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In this work, electrical and optical performances for 250 nm AlGaN-based flip-chip deep ultraviolet light emitting diodes (DUV LEDs) with different chip sizes are studied. Reduced chip size helps increase the light extraction efficiency (LEE) with the cost of increased surface nonradiative recombination. Nevertheless, a thin p-Al 0.67-0 Ga 0.33-1 N layer of 10 nm can manage current distribution while suppressing surface recombination and reducing light absorption simultaneously, which results in the increased optical power density. Thanks to the better current management and reduced optical self-absorption effect, the reduced Joule heating effect suppresses the thermal droop of the optical power density for a small DUV LED chip. We also find that the p-Al 0.67-0 Ga 0.33-1 N layer thickness shows very significant impact on device resistance especially for the small DUV LED chip, such that the device resistance has a remarkable increase when the p-Al 0.67-0 Ga 0.33-1 N layer is thickened to 100 nm.

In this Letter, an omni-directional reflector (ODR) with a thin hybrid dielectric layer (hybrid-ODR) is proposed to enhance the light extraction efficiency (LEE) for inclined-sidewall-shaped AlGaN-based deep ultraviolet light-emitting diode (DUV LED) by inserting a thin diamond with high refraction index into a conventional Al/Al 2 O 3 -based ODR. The three-dimensional finite-difference time-domain (3D FDTD) simulation results show that the LEE of TM-polarized light for the DUV LED with hybrid-ODR is enhanced by 18.5% compared with Al/Al 2 O 3 -based ODR. It is because the diamond can transform the evanescent wave in Al 2 O 3 into the propagating light wave in diamond, thereby preventing effective excitation of the surface plasmon polariton (SPP) on the surface of the metal Al. Moreover, the Brewster's angle effect causes the TM-polarized light in diamond to propagate effectively into AlGaN. Furthermore, decreasing the total thickness of the dielectric layer also improves the scattering effect of the inclined sidewall. However, the utilization of hybrid-ODR results in a slight reduction in the LEE for transverse electric (TE) polarized light because the light is confined to the diamond layer and eventually absorbed by the metal Al.

It is known that light extraction efficiency (LEE) for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) can be enhanced by using an inclined sidewall of mesa. However, the reported optimal inclined angles are different. In this work, to explore the origin for enhancing the LEE of DUV LED by using inclined sidewalls, we investigate the effect of an inclined sidewall angle on the LEE for AlGaN-based DUV LEDs with different mesa diameters by using ray tracing. It is found that when compared to large-size DUV LEDs with inclined sidewall, the LEE of small-size DUV LEDs with inclined sidewall is enhanced from both the bottom and side surfaces due to the reduced scattering length and material absorption. Additionally, the optimal inclined sidewall angle is recommended within the range of 25°-65°, and the optimal angle for DUV LEDs decreases as the chip size increases. It can be attributed to the fact that there are two scattering mechanisms for the inclined sidewall. For smaller chip sizes, most of the light is directly scattered into escape cones by the inclined sidewall, resulting in a larger optimal angle. For larger chip sizes, the light firstly experiences total internal reflections by the out-light plane and then is scattered into escape cones by the inclined sidewalls, leading to a smaller optimal angle.

In this Letter, beveled mesas for 30 × 30 µm 2 GaN-based micro-light-emitting diodes (µLEDs) with different inclination angles are designed, fabricated, and measured. We find that µLED with a mesa inclination angle of 28° has the lowest internal quantum efficiency (IQE) and the highest injection current density at which the peak IQE is obtained. This is due to the increased quantum confined Stark effect (QCSE) at the mesa edge. The increased QCSE results from the strong electric field coupling effect. Instead of radiative recombination, more nonradiative recombination and leakage current will be generated in the sidewall regions. Besides, the smallest angle (28°) also produces the lowest light extraction efficiency (LEE), which arises from the optical loss caused by the sidewall reflection at the beveled surface sides. Therefore, the inclination angle for the beveled mesa has to be increased to 52° and 61° by using Ni and SiO 2 as hard masks, respectively. Experimental and numerical results show that the external quantum efficiency (EQE) and the optical power can be enhanced for the fabricated devices. Meanwhile, the reduced surface recombination rate also decreases the leakage current.

In this work, we have proposed and fabricated a metal/Ga 2 O 3 /GaN hybrid structure metal-semiconductor-metal ultraviolet photodetector with low dark current and high responsivity. The Schottky contact of Ni/Ga 2 O 3 makes the Ga 2 O 3 layer fully depleted. The strong electric field in the Ga 2 O 3 depletion region can push the photo-induced electrons from the Ga 2 O 3 layer into the GaN layer for more efficient carrier transport. Therefore, the hybrid structure simultaneously utilizes the advantage of the absorption to solar-blind ultraviolet light by the Ga 2 O 3 layer and the high electron mobility of the GaN layer. Thus, the dark current and the photocurrent for the proposed device can be greatly improved. As a result, an extremely high photo-to-dark-current ratio of 1.46 × 10 6 can be achieved. Furthermore, quick rise and fall times of 0.213 s and 0.027 s at the applied bias of 6 V are also obtained, respectively.

In this work, a 280-nm-wavelength deep-ultraviolet light-emitting diode (DUV LED) with a p + -GaN/SiO 2 /ITO tunnel junction is fabricated and investigated. Due to the decreased tunnel region width and enhanced electric field intensity in the 1-nm-thick SiO 2 layer, the interband tunneling efficiency and the corresponding hole injection efficiency are promoted. Therefore, the external quantum efficiency (EQE) for the proposed device is increased when compared with a traditional DUV LED. In addition, an improved current spreading effect is observed for our proposed device. As a result, improved wall-plug efficiency (WPE) is obtained owing to the increased optical power and decreased forward operating voltage. Meanwhile, the enhanced electric field intensity in the SiO 2 layer reduces the voltage drop in the p-n junction region for the proposed device, and thus the leakage current is reduced.

The ${{\rm MoO}_{\rm x}}/{\rm Al}$MoO x /Al electrode was designed and fabricated on p-GaN and sapphire with good ohmic behavior and decent deep ultraviolet (DUV) reflectivity, respectively. The influences of ${{\rm MoO}_{\rm x}}$MoO x thickness and annealing condition on the electrical and optical behaviors of the ${{\rm MoO}_{\rm x}}/{\rm Al}$MoO x /Al structure were investigated. Surface morphology of ${{\rm MoO}_{\rm x}}$MoO x with different thicknesses reveals a 3D growth mode. Partial decomposition of ${{\rm MoO}_{\rm x}}$MoO x was discovered, which helps in the formation of ohmic contact between ${{\rm MoO}_{\rm x}}$MoO x and Al. The potential for application in deep ultraviolet light-emitting-diodes (DUV-LEDs) has also been demonstrated.