White-Light GaN-μLEDs Employing Green/Red Perovskite Quantum Dots as Color Converters for Visible Light Communication

GaN-based μLEDs with superior properties have enabled outstanding achievements in emerging micro-display, high-quality illumination, and communication applications, especially white-light visible light communication (WL-VLC). WL-VLC systems can simultaneously provide white-light solid-state lighting (SSL) while realizing high-speed wireless optical communication. However, the bandwidth of conventional white-light LEDs is limited by the long-lifetime yellow yttrium aluminum garnet (YAG) phosphor, which restricts the available communication performance. In this paper, white-light GaN-μLEDs combining blue InGaN-μLEDs with green/red perovskite quantum dots (PQDs) are proposed and experimentally demonstrated. Green PQDs (G-PQDs) and red PQDs (R-PQDs) with narrow emission spectrum and short fluorescence lifetime as color converters instead of the conventional slow-response YAG phosphor are mixed with high-bandwidth blue InGaN-μLEDs to generate white light. The communication and illumination performances of the WL-VLC system based on the white-light GaN-based μLEDs are systematically investigated. The VLC properties of monochromatic light (green/red) from G-PQDs or R-PQDs are studied in order to optimize the performance of the white light. The modulation bandwidths of blue InGaN-μLEDs, G-PQDs, and R-PQDs are up to 162 MHz, 64 MHz, and 90 MHz respectively. Furthermore, the white-light bandwidth of 57.5 MHz and the Commission Internationale de L’Eclairage (CIE) of (0.3327, 0.3114) for the WL-VLC system are achieved successfully. These results demonstrate the great potential and the direction of the white-light GaN-μLEDs with PQDs as color converters to be applied for VLC and SSL simultaneously. Meanwhile, these results contribute to the implementation of full-color micro-displays based on μLEDs with high-quality PQDs as color-conversion materials.


Introduction
Group III-nitride semiconductors are among the most popular wide-bandgap semiconductors owing to their superior advantages of high electron mobility, wide bandgap, high stability, and high breakdown voltage [1][2][3]. The wide direct bandgap, ranging from deep ultraviolet (~6.2 eV) to near-infrared (~0.7 eV), can be tuned to span the entire UV and visible spectrum. Therefore, group III-nitride semiconductors have gathered enormous attention and undergone rapid development in varied applications of optoelectronics and

Fabrication of InGaN-µLED Device
Blue-emitting InGaN-µLEDs were fabricated from a commercial GaN epitaxy wafer grown on c-plane sapphire substrates. An n-GaN layer, an InGaN/GaN multiple quantum well (MQW) layer, an AlGaN electron blocking layer, and a p-GaN layer were deposited on the patterned sapphire substrates through metal-organic chemical vapor deposition.
A series of fabrication processes including photolithography, wet and dry etching, film deposition, lift-off, etc. were performed in a cleanroom. The following briefly describes the typical fabrication processes of the InGaN-µLED, which is shown in Figure 1a. The Ni/Au (10 nm/25 nm) metal was deposited as a current spreading layer by magnetron sputtering or electron-beam evaporation before fabrication of the InGaN-µLED device. Next, utilizing inductively coupled plasma etching (ICP), the epitaxial structure was etched to the n-GaN layer, and the µLED mesa was manufactured. This was followed by rapid thermal annealing in nitrogen at 500 • C to form an ohmic contact. Then, a 290 nm SiO 2 layer was deposited as a standard isolation layer by plasma-enhanced chemical vapor deposition (PECVD). Lithographic patterning and wet/dry etching processes were used to etch SiO 2 to open apertures on the µLED mesas and the n-contact area to deposit metal. Finally, via photolithography and metallization, the Ti/Au (50 nm/200 nm) metal was deposited as n-track and p-track. The fabrication details can be found in previous work [19,35,[38][39][40][41][42]. In this way, the µLED arrays with square and circular mesas and various sizes were fabricated successfully. The diameters for circular µLED pixels were 40, 60, and 100 µm. For square µLED pixels, the side lengths were 40, 60, and 80 µm. The 2D epitaxial structure of a single square InGaN-µLED is shown in Figure 1b. Figure 1c presents a 3D schematic diagram of the InGaN-µLED arrays with square pixels.

