Beyond 53% internal quantum efficiency in a AlGaN quantum well at 326  nm UVA emission and single-peak operation of UVA LED

Khan, M. Ajmal; Takeda, Ryohei; Yamada, Yoichi; Maeda, Noritoshi; Jo, Masafumi; Hasegawa, Hideki

doi:10.1364/ol.376894

Cited by 35 publications

(41 citation statements)

References 19 publications

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“…The relaxation ratio in the n-AlGaN EIL underneath the MQWs increases with an increase in the thickness of the BL ( T BL ), shown in Figure c. Such a relaxation is critical for the suppression of extended defects as well as nonuniformity in the Al-alloy distribution in the active region (MQWs) for achieving the desired target of NB-UVB emission wavelength. , Theoretically speaking, one can see that there is a simple evaluation method to investigate the electric field dependence in MQWs on the strain-relaxation conditions. ,, The total polarization field of an AlGaN layer can be expressed as given in the following eq where P sp and P pz are the spontaneous and piezoelectric polarization. Similarly, a e : the in-plane lattice constant of the unstrained epilayer and a s : the in-plane lattice constant of a virtual substrate.…”

Section: Results and Discussionmentioning

confidence: 82%

“…It can be speculated that the highly relaxed n-AlGaN EIL up to 80–90% might be useful for the UVA emitters, and partially relaxed n-AlGaN EIL up to 50% might be useful for the UVB emitters (this work). As a proof of concept, using 84% relaxed n-AlGaN EIL underneath the UVA-MQWs, we successfully achieve a η int of 53% for UVA emitters . In more simple words, we need different relaxation conditions for the n-AlGaN EIL underneath the UVA, UVB, and UVC-MQWs for different peak positions of wavelength emission from UVA, UVB, and UVC emitters.…”

Section: Results and Discussionmentioning

confidence: 82%

“…Previously, we observed some nonlinear behavior in the emitted L at 310 nm emission in UVB LEDs, , as well as at 326 nm emission in UVA LED devices too in ref and it was determined that the nonlinearity in the I – L and I –η ext characteristics was not influenced either by varying the number of stacking layers in AlGaN BLs or by using different materials (Ni/Au, Ni/Mg, and Ni/Al) for p-electrodes. , However, such a nonlinear behavior in the I – L and I –η ext characteristics was attributed to the 25-nm-thicker QWBs in the MQWs. Such thicker QWBs prevent the uniform distribution of e–h pairs in the MQWs region, which ultimately causes inefficient radiative recombination in the active region of UVB LEDs .…”

Section: Results and Discussionmentioning

confidence: 99%

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External Quantum Efficiency of 6.5% at 300 nm Emission and 4.7% at 310 nm Emission on Bare Wafer of AlGaN-Based UVB LEDs

Khan

Itokazu

Maeda

et al. 2020

ACS Appl. Electron. Mater.

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124

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As per the Minamata Convention on Mercury, regulation on mercury use will be stricter from the year of 2020, and safe AlGaNbased ultraviolet (UV) light sources are urgently needed for killing SARS-CoV-2 (corona virus). AlGaN-based ultraviolet-B (UVB) light-emitting diodes (LEDs) and UVB laser diodes (LDs) have the potential to replace toxic mercury UV lamps. Previously, the internal-quantum-efficiency (η int ) was enhanced from 47 to 54% in AlGaN UVB multiquantum wells (MQWs). However, some nonlinear behaviors in both light output power (L) and external quantum efficiency (η ext ) in the 310 nm band UVB LEDs were observed, and later on, such nonlinearities were overcome by reducing the thicknesses of quantum well barriers (T QWB ) in MQWs. After relaxing the n-AlGaN electron injection layer up to 50% underneath the MQWs and using a highly reflective Ni/Al p-electrode, L and η ext of the 310 nm band UVB LED were greatly improved from 12 mW and 2.3% to record values of 29 mW and 4.7%, respectively. Similarly, for the 294 nm band UVB LED, η ext and L values, respectively, were also remarkably improved up to 6.5% and 32 mW at room temperature under bare wafer conditions using a better carrier confinement scheme in the MQWs as well as using a moderately Mgdoped p-type multiquantum-barrier electron-blocking layer (p-MQB EBL). The moderately doped p-MQB EBL was used to achieve better hole transport to enhance the hole injection toward the MQWs as well as to block the high-energy electron from overshooting. Possible explanations and recommendations for the improvements in the performances of 294−310 nm UVB LEDs are broadly discussed. Most importantly, such controllable multi-UVB-wavelength emitters may extend nitride-based LEDs to previously inaccessible areas, for example, electrically pumped AlGaN-based UVB LDs.

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Section: Results and Discussionmentioning

confidence: 82%

Section: Results and Discussionmentioning

confidence: 82%

Section: Results and Discussionmentioning

confidence: 99%

See 1 more Smart Citation

External Quantum Efficiency of 6.5% at 300 nm Emission and 4.7% at 310 nm Emission on Bare Wafer of AlGaN-Based UVB LEDs

Khan

Itokazu

Maeda

et al. 2020

ACS Appl. Electron. Mater.

