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Visible light communication (VLC) possesses abundant spectrum resource and can be a promising candidate in the 6G network for its ability of combining positioning, lighting and communication. At present, the high-speed VLC is mainly based on blue and red light. The data rate of green light is still low. In order to develop the bandwidth and data rate of green micro-LEDs in wavelength division multiplexing (WDM) applications of VLC system, we designed high-bandwidth green micro-LEDs with high light output power. A 10-$\mu$ m green micro-LED achieved an E-E bandwidth and an E-O bandwidth up to 1.27 GHz and 2.19 GHz at a current density of 40 kA/cm$^{2}$. In order to increase the light output power to milliwatts, whereas mostly maintaining the same bandwidth as single pixel, series-connected micro-LED array was fabricated. We also utilized a bit and power loading algorithm for VLC system, which can obtain high spectral efficiency below the 3.8 $\times 10^{-3}$ forward error correction (FEC) threshold by adopting signal-to-noise ratio (SNR) gap adjustment. Record data rates of 9.06 Gbps, 8.14 Gbps and 6.23 Gbps with bit error rates (BER) of 3.7 $\times 10^{-3}$, 3.4 $\times 10^{-3}$ and 3.2 $\times 10^{-3}$ were achieved at 0.2 m, 1 m and 3 m, respectively. These are the highest data rates achieved by green micro-LED in free space, which will promote the applications of high-speed WDM in VLC.

The optimal photodetector (PD) size is an important topic in realizing high-speed VLC systems, but there is still a lack of sufficient research. In this work, we studied the size-dependent characteristics of GaN micro-LED/LED-based PDs. We first tested the photocurrent and bandwidth characteristics of eight different sizes of PDs, including 20, 40, 60, 80, 100, 150, 200, and 300 $\boldsymbol{\mu}$m. We find that the 300 $\boldsymbol{\mu}$m device has the highest photocurrent when the light incident power of the device is the same, and the 20 $\boldsymbol{\mu}$m device has the highest bandwidth of 166.8 MHz. To take into account the effects of bandwidth and photocurrent on VLC, we studied the size-dependent VLC performance of micro-LED/LED-based PDs by using the OFDM scheme. An 80-$\mu$m micro-LED-based PD has the potential to balance the photocurrent and bandwidth to achieve the highest data rate of 7.1291 Gbps. Our work has proved that the optimal size of micro-LED-based PD is 80 $\boldsymbol{\mu}$m which will guide the design of micro-LED/LED-based PD. Furthermore, our micro-LED-based PD array has the potential to be used in multiple-input multiple-output (MIMO) VLC, with a potential data rate up beyond tens of Gbps, which opens a new way to achieve ultra-high-speed communication.

In this paper, 10.5 Gbps visible light communication systems based on a single c-plane freestanding GaN micro-LED are presented. We investigate the size effect on blue GaN-substrate micro-LED from optoelectronic properties to communication performance. Larger-size micro-LED can obtain higher light output power (LOP), but smaller-size micro-LED can achieve higher modulation bandwidth. An ultra-high −3 dB bandwidth of $\sim$ 2.3 GHz was obtained by 20 $\mu$m micro-LED. Meanwhile, we further improved the OFDM signal bandwidth to effectively utilize the high bandwidth of GaN-substrate micro-LED, and the maximum data rates of 20, 40, 60, 80, 100, and 300 $\mu$m micro-LEDs are 8.294, 8.394, 10.547, 10.019, 9.398, and 7.294 Gbps, respectively. 60 $\mu$m micro-LED has the potential to balance the LOP and bandwidth to achieve a record data rate of 10.547 Gbps, which is the highest data rate obtained by a single micro-LED to the best of our knowledge. Based on the optimum size of 60 $\mu$m micro-LED, extended distances of 0.5, 1, 5, and 10 m with data rates of 10.045, 9.668, 9.277, and 8.649 Gbps are also presented.

With ever-growing demand for 6G networks technology, visible light communication (VLC) as a vital component of 6G has challenging requirement for superior performance of light source. Herein, 20 $\mu$m blue micro-LED fabricated on 2-inch c-plane GaN freestanding substrate with high bandwidth over 1 GHz was first demonstrated, and the systematic comparisons to sapphire-substrate micro-LED from the epitaxial materials characterization to optoelectronic properties as well as communication performance were further conducted. GaN-substrate LED wafer has high crystal quality with lower threading dislocation density (TDD), which is two orders of magnitude lower than that of sapphire-substrate LED wafer. The much lower TDD of GaN-substrate LED wafer and better heat dissipation of GaN substrate offer GaN-substrate micro-LED great advantages in optoelectronic properties as well as communication performance. GaN-substrate micro-LED exhibited higher light output power (LOP) especially at high operating current, which is improved by $\sim$21% at 96 mA compared to that of sapphire substrate. Also, GaN-substrate micro-LED achieved a relatively higher bandwidth of 1.282 GHz while maintaining higher LOP with lower operating voltage. And a higher data rate of 4.48 Gbps under 1 m free space link was obtained by GaN-substrate micro-LED, proving it being a promising candidate in high-speed VLC.