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Abstract
This work presents a novel all-in-one Micro-LED pixel (µLEDP) technology by integrating red-green-blue super-pixels (RGBSP) in a single unit cell. Measurement results show that the proposed µLEDP delivers excellent optical and electrical characteristics, including wide color gamut (109% NTSC), wide correlated color temperature range (2831.7-10016.8 K), and high modulation system bandwidth (58-62 MHz). To the best of our knowledge, the proposed integrated µLEDP achieves the highest data rate compared to published results based on other multi-color low-capacitance high-bandwidth LEDs. The maximum simulated non-return-to-zero (NRZ) and 4-level pulse-amplitude-modulation (PAM-4) data rates of 0.3-Gb/s and 1.1-Gb/s, respectively.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapidly growing demand for cost-effective ultra-fine resolution display, GaN-based Micro-LEDs have garnered wide attention and research effort [1–5]. As shown in Fig. 1, the screen pixel pitch of popular electronic products, such as iPad and Apple Watch, is steadily scaling smaller to offer an improved visual experience and hence has become an inevitable market trend. On the other hand, due to its small size, high current density, and short carrier lifetime, Micro-LED exhibits a superior high modulation bandwidth. Micro-LED-based display can also support novel communication functionalities for ONFC. It makes it possible to realize high-quality display and high-speed ONFC function on the same consumer electronic product simultaneously.
Fig. 1. An example of RGB Micro-LED array-based smartwatch for simultaneous display and optical near field communication (ONFC) applications. The market trend of pixel pitch in popular electronic products.
In recent years, the reports on ONFC have mainly focused on the custom design of the driving circuit or the performance improvement of the light source. They have not been deeply explored from the system level of practical applications. In terms of driving circuit design, researchers have made great attempts [6–10]. By improving the structure of the driving circuit and applying various bandwidth expansion techniques, the data rate has been improved to a certain extent. However, due to the limitation of light sources, the finally realized data rate is still low and cannot meet the standard of high-speed ONFC. In the report of [11–14], the performance of Micro-LED as a communication light source has been significantly improved. But the design of the corresponding driving circuit is not considered, and the impact of the parasitic parameters that the driving circuit may bring in practical deployments are ignored. Moreover, the proposed light sources are monochromatic or dual-color, which cannot achieve full-color display. The ONFC function is challenging to adapt to the existing consumer display devices, which greatly reduces the possibility of practical application. Compared with the green and blue Micro-LEDs, the substrates of red Micro-LEDs are different. After the monochromatic light source is fabricated, it needs to be transferred to a common panel through fine transfer technology, making the integration technology very challenging. There is research on the transfer printing technology of red Micro-LED and its application in communication [15]. However, there is no relevant report on deploying integrated RGB super-pixels in high-speed ONFC communication to the best of our knowledge.
This work proposes a novel µLEDP technology to address market needs and emerging applications. Superior pixel-level optical and electrical properties are reported based on measurement data. Furthermore, an in-house cross-domain co-simulation tool Integrated System Evaluation Engine (ISEE) [16] is used to simulate an ONFC system based on the compact equivalent circuit model of super-pixels to verify the ONFC functionality. The results show that this µLEDP technology with integrated RGB super-pixels has the potential to realize high-quality display and high-speed ONFC communication simultaneously.
2. Fabrication process
The 200 µm × 100 µm red super-pixels are fabricated using AlGaInP MQWs on a GaAs substrate by metalorganic chemical vapor deposition (MOCVD). As shown in Fig. 2(a), the glue is used to bond the epilayer of sapphire substrate on the top of p-type GaP. The GaAs substrate is removed by wet chemical etching to expose the n-type GaP layer, as shown in Fig. 2(b). The following steps are shown in Fig. 2(c), where a layer of SiO2 as hard mask photolithography defines the mesa and inductively coupled plasma (ICP) etching is used to control the thickness of p-type GaP layer. After that, a 200 nm Au/Ge/Ni layer and a 200 nm Au/Zn layer are evaporated by electron-beam evaporation on the n-type GaP layer and p-type GaP layer to form the cathode and anode. Using plasma-enhanced chemical vapor deposition (PECVD), a 600 nm SiO2 layer is deposited as the passivation layer. Finally, the indium n-pad and p-pad are deposited on n-GaP and p-GaP as electrodes by electron-beam evaporation.
Fig. 2. (a-f) Fabrication process flow of AlGaInP red super-pixel (top) and InGaN green/blue super-pixel (bottom) in the proposed all-in-one µLEDP.
