240 nm AlGaN-based deep ultraviolet micro-LEDs:size effect versus edge effect

240 nm AlGaN-based micro-LEDs with different sizes are designed and fabricated.Then,the external quantum efficiency(EQE)and light extraction efficiency(LEE)are systematically investigated by comparing size and edge effects.Here,it is revealed that the peak optical output power increases by 81.83%with the size shrinking from 50.0 to 25.0μm.Thereinto,the LEE increases by 26.21%and the LEE enhancement mainly comes from the sidewall light extraction.Most notably,transversemagnetic(TM)mode light intensifies faster as the size shrinks due to the tilted mesa side-wall and Al reflector design.However,when it turns to 12.5μm sized micro-LEDs,the output power is lower than 25.0μm sized ones.The underlying mechanism is that even though protected by SiO2 passivation,the edge effect which leads to current leakage and Shockley-Read-Hall(SRH)recombination deteriorates rapidly with the size further shrinking.Moreover,the ratio of the p-contact area to mesa area is much lower,which deteriorates the p-type current spreading at the mesa edge.These findings show a role of thumb for the design of high efficiency micro-LEDs with wavelength below 250 nm,which will pave the way for wide applications of deep ultraviolet(DUV)micro-LEDs.

Increasing the hole energy by grading the alloy composition of the p-type electron blocking layer for very high-performance deep ultraviolet light-emitting diodes

It is well known that the p-type AlGaN electron blocking layer(p-EBL) can block hole injection for deep ultraviolet light-emitting diodes(DUV LEDs). The polarization induced electric field in the p-EBL for [0001] oriented DUV LEDs makes the holes less mobile and thus further decreases the hole injection capability. Fortunately,enhanced hole injection is doable by making holes lose less energy, and this is enabled by a specifically designed p-EBL structure that has a graded AlN composition. The proposed p-EBL can screen the polarization induced electric field in the p-EBL. As a result, holes will lose less energy after going through the proposed p-EBL, which correspondingly leads to the enhanced hole injection. Thus, an external quantum efficiency of 7.6% for the 275 nm DUV LED structure is obtained.

Large scale synthesis of full-color emissive carbon dots from a single carbon source by a solvent-free method

Full-color emissive carbon dots(CDs)hold a great promise for various applications,especially in light emitting diodes(LEDs).However,the existing synthetic routes for CDs are carried out in solutions,which suffer from low yields,high pressures,various byproducts,large amounts of waste solvents,and complicated photoluminescence(PL)origins.Therefore,it is necessary to explore large scale synthesis of CDs with high quantum yield(QY)across the entire visible range from a single carbon source by a solvent-free method.In this work,a series of CDs with tunable PL emission from 442 to 621 nm,QY of 23%-56%,and production yield within 34%-72%,are obtained by heating o-phenylenediamine with the catalysis of KCl.Detailed characterizations identify that,the differences between these CDs with respect to the graphitization degree,graphitic nitrogen content,and oxygen-containing functional groups,are responsible for their distinct optical properties,which can be modulated by controlling the deamination and dehydrogenation processes during reactions.Blue,green,yellow,red emissive films,and LEDs are prepared by dispersing the corresponding CDs into polyvinyl alcohol(PVA).All types of white LEDs(WLEDs)with high colorrendering-index(CRI),including warm WLEDs,standard WLEDs,and cool WLEDs,are also fabricated by mixing the red,green,and blue emissive CDs into PVA matrix by the appropriate ratios.

Polarization-enhanced AlGaN solar-blind ultraviolet detectors

AlGaN solar-blind ultraviolet detectors have great potential in many fields,although their performance has not fully meet the requirements until now.Here,we proposed an approach to utilize the inherent polarization effect of AlGaN to improve the detector performance.AlGaN heterostructures were designed to enhance the polarization field in the absorption layer,and a high built-in field and a high electron mobility conduction channel were formed.As a result,a high-performance solar-blind ultraviolet detector with a peak responsivity of 1.42 A/W at 10 V was achieved,being 50 times higher than that of the nonpolarization-enhanced one.Moreover,an electron reservoir structure was proposed to further improve the performance.A higher peak responsivity of 3.1 A/W at30 V was achieved because the electron reservoir structure could modulate the electron concentration in the conduction channel.The investigation presented here provided feasible approaches to improve the performance of the AlGaN detector by taking advantage of its inherent property.

