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Abstract
We propose and demonstrate a green semipolar (20-21) micro-light emitting diode (LED) acting as a high speed visible light communication (VLC) photodiode (PD). The micro-LED PD has the optical-to-electrical (OE) response of 228 MHz. A record data rate of 540 Mbit/s in on-off-keying (OOK) format with free-space transmission distance of 1.1 m was achieved, fulfilling the pre-forward error correction (FEC) limit. Many transmitters (Txs) and receivers (Rxs) is required to support the high density pico/femto-cells in future wireless networks, as well as the Internet-of-Things (IOT) networks. The proposed work could allow the realization of a low-cost, small-footprint and a high level of integration of VLC Txs and Rxs on the same platform.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The wireless traffic is increasing very rapidly in recent years due to the applications of 5G/6G mobile communications, Internet-of-Things (IOT), virtual reality/augmented reality (VR/AR), etc [1]. In order to support this increasing wireless traffic, higher radio-frequency (RF) bands are adopted. Carrier frequencies in millimeter wave (mm-wave) regions, such as 60 GHz or even up to 100 GHz will be necessary in the future wireless networks [2]. Besides, increasing the number of network cells and minimizing the cell size (i.e. using pico/femto-cells) can also increase the wireless network capacity and coverage. As a result, a large number of transmitters (Txs) and receiver (Rxs) is required. These Txs and Rxs should be simple and low-cost. Besides, bi-directional and parallel transmissions of highly directional pencil-like signal beams are desirable [1]. In order to provide efficient wireless transmission in the heavily loaded RF spectrum, optical wireless communication (OWC) and visible light communication (VLC) are proposed. They use the optical regions in the electromagnetic spectrum, which can provide license-free and abundant bandwidths. Besides, VLC can co-exist with the RF communication systems without introducing electromagnetic interference (EMI) to the RF devices. It can also provide highly directional, pencil-like and secure transmission beams for communication. Hence, VLC has been considered as a promising technology for the future wireless communications [3–5]. Due to low power consumption, low cost and long lifetime, light emitting diode (LED) is gradually replacing the conventional lighting devices for illumination. The relatively high modulation speed of the LED also allows the implementation of simultaneous lighting and VLC [6]. However, these LED based VLC systems have difficulty to achieve Gbit/s transmission due to the LED low intrinsic modulation bandwidth and nonlinear electrical-to-optical (EO) response. Different approaches, such as digital equalizations/filters [6,7], spectral efficient modulation formats [8–12], multi-input and multi-output (MIMO) [13,14], and multiplexing [15,16] have been proposed to increase the VLC transmission capacity.
Apart from these approaches, increasing the LED intrinsic modulation response is important. This can be realized by minimizing the chip size; hence lower capacitance and higher current density can be achieved. The micro-LED has a typical size from 1 to 100 μm, and it has been employed in information displays, as well as in VR/AR applications. Micro-LED offers the advantages of low power consumption, high brightness, low resistance-capacitance (RC) constant and high reliability; hence, it can be a promising candidate for the high data rate VLC systems. In 2017, Tian et al. reported a 300 Mbit/s and 3-m underwater VLC transmission using on-off-keying (OOK) and employing a 160 MHz bandwidth blue micro-LED [17]. In 2019, Xie et al. demonstrated a 4.81 Gbit/s and 0.3-m free space transmission using OFDM and employing a 285 MHz bandwidth blue micro-LED array [18]. In 2020, Lan et al. reported a 1 Gbit/s OOK VLC transmission using a violet micro-LED array with 615 MHz bandwidth [19]. In the same year, Chen et al. demonstrated a 1.5 Gbit/s OOK VLC transmission based on InGaN/GaN green micro-LED with bandwidth of 756 MHz [20]. In 2021, Chang et al. achieved a 4.343 Gbit/s OFDM VLC transmission using green micro-LED array with bandwidth of 1.1 GHz [21].
