Significance Recently, deep-ultraviolet (DUV) communications based on DUV micro-LED technology have drawn a significant interest. This is because deep-ultraviolet (DUV) communications possess a number of advantages such as the low back-ground noise, a non-line-of-sight (NLOS) link and high security. However, its development is constrained by the lack of light sources with high power and high modulation bandwidth. In recent years, the rapid advancement of low-cost, high-output power AlGaN-based DUV LEDs has greatly accelerated the development of UVC communication and its application in various fields. Moreover, the DUV μLEDs with small chip size have the advantages of high modulation speed and low power consumption, making them attractive for implementing high-speed UVC systems. However, the low luminous efficiency of AlGaN-based μLED seriously affects the data transmission rate in deep ultraviolet communication. Therefore, we provide a review and comprehensive analysis of the size effect on the optical, electrical, thermal and modulation properties for AlGaN-based μLED, including its underlying physics mechanism. In addition, we also review various approaches to improve the light extraction efficiency and thermal characteristics of DUV μLED, which is of great significance for the study of DUV μLED.
Progress Firstly, the current research status of DUV μLED as a solar blind UV communication source is introduced. The performance for UV communication system utilizing LED as a light source is summarized (Tab.1). It can be seen that under the same modulation mode, larger bandwidth and higher data transmission rate can be achieved with DUV μLED. In addition, due to the rapid attenuation of ultraviolet light power in the atmosphere, the transmission rate decreases for long distance communication. Therefore, ensuring both a large modulation bandwidth and a high optical output power for DUV μLED are very crucial for the high-speed propagation of DUV μLED optical communication system. The optical and electrical properties of DUV μLED are significantly affected by its size. The smaller size of the μLED enable them to withstand a higher current density, while the capacitance decreases as the size decreases. Consequently, the μLED with smaller size exhibits a higher modulation bandwidth. However, the reduction of the active area results in a decrease in output power as the size decreases. Additionally, the severe self-heating effect induces a thermal droop in EQE, making it challenging to achieve high power with high work currents. The low light extraction efficiency (LEE) and increased series resistor further deteriorate the self-heating effect. Therefore, to break the bottle of the light output power of μLED, it is necessary to improve the LEE, the series resistor and heat dissipation. Various micro-nano structures for nAlGaN, pAlGaN and sapphire can be used as scatter centers to improve the LEE. The patterned pAlGaN exhibits the most significant effect in improving LEE due to its proximity to the active region. However, it generally brings in a higher work voltage. Increasing the ohmic contact area and only patterning the area around the p-electrode can avoid the disadvantage. In addition, the inclined sidewall technology shows a significant potential for enhancing the LEE of DUV LED. And the shape and the sidewall reflector for the inclined sidewall have a substantial influence on the LEE of DUV μLED. Furthermore, to mitigate the self-heating effects of the device, the ohmic contact resistivity of DUV μLED device should be decreased, and the reflectivity of electrode should be increased. Therefore, the designed electrode needs to possess excellent ohmic contact and high reflectivity. Meanwhile, the device heat dissipation can be improved by increasing the electrode contact area and the device side wall area. Various technologies, such as a rectangle chip shape and a metal radiator can be utilized to enhance the device heat dissipation of DUV μLED.
Conclusions and Prospects This paper presents a systematically review of the research status of DUV μLED in the field of wireless optical communication. And the size effect on the modulation characteristics, light extraction efficiency, current and voltage characteristics, optical power characteristics and side wall defect ratio are comprehensive analyzed and its underlying physical mechanism is also shown. Various technologies for improving the efficiency of light extraction and heat dissipation are summarized and discussed in detail. Although a great progress have been made in the development of DUV μLED, further research should be dedicated to enhancing the LEE and heating dissipation of DUV μLED. Especially, the electrode and the chip shape need to be designed to ensure high reflectivity and excellent ohmic contact, high efficiency scatter and good heating dissipation.
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