Fabrication of GaN-Based White-Light µLEDs
GaN-based white-light micro-LEDs were fabricated by employing blue InGaN-µLEDs as light sources and PQDs as the color converters. PQDs were synthesized through a modified hot-injection method based on a previous study [43]. Under a nitrogen atmosphere and at a reaction temperature of 150 • C, Cs-oleate solution was quickly injected into a mixture of octadecene, oleylamine, oleic acid, and PbX 2 (X = Br or I). The as-prepared green CsPbBr 3 QDs (G-PQDs) were extracted after discarding the supernatant. The red CsPb(Br/I) 3 QDs (R-PQDs) were fabricated in the same way. The fabricated G-PQDs and R-PQDs were encapsulated in epoxy resin, which isolated them from oxygen and moisture. The detailed synthesis processes can be found in our previous study [38]. The prepared PQD films acting as color converters were overlaid on the blue-emitting InGaN-µLEDs, and the white light was obtained under excitation of the µLEDs. Then, the InGaN-µLED chip covered with PQDs was bonded to a printed circuit board. Finally, the hybrid white-light GaN-based devices were implemented.

Construction of WL-VLC System
As presented in Figure 1d, the main part of the VLC system consisted of blue InGaN-µLED, G-PQDs and R-PQDs, a transmitter lens (Tx lens), a receiver lens (Rx lens), and a photodetector. The blue-emitting µLEDs were employed as light sources to excite different PQDs which could adjust the color component of light required by the VLC system. Using a non-return-to-zero on-off key (NRZ-OOK) modulation scheme, the signals were modulated to the µLEDs by the bias tee. The modulated light propagated through the free-space transmission link. Color converters were put into the light path to fulfill the experimental requirements. The Tx lens and Rx lens were used to collimate and focus the modulated light generated from the light-emitter. The light was received and recorded by a photodetector, and then the light signals were converted to electric signals for further analysis of the frequency response, BER, and eye diagrams. Based on the above VLC link, a network analyzer (Agilent, N5225A, 10 MHz-50 GHz) was used to measure the frequency response to obtain the modulation bandwidth under different currents. The BER was measured by an error-detector module built into the signal quality analyzer (Anritsu, MP1800A, 0.1-14 GHz). A wide-bandwidth oscilloscope (Agilent 86100A, 14 GHz) was used to capture eye diagrams. All measurements were carried out at ambient temperature in air.
μLEDs, and the white light was obtained under excitation of the μLEDs. Then, the InGaN-μLED chip covered with PQDs was bonded to a printed circuit board. Finally, the hybrid white-light GaN-based devices were implemented.

Construction of WL-VLC System
As presented in Figure 1d, the main part of the VLC system consisted of blue InGaN-μLED, G-PQDs and R-PQDs, a transmitter lens (Tx lens), a receiver lens (Rx lens), and a photodetector. The blue-emitting μLEDs were employed as light sources to excite different PQDs which could adjust the color component of light required by the VLC system. Using a non-return-to-zero on-off key (NRZ-OOK) modulation scheme, the signals were modulated to the μLEDs by the bias tee. The modulated light propagated through the free-space transmission link. Color converters were put into the light path to fulfill the experimental requirements. The Tx lens and Rx lens were used to collimate and focus the modulated light generated from the light-emitter. The light was received and recorded by a photodetector, and then the light signals were converted to electric signals for further analysis of the frequency response, BER, and eye diagrams. Based on the above VLC link, a network analyzer (Agilent, N5225A, 10 MHz-50 GHz) was used to measure the frequency response to obtain the modulation bandwidth under different currents. The BER was measured by an error-detector module built into the signal quality analyzer (Anritsu, MP1800A, 0.1-14 GHz). A wide-bandwidth oscilloscope (Agilent 86100A, 14 GHz) was used to capture eye diagrams. All measurements were carried out at ambient temperature in air.

