Visible light communication (VLC) is a promising solution for the next-generation high-speed access technology. As an important supplement to radio frequency (RF) communication, the available spectrum of VLC is over 3 orders of magnitude wider than the RF one. VLC can be combined with solid-state lighting, which has been widely implemented in many fields. In addition, VLC exhibits the advantages of low power consumption, no electromagnetic interference, eye-safety, and strong confidentiality [1]. It is simultaneously suitable for high-speed communication and illumination applications in special environments such as airports, hospitals, nuclear power plants, underwater [2], and deep space [3]. The electrical-to-optical (E-O) bandwidth of the light-emitting device is critical in a bandwidth-limited VLC system, although there are alternative approaches to optimize the responsivity and detectivity of photodetectors such as germanium/perovskite heterostructures and InGaN multiple quantum well (QW) micro-size photodetectors [4,5]. In comparison with commercial light-emitting diodes (LEDs), micro-size LEDs (micro-LEDs) based on III-nitride semiconductors with smaller active area and lower RC delay provide a promising approach to improve the E-O bandwidth [6,7]. Beneficial from the high E-O bandwidth, the micro-LED has great potential for high-speed VLC implementation [8,9]. The data rate of micro-LED-based VLC using non-return-to-zero on-off keying (NRZ-OOK) can reach up to 1 Gbps, and multi-Gbps data rates can be achieved by advanced modulation formats such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), or orthogonal frequency-division multiplexing (OFDM) [10–17]. Tsonev et al. have demonstrated a VLC link with data rate over 3 Gbps based on a micro-LED using OFDM and then the data rate is extended to 5 Gbps afterwards [10–12]. Currently, single-pixel GaN-based micro-LEDs have been able to achieve a data rate of 10 Gbps by using bit-loading OFDM [13]. In addition, green-band, deep-ultraviolet, and perovskite materials have also been introduced for free-space communication applications [14–16]. Xie et al. adopted a series-structure micro-LED array with 18-mW emitting power to realize a VLC system with a data rate of 5.18 Gbps [17].

However, traditional c-plane InGaN-based QW LED devices have long suffered from the limitation of the polarization-field-induced quantum-confined Stark effect (QCSE) [18]. The QCSE leads to a longer carrier lifetime in the QW at low current density, which limits the E-O bandwidth of devices. Therefore, high carrier concentration is required to screen the polarization field and shorten the carrier lifetime. A high E-O bandwidth of near 1 GHz has already been presented in micro-LEDs operating at a high current density [12,19]. However, the high current density will sacrifice the luminous efficiency of the LED which is well known as efficiency droop. Although there have been efforts to develop LED epitaxial structures optimized for shortening carrier lifetime under lower current density, the typical E-O bandwidth and operating current density of a c-plane micro-LED are still below 1 GHz and beyond $1\mathrm{kA/}{\mathrm{cm}}^2$, respectively. Achieving high E-O bandwidth LEDs under lower current density by growing InGaN-based QW devices on semi-polar or non-polar surfaces to suppress the QCSE is considered a fundamental solution [20–23]. For QW samples, the QCSE of semi-polar and non-polar samples will be significantly smaller than that of polar ones. High E-O bandwidth micro-LEDs beyond 1 GHz based on semi-polar and non-polar substrates have been demonstrated successfully [21,24]. Even though the injection current density for high E-O bandwidth of the semi-polar and non-polar micro-LEDs is significantly reduced, the high cost and small size of semi-polar and non-polar substrates limit the mass production of high-speed LEDs. In recent years, GaN-based quantum dots (QDs), as nanomaterials with extremely strong three-dimensional quantum confinement capabilities, have received tremendous attention and have been applied in various optoelectronic devices [25–30]. For QDs, the carrier lifetime can be efficiently decreased since the reduced dimensionality of the active region [31]. In addition, the strain can usually be relaxed during the QD formation process, so the QCSE will also be significantly smaller than that of the polar QW samples [32,33]. In traditional Stranski-Krastanov (SK) mode growth of InGaN QDs, an InGaN wetting layer is unavoidably formed beneath the QD layer [26]. However, light-emitting devices utilizing the InGaN wetting layer beneath the SK QDs are rarely reported. Here, we try to use the InGaN wetting layer as the active region of high-speed LEDs based on the traditional mature c-plane GaN growth and device fabrication process. Since the formation of the wetting layer is accompanied by the three-dimensional formation of QDs, the wetting layer also undergoes sufficient strain relaxation. Hence, the carrier lifetime in the wetting layer will be much shorter due to significantly suppressed QCSE, which is beneficial for the realization of high speed LEDs at lower current densities.

