Micro-LED, also called μ LED, refers to the LED chip with a table size of 1~100 μm. Compared with traditional deep ultraviolet LED chips, it has good current scalability, fast heat dissipation, short carrier life and small RC time constant, which is conducive to improving communication bandwidth. Compared with the traditional deep ultraviolet LED with the same luminous area, the light extraction efficiency is obviously improved after the array arrangement. Micro-LED has been widely used in visible light communication (such as blue light and green light), but the Micro-LED process is still in the development stage in the solar blind area ultraviolet light communication. The summary and in-depth analysis of deep ultraviolet Micro-LED devices and their applications are of great significance to further improve their performance.

Figure 1: Flip-chip UV LED structure

Blind LED for Ultraviolet Communication

Micro-LED the light extraction of the side wall can be optimized, and its size is small, and the modulation bandwidth mainly controlled by the carrier lifetime is relatively high, so it is very beneficial for the application of optical communication.

Since the University of Strathclyde reported in 2019 that a communication rate of 1 Gb/s was achieved using a 262 nm UV Micro-LED communication array, physical phenomena such as high injection current density, blue shift of wavelength and narrowing of half-peak width in UV Micro-LED have been extensively studied. At present, the main research direction of ultraviolet Micro-LED is the preparation process of the device, the basic physical phenomenon, and its application technology to communication and display.

For communication applications, the 285 nm wavelength Micro-LEDs communication array chip reported by Daniel M. Maclure team of Strcthclyde University has achieved communication rates of 6.5 Gb/s and> 1 Gb/s at a distance of 10 m and 116 m respectively, which is the longest distance and rate currently achieved using ultraviolet Micro-LED.

Figure 2 (a) Electrode part of Micro-LED array for ultraviolet communication, (B) Micro-LED chip part

The on-chip integration of ultraviolet communication usually refers to the technology of connecting the ultraviolet LED (or Micro-LED) and the ultraviolet detector (PD) of the same MQWs structure through the coupling waveguide, and using the luminescence-detection phenomenon to carry out the ultraviolet communication on the chip to realize the optical interconnection. This technique makes full use of the transverse propagating TM mode photons emitted by high Al-component MQWs.

In 2022, the on-chip integration of 274 nm LED, self-driven PD and waveguide reported by Wei Tongbo team of Semiconductor Research Institute of Chinese Academy of Sciences will achieve a communication bandwidth of 600 μm from 1MHz in on-chip communication experiments. The self-driven PD exhibits a rise/fall response time of 127/131 ns, which is the highest performance of the self-driven PD reported so far.

Figure 3(a) The transmitted signal loaded by the S1-LED, the received signal captured by the S1-PD, (B) the rise/fall time fitted from the received signal obtained on the S1-PD,(c) the communication eye diagram measured at 1MHz

At present, the research direction of mass production of UVC LED in the market focuses on the optimization of electro-optical conversion efficiency (WPED) and light extraction efficiency. The light extraction efficiency of mass-produced chips is increased from 6% ~ 12% to 25%, and WPE is increased from 3% to 6%. The challenges of improving the performance of mass-produced products come from the fields of epitaxy, chips and packaging, including improving material quality, optimizing packaging materials, improving ohmic contacts and reducing production costs.

At present, the WPE of some laboratory prepared devices can reach 6% ~ 10%, and is expected to exceed 15% in 2026. The integration of UV LED and self-driving PD is expected to achieve multi-functional systems, such as real-time detection of light intensity communication, lighting chips, etc., but the corresponding research is currently in the development stage.

Vigorously developing Micro-LED is an effective solution to improve the poor light extraction efficiency and low brightness of the chip. At present, its research focuses on the field of device physics, which verifies its potential in improving LED performance, but its performance is still far from visible light Micro-LED. The subsequent preparation process should be improved, taking into account cost and reliability, in order to achieve its high-level application in the commercial field.