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Abstract
In this work, we demonstrated a convenient and reliable method to realize the vertical stack integration of the blue and yellow InGaN micro-LED arrays. The standard white and color-tunable micro-light sources can be achieved by adjusting the current densities injection of the micro-LEDs. The spectra cover violet, standard white, cyan, etc., showing an excellent color-tunable property. And the mixed standard white light can be separated into red-green-blue three primary colors through the color filters to realize full-color micro-LED display with a color gamut of 75% NTSC. Besides, the communication capability of the integrated micro-LED arrays as visible light communication (VLC) transmitters is demonstrated with a maximum total data rate of 2.35 Gbps in the wavelength division multiplexing (WDM) experimental set-up using orthogonal frequency division multiplexing modulation. In addition, a data rate of 250 Mbps is also realized with the standard white light using on-off keying (OOK) modulation. This integrated device shows great potential in full-color micro-LED display, color-tunable micro-light sources, and high-speed WDM VLC multifunctional applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, micro-light emitting diode (LED) has attracted considerable attention due to the excellent optoelectronic properties of high brightness, fast response time, high modulation bandwidth, and low power consumption, which promotes its application, including micro-display, wireless optical communication, maskless photolithography, and micro-lighting [1–4]. However, although micro-LEDs have made significant progress in the above applications, conventional micro-LEDs mainly emit monochromatic light, which limits the desire for multi-color emission on a single-chip platform, such as full-color micro-LED display, multi-channel visible light communication (VLC), and color-tunable lighting, etc. Therefore, expanding the multi-color emission of micro-LEDs is potential and crucial. The latest studies to achieve multi-color emission of single-chip mainly include color conversion, epitaxial growth, and transfer printing techniques. The color conversion techniques mainly use the UV or blue micro-LEDs to excite red and green quantum dots (QDs) to obtain red and green light [5–7], but the long response time and poor stability of the color conversion materials limit the applications in many areas. The epitaxial growth techniques aim to obtain different color emitters, such as nanowire micro-LED arrays with different diameters or nanoring micro-LEDs with different ring thicknesses [8,9]. Nevertheless, non-uniform composition, narrow wavelength coverage, and low internal quantum efficiency are great challenges. Concerning the transfer printing technique, the process is relatively complex and high-cost [10]. Herein, some other integration techniques to realize monolithic multi-color emitting are expected.
The vertical stack integration technology seems to be promising to achieve multi-color emission. Choi et al. realized the high-resolution full-color active-matrix organic light emitting diode (AMOLED) display with TFT-driving by stacking integration technology, proving the feasibility of this method [11]. Geum et al. adopted the bonding interface engineering monolithic integration method, realizing vertically stacked integration and high-resolution micro-LED display using distributed bragg reflectors (DBR) structure as an adhesive layer and a color filter [12]. Kang et al. demonstrated the vertically stacked integration of blue and green micro-LEDs with SU8 as a passivation layer, obtaining independent controls of four-color modes [13]. However, the above-mentioned vertical stack integration schemes have some drawbacks, such as complex semiconductor processes and cumbersome steps. Here, we propose a convenient and reliable vertically stacked integration method. Firstly, high-precision up-down self-aligned equipment was utilized to focus micro-LED arrays back to back under an optical microscope, and then the integrated micro-LED arrays were bonded and packaged on a double-sided printed circuit board (PCB) to achieve the independent control, which can avoid the disadvantages of the above integration technology as much as possible.