The 200 µm × 100 µm green and blue super-pixels are fabricated using InGaN MQWs grown on a sapphire substrate by MOCVD, as shown in Fig. 2(d). The mesa structures are defined by photolithography and ICP etching with a hard SiO2 mask. Then, a 10 nm Ni/Au layer is evaporated as the current spreading layer (CSL). An ohmic contact between p-GaN and CSL is achieved by rapid thermal annealing (RTA) technology at 570 °C for 5 min with N2:O2 = 4:1 (10 sccm for both N2 and O2), which is shown in Fig. 2(e). A layer of 200 nm thickness Ti/Al/Ti/Au is subsequently evaporated by electron-beam evaporation on the CSL layer and n-GaN layer to form the p- and n-electrodes. The SiO2 layer is deposited as the passivation layer, and the indium p-pad and n-pad are deposited on p-GaN and n-GaN, as shown in Fig. 2(f). The detailed fabrication process can refer to [17].
After fabricating the solder bumps and dicing the wafer into individual chips using laser, the RGBSP are individually transferred and bonded at 50 µm pitch on the metal pattern of a common p-pad structure to integrate the µLEDP with a flip-chip configuration, as depicted in Fig. 3. The size of packaging unit is 0.79 mm × 0.79 mm × 0.65 mm. The encapsulation material is black silicone to improve the contrast ratio of the display.
Fig. 3. (a) 3D perspective view of the µLEDP internal package structure illustrates the red/green/blue emitting output light and the package dimension. (b) The microscope graph of the integrated RGB super-pixels in the proposed all-in-one µLEDP.
3. Results and discussion
3.1 Electrical characteristics
Figure 4 shows the voltage-current (V-I) and light output power-current (L-I) characteristics of the µLEDP measured by Keysight B2902A precision source/measure unit and GL SPECTIS 1.0 Touch handheld spectrometer, respectively. The turn-on voltage of RGBSP is 1.44, 2.00, and 2.33 V when current density is 10−4 A/cm2, and the ideal factor can reach 1.79, 1.76, and 1.74 under the bias condition of 0-1.66, 0-2.43, and 0-2.70 V, respectively. When RGBSPs are biased at 20/55/55 mA, the peak light power recorded is 1.86/2.08/4.33 mW, respectively.
3.2 Optical characteristics
Figure 5(a) illustrates the electroluminescence (EL) of the RGBSP measured by JETI Spectraval 1501 spectroradiometer. The wavelength and full-width half of maximum (FWHM) are 640 nm and 23 nm for red pixel biased at 30 mA, 515 nm and 38 nm for green pixel biased at 55 mA, and 463 nm and 28 nm for blue pixel biased at 55 mA, respectively. Due to its narrow emission wavelength, the color gamut formed by these three-color pixels can reach 109% NTSC, which is very good performance for display application, as shown in Fig. 5(b). In the case of fine bias control to adjust the ratio of each color intensity, the µLEDP can achieve different correlated color temperatures (CCT). Figure 5(b) shows the example: the µLEDP can light as warm white (CCT: 2831.7 K) (LR:LG:LB = 1:0.28:0.43), neutral white (CCT: 4416.1 K) (LR:LG:LB = 1:0.33:0.50) and cool white (CCT: 10016.8 K) (LR:LG:LB = 1:0.33:0.75). This implies that the µLEDP can also be applied to a wide range of lighting scenarios for the next generation of smart lighting applications [18]. Figure 6 demonstrates the measured light distribution curve of each color pixel showing their great half-peak angles reaching almost 50°. The asymmetric light distribution is due to the pitch between the RGBSP.
Fig. 5. (a) Measured electroluminescence and FWHM of the RGBSP. (b) Color gamut and color temperature curve of the µLEDP. The color gamut can achieve 109% NTSC, 114% Adobe RGB, and 154% sRGB.
3.3 Performance of ONFC system
Figure 7 shows the experimental setup of the system S-parameter measurement. The µLEDP is wire-bonded on the PCB test fixture and connected to an SMA connector. A precision source/measure unit Keysight B2902A provides a DC bias current for the µLEDP for setting a stable operating point. A R&S ZVB8 vector network analyzer (VNA) generates an AC signal for measuring the small-signal frequency response. The DC bias and AC signal are coupled through a bias-T (ZFBT-6GW+) and sent to the µLEDP through the SMA connector. The light emitted from µLEDP is collimated by an optical lens, and the transmission distance is 15 cm. In the receiver end, a photodetector (PD) (Hamamatsu Photonics APD module C12702-11) is used to detect the light signal, convert it to the electrical signal, and feed it into the VNA. The VNA is calibrated before the measurement to eliminate the effect of equipment, ensuring that the results are only affected by the entire ONFC system.
The measured system bandwidth of RGBSP is shown in Fig. 8(a). The bandwidth of ONFC system increases with the diode bias current. When the RGBSP are biased at 45 mA, 70 mA and 75 mA, the system bandwidth reaches 62 MHz, 58 MHz, and 61 MHz, respectively, which are sufficiently high for practical ONFC system implementation. It is noted that due to severe parasitic existing in the PCB test fixture and the package, the intrinsic bandwidth of each RGBSP is estimated to be over 100 MHz. Further measurements, with improved PCB test fixture, will be conducted to allow more detailed characterization. The modulation bandwidth of RGBSPs in this report still has room to be improved due to the limitation of transferring sub-pixels from different substrates. The size of RGBSPs was chosen based on the consideration of display and communication link performance.
