Direct integration of micro-LEDs and a SPAD detector on a silicon CMOS chip for data communications and time-of-flight ranging

Enhanced contact performance of high-brightness micro-LEDs via ITO/Al anode stack and annealing process

Micro-light-emitting diodes (Micro-LEDs) are a new type of display device based on the third-generation semiconductor gallium nitride (GaN) material which stands out for its high luminous efficiency, elevated brightness, short response times, and high reliability. The contact between anode layers and P-GaN is one of the keys to improving the performance of the devices. This study investigates the impact of electrode structure design and optimized annealing conditions on the anode contact performance of devices. The Micro-LED device with the size of 9.1 μm whose electrode structure is ITO/Ti/Al/Ni/Cr/Pt/Au (100/50/350/100/500/500/5000 Å) exhibits a significant improvement in contact performance after annealing under the Ar gas atmosphere at 500 °C for 5 min. The optimized device exhibited a current of 10.9 mA and a brightness of 298,628 cd/m2 under 5 V. The EQE peak value of Device A is 10.06% at 400 mA.

Proximity-Based Optical Camera Communication with Multiple Transmitters Using Deep Learning

In recent years, optical camera communication (OCC) has garnered attention as a research focus. OCC uses optical light to transmit data by scattering the light in various directions. Although this can be advantageous with multiple transmitter scenarios, there are situations in which only a single transmitter is permitted to communicate. Therefore, this method is proposed to fulfill the latter requirement using 2D object size to calculate the proximity of the objects through an AI object detection model. This approach enables prioritization among transmitters based on the transmitter proximity to the receiver for communication, facilitating alternating communication with multiple transmitters. The image processing employed when receiving the signals from transmitters enables communication to be performed without the need to modify the camera parameters. During the implementation, the distance between the transmitter and receiver varied between 1.0 and 5.0 m, and the system demonstrated a maximum data rate of 3.945 kbps with a minimum BER of 4.2×10-3. Additionally, the system achieved high accuracy from the refined YOLOv8 detection algorithm, reaching 0.98 mAP at a 0.50 IoU.

Nanosecond pulsed CMOS LED for all-silicon time-of-flight ranging

Light detection and ranging (LIDAR) is a widely used technique for measuring distance. With recent advancements in integrated photonics, there is a growing interest in miniaturizing LIDAR systems through on-chip photonic devices, but a LIDAR light source compatible with current integrated circuit technology remains elusive. In this letter, we report a pulsed CMOS LED based on native Si, which spectrally overlaps with Si detectors' responsivity and can produce optical pulses as short as 1.6 ns. A LIDAR prototype is built by incorporating this LED and a Si single-photon avalanche diode (SPAD). By utilizing time-correlated single-photon counting (TCSPC) to measure the time-of-flight (ToF) of reflected optical pulses, our LIDAR successfully estimated the distance of targets located approximately 30 cm away with sub-centimeter resolution, approaching the Cramér-Rao lower bound set by the pulse width and instrument jitter. Additionally, our LIDAR is capable of generating depth images of natural targets. This all-Si LIDAR demonstrates the feasibility of integrated distance sensors on a single photonic chip.

Integration Technology of Micro-LED for Next-Generation Display

Inorganic micro light-emitting diodes (micro-LEDs) based on III-V compound semiconductors have been widely studied for self-emissive displays. From chips to applications, integration technology plays an indispensable role in micro-LED displays. For example, large-scale display relies on the integration of discrete device dies to achieve extended micro-LED array, and full color display requires integration of red, green, and blue micro-LED units on the same substrate. Moreover, the integration with transistors or complementary metal-oxide-semiconductor circuits are necessary to control and drive the micro-LED display system. In this review article, we summarized the 3 main integration technologies for micro-LED displays, which are called transfer integration, bonding integration, and growth integration. An overview of the characteristics of these 3 integration technologies is presented, while various strategies and challenges of integrated micro-LED display system are discussed.

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.