Self Cite

124

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“…The (105) plane RSM shown in Figure 3f was performed to demonstrate the epitaxy quality of the n ‐AlGaN layer on the modified HTA‐AlN. [ 53,54 ] It is observed that the broadening of the n‐ AlGaN diffraction pattern is close to that of the AlN pattern and is narrower than that of the untreated sample (Figure 3e). Accordingly, a relaxation degree of only 9% is estimated from the n ‐AlGaN diffraction pattern.…”

Section: Resultsmentioning

confidence: 99%

Drive High Power UVC‐LED Wafer into Low‐Cost 4‐Inch Era: Effect of Strain Modulation

Liu

Yuan

Huang

et al. 2022

Adv Funct Materials

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Ultraviolet-C light-emitting diodes (UVC-LEDs) have great application in pathogen inactivation under various kinds of situations, especially in the fight against COVID-19. Unfortunately, its epitaxial wafers are so far limited to a size of 2 inches, which greatly increases the cost of massive production. In this work, a 4-inch crack-free high-power UVC-LED wafer is reported. This achievement relies on a proposed strain-tailored strategy, where a 3D to 2D (3D-2D) transition layer is introduced during the homo-epitaxy of AlN on the high temperature annealed (HTA)-AlN template, which successfully drives the original compressive strain into a tensile one and thus solves the challenge of realizing a high-quality Al 0.6 Ga 0.4 N layer with a flat surface. This smooth Al 0.6 Ga 0.4 N layer is nearly pseudomorphically grown on the strain-tailored HTA-AlN template, leading to 4-inch UVC-LED wafers with outstanding performances. The strategy succeeds in compromising the bottlenecked contradictory in producing a large-sized UVC-LED wafer on pronounced crystalline AlN template: The compressive strain in HTA-AlN allows for a crack-free 4-inch wafer, but at the same time leads to a deterioration of the AlGaN morphology and crystal quality. The launch of 4-inch wafers makes the chip fabrication process of UVC-LEDs match the mature blue one, and will definitely speed up the universal application of UVC-LED in daily life.

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“…The double peak emission was reported in previous studies of AlGaN-based UVC and UVB LEDs. [17][18][19][20] Because of higher surface mobility than Al adatoms, Ga atoms are much easier to incorporate into the steps of the dislocation outcrops, which is accompanied by the formation of hexagonal spiral hillocks. [17,21] Therefore, this leads to the growth of relatively Ga-richer AlGaN wells at the steps of the hillocks, resulting in a longer wavelength emission at 295 nm.…”

Section: Uvc Leds Manufacturingmentioning

confidence: 99%

Effect of a Lateral Overgrowth Process on the Strain Evolution of AlN Films Grown on a Nanopatterned Sapphire Substrate for Ultraviolet‐C Light‐Emitting Diode Applications

Liu

Hoo

Chen

et al. 2021

Physica Rapid Research Ltrs

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Ultraviolet-C (UVC) light-emitting diodes (LEDs) based on AlGaN have been confirmed as an excellent potential candidate for various sterilization applications such as water/air purification, medical diagnostics, security, and so on. [1][2][3][4][5] In particular, the global outbreak of the COVID-19 has sparked even more research enthusiasm to UVC LEDs in both research and industry field due to their superior ability to eliminate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within second level. [6] However, in terms of device performance and mass production costs, UVC LEDs are still not comparable with GaN-based blue/green LEDs, and the main challenge is to achieve high crystalline quality and large-size AlN templates. The relatively large mismatch of lattice and thermal expansion between AlN and sapphire substrates results in the generation of numerous dislocations whose density is up to 10 10 cm À2 at the AlN/sapphire interface, which intensively affects the radiative property and reliability of UVC LEDs. [7,8] However, to realize high-performance UVC LED devices, AlN templates with threading dislocation (TD) density below 10 9 cm À2 are preferred. [1] To address this problem, epitaxial lateral overgrowth (ELOG) was proposed through the use of nanopatterned sapphire substrates (NPSSs), which subsequently became widespread due to the successful annihilation of TDs to %10 8 cm À2 . To further investigate the contribution of ELOG, NPSSs with various types of patterns were designed to eliminate dislocations. [9] Although the crystal quality has satisfied the subsequent requirements for epitaxy, the residual strain of the AlN template on NPSS is also quite important, because it greatly affects the warpage of wafer: excessive warpage leads to surface cracks and damages the uniformity of the chips on wafer. Previous studies have found that AlN on NPSS with hole-type pattern presents tensile strain which causes horrendous cracks. [10,11] In addition, the strain state also affects the surface roughness and the relaxation ratio of the upper AlGaN layer, which is accompanied by freshly generated dislocations. Thus, it is meaningful to understand how the NPSS pattern affects the strain evolution and further contributes to UVC LED light emission, which is crucial to optimizing device performance by modulating NPSS patterns.In this work, strain evolution of AlN templates grown on two types of NPSSs (hole type [HT] and pillar type [PT], respectively) is investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) reciprocal space map (RSM). It was found that the crystal merging of the AlN on HT-NPSS during ELOG leads to tensile strain, whereas the AlN on PT-NPSS, in which there is

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Beyond 53% internal quantum efficiency in a AlGaN quantum well at 326 nm UVA emission and single-peak operation of UVA LED

Cited by 35 publications

References 19 publications

External Quantum Efficiency of 6.5% at 300 nm Emission and 4.7% at 310 nm Emission on Bare Wafer of AlGaN-Based UVB LEDs

External Quantum Efficiency of 6.5% at 300 nm Emission and 4.7% at 310 nm Emission on Bare Wafer of AlGaN-Based UVB LEDs

Drive High Power UVC‐LED Wafer into Low‐Cost 4‐Inch Era: Effect of Strain Modulation

Effect of a Lateral Overgrowth Process on the Strain Evolution of AlN Films Grown on a Nanopatterned Sapphire Substrate for Ultraviolet‐C Light‐Emitting Diode Applications

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