Electrical and Optical Properties of InGaN-μLEDs
We studied the electrical and optical properties of the fabricated InGaN-μLEDs before they were embedded into the system. The relationship between current density and

Electrical and Optical Properties of InGaN-µLEDs
We studied the electrical and optical properties of the fabricated InGaN-µLEDs before they were embedded into the system. The relationship between current density and bias voltage (J-V) and that between light output power and injection current (L-I) of circular µLEDs with different diameters are illustrated in Figure 2. The figure indicates a dependent relationship between output performance and device scale. That is, the smaller the µLEDs, the higher the current density that the µLEDs could sustain, and a lower light output that the µLEDs could produce. These results were in accordance with the J-V and L-I characteristics of the InGaN-µLEDs with square pixels, as shown in Figure S1. Therefore, there was a trade-off between modulation bandwidth and light-output power, which will be discussed in the following section.
bias voltage (J-V) and that between light output power and injection current (L-I) of circular μLEDs with different diameters are illustrated in Figure 2. The figure indicates a dependent relationship between output performance and device scale. That is, the smaller the μLEDs, the higher the current density that the μLEDs could sustain, and a lower light output that the μLEDs could produce. These results were in accordance with the J-V and L-I characteristics of the InGaN-μLEDs with square pixels, as shown in Figure S1. Therefore, there was a trade-off between modulation bandwidth and light-output power, which will be discussed in the following section.  Figure 3 demonstrates the normalized electroluminescence (EL) spectra of the circular InGaN-μLEDs, taking 80 μm μLED as an example. In Figure 3a, with increasing injection current, the peak wavelength of the EL spectrum moved to the shorter wavelength direction and the full width at half maximum (FWHM) increased, then both kept saturation. Both the movement of peak wavelength and the increase of FWHM were on a small scale, ensuring good monochromaticity for SSL. Figure 3b shows the peak wavelength as a function of the injection current, which was extracted from Figure 3a. The blueshift of the emission wavelength observed is attributed to the carrier-screening effect of the quantum-confined Stark effect and/or the band-filling effect [44], which degrades the performance of μLEDs (e.g., aging, reliability, and modulation bandwidth) [45,46]. As a result, to obtain optimal modulation bandwidth and light-output power, the appropriate working current was chosen to drive the μLEDs. For InGaN-μLEDs with square pixels, the EL spectra were similar to those of InGaN-μLEDs with circular pixels, as shown in Figure S2.   Figure 3a, with increasing injection current, the peak wavelength of the EL spectrum moved to the shorter wavelength direction and the full width at half maximum (FWHM) increased, then both kept saturation. Both the movement of peak wavelength and the increase of FWHM were on a small scale, ensuring good monochromaticity for SSL. Figure 3b shows the peak wavelength as a function of the injection current, which was extracted from Figure 3a. The blueshift of the emission wavelength observed is attributed to the carrier-screening effect of the quantum-confined Stark effect and/or the band-filling effect [44], which degrades the performance of µLEDs (e.g., aging, reliability, and modulation bandwidth) [45,46]. As a result, to obtain optimal modulation bandwidth and light-output power, the appropriate working current was chosen to drive the µLEDs. For InGaN-µLEDs with square pixels, the EL spectra were similar to those of InGaN-µLEDs with circular pixels, as shown in Figure S2.