Micro-LED does not only provide merits in information displays and VLC Tx as discussed above, but also acts as a fast photodetector (PD). When micro-LED acts as the Tx, EO conversion is achieved by high efficient radiative recombination in the InGaN/GaN multiple quantum wells (MQWs). When the same device acts as the PD, there is no need to change the MQW structure and materials. Due to the photoelectric effect, when the MQWs are excited by input optical signal, optical-to-electrical (OE) conversion can be achieved by the photo-generated electron-hole pairs. There are few studies of employing broad area LED or micro-LED as the PD in the literatures. In 2012, Giustiniano et al. demonstrated a simple and low cost LED-to-LED communication at data rate of 870 bit/s using off-the-shelf broad area blue and red LEDs [22]. In 2014, Chun et al. demonstrated a 15 Mbit/s OOK LED-to-LED communication using RGB LEDs [23]. In 2015, Stepniak et al. reported a 100 Mbit/s OOK LED-to-LED transmission using red LED Rx with digital equalization [24]. In 2016, Miawarni et al. reported a 10 kHz square wave transmission using red laser diode (LD) Tx and parallel red LED Rx [25]. Micro-LED serving as Rx is also employed to increase the Rx data rate. In 2019, Liu et al. demonstrated a 185 Mbit/s OOK transmission using a 405 nm violet LD and 405 nm micro-LED PD, which has the OE response of 56.8 MHz [26]. In 2021, Lin et al. achieved a 60 Mbit/s underwater OOK transmission using a 450 nm blue LD and micro-LED PD with OE response of 13.98 MHz [27]. The performances of using broad area LEDs and micro-LEDs as PDs are summarized in Table 1.
Table 1. Performances of using broad area LEDs and micro-LEDs as PDs
In this work, we propose and demonstrate a green semipolar (20-21) micro-LED acting as high speed VLC PD. The micro-LED grown on semipolar or nonpolar orientations could provide higher modulation response [21,28]. 540 Mbit/s at a free-space transmission distance of 1.1 m is achieved using OOK format, fulfilling the pre-forward error correction (pre-FEC) bit-error-rate (BER) threshold (BER = 3.8 × 10−3). The result reported here offers much higher detection data rate than other works reported in the literatures. As discussed above, a large number of Txs and Rxs is required to support the high density pico/femto-cells in future wireless networks, as well as the IOT networks. The proposed work could allow the realization of low-cost, easy of mass production, small-footprint and high-level of integration VLC Txs and Rxs on the same micro-LED array platform, enabling spatial multiplexing transmissions.
2. Device structure and experiment
As discussed before, micro-LED acting as PD offers the advantages of low-cost, easy of mass production and high-level of Tx/Rx integration. As discussed above, a large number of Txs and Rxs is required to support the high density pico/femto-cells in future wireless networks. The micro-LED acting as Tx and Rx could allow the realization of low-cost VLC access point on the same micro-LED array platform. The concept of using the micro-LED array platform as Txs and Rxs providing uplink, downlink or spatial multiplexing transmissions is illustrated in Fig. 1.
Fig. 1. Concept of using the micro-LED array platform as Txs and Rxs providing parallel transmissions.
Figure 2 illustrates the cross section of our developed semipolar green micro-LED acting as PD. The micro-LED array platform consists of 2 × 4 micro-LEDs with diameter of 30 μm each. It is worth to note that the micro-LED is optimized for light emitting, and here, we use the same device for photo-detection. The LED grown on semipolar or nonpolar orientations can provide higher modulation response due to a larger overlap of electron-hole wave-function as well as shortened carrier lifetimes [21]. Moreover, lower efficiency droop due to a higher percentage of radiative to Auger recombination can be observed [29].
