High‐quality epitaxy consisting of Al1−xGaxN/Al1−yGayN multiple quantum wells (MQWs) with sharp interfaces and emitting at ≈280 nm is successfully grown on sapphire with a misorientation angle as ...large as 4°. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat‐terrace is observed on 4° misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer‐level output‐power of the ultraviolet light emitting diodes on 4° misoriented substrate is nearly increased by 2–3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step‐bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high‐efficient ultraviolet emitters.
Enhanced ultraviolet luminescence of AlGaN wavy‐quantum‐wells grown on large misoriented sapphires is demonstrated, enabled by the successful introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation and thus forming a 3D carrier confinement for radiative recombination. Herein, the compositional modulation in active region arising from large misoriented‐substrate offers a promising approach in the pursuit of high‐efficient ultraviolet emitters.
Mercury-free semiconductor deep ultraviolet light-emitting diodes (DUV-LEDs) still suffer from low power efficiencies due to extremely poor light extraction efficiency. In this paper, a ...high-reflective electrode is proposed and carefully optimized for making an ohmic contact with the n-type AlGaN layer in the DUV-LEDs (~280 nm). The electrode is made of Cr/120-nm Al metal stack with a different thickness of the Cr layer in a range of 1-20 nm followed by the complete LED fabrication process. We found that the DUV-LED with a 1-nm Cr/120-nm Al electrode obtains maximum light output power and the highest external quantum efficiency, showing an enhancement factor of 40.9% and 25.4%, respectively, in comparison to a conventional LED with an electrode made of 20-nm Cr/120-nm Al. This enhancement is mainly attributed to a higher UV reflectivity of the 1-nm Cr/120-nm Al electrode. Furthermore, a better ohmic contact was achieved in such electrode due to the formation of higher quality Al-Cr and Cr-N alloys in such metal/n-AlGaN junction.
To improve the light extraction efficiency (LEE) of AlGaN-based deep ultraviolet LEDs (DUV-LEDs) by simple and effective method, this has greatly attracted ever-growing attention in DUV-LEDs field. ...Here, nanolens arrays (NLAs) fabricated by nanophotolithography and wet-etched technique is proposed to improve the LEE of DUV-LEDs, and its theoretical feasibility is verified by the Monte Carlo Ray-Trace method and finite element analysis which show an obvious improvement of light and electric field distribution benefit from NLAs structure. By controlling the time of wet etching, the effect of the uniform patterns of NLAs with adjustable morphology on the LEE of DUV-LEDs is systematically studied. Compared with the sandwiched flat lens, experimental results show that the obviously enhancements of the light output power are achieved by using the proposed NLAs to be 13.0%, 21.1%, 24.7%, 13.0%, and 11.5%, respectively, corresponding to their radius of 325, 340, 350, 360, and 400 nm, under the driving current of 300 mA. Therefore, the highest light output power of DUV-LEDs is used the optimized NLAs with radius of 350 nm, and its emission angle also shows the largest improvement of ~14°, suggesting the best LEE.
This paper investigated the optical and thermal performance of the nitride-based ultraviolet light-emitting diodes fabricated by the eutectic flip-chip method. A new packaging structure was proposed ...by introducing a thin encapsulation layer doped with 0.4 wt% AlN nanoparticles (NPs) and uniform quartz lens simultaneously. Experimental results showed that the packaging structure proposed in this paper could significantly enhance the light output power, reduce the junction temperature, and increase the emission angle compared with the encapsulation layer consisting silicone only. When the NPs concentration increased from 0.1 to 0.4 wt%, the light output power increased from 7.6% to 17.4% at the forward current of 800 mA. Meanwhile, the junction temperature decreased by 5.7 °C, while the emission angle increased by 11.3°. What is more, it was found that the enhancement of light output power depended on the NPs concentration and showed the maximum at the concentration of 0.4 wt%. The enhanced light output power was attributed to the additional light scattering and the increased average refractive index resulted from the NPs introduced in the proposed package structure.
Flip-chip ultraviolet light-emitting diode (FC UV-LED) fabricated by direct AuSn eutectic package is of high interest in Research and Development due to its excellent thermal performance and good ...reliability. However, the voids in eutectic bonding layer due to the lack of AuSn filled have a big influence on the thermal management and optical performance of FC UV-LEDs, and it is believed that the eutectic voids can affect the thermal-conduction resistance (the following unified called thermal resistance) and the junction temperature of FC UV-LEDs. In this paper, modeling and thermal simulation using finite element analysis is developed by considering the geometrical model of eutectic FC UV-LEDs with 3%, 10%, 20%, and 30% bonding voidage. Meanwhile, to validate the simulation, the thermal parameters of FC UV-LEDs are determined and measured using thermal transient tester, and it is found that UV-LED with 3% voidage shows lowest thermal resistance and junction temperature compared with the other samples in both simulation and experiment. Moreover, the optical performance of UV-LEDs is evaluated via the photoelectric analysis system, and the results confirm that the lowest thermal resistance leads to the lowest junction temperature but the highest light output power.
AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) suffer from severe quantum confined Stark effect (QCSE) due to the strong polarization field in the quantum wells (QWs) grown on c-plane ...substrates. In this paper, we propose a novel DUV LED structure embedded with graded QWs in which the Al composition was linearly changed to screen the QCSE. A significant increase of the internal quantum efficiency and thus an enhancement of the light output power by nearly 67% can be achieved, attributing to the improvement of the electron-hole wave function overlap (Γ e-hh ) to 58.6% in the Increased-Al-composition graded QWs, as compared to the QW without grading (Γ e-hh = 40.4%) and reverse grading (Γ e-hh = 33.6%). Further investigations show that the grading profile of the Al composition in the QWs, including either linearly increases or decreases along the growth direction and the thickness of graded QWs, determine the polarization electrical field in the QWs and as a result, significantly affecting the performance of the devices. In the end, a careful optimization of the graded QWs is called. The proposed structure with such unique graded QWs provides us an effective solution to suppress the QCSE effect in the pursuit of high-performance DUV emitters.
Indium gallium nitride (InGaN) based blue light-emitting diodes (LEDs) suffer from insufficient crystal quality and serious efficiency droop in large forward current. In this paper, the InGaN-based ...blue LEDs are grown on sputtered aluminum nitride (AlN) films to improve the device light power and weaken the efficiency droop. The effects of oxygen flow rate on the sputtering of AlN films on sapphire and device performance of blue LEDs are studied in detail. The mechanism of external quantum efficiency improvement is related to the change of V-pits density in multiple quantum wells. The external quantum efficiency of 66% and 3-V operating voltage are measured at a 40-mA forward current of with the optimal oxygen flow rate of 4 SCCM.
The novel honeycomb hole-shaped electrode (HHSE) structure was proposed for 280-nm AlGaN-based flip-chip deep-ultraviolet light emitting diode (DUV-LED), which was demonstrated to be beneficial to ...the current spreading, optical emission, and heat dispersion. By means of the finite element method, we investigated the current density distribution at an injection current density of 67.5 A/cm 2 and found that the effective current spreading area was approximately improved by 35.3% for the light emitting diode (LED) with a novel electrode compared with the LED with the conventional finger-shape electrode. As a result, the wall-plug efficiency (WPE) of the LED with a novel electrode was increased by 28% at an injection current of 280 mA. Meanwhile, the symmetrical distribution of optical emission and temperature was realized. This paper paves the way to solve the current crowding effect due to the poor conductivity of the n-AlGaN layers with high Al fraction and significantly develop high efficiency in the AlGaN-based DUV-LED.
In this paper, we reported on wafer-scale nanoporous (NP) AlGaN-based deep ultraviolet (DUV) distributed Bragg reflectors (DBRs) with 95% reflectivity at 280 nm, using epitaxial periodically stacked ...n-Al
Ga
N/u-Al
Ga
N structures grown on AlN/sapphire templates via metal-organic chemical vapor deposition (MOCVD). The DBRs were fabricated by a simple one-step selective wet etching in heated KOH aqueous solution. To study the influence of the temperature of KOH electrolyte on the nanopores formation, the amount of charge consumed during etching process was counted, and the surface and cross-sectional morphology of DBRs were characterized by Scanning electron microscopy (SEM) and atomic force microscopy (AFM). As the electrolyte temperature increased, the nanopores became larger while the amount of charge reduced, which revealed that the etching process was a combination of electrochemical and chemical etching. The triangular nanopores and hexagonal pits further confirmed the chemical etching processes. Our work demonstrated a simple wet etching to fabricate high reflective DBRs, which would be useful for AlGaN based DUV devices with microcavity structures.
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► Zn powder as precursor template for synthesis ZnO hollow spheres. ► Different precursor templates result in different ZnO nanostructures. ► Different experimental conditions enable ...growth of different surface morphologies of ZnO sphere. ► ZnO hollow sphere materials have good gas sensing performance for detecting ethanol gas.
Using Zn powder as precursor templates, ZnO hollow microspheres were successfully prepared by thermal evaporation method and characterized by X-ray diffraction analysis, scanning electron microscope and transmission electron microscope. It was found that different size and shape of precursor resulted in different ZnO nanostructures. When varying experimental conditions, such as air flow rate and working pressure, ZnO hollow spheres with different surface morphologies could be obtained. The advantages of the present synthetic technology are simple, relatively low cost, and high reproducibility. A gas sensor was fabricated from the as-prepared ZnO hollow microspheres and tested to the ethanol gas at different operating temperatures.
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