## 3 Results and Discussion
### Frequency Responses of InGaN-\(\mu\)LEDs  
To study the modulation bandwidth of the InGaN-\(\upmu\)LED for high-speed VLC, we evaluated the normalized electrical-to-optical frequency response of the circular \(\upmu\)LEDs with different sizes under the injection current varying roughly in the region from approximately 0 mA (\(\sim\)hundreds of nA) to 150 mA (Figure 4a-c). As shown in Figure 4a, with  
Figure 3: (**a**) The normalized EL spectra of circular \(\upmu\)LED with a diameter of 80 \(\upmu\)m under different currents. (**b**) Peak wavelength extracted from the EL spectra at different injection currents.  
Figure 2: (**a**) J–V and (**b**) L–I characteristics of circular InGaN-\(\upmu\)LEDs with different diameters.  
the increase of injection current, the modulation bandwidth first increased then saturated for the circular \(\upmu\)LED with a diameter of 40 \(\upmu\)m. In Figure 4b,c, the characteristics of the modulation bandwidth versus injection current for circular \(\upmu\)LEDs with diameters of 60 \(\upmu\)m and 100 \(\upmu\)m were consistent with that of 40 \(\upmu\)m \(\upmu\)LEDs (Figure 4a). The higher bandwidth was obtained at a higher current, which can be attributed to the reduced differential carrier lifetime at higher injection currents [42]. The bandwidth saturation might be attributed to the impedance mismatch between the \(\upmu\)LED and the package [39]. The highest bandwidths of \(\upmu\)LEDs with different sizes are shown in Figure 4d. It is evident that attainable modulation bandwidth increased as the size of the \(\upmu\)LEDs decreased, further indicating that InGaN-\(\upmu\)LEDs have greater potential in the field of VLC than broad-area LEDs. For InGaN-\(\upmu\)LEDs with square pixels, the frequency response of the \(\upmu\)LED with a side length of 80 \(\upmu\)m has been studied in previous work [38; 39]. The change trend of bandwidth versus current was consistent with that of the e circular InGaN-\(\upmu\)LEDs.  
Although smaller \(\upmu\)LEDs possessed higher bandwidth (Figure 4), the output power was decreased owing to the smaller mesa area (Figure 2), leading to a higher BER that limits communication speed and transmission distance. In this work, the InGaN-\(\upmu\)LEDs as light sources excited PQDs and mixed them to generate white light for SSL and VLC, so a higher light-output power was desired. Comparing Figure 2b with Figure S1b, it is evident that the square InGaN-\(\upmu\)LED with the side length of 80 \(\upmu\)m had the maximum light-output power (up to 1.3 mW) among the samples. In addition, note that in Figure S3, the square  
Figure 4: Frequency responses of the circular InGaN-\(\upmu\)LEDs with the diameters of (**a**) 40 \(\upmu\)m, (**b**) 60 \(\upmu\)m, and (**c**) 100 \(\upmu\)m under different injection currents. The -3 dB modulation bandwidth is marked by a dashed line. (**d**) The maximum bandwidths of InGaN-\(\upmu\)LEDs with different sizes.  
\(\mu\)LED with the side length of 80 \(\upmu\)m presented a maximum bandwidth bandwidth of about 162 MHz. Therefore, to balance the bandwidth of the \(\mu\)LED for VLC and the light-output performance for the SSL, the square InGaN-\(\mu\)LED with the side length of 80 \(\upmu\)m was applied for the WL-VLC system.