In this work, the blue and yellow micro-LED arrays were successfully fabricated, integrated and packaged through the vertical integration method. Compared with the integration of yellow color conversion material, this integrated device mainly includes the following advantages. It is non-toxic and stable compared with the QDs, and has higher modulation bandwidth for high-speed communication [14,15]. Compared with the QDs spin coating process, the integration process in this work is convenient and reliable [16]. And the InGaN based blue and yellow micro-LEDs have good consistency and good color space uniformity while forming white light. The integrated micro-LEDs can emit light with varying colors, such as violet, standard white, cyan , etc. with the increase of injection current density due to the special multiple quantum wells (MQWs) structure of the yellow micro-LED [1]. Besides, the standard white can be separated into red-green-blue (RGB) three primary colors through color filters, enabling the device great application potential in color-tunable micro-light sources and full-color micro-LED display. Further the communication capability of the integrated device as VLC transmitters is demonstrated with a maximum total data rate of 2.35 Gbps in the wavelength division multiplexing (WDM) experimental set-up using orthogonal frequency division multiplexing (OFDM) modulation, while it is 250 Mbps using on-off keying (OOK) modulation with the standard white. This integrated device shows great potential in color-tunable micro-light sources, full-color micro-LED display and high-speed WDM VLC multifunctional applications.
2. Fabrication and characterization of the integrated micro-LEDs
The schematic diagram of the integrated blue and yellow micro-LED arrays is shown in Fig. 1(a) with a pixel diameter of 80 $\mu$m. Both the blue and the yellow micro-LEDs were epitaxially grown with a GaN buffer layer, a n-GaN layer, InGaN/GaN MQWs, an AlGaN electron block layer, and a p-GaN layer. The MQWs structure of the yellow micro-LED includes high indium content QWs (In$_{0.55}$Ga$_{0.45}$N/GaN) near the p-GaN side and low indium content QWs (In$_{0.1}$Ga$_{0.9}$N/GaN) near the n-GaN side. Before we carried out the micro-LED semiconductor process, we polished and thinned the sapphire substrates (4 inch) from 600 $\mu$m to 300 $\mu$m to optimize the fabrication processes [17]. And then indium tin oxide (ITO) thin film was deposited to form an ohmic contact after rapid thermal annealing at 550 $^{\circ }$C under nitrogen. After that, the SiO$_2$ (300 nm) was deposited as the passivation layer and the Ti/Au (50/250 nm) bilayer was deposited by magnetron sputtering as the metal electrodes. The detailed fabrication process for the micro-LEDs can be referred to our previous work [18]. Then the prepared blue and yellow micro-LED arrays were pasted back-to-back for integration. First, a thin layer of glue (Norland Optical Adhesives, NOA) was spin coated on the sapphire substrate of the yellow micro-LED array, and then the yellow and the blue micro-LED arrays were aligned under the microscope platform with high-precision vertical alignment function. The glue is transparent and colorless, and can be cured by UV light at room temperature, which is very suitable for adhesives. Then, the integrated device was embedded into the double-sided PCB with the designed aperture for fixation. Finally, the double-sided wire bonding was performed through the wire bonding machine. Figure 1(b) shows the electroluminescence (EL) spectra of the integrated blue and yellow micro-LEDs with the peak wavelengths of 456 nm and 568 nm at the current densities of 0.8 A/$cm^2$ and 35.8 A/$cm^2$, respectively. The inset is the corresponding light emission image, rendered in standard white.
Fig. 1. (a) Schematic illustration of the structure of the integrated blue and yellow micro-LED arrays with a pixel diameter of 80 $\mu$m, where the color filters are used to separate the standard white to form RGB three primary colors. (b) The emission spectra for the integrated blue and yellow micro-LEDs with the current densities of 0.8 A/$cm^2$ and 35.8 A/$cm^2$, respectively. Inset: the optical microscope image of standard white light emission for the integrated micro-LEDs.
Figures 2(a)–(b) present the voltage versus current (I-V) and light output power versus current (L-I) characteristics of the blue and the yellow micro-LEDs, respectively. The I-V characteristics tested by a current source (Keithley, 2614B) show good linearity after the turn-on for the blue and the yellow micro-LEDs. The leakage current reaches nA level under the reverse bias for both the blue and the yellow micro-LEDs, denoting the high quality of the micro-LED epilayers and the good insulation passivation during the device fabrication. The L-I characteristics were measured by an optical power meter (Thorlabs PM100D), with the light output power of 3.15 mW for the blue micro-LED and 0.104 mW for the yellow micro-LED at 50 mA. Modulation bandwidth is one of the basic parameters for VLC. AC frequency sweep signals generated by a vector network analyzer (VNA, PicoVNA106) were combined with DC signals via a bias-tee to drive the micro-LEDs and then the optical response was fed to the VNA through a Si avalanche photodiode (APD, Hamamatsu C5658). The extracted −3 dB optical modulation bandwidth versus current density of the blue and the yellow micro-LEDs are shown in Fig. 2(c), which are 87.4 and 308 MHz at the current density of 1000 A/$cm^2$, respectively. The increase in bandwidth with the increase of current density for both the blue and the yellow micro-LEDs may be attributed to the decrease in carrier lifetime at higher current density, which is consistent with the trend of blue micro-LEDs in our previous study [19].