Frequency Responses of InGaN-μLEDs
To study the modulation bandwidth of the InGaN-μLED for high-speed VLC, we evaluated the normalized electrical-to-optical frequency response of the circular μLEDs with different sizes under the injection current varying roughly in the region from approximately 0 mA (~hundreds of nA) to 150 mA (Figure 4a-c). As shown in Figure 4a, with the increase of injection current, the modulation bandwidth first increased then saturated for the circular μLED with a diameter of 40 μm. In Figure 4b-c, the characteristics of the modulation bandwidth versus injection current for circular μLEDs with diameters of 60 μm and 100 μm were consistent with that of 40 μm μLEDs (Figure 4a). The higher

Frequency Responses of InGaN-µLEDs
To study the modulation bandwidth of the InGaN-µLED for high-speed VLC, we evaluated the normalized electrical-to-optical frequency response of the circular µLEDs with different sizes under the injection current varying roughly in the region from approximately 0 mA (~hundreds of nA) to 150 mA (Figure 4a-c). As shown in Figure 4a, with the increase of injection current, the modulation bandwidth first increased then saturated for the circular µLED with a diameter of 40 µm. In Figure 4b,c, the characteristics of the modulation bandwidth versus injection current for circular µLEDs with diameters of 60 µm and 100 µm were consistent with that of 40 µm µLEDs (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 µLED and the package [39]. The highest bandwidths of µLEDs with different sizes are shown in Figure 4d. It is evident that attainable modulation bandwidth increased as the size of the µLEDs decreased, further indicating that InGaN-µLEDs have greater potential in the field of VLC than broad-area LEDs. For InGaN-µLEDs with square pixels, the frequency response of the µLED with a side length of 80 µm has been studied in previous work [38,39]. The change trend of bandwidth versus current was consistent with that of the e circular InGaN-µLEDs.  Although smaller μLEDs 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-μLEDs 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-μLED with the side length of 80 μm had the maximum light-output power (up to 1.3 mW) among the samples. In addition, note that in Figure S3, the square μLED with the side length of 80 μm presented a maximum bandwidth bandwidth of about 162 MHz. Therefore, to balance the bandwidth of the μLED for VLC and the light-output performance for the SSL, the square InGaN-μLED with the side length of 80 μm was applied for the WL-VLC system. Although smaller µLEDs 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-µLEDs 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-µLED with the side length of 80 µm had the maximum light-output power (up to 1.3 mW) among the samples. In addition, note that in Figure S3, the square µLED with the side length of 80 µm presented a maximum bandwidth bandwidth of about 162 MHz. Therefore, to balance the bandwidth of the µLED for VLC and the light-output performance for the SSL, the square InGaN-µLED with the side length of 80 µm was applied for the WL-VLC system.