In this paper, we present a 1-GHz modulation bandwidth VLC system based on a single-pixel micro-LED with an E-O bandwidth of 1.3 GHz under a current density of $528.5\mathrm{A}/{\mathrm{cm}}^2$. A 3-m VLC system with a 2-Gbps NRZ-OOK data rate with bit error rate (BER) of $1.2\times {10}^{-3}$ and with 4-Gbps QPSK-OFDM with BER of $3.2\times {10}^{-3}$ is experimentally demonstrated and analyzed. To the authors’ best knowledge, as for all the single-pixel LED-based point-to-point VLC systems, the present one achieved the highest distance-bandwidth product of 3 GHz·m and the highest distance-rate product of 12 Gbps·m using QPSK-OFDM. This work paves the way to next-generation illumination devices with InGaN nano-materials for high-speed VLC and shows great potential in actual free-space optical communication application.

In order to observe the morphology of the QDs and wetting layer, another sample without capping layer was grown, which maintained the same structure beneath the active region. The surface morphology of the sample was measured by a Bruker Dimension Icon atomic force microscope (AFM).

A 10 μm × 10 μm AFM image of the sample is shown as Fig. 2(b). In previous work, green InGaN QDs with a density around $3\times {10}^8–9\times {10}^8{\mathrm{cm}}^{-2}$ can be grown by the two-step growth interruption method [26,34]. The nominal thickness of the InGaN film is usually 2‒3 nm, and the growth temperature is between 640°C and 650°C. In this work, in order to reduce the density of QDs formed on the sample surface, the nominal thickness of the InGaN layer was reduced to 1.5 nm, and the growth temperature was also increased to 655°C to decompose the upper QDs as much as possible. The green ellipses in Fig. 2(b) indicate the remaining InGaN QDs on the sample surface, and the density of QDs is reduced to $2\times {10}^7{\mathrm{cm}}^{-2}$. Therefore, the wetting layer is distributed over most of the sample surface, and the luminescence of the sample is dominated by the wetting layer. Subsequent photoluminescence test also found that the peak wavelength of the sample is almost consistent with the wetting layer peak in the previous green QD samples [26].

In order to observe the morphology of the wetting layer sample in depth, a 1.5 μm × 1.5 μm AFM image is shown in Fig. 2(c). It can be seen that the nano-structured InGaN wetting layer appears like a broken nano-carpet. This unique nanostructure allows the wetting layer to fully release the compressive strain, thereby suppressing the QCSE and shortening the carrier life in the wetting layer. Figure 2(d) shows a high-angle annular dark-field scanning transmission electron microscope (STEM) image of the LED sample, which demonstrates that the actual thickness of each layer of the sample is quite consistent with the design. A larger magnification bright-field STEM image shows that the wetting layer has a height around 2 nm, as shown in Fig. 2(e).

The QPSK-OFDM experimental demonstration can be divided into real-time communication and off-line processing. The signal is modulated and demodulated off-line, as shown in Fig. 5. First, the NRZ-OOK data stream is generated and mapped into the QPSK-OFDM signal format with 256 carriers via a MATLAB program. The modulated serial signal is converted into parallel and then Hermitian symmetry is imposed before performing an inverse fast Fourier transform. The cyclic prefix (CP) of 1/16 is inserted into the low-speed parallel blocks which are then converted back into a serial format. In addition, in order to obtain a suitable format for demodulation, a synchronization sequence is added in front of the frame, which is then uploaded into an arbitrary waveform generator (AWG, AWG7000A, Tektronix). The AWG generates an up-sampled RF signal to conduct the real-time communication experiments. At the receiver, a high-speed sampling oscilloscope (DPO75902SX, Tektronix) is used for recording the down-sampled signal with different data rates under various injection current densities. Meanwhile, a signal analyzer (N9030B, Keysight) is used to observe the signal spectrum. Recorded data have been further processed in MATLAB. After synchronization, the high-speed serial data stream is converted into low-speed parallel data blocks and the CP has been removed. Parallel time-domain signals are transformed into frequency-domain signals by a fast Fourier transform. After further performing equalization through channel estimation, the serial QAM signal is de-mapped into a baseband signal that has been then compared with the original input signal to evaluate the BER.