Fig. 2. (a)-(b) I-V and L-I characteristics of the blue and yellow micro-LEDs. (c) −3 dB optical modulation bandwidth of the blue and yellow micro-LEDs as a function of current density.
3. Integrated device for color-tunable micro-light sources, full-color display and WDM VLC
With the successful stack integration of the blue and the yellow micro-LED arrays, we studied the characteristics of this integrated device as color-tunable micro-light sources and RGB full-color micro-LED display in detail. As shown in Fig. 3(a), the corresponding peak wavelength is 456 nm when the current density of the blue micro-LED (J$_B$) is set to 0.8 A/$cm^2$. When the current density of the yellow micro-LED (J$_Y$) increases from 10 A/$cm^2$ to 100 A/$cm^2$, the MQWs (In$_{0.55}$Ga$_{0.45}$N/GaN) near the p-GaN side participate in radiation recombination, and the peak wavelength of yellow micro-LED shows an obvious blue-shift from 600 nm to 553 nm, and with the further increase of the injection current density (J$_Y$) to 1000 A/$cm^2$, the MQWs (In$_{0.1}$Ga$_{0.9}$N/GaN) near the n-GaN side start to participate in radiation recombination, corresponding to a peak wavelength of 447 nm. The blueshift of the spectrum is mainly attributed to the screening of the QCSE and band filling effect [20,21]. Therefore, as the injection current density increases, the integrated device will have a wide spectral shift. The standard white light can be achieved when the current densities of the blue and the yellow micro-LEDs are 0.8 A/$cm^2$ and 35.8 A/$cm^2$ respectively, with color coordinates of (0.33, 0.33). The luminous flux of the white light is low for one micro-LED pixel, but a micro-LED array can be employed to meet the practical indoor lighting and VLC requirements. As shown in Fig. 3(b), it can be seen that with the increase of the injection current density of the yellow micro-LED, the color of the emitting light of the integrated micro-LEDs can be shifted from violet, standard white to cyan, and the color coordinates and the color temperature also show large shifts, indicating excellent color-tunable properties.
Fig. 3. (a) The spectrum of the integrated micro-LEDs when the current density of blue micro-LED is 0.8 A/$cm^2$ and the current density of yellow micro-LED increases from 10 A/$cm^2$ to 1000 A/$cm^2$. (b) The corresponding color coordinate distribution and luminous color diagram in CIE 1931. (c) The RGB spectrum is separated by color filters from standard white. (d) Color coordinates and color gamut distribution of the separated RGB spectrum in CIE 1931.
Furthermore, to explore the potential of RGB full-color micro LED display, the color filters with central wavelengths of 600 nm, 520 nm, 450 nm, and the bandwidth of 15 nm were used to separate the standard white light of the integrated micro-LED arrays. The color filters were placed on the blue micro-LED side, mainly considering the light output power of the blue micro-LED side is larger than the yellow micro-LED side, and the blue micro-LED will not absorb the emitted yellow light from the yellow micro-LED, which will lead to better display and communication performance. As shown in Fig. 3(c), the peak wavelengths of the separated spectrum bands are 590 nm, 524 nm and 455 nm with the corresponding full width at half maximums (FWHMs) of 36, 13 and 15 nm, respectively. The color coordinate distribution and the corresponding color emission are shown in Fig. 3(d), and the color gamut in CIE 1931 is 75$\%$ NTSC, indicating great potential in RGB full-color micro-LED display of this integrated device.