Visible Light Communication and Solid-State Lighting
Before the construction of the white-light system, two kinds of color-conversion materials (R-PQDs and G-PQDs) were utilized separately to generate monochromatic light for communication, and the properties of both PQDs are discussed in the following. Figure 5a presents the spectrum of R-PQDs under excitation of the µLED, where one peak wavelength was at 445 nm from the µLED and another peak wavelength was at 611 nm from R-PQDs. Likewise, the spectrum of G-PQDs excited by µLED is shown in Figure 5b, with two peak wavelengths representing µLED and G-PQDs at 445 nm and 531 nm, respectively. The FWHMs of the R-PQDs and G-PQDs were 34 nm and 21 nm, manifesting the outstanding narrow emission of PQDs.  The normalized responses of R-PQDs and G-PQDs excited by the blue InGaN-μLEDs were measured, which are exhibited in Figure 6. The bandwidth of the square InGaN-μLED with the size of 80 μm was up to ~162 MHz, and under the excitation of the μLED, the bandwidths of R-PQDs (μLED + R-PQDs) and G-PQDs (μLED + G-PQDs) were ~90 MHz and ~64 MHz, respectively. Meanwhile, the R-PQDs showed better stability and higher bandwidth than the G-PQDs as the curve declined more slowly and smoothly. The superiority of PQDs can be supported by drawing a comparison with the slow frequency responses of conventional YAG:Ce 3+ phosphors (2.5 MHz) [25].  The normalized responses of R-PQDs and G-PQDs excited by the blue InGaN-µLEDs were measured, which are exhibited in Figure 6. The bandwidth of the square InGaN-µLED with the size of 80 µm was up to~162 MHz, and under the excitation of the µLED, the bandwidths of R-PQDs (µLED + R-PQDs) and G-PQDs (µLED + G-PQDs) were~90 MHz and~64 MHz, respectively. Meanwhile, the R-PQDs showed better stability and higher bandwidth than the G-PQDs as the curve declined more slowly and smoothly. The superiority of PQDs can be supported by drawing a comparison with the slow frequency responses of conventional YAG:Ce 3+ phosphors (2.5 MHz) [25].
As shown in Figure 7a,b, the eye diagrams were measured at a data rate of 100 Mbps for µLED + R-PQDs and µLED + G-PQDs, respectively. Attributed to the higher frequency response and signal-to-noise ratio of the R-PQDs, the eye diagram is not only open and clear but also less noisy in comparison with that of the G-PQDs. The maximum data rate of the R-PQDs shown in Figure 7c was achieved at 200 Mbps with a BER of 3.3 × 10 −3 , beneath the forward error correction (FEC) criterion of 3.8 × 10 −3 . Similarly, in Figure 7d, the maximum achievable data rate of the G-PQDs system was 170 Mbps with a BER of 3.5 × 10 −3 , below the FEC threshold. Therefore, both R-PQDs and G-PQDs show excellent potential in optical wireless communication.
μLED with the size of 80 μm was up to ~162 MHz, and under the excitation of the μLED, the bandwidths of R-PQDs (μLED + R-PQDs) and G-PQDs (μLED + G-PQDs) were ~90 MHz and ~64 MHz, respectively. Meanwhile, the R-PQDs showed better stability and higher bandwidth than the G-PQDs as the curve declined more slowly and smoothly. The superiority of PQDs can be supported by drawing a comparison with the slow frequency responses of conventional YAG:Ce 3+ phosphors (2.5 MHz) [25]. As shown in Figure 7a,b, the eye diagrams were measured at a data rate of 100 Mbps for μLED + R-PQDs and μLED + G-PQDs, respectively. Attributed to the higher frequency response and signal-to-noise ratio of the R-PQDs, the eye diagram is not only open and clear but also less noisy in comparison with that of the G-PQDs. The maximum data rate of the R-PQDs shown in Figure 7c was achieved at 200 Mbps with a BER of 3.3 × 10 −3 , beneath the forward error correction (FEC) criterion of 3.8 × 10 −3 . Similarly, in Figure 7d, the maximum achievable data rate of the G-PQDs system was 170 Mbps with a BER of 3.5  To verify high-efficiency illumination and high-speed wireless communication same time, the characteristics of the WL-VLC system based on GaN-based whit LED were further studied. Based on the abovementioned systems, the green ligh emitted from the G-PQDs under excitation of the blue light from μLED, and th mixed light was used to excite the R-PQDs. Consequently, as the blue light, green and red light merged, the white light was obtained. The performances of the whit system were measured and discussed in our further experiments.
As exhibited in Figure 8a, the CIE color coordinates of the WL-VLC system (0.3327, 0.3114) and the color temperature was 5474 K. Note that compared with th of (0.27, 0.30) in our previous study [38], a better-quality white light of our WL-VL tem was generated, which is very close to the CIE of (0.33, 0.33) for standard white As presented in Figure 8b, the maximum bandwidth achieved for the proposed W system was 57.5 MHz. The eye diagram of the WL-VLC system at 90 Mbps is sho the inset of Figure 8b. The above results reveal that the WL-VLC system proposed work implemented a high-performance SSL and VLC based on the white-light GaN To verify high-efficiency illumination and high-speed wireless communication at the same time, the characteristics of the WL-VLC system based on GaN-based white-light LED were further studied. Based on the abovementioned systems, the green light was emitted from the G-PQDs under excitation of the blue light from µLED, and then the mixed light was used to excite the R-PQDs. Consequently, as the blue light, green light, and red light merged, the white light was obtained. The performances of the white-light system were measured and discussed in our further experiments.
As exhibited in Figure 8a, the CIE color coordinates of the WL-VLC system were (0.3327, 0.3114) and the color temperature was 5474 K. Note that compared with the CIE of (0.27, 0.30) in our previous study [38], a better-quality white light of our WL-VLC system was generated, which is very close to the CIE of (0.33, 0.33) for standard white light. As presented in Figure 8b, the maximum bandwidth achieved for the proposed WL-VLC system was 57.5 MHz. The eye diagram of the WL-VLC system at 90 Mbps is shown in the inset of Figure 8b. The above results reveal that the WL-VLC system proposed in this work implemented a high-performance SSL and VLC based on the white-light GaN-based µLED. Compared with the monochromatic light system, the eye diagram is noisy and relatively close. Consequently, substantial efforts will be made to further optimize the InGaN-µLEDs and PQDs to achieve high bandwidth, high light-output power, and better linearity.