Temperature-dependent time-resolved photoluminescence (TRPL) measurement is carried out to reveal the carrier dynamics of the LED sample, and the detection wavelength is set according to the emission peak. The TRPL is performed using a tunable femtosecond laser with a 380-nm excitation wavelength and 100-fs FWHM pulse width. The repetition rate and average energy of each pulse are recorded to be 8 MHz and 62.5 pJ, respectively. Figure 7(c) shows the temperature-dependent TRPL results, which show that as the temperature increases, the carrier lifetime of the sample does not change much at first, but when the temperature exceeds 150 K the carrier lifetime begins to increase. Furthermore, TRPL curves can be fitted using a bi-exponential decay model, which is defined as follows: $$I=A_1{e}^{-t/{\tau }_{\mathrm{fast}}}+A_2{e}^{-t/{\tau }_{\mathrm{slow}}},$$(1)where $I$ represents the intensity, $t$ represents time, $\tau$ is the carrier recombination lifetime, and ${\tau }_{\mathrm{fast}}$ and ${\tau }_{\mathrm{slow}}$ refer to the fast decay lifetime and the slow decay lifetime in the TRPL curve, respectively. Figure 7(d) shows ${\tau }_{\mathrm{fast}}$ and ${\tau }_{\mathrm{slow}}$ at different temperatures. According to the results, as the temperature increases from 5 to 300 K, ${\tau }_{\mathrm{fast}}$ is always maintained at around 1.4 ns, while ${\tau }_{\mathrm{slow}}$ stays at 4 ns at first, but after 150 K it quickly increases to 11.7 ns. Carrier lifetime is a key parameter for VLC devices. When it is small enough, it signifies that carriers can quickly recombine, meaning that the modulation speed of the devices can also increase. Compared to the conventional QW devices with a $\tau$ as long as several nanoseconds, which limits the device E-O bandwidth to the order of megahertz, the $\tau$ as short as 1 ns of the sample demonstrates a great potential for gigahertz-range VLC.

In our work, the following bandwidth measurements are strictly distinguished in different situations, which include the E-O bandwidth of the micro-LED device tested on wafer, the E-O bandwidth of the packaged micro-LED device, and the modulation bandwidth of the VLC system. Obviously, considering the actual application, the values of bandwidth will decrease from the micro-LED chip to the packaged micro-LED device, and then decrease from the packaged micro-LED device to the modulation bandwidth of the VLC system. Therefore, the back-to-back data transmission can obtain maximum data rate over an optical fiber. For real experimental demonstration of the VLC system, the data rate is lower than back-to-back limited by communication distance, packaging of devices, optical power, channel loss, and other factors.

The abovementioned E-O bandwidth value of 1.3 GHz for c-plane polar micro-LEDs before packaging is tested on wafer using a plastic optical fiber to export light which exhibits the LED under a radio-frequency ground-signal (RF GS) micro-probe. It is not difficult to notice that there is no communication distance in some publications, which is impossible in the real application of VLC systems [23]. Therefore, the data rate from testing on wafer by the GS micro-probe with collecting by plastic optical fiber over 0 m free-space communication distance as shown in Fig. 8(b) is totally different from a real system compared with the micro-LED-based VLC system in Fig. 6(a). The measured modulation bandwidth of 1 GHz is limited by the bandwidth of the APD receiver and influenced by the free-space channel conditions and the attenuation of the optical power, which results in the decline of bandwidth from 1.3 to 1 GHz.

Finally, we compare our wetting layer micro-LED-based VLC system with other point-to-point VLC systems based on single-pixeled LEDs which have been reported in other literature. As shown in Table 2, compared with the works of Islim et al. with a distance-rate product of $2.0566\mathrm{G}\mathrm{b}\mathrm{p}\mathrm{s}·\mathrm{m}$ and the works of Xie et al. with a distance-rate product of $1.554\mathrm{G}\mathrm{b}\mathrm{p}\mathrm{s}·\mathrm{m}$ [13,17], combining with communication distance over 3 m, our micro-LED based VLC system has obvious performance advantages including the highest distance-bandwidth product of 3 GHz·m and the highest distance-rate product of 12 Gbps·m based on the QPSK-OFDM format, and all these records can only be obtained by taking advantage of the superior performance of our micro-LED device.Table 2.

Performance of VLC Systems Based on Single-Pixeled Micro-LED (Summary of Part of Existing Works)

In conclusion, for solving the intrinsic bandwidth limitation of luminescent devices in the VLC systems, self-assembly grown nano-structured InGaN wetting layers were adopted as the active layers of a high-speed LED. A 480-nm blue micro-LED with 1.3 GHz E-O bandwidth on c-plane GaN is presented in this paper which is much higher than any other reports based on the c-plane epitaxy. The VLC system modulation bandwidth can reach 1 GHz after using the packaged micro-LED as transmitter. In addition, to give a comprehensive evaluation, we employed the micro-LEDs in a VLC system and demonstrate a 3-m data transmission over air channel, which has the highest distance-bandwidth product of 3 GHz·m among all the point-to-point VLC systems based on a single-pixel LED. By employing a $2^{31}–1$ bits NRZ-OOK signal without using non-linearity mitigation or equalization techniques, the received eye diagrams are clear when observed at 2 Gbps data rates with a low current density of $407\mathrm{A}/{\mathrm{cm}}^2$ and a BER of $1.2\times {10}^{-3}$. We achieved a record-breaking data rate among all the single-pixel LED-based point-to-point VLC systems using simple NRZ-OOK modulation. As for QPSK-OFDM, we demonstrated a data rate of 4 Gbps with a BER of $3.2\times {10}^{-3}$, which is the highest distance-rate product of 12 Gbps·m among all the point-to-point single-pixel LED-based VLC systems. It further proves the promising potential of our proposed InGaN wetting layer micro-LED in the next generation of high-speed VLC.