In the fabrication process of the proposed micro-LED array, epitaxial growth of semipolar (20−21) GaN on a (22−43) pattern sapphire substrate (PSS) was performed using low-pressure metal organic chemical vapor deposition (MOCVD). Here, inductively coupled plasma reactive-ion etching (ICP-RIE) was employed to produce the PSS. After the PSS preparation, semipolar (20−21) Ge-doped GaN was grown on top of it. An un-doped GaN layer with thickness of 10 μm was grown on it acting as a bulk layer. Then, it was planarized by chemical-mechanical planarization to form a flat surface. After this, a 1.5 μm n-type GaN, followed by an active region of three pairs of InGaN/GaN MQWs, and a 100 nm p-type GaN layer was deposited. On the p-type GaN, a 200 nm thick indium tin oxide (ITO) layer was deposited. Hydrogen chloride (HCl) and ICP-RIE were utilized to etch the ITO film and a 1 μm depth mesa. Then 400°C annealing by rapid thermal process was performed to produce a p-type ohmic contact. Then, titanium/aluminum/nickel/gold (Ti/Al/Ni/Au) were deposited as the electrodes. They have the thickness of 25/125/45/75 nm respectively. After this, a 30 nm aluminum oxide (Al2O3) passivation layer and a 200 nm silicon dioxide (SiO2) layer were grown. Then, the via holes were etched by ICP-RIE. After this, titanium/aluminum/gold (Ti/Al/Au) was deposited as sidewall reflectors and metal pads. Finally, at the back side of the device, a distributed Bragg reflector (DBR) was constructed. It is worth to note that the proposed micro-LED PD array is originally designed for light emission, and it has the similar fabrication process in [21]. Then, the fabricated 2 × 4 micro-LED array with diameter of 30 μm each was mounted on a TO-package. Figures 3(a) to (c) show the photographs of the micro-LED PD array mounted on a TO-package at different magnifications. The 30 µm micro-LED is the smallest micro-LED that can be fabricated by us. It is worth to mention that three-pair of InGaN/GaN MQWs used in the micro-LED is originally designed for light emission. Here, we use the same device for photo-detection without the need of modifying the micro-LED architecture, and using more pairs of MQW may increase the light absorption.
Fig. 3. Photographs of the 2 × 4 micro-LED PD array mounted on a TO-package at different magnifications.
Figure 4 shows the experiment setup of using the proposed semipolar micro-LED array as high speed VLC PD. At the Tx side, a bit-error-rate tester (BERT, Anritsu MP1800A) was used to generate the electrical OOK data to drive a green LD. The BERT consisted of a pulse pattern generator (PPG) generating high speed (∼12 ps rise/fall time) and low jitter (∼8 ps) pseudorandom binary sequence (PRBS) OOK waveform. It also consisted of a high sensitivity error detector (ED) (∼ 10 mV typical). The OOK data was used to drive a green LD (Thorlabs PL520), which had a peak wavelength of 520 nm via a bias-tee circuit. The produced optical signal was then received by the proposed semipolar micro-LED PD array after 1.1 m free-space transmission distance. In the experiment, the received optical power was measured by a power meter (Thorlabs PM100D). A pair of lenses was used for focusing. Here, we adjusted the optical spot size of the Rx lens so that the optical signal was launched onto the 2 × 4 micro-LED PD array. The received signal after the micro-LED PD array was amplified by a transimpedance amplifier (TIA, Maxim Integrated MAX3665). The TIA transimpedance gain is 8 kΩ and the 3-dB bandwidth is 470 MHz. It converted the small photodiode current to a differential voltage. The differential voltage was then amplified by a differential amplifier (Analog Devices ADA4937-1). The Rx circuit design was similar to the VLC Rx architecture reported in [30,31]. Then the output of the differential amplifier was connected to a digital sampling oscilloscope (DSO) (Agilent 86100A) for eye diagram detection; and also to the ED of BERT for BER measurement as illustrated in Fig. 4. Figure 4 also illustrates how the reverse bias is applied to the micro-LED PD array at the Rx side.