Conclusions
In this study, a WL-VLC system based on white-light GaN-based μLEDs with green and red PQDs as color converters was proposed and demonstrated experimentally for both communication and illumination. The essential optical and electrical performances of monochromatic light from the InGaN-μLEDs, G-PQDs, and R-PQDs, as well as the white light, were studied. The corresponding communication and illumination performances were systematically investigated in detail. We achieved the maximum bandwidths of 162 MHz, 90 MHz, 64 MHz, and 57.5 MHz for InGaN-μLEDs, R-PQDs, G-PQDs, and white-light in our WL-VLC system, respectively. High-quality white light was generated which possessed CIE of (0.3327, 0.3114) and a correlated color temperature of 5474 K. It is worth noting that using R-PQDs and G-PQDs as color converters, the available CIE of (0.3327, 0.3114) is closer to the CIE of (0.33, 0.33) for standard white light, compared with that (0.27, 0.30) of white light using yellow PQDs as color converters in our previous study. These results shed new light on the potential of InGaN-μLEDs combined with PQDs for both high-speed VLC and high-efficiency SSL.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Figure S1: (a) J-V and (b) L-I characteristics of square InGaN-μLEDs with different sizes. Figure S2: The normalized EL spectra of the square μLED with a side length of 80 μm under different applied voltages.

Conclusions
In this study, a WL-VLC system based on white-light GaN-based µLEDs with green and red PQDs as color converters was proposed and demonstrated experimentally for both communication and illumination. The essential optical and electrical performances of monochromatic light from the InGaN-µLEDs, G-PQDs, and R-PQDs, as well as the white light, were studied. The corresponding communication and illumination performances were systematically investigated in detail. We achieved the maximum bandwidths of 162 MHz, 90 MHz, 64 MHz, and 57.5 MHz for InGaN-µLEDs, R-PQDs, G-PQDs, and white-light in our WL-VLC system, respectively. High-quality white light was generated which possessed CIE of (0.3327, 0.3114) and a correlated color temperature of 5474 K. It is worth noting that using R-PQDs and G-PQDs as color converters, the available CIE of (0.3327, 0.3114) is closer to the CIE of (0.33, 0.33) for standard white light, compared with that (0.27, 0.30) of white light using yellow PQDs as color converters in our previous study. These results shed new light on the potential of InGaN-µLEDs combined with PQDs for both high-speed VLC and high-efficiency SSL.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano12040627/s1. Figure S1: (a) J-V and (b) L-I characteristics of square InGaN-µLEDs with different sizes. Figure S2: The normalized EL spectra of the square µLED with a side length of 80 µm under different applied voltages. Figure S3: The normalized response of the square µLED with a side length of 80 µm under a current of 70 mA. The dashed line represents the forward error correction (FEC) threshold.