As an important supplement to wireless radio frequency communication in free space and acoustic wave communication in underwater environment, visible light communication (VLC) is considered to be a strong contender for the next generation mobile wireless "last mile" access technology, which has great potential to be widely used in 5G+ or 6G.
VLC has many advantages, such as the combination of illumination and communication, anti-electromagnetic interference, no spectral licensing, and good privacy within a confined space. However, the bandwidth limitation of VLC systems based on commercial white LED will not be able to support the need for high-speed data communications in 5G+ or 6G.
So far, almost all reported GaN-based high-speed LEDs are based on InGaN/GaN quantum well (QW) active region. For traditional polar InGaN/GaN QW LEDs, due to the existence of the polarization electric field inside the QWs, the electron and hole wave-functions are spatially separated, and the radiative recombination rate is reduced, so the carrier lifetime becomes longer, which limits the modulation bandwidth.
In this case, the only way to increase the modulation bandwidth of the device is increasing the injection current density. Under high injection, the carrier concentration increases and the carrier recombination rate also becomes faster. Besides, the high carrier concentration can screen the influence of the polarization field, thereby shortening the carrier lifetime.
At present, the typical polar QW high-speed LEDs can achieve modulation bandwidths of hundreds of MHz under injection current density up to 5-10 kA/cm2. However, the device faces the problem of serious efficiency droop and heat generation under high injection.
Adopting non-polar/semi-polar GaN homogeneous substrates with high crystal quality can suppress the effect of polarization field and obtain a high bandwidth at relative low current density. However, as such non-polar/semi-polar GaN substrates are so expensive and small in size, it is difficult to practically apply them in industry.
With the support of the Cross-Strait Tsinghua Collaboration Program, the research groups from Tsinghua University (Beijing), National Tsing Hua University (Hsinchu), and Tsinghua-Berkeley Shenzhen Institute, start to collaboration on high-speed LEDs, including material epitaxy, chip process, as well as VLC systems.
They obtained a high-bandwidth bule LED through the innovation of active region structure, and verified its applications in high-speed VLC system. The results were recently published in Photonics Research, Volume 9, Issue 5, 2021 (Lei Wang, Zixian Wei, Chien-Ju Chen, et al., 1.3 GHz E-O bandwidth GaN-based micro-LED for multi-gigabit visible light communication).
By using metal organic chemical vapor deposition (MOCVD) combined with the growth interruption method, InGaN quantum dots with a wetting layer were grown on traditional c-plane sapphire substrates. Then, by eliminating the upper quantum dots and retaining the lower wetting layer, a blue LED wafer with ultrashort carrier lifetime (~1 ns) is obtained.
Based on the LED wafer, a 75-μm-size 480-nm-wavelength GaN-based micro-LED with 1.3 GHz E-O bandwidth under a relatively low current density of 528 A/cm2 on c-plane GaN is presented. This result is much better than any other reports based on the c-plane LEDs and is comparable to the best results based on the non-polar/semi-polar LEDs. Fig. 1 shows the photograph of testing high-bandwidth GaN-based blue micro-LED on wafer.
Furthermore, this work experimentally demonstrated a high-speed VLC system over 3 m transmission link using the packaged high-bandwidth LED with non-return-to-zero on-off-keying (NRZ-OOK) and quadrature phase shift keying-orthogonal frequency division multiplexing (QPSK-OFDM) modulation format implemented, respectively. The schematic diagram of the proposed VLC system based on GaN-based blue micro-LED is shown in Fig. 2.
The modulation bandwidth of the proposed VLC system is up to 1 GHz over a 3-m communication distance which is the highest around all the existing VLC systems based on single LED source, as shown in Fig. 2. Then, a 2 Gbps NRZ-OOK VLC link, which has the highest data rate among the ones using sample NRZ-OOK modulation format. By using higher-order modulation and OFDM, the data rate of this VLC system can reach 4 Gbps. This result provides a new solution for the construction of high-speed VLC systems in the future.
Fig.1 Photograph of testing high-bandwidth GaN-based blue micro-LED on wafer
Fig.2 Schematic of the proposed VLC system based on GaN-based blue micro-LED