Figures 5(a)–5(c) show the photographs of the experimental setup, side-view and top-view of the Rx circuit respectively. As illustrated in Fig. 5(c), the micro-LED PD array was first mounted on a TO-package, which was soldered on a printed circuit board (PCB) with the TIA and differential amplifier acting as two stages amplification. The detail of the Rx circuit has been discussed above.
3. Results and discussions
We first characterized our proposed micro-LED PD array by measuring the OE responses and short-circuit current. Figure 6(a) shows the measured OE responses of the micro-LED PD at different direct-current (DC) biases. The frequency responses were measured by a vector network analyzer (VNA, Rohde & Schwarz ZND). The measured 3-dB OE bandwidths are 208 MHz and 228 MHz at bias voltages of 3.3 V and 5 V respectively. The use of 3.3 V or 5 V DC is recommended by the data sheet of TIA MAX3665. In Fig. 6(a), a peak at frequency of 388 MHz at bias voltage of 5 V is observed. This is due to differential amplifier gain characteristic, which can be observed in the data sheet of the differential amplifier ADA4937-1. As the semipolar micro-LED structure is used, the measured 3-dB OE frequency responses is much larger than the broad area LED PD and other micro-LED PD as shown in Table 1. Figure 6(b) shows the short circuit current of the micro-LED PD array. The short circuit current was measured by connecting a current meter across the two terminals of the micro-LED PD without any DC bias or external amplifier circuit. Besides, the dark current of micro-LED PD array is 3.5 × 10−10 A @ 0 V bias.
Fig. 6. Measured (a) frequency responses and (b) short circuit current of micro-LED PD array at different biases.
Finally, we evaluated the received signal quality using BER measurements. Figures 7(a)–7(b) show the experimental BER curves received by the micro-LED PD array at data rates of 200 Mbit/s and 540 Mbit/s respectively. The insets illustrate the corresponding detected OOK eye-diagrams. At the data rate of 200 Mbit/s, when the received optical power is 1.92 mW, BER of 2.91 × 10−3 is achieved, fulfilling the pre-FEC BER threshold. When the data rate is increased to 540 Mbit/s, BER of 1.64 × 10−3 is achieved at received optical power of 3.20 mW, also fulfilling the pre-FEC BER threshold. Figures 8(a)-(d) illustrate the detected OOK eye-diagrams at 200 Mbit/s, 300 Mbit/s, 400 Mbit/s and 540 Mbit/s, respectively by the micro-LED PD array. All the data rates can satisfy the pre-FEC BER threshold.
Fig. 7. Measured BERs by the micro-LED PD array at data rates of (a) 200 Mbit/s and (b) 540 Mbit/s. Insets: corresponding OOK eye diagrams.
Fig. 8. Detected OOK eye-diagrams at (a) 200 Mbit/s, (b) 300 Mbit/s, (c) 400 Mbit/s, (d) 540 Mbit/s.
4. Conclusion
We proposed and demonstrated a green semipolar (20-21) micro-LED array acting as high speed VLC PD. The measured 3-dB OE bandwidths were 228 MHz and 208 MHz at bias voltage of 5 V and 3.3 V respectively. A record data rate of 540 Mbit/s in OOK format with free-space transmission distance of 1.1 m was achieved, fulfilling the pre-FEC BER threshold. The measured BER was 1.64 × 10−3 at the received optical power of 3.20 mW. We also discussed in detail the fabrication of the semipolar green 2 × 4 micro-LED PD array and the Rx architecture. The proposed work could allow the realization of low-cost, easy of mass production, small-footprint and high-level of integration VLC Txs and Rxs on the same micro-LED array platform, enabling spatial multiplexing transmissions.
Funding
Ministry of Science and Technology, Taiwan (MOST-109-2221-E-009-155-MY3, MOST-110-2221-E-A49-057-MY3, MOST-110-2224-E-A49-003).
Acknowledgments
The authors would like to acknowledge Prof. Jun Han of Yale University and Prof. Po-Tsung Lee of National Yang Ming Chiao Tung University (NYCU).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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