Besides the application in color-tunable micro-light sources and full-color micro-LED display, the integrated device has great potential in WDM VLC. The communication capability of the integrated device to operate as double-color WDM transmitters was explored in the following part. We tested the communication data rate under two conditions. One is under the optimal currents which maximize the data rate, and the other is under the specific currents at which the blue and the yellow micro-LEDs form standard white light emission. OFDM modulation is used for communication under the optimal currents, and OOK for the white light communication.
The schematic diagram of the experiment setup using OFDM signals is shown in Fig. 4. The point to point transmission is an optimistic scenario for VLC application. In practical mobile communication applications, the receiving direction is usually random, and such issue could be addressed by many techniques such as adopting tracking system [22,23]. The OFDM digital signals were offline generated in MATLAB, and then was uploaded into an arbitrary waveform generator (AWG, Tektronix AWG710B), which converted the digital signals to analog signals. An electrical amplifier (EA, Mini-circuit ZHL-2-12X+) was used to amplify the output signals from AWG for optimal performance. Through a bias-tee (Mini-circuit ZFBT-6GW), the OFDM analog signals were biased by a direct current (DC) signals from a DC source (Yokogawa GS610) to become positive to drive the integrated blue and yellow micro-LEDs. The optical signals were collimated and focused by two lenses to be collected onto a high sensitivity 1 GHz APD (Hamamatsu C5658). The APD converted the light signals into the electrical signals, which were captured by a high-speed digital signal analyzer (DSA, Agilent DSA90604A). Finally, the received data were offline demodulated and analyzed with a MATLAB program to calculate the bit error rate (BER), constellation diagram, and other parameters. The main parameters based on the proposed WDM VLC system are listed in the Table 1.
Fig. 4. Photograph and schematic diagram of the OFDM experimental setup of the 0.3 m free-space VLC system based on integrated blue and yellow micro-LEDs.
Table 1. The main parameters based on the proposed WDM VLC system.
The OFDM transmitter (Tx) module and receiver (Rx) module were also demonstrated in Fig. 4. For the Tx module, the binary data were firstly mapped into quadrature amplitude modulation (QAM) format. We used 16 QAM for the blue micro-LED and 8 QAM for the yellow micro-LED. Real-value signals were obtained by Hermitian symmetry. The zero forcing pre-equalization was utilized in the frequency domain to expand the signal bandwidth [24]. First, we obtained the frequency response function of the channel through channel estimation. Then, before the next data transmission, we multiplied the inverse transform of the frequency response by the data to offset the channel frequency response. Then up-sampling at 2x, inverse fast Fourier transform (IFFT), and adding circle prefix (CP) of 1/32 OFDM frame’s length were performed on the time domain signals. For the Rx module, the signals from DSA were processed by removing CP, FFT, down-sampling, and QAM demapping. Channel estimation was also used to reduce BER.
We tested the communication performance of the blue and the yellow micro-LEDs under the transmission distance of 0.3 m. The distance could be extended by using a micro-LED array with higher light output power [25]. Driving currents and peak-to-peak voltages (V$_{pp}$) were optimized to obtain the lowest BER. The light output power of the micro-LED increase with current or voltage is not linear and higher driving current leads more serious nonlinear effect. However, our micro-LEDs have higher bandwidth at higher currents. Hence, the current has an optimal value in considering the balance of reducing the nonlinear effect and improving the bandwidth. The V$_{pp}$ of the received signal increases as the V$_{pp}$ of the signal voltage increases, but it will also bring more nonlinear effects. Therefore, the V$_{pp}$ has an optimal value in considering the balance of reducing the nonlinear effect and increasing the strength of received signal. The tested BER under different currents and V$_{pp}$ of the OFDM temporal signal is presented in Fig. 5. The lowest points in Figs. 5(a) and 5(b) were chosen as the optimal drive current and V$_{pp}$ of OFDM signals for the subsequent VLC test, which were 35 mA and 0.16 V for the blue micro-LED, and 35 mA and 0.35 V for the yellow micro-LED, respectively.
Fig. 5. (a) BER versus drive current and (b) BER versus V$_{pp}$ for the blue and the yellow micro-LEDs, respectively.