Direct demonstration of carrier distribution and recombination within step-bunched UV-LEDs

AlGaN-based solid state UV emitters have many advantages over conventional UV sources. However, UV-LEDs still suffer from numerous challenges, including low quantum efficiency compared to their blue LED counterparts. One of the inherent reasons is a lack of carrier localization effect inside fully miscible AlGaN alloys. In the pursuit of phase separation and carrier localization inside the active region of AlGaN UV-LED, utilization of highly misoriented substrates proves to be useful, yet the carrier distribution and recombination mechanism in such structures has seldom been reported. In this paper, a UV-LED with step-bunched surface morphology was designed and fabricated, and the internal mechanism of high internal quantum efficiency was studied in detail. The correlation between microscale current distribution and surface morphology was provided, directly demonstrating that current prefers to flow through the step edges of the epitaxial layers. Experimental results were further supported by numerical simulation. It was found that efficient radiative recombination centers were formed in the inclined quantum well regions. A schematic three-dimensional energy band structure of the multiple quantum wells(MQWs) across the step was proposed and helps in further understanding the luminescence behavior of LEDs grown on misoriented substrates. Finally, a general principle to achieve carrier localization was proposed, which is valid for most ternary Ⅲ-Ⅴ semiconductors exhibiting phase separation.

Polarization assisted self-powered GaN-based UV photodetector with high responsivity

In this work, a self-powered GaN-based metal-semiconductor-metal photodetector(MSM PD) with high responsivity has been proposed and fabricated. The proposed MSM PD forms an asymmetric feature by using the polarization effect under one electrode, such that we adopt an AlGaN/GaN heterojunction to produce the electric field,and by doing so, an asymmetric energy band between the two electrodes can be obtained even when the device is unbiased. The asymmetric feature is proven by generating the asymmetric current-voltage characteristics both in the dark and the illumination conditions. Our results show that the asymmetric energy band enables the selfpowered PD, and the peak responsivity wavelength is 240 nm with the responsivity of 0.005 A/W. Moreover, a high responsivity of 13.56 A/W at the applied bias of 3 V is also achieved. Thanks to the very strong electric field in the charge transport region, when compared to the symmetric MSM PD, the proposed MSM PD can reach an increased photocurrent of 100 times larger than that for the conventional PD, even if the illumination intensity for the light source becomes increased.

Impact of p-AlGaN/GaN hole injection layer on GaN-based vertical cavity surface emitting laser diodes[Invited]

The hole injection capability is essentially important for GaN-based vertical cavity surface emitting lasers[VCSELs]to enhance the laser power.In this work,we propose GaN-based VCSELs with the p-AlGaN/p-GaN structure as the p-type hole supplier to facilitate the hole injection.The p-AlGaN/p-GaN heterojunction is able to store the electric field and thus can moderately adjust the drift velocity and the kinetic energy for holes,which can improve the thermionic emission proc-ess for holes to travel across the p-type electron blocking layer[p-EBL].Besides,the valence band barrier height in the p-EBL can be reduced as a result of usage of the p-AlGaN layer.Therefore,the better stimulated radiative recombination rate and the increased laser power are obtained,thus enhancing the 3 dB frequency bandwidth.Moreover,we also inves-tigate the impact of the p-AlGaN/p-GaN structure with various AIN compositions in the p-AlGaN layer on the hole injection capabilit,the laser power,and the了dB frequency bandwidth.

On the polarization self-screening effect in multiple quantum wells for nitride-based near ultraviolet light-emitting diodes

The tilted energy band in the multiple quantum wells（MQWs） arising from the polarization effect causes the quantum confined Stark effect（QCSE） for [0001] oriented III-nitride-based near ultraviolet light-emitting diodes（NUV LEDs）. Here, we prove that the polarization effect in the MQWs for NUV LEDs can be self-screened once the polarization-induced bulk charges are employed by using the alloy-gradient InxGa1-xN quantum barriers. The numerical calculations demonstrate that the electric field in the quantum wells becomes weak and thereby flattens the energy band in the quantum wells, which accordingly increases the spatial overlap for the electron-hole wave functions. The polarization self-screening effect is further proven by observing the blueshift for the peak emission wavelength in the calculated and the measured emission spectra. Our results also indicate that for NUV LEDs with a small conduction band offset between the quantum well and the quantum barrier,the electron injection efficiency for the proposed structure becomes low. Therefore, we suggest doping the proposed quantum barrier structures with Mg dopants.