Visible light communication (VLC) has emerged as a promising communication method in 6G. Compared to traditional wireless communications, VLC operates in the 400–800 THz range, and has different physical features, such as great electromagnetic interference resistance, high confidentiality, and high data rate.

Nowadays, signal communication at such a short wavelength (about 400~780nm) presents significant hurdles to both transmitting and receiving devices. During the last decade, the development of the transmitting devices, including LED, SLD and LD, have achieved noteworthy results. However, visible light receiving devices have developed slowly. Si and GaAs-based detectors in the near infrared are still being used. Their minimum peak frequency response is also at 800 nm wavelength and then decreases rapidly in the visible band (400-780 nm), which leads to the development of a very competitive underwater visible light communication scenario (blue-green 400-570 nm) is blocked.

Reverse biasing GaN LEDs enables visible band photodetectors. However, almost all reported GaN-based LEDs are grown on sapphire substrates or GaN substrates. Due to the non-conductivity and low thermal conductivity of sapphire substrates, sapphire-based LEDs are often fabricated in lateral structures, which can bring current crowding and current droop under a high injection current density or high incident light power. A GaN substrate is the most ideal substrate for growing GaN-based LEDs, but it faces problems such as difficulty in production and high price for commercial use.

To address the problems and drive the LED-based photodetector into practical applications, a joint research group led by Prof. Nan Chi, Prof. Chao Shen, and Dr. Jianyang Shi from Fudan University, Prof. Fengyi Jiang and Prof. Jianli Zhang from Nanchang University reported the generation of Si-substrate vertical-structure InGaN/GaN micro-LED-based photodetector. Using this proposed photodetector, 10Gbps visible light communication has been successfully achieved. The relevant research results were published in Photonics Research, Volume 10, 2022 (Jianyang Shi, Zengyi Xu, Wenqing Niu, Dong Li, Xiaoming Wu, Ziwei Li, Junwen Zhang, Chao Shen, Guangxu Wang, Xiaolan Wang, Jianli Zhang, Fengyi Jiang, Shaohua Yu, Nan Chi. Si-substrate vertical-structure InGaN/GaN micro-LED-based photodetector for beyond 10 Gbps visible light communication[J]. Photonics Research, 2022, 10(10): 2394).

The schematic of the vertical structure of the Si-substrate micro-LED is shown in Fig. 1(a). Three chip sizes of 10 μm × 10 μm, 50 μm × 50 μm and 100 μm×100 μm are designed simultaneously in the experiment. The scanning electron micro-scope (SEM) images of 50 μm × 50 μm is demonstrated in Fig. 1(b). On the basis of the above micro-LED chips, 4×4 LED arrays are manufactured as shown in Fig. 1(c). Figures 1(d) shows the optical microscope images of 50 μm micro-LED arrays.

Fig. 1 (a) Schematic of the vertical structure of Si-substrate micro-LED-based photodetector; (b)SEM images of 50μm chips; (c) Schematic of layout for 4×4 Si-substrate micro-LED array; (d) optical microscope images of 50μm micro-LED array

Based on this Si-substrate GaN Micro-LED, a visible light photodetector with a peak responsivity of 400nm and a full-widths at half maxima of 72 nm has been successfully achieved. When a forward bias voltage is given to the device, visible light of about 573 nm can be emitted, and when a reverse bias voltage is given, the device enables the detection of visible light signals. By increasing the reverse bias voltage, the -20 dB bandwidth can reach a maximum of 822 MHz, and by using this detector, visible light communication with a transmission distance of 1m and a transmission rate of 10.14 Gbps has been successfully achieved.

Compared to conventional sapphire-substrate GaN micro-LEDs, Si-substrate micro-LEDs have high crystal quality, better conductivity and great thermal conductivity. It is also compatible with existing silicon-based circuits and has the advantage of becoming more miniaturized and highly integrated. The visible light photodetector constructed in this way is suitable for future cost- and rate-sensitive 6G visible light communication application scenarios. The joint research group from Fudan University and Nanchang University has realized Si-substrate GaN Micro-LED as a high-speed visible light photodetector, and will further carry out research on higher response sensitivity and higher transmission bandwidth to bring the device to practicality and finally realize the integrated design and application of visible light communication transceiver.