Noise and Breakdown Characterization of SPAD Detectors with Time-Gated Photon-Counting Operation

Being ready-to-detect over a certain portion of time makes the time-gated single-photon avalanche diode (SPAD) an attractive candidate for low-noise photon-counting applications. A careful SPAD noise and performance characterization, however, is critical to avoid time-consuming experimental optimization and redesign iterations for such applications. Here, we present an extensive empirical study of the breakdown voltage, as well as the dark-count and afterpulsing noise mechanisms for a fully integrated time-gated SPAD detector in 0.35-μm CMOS based on experimental data acquired in a dark condition. An "effective" SPAD breakdown voltage is introduced to enable efficient characterization and modeling of the dark-count and afterpulsing probabilities with respect to the excess bias voltage and the gating duration time. The presented breakdown and noise models will allow for accurate modeling and optimization of SPAD-based detector designs, where the SPAD noise can impose severe trade-offs with speed and sensitivity as is shown via an example.

Optical design of InGaN/GaN nanoLED arrays on a chip: toward: highly resolved illumination

The physical laws of diffraction limit the spatial resolution of optical systems. In contrary to most superresolution microscopy approaches used today, in our novel idea we are aiming to overcome this limit by developing a spatially resolved illumination source based on semiconductor nanoscale light emitting diode (nanoLED) arrays with individual pixel control. We present and discuss the results of optical simulations performed for such nanoLED emitter arrays and analyze the theoretical limits of this approach. As possible designs we study arrays of GaN nanofins and nanorods (obtained by etching nanofin arrays), with InGaN/GaN multi quantum wells embedded as active regions. We find that a suitable choice of the array dimensions leads to a reasonably directed light output and concentration of the optical power in the near field around an activated pixel. As a consequence, the spatial resolution for this type of microscopy should only be limited by the pixel pitch, and no longer by the optical diffraction. Realization of optimized nanoLED arrays has a potential to open new field of chip based superresolution microscopy, making super-high spatial resolution ubiquitously available.

Fully Integrated Time-Gated 3D Fluorescence Imager for Deep Neural Imaging

This paper presents a device for time-gated fluorescence imaging in the deep brain, consisting of two on-chip laser diodes and 512 single-photon avalanche diodes (SPADs). The edge-emitting laser diodes deliver fluorescence excitation above the SPAD array, parallel to the imager. In the time domain, laser diode illumination is pulsed and the SPAD is time-gated, allowing a fluorescence excitation rejection up to O.D. 3 at 1 ns of time-gate delay. Each SPAD pixel is masked with Talbot gratings to enable the mapping of 2D array photon counts into a 3D image. The 3D image achieves a resolution of 40, 35, and 73 μm in the x, y, and z directions, respectively, in a noiseless environment, with a maximum frame rate of 50 kilo-frames-per-second. We present measurement results of the spatial and temporal profiles of the dual-pulsed laser diode illumination and of the photon detection characteristics of the SPAD array. Finally, we show the imager's ability to resolve a glass micropipette filled with red fluorescent microspheres. The system's 420 μm-wide cross section allows it to be inserted at arbitrary depths of the brain while achieving a field of view four times larger than fiber endoscopes of equal diameter.

Gigabit per second visible light communication based on AlGaInP red micro-LED micro-transfer printed onto diamond and glass

Full-color smart displays, which act both as a display and as a high-speed visible light communication (VLC) transmitter, can be realized by the integration of red-green-blue micron-sized light emitting diodes (micro-LEDs) onto a common platform. In this work, we report on the integration of aluminum gallium indium phosphide red micro-LEDs onto diamond and glass substrates by micro-transfer printing and their application in VLC. The device on-diamond exhibits high current density and bandwidth operation, enabled by diamond's superior thermal properties. Employing an orthogonal frequency division multiplexing modulation scheme, error-free data rates of 2.6 Gbps and 5 Gbps are demonstrated for a single micro-LED printed on-glass and on-diamond, respectively. In a parallel configuration, a 2x1 micro-LED array achieves error-free data rates of 3 Gbps and 6.6 Gbps, on-glass and on-diamond, respectively.