Under the optimal operating condition, the BER versus data rate of the WDM VLC system for the yellow micro-LED is shown in Fig. 6(a) with the constellation diagrams corresponding to the data rates of 1.35 Gbps, 1.5 Gbps, 1.65 Gbps and 1.8 Gbps, while the Fig. 6(b) is the BER versus data rate for the blue micro-LED with the constellation diagrams corresponding to the data rates of 400 Mbps, 600 Mbps, 700 Mbps and 800 Mbps. It can be seen from both Figs. 6(a) and 6(b) that the measured BER becomes higher with the increase of data rate. The data rate is 1.65 Gbps with a BER of $2.9{\times }10^{-3}$ for the yellow micro-LED, while it is 700 Mbps with a BER of $3.4{\times }10^{-3}$ for the blue micro-LED. Thus, a WDM-OWC system with an aggregated data rate of 2.35 Gbps can be obtained. Figures 6(c) and 6(d) show the spectrum of the OFDM signal with a signal bandwidth of 550 MHz for the yellow micro-LED and that with a bandwidth of 175 MHz for the blue micro-LED, respectively. Because the −3 dB optical modulation bandwidth of the blue micro-LED is much lower than that of the yellow micro-LED, the signal bandwidth and data rates are also lower than that of the yellow micro-LED. As shown in Figs. 6(c) and 6(d), the light output powers at the receiver side were 1.28 mW for the blue micro-LED and 31.3 $\mu$W for the yellow micro-LED. Here, the light out power of the yellow micro-LED is relatively low, mainly due to the high crystal defects and strong quantum-confined stark effect (QCSE) of the high indium MQWs [26]. Techniques including employing DBR and developing a micro-LED array have been reported to improve the micro-LED efficiency and VLC performance [25,27].
Fig. 6. In WDM VLC system with the yellow and blue micro-LED as the transmitters: (a)-(b) the tested BER versus data rate, (c)-(d) the frequency spectrum. respectively.
We also measured the communication performance under the specific currents with standard white emission for the integrated device. The currents were 0.04 mA for the blue micro-LED and 1.8 mA for the yellow micro-LED, respectively. Because the currents of both micro-LEDs are relatively small in this case, small light output power and large nonlinear effect will significantly degrade the communication performance of OFDM signals, so the OOK signals were adopted here. Data rates of 200 Mbps and 50 Mbps with the corresponding BERs of $4.1{\times }10^{-4}$ and $3.7{\times }10^{-4}$ were achieved for the yellow and the blue micro-LEDs, respectively. The eye diagrams are presented in Fig. 7(a) and 7(b), where the lower half of OOK signals are cut off due to the minuscule current for blue micro-LED. However, the signals can be demodulated, because the low level is still below the decision threshold. Based on the blue micro-LED and the yellow micro-LED, the white light WDM-OWC system can achieve an aggregated data rate of 250 Mbps.
4. Conclusions
In summary, we have successfully realized vertical stack integration of the blue and yellow InGaN micro-LED arrays through a convenient and reliable alignment method. The integrated device shows great potential for color-tunable micro-light sources with broad-spectrum and adjustable color temperature. The white light of each micro-LED pixel can be separated into RGB three primary colors through color filters to realize RGB full-color micro-LED display. Moreover, the integrated device as VLC transmitters in the WDM experimental set-up is demonstrated with a maximum data rate of 2.35 Gbps. For future practical applications, especially for indoor scenario, a micro-LED array may be employed to provide high light output power and large cover areas from the ceiling for mobile VLC. This work suggests the promising applications of the vertically integrated multi-color micro-LED arrays in color-tunable light source, full-color display and visible light communication.
Funding
National Key Research and Development Program of China (2021YFB3601000, 2021YFB3601003, 2021YFE0105300); National Natural Science Foundation of China (61974031); Science and Technology Commission of Shanghai Municipality (21511101303); Leading-edge Technology Program of Natural Science Foundation of Jiangsu Province (BE2021008-2).
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 may be obtained from the authors upon reasonable request.
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