Fig. 8. (a) Measured ONFC system bandwidth under different current biases of the RGBSPs. (b) Measured BERs under different data rates (NRZ) of the RGBSPs.
A pseudo-random binary sequence-11 (PRBS-11) is generated by a bit-error-rate (BER) tester (Agilent E8403A) to measure the BERs of RGBSP separately. The results under different data rates are demonstrated in Fig. 8(b). At the forward-error-correction (FEC) limit of 3.8e-3 [19], the results indicate that the highest achievable transmission data rates of the red/green/blue diode in RGBSP are 220-Mb/s, 170-Mb/s and 195-Mb/s, respectively. At BER of 10e-9, the measured data rates of RGBSP are 201/148/179-Mb/s, respectively.
4. Equivalent model of µLEDP
A compact equivalent circuit model [20–25] of µLEDP is developed based on each RGBSP to help evaluate the overall ONFC link performance under different circuit architectures and signaling schemes. Figure 9(a) shows the schematic of the compact model, which consists of two parts: the input electrical part described by an equivalent circuit, and the output optical stage defined by a first-order low-pass filter. The current-independent components in the equivalent circuit represent the PCB test fixture, wire bonds, and the package, while the current-dependent ones model the contact resistor Rs, the active region resistance Rd and capacitance Cj. The time constant τ of the low-pass filter is also a function of the current. The values of these components and their parameters are extracted and curve-fitted from the measured S-parameters under different bias conditions. The measured and modeled small-signal frequency responses of the green super-pixel are in excellent agreement as shown in Fig. 9(b), under all bias conditions of interest.
Fig. 9. (a) The proposed compact equivalent circuit model of the RGBSPs in the µLEDP. (b) Measured vs. modeled S21 of green super-pixel under bias of 20 mA and 70 mA.
To evaluate the data transmission performance of the µLEDP, a NRZ and a PAM-4 ONFC system is designed and implemented using ISEE [16] using the compact models described above as the light sources. In the NRZ ONFC system, the RGBSP are driven separately using three transmitters (NRZ-TXs). The NRZ-TX consists of a pre-driver (Pre-Drv), a main driver, and a current-mode equalizer (CMEQ), as shown in Fig. 10(a)-(b). In the PAM-4 case, each super-pixel is driven by the combined current from three NRZ-TXs controlled by 3-bit thermometer codes. With the CMEQ, an error-free NRZ eye and PAM-4 eye can be attained using the red, green and blue super-pixel at 200-Mb/s and 300-Mb/s, respectively, while producing an optical modulation amplitude of 0.97 dBm, as presented in Fig. 10(c)-(e). The optical power over different data rates and transmission distances are computed for both NRZ and PAM-4 cases. Figure 11 shows the calculation results for the red super-pixel. With reference to a typical optical receiver sensitivity of -32 dBm [26,27], the relation between the highest achievable data rate versus the transmission distance is shown in the projected curve on the X-Y plane in Fig. 11. As the transmission distance decreases, the received optical power increases, and therefore the highest achievable data rate increases. This trend is consistent with Shannon’s theory. At 4.0 cm, the µLEDP ONFC link distance, the aggregated data rate from the RGBSP can reach 1.1-Gb/s using PAM-4. Alternatively, the range can be increased to 8.0 cm by using simpler NRZ modulation, however, the data rate drops to 0.3-Gb/s.
Fig. 10. (a) NRZ and (b) PAM-4 ONFC system block diagram in ISEE simulation [14]. (c-e) Comparison of 200-Mb/s NRZ eyes and 300-Mb/s PAM-4 eyes with and without enabling current mode equalization, using the red, green and blue super-pixel.
Fig. 11. The received optical power at different data rates and distances, using red super-pixel, assuming -32-dBm optical receiver sensitivity: (a) NRZ; (b) PAM-4.
5. Conclusion
In this work, we demonstrated a novel all-in-one µLEDP technology by integrating RGBSP in a single unit cell. It has 109% NTSC color gamut, 2831.7-10016.8 K CCT range and 58-62 MHz modulation system bandwidth. With the compact equivalent model of µLEDP, the NRZ and PAM-4 ONFC system are simulated. The maximum NRZ and PAM-4 data rates can be recorded as 0.3-Gb/s and 1.1-Gb/s. These excellent performances show that the µLEDP can achieve simultaneous display and ONFC functions. Furthermore, with its wide CCT range, µLEDP is also well suited for general lighting applications.
Funding
Hong Kong Research Grants Council under General Research Fund (GRF) project (No. 16200419); Foshan-HKUST Project (No. FSUST20-SHCIRI05C); Hetao Shenzhen-Hong Kong Science and Technology Innovation Cooperation Zone (HZQB-KCZYB-2020083).
Acknowledgments
The authors would like to thank the HKUST Nano Fabrication Facilities and Shenzhen Sitan Technology Limited for providing the fabrication and testing facilities.
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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