing the unique intrinsic property of AlGaN DUV LEDs is strongly desired. In this study, an elegant approach based on a DUV LED having multiple mesa stripes whose inclined sidewalls are covered by a MgF2/Al omni-directional mirror to take advantage of the strongly anisotropic transverse-magnetic polarized emission pattern of AlGaN quantum wells is presented. The sidewall-emission-enhanced DUV LED breaks through the fundamental limitations caused by the intrinsic properties of AlGaN, thus shows a remarkable improvement in light extraction as well as operating voltage simultaneously. Furthermore, an analytic model is developed to understand and precisely estimate the extraction of DUV photons from AlGaN DUV LEDs, and hence to provide promising routes to maximize the power conversion efficiency.

In this report, we investigate the impact of a thin p-GaN layer on the efficiency for AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs). According to our results, the light extraction efficiency (LEE) becomes higher with the decrease of the p-GaN layer thickness, which can be ascribed to the decreased absorption of DUV emission by the thin p-GaN layer. Moreover, we also find that the variation trend of external quantum efficiency (EQE) is consistent with that of LEE. Therefore, we can speculate that high-efficiency DUV LEDs can be achieved by using thin p-GaN layer to increase the LEE. However, a thin p-GaN layer can also cause severe current crowding effect and the internal quantum efficiency (IQE) will be correspondingly reduced, which will restrict the improvement of EQE. In this work, we find that the adoption of a current spreading layer for such DUV LED with very thin p-GaN layer can facilitate the current spreading effect. For the purpose of demonstration, we then utilize a well-known p-AlGaN/n-AlGaN/p-AlGaN (PNP-AlGaN) structured current spreading layer. Our experimental and numerical results show that, as long as the current crowding effect can be suppressed, the DUV LED with thin p-GaN layer can significantly increase the EQE and the optical power thanks to the enhanced LEE.

Low light extraction efficiency (LEE) is one of the most challenging points for light-emitting diode (LED) array packaging. Simulation and Experimental analysis of LEE enhancement of LED array packaging with microstructures on packaging silicone gel is presented. To obtain high LEE, microstructures including inversed spherical cap, cylinder-grooves, and V-grooves, are introduced on top surface of packaging silicone gel. Conventional chip (CC) and vertical thin film chip (VTFC) are adopted. Simulation results demonstrate that microstructures are able to effectively improve the LEE of LED array packaging. Experimental results show that inversed spherical cap, cylinder-grooves, and V-grooves structures which are manufactured by molding process can increase the LEE to 12.13%, 10.23%, and 7.44% respectively. This method would be a potential way in improving the LEE of LED array packaging with low profile and high luminous efficiency.

Al-rich AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have a low light extraction efficiency, especially when the emission wavelength is shorter than 280 nm, and this is partially because of the dominant transverse-magnetic polarized light. Our results show that the transverse-electric (TE) polarized light can be obtained even if the emission wavelength becomes even shorter by reducing the quantum well thickness. The ultrathin quantum well enables the enhanced TE-polarized emission that arises from the redistributed subbands for holes. On the contrary to the common belief, we observe a blueshift for the emission wavelength when the AlN composition in the quantum barrier increases. The internal quantum efficiency (IQE) for DUV LEDs with ultrathin quantum wells is no longer determined by the quantum-confined Stark effect, while quantum barrier with high AlN composition is vitally important to improve the electron injection efficiency and thus enhance the IQE.

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) have great application prospects in sterilization, UV phototherapy, biological monitoring and other aspects. Due to their advantages of energy conservation, environmental protection and easy miniaturization realization, they have garnered much interest and been widely researched. However, compared with InGaN-based blue LEDs, the efficiency of AlGaN-based DUV LEDs is still very low. This paper first introduces the research background of DUV LEDs. Then, various methods to improve the efficiency of DUV LED devices are summarized from three aspects: internal quantum efficiency (IQE), light extraction efficiency (LEE) and wall-plug efficiency (WPE). Finally, the future development of efficient AlGaN-based DUV LEDs is proposed.

AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs) have wide applications in many fields, such as air and water purification, disinfection, and biochemical inspection. However, the wall plug efficiency (WPE) of DUV LEDs is relatively low for these applications, which is due to the low light extraction efficiency (LEE) yielding limited light output from LED package. In this work, we proposed a quartz lens with three-dimensional (3D) structure to enhance the light extraction of DUV LEDs. The optical models of DUV LEDs were established and the optical performances of DUV LEDs were investigated by using optical simulation. The DUV LEDs with the 3D quartz lens exhibit higher light efficiency than those with traditional quartz lens structure. Compared with the traditional structure, the light output power of the 3D lens structure is enhanced by 25.93% at the current of 140 mA and the packaged DUV LED can achieve a wide light angle of 153.2°. It is attributed to the significant light extraction enhancement of the sidewall emission from DUV LED chip.

DUV LEDs have very low light extraction efficiency (LEE), which is caused by the unique optical polarization and the optically absorptive semiconductor and metal layers. This chapter reviews and analyzes the approaches that have ever been used to improve the LEE. This chapter also points out that, the removal of the p-GaN layer can yield a high LEE without guaranteeing the enhanced wall plug efficiency in the same time. Thus, even more effort shall be made to achieve excellent ohmic contact for DUV LEDs.

Total internal reflection (TIR) effect leads to low light extraction efficiency (LEE) of the GaN LEDs on SiC and sapphire substrates. The LEE enhancements of the GaN based flip-chip light-emitting diodes (FC-LEDs) on patterned substrates are investigated by experiments and simulations. The optical output power of the FC-LEDs on patterned SiC substrate increases by 8.15% while the optical output power of the FC-LEDs on patterned sapphire substrate improves 48% from the experimental results. According to the simulation data, the LEE enhancements of the FC-LEDs on patterned SiC and sapphire substrates cause the improvements of the light output power.