Wing Cheung Chong

A Novel BLU-Free Full-Color LED Projector Using LED on Silicon Micro-Displays

In this letter, we have described the design and fabrication of a novel backlight-unit (BLU)-free full-color light emitting diode (LED) based projector. The prototype used three active matrix addressable light emitting diode on silicon (LEDoS) micro-displays with peak emission wavelengths of 630, 535, and 445 nm. The LEDoS micro-displays were realized by integrating monolithic micro-LED arrays and silicon-based integrated circuits using a flip-chip bonding technique. Since the LEDoS micro-displays are self-emitting, conventional BLUs used in liquid crystal displays were not needed. Using a trichroic prism to combine the light from the three LEDoS chips, we have produced the worlds first three-LEDoS projector. This BLU-free three-LEDoS projector consists of much fewer optical components and has significantly higher light utilization efficiency compared with conventional projectors.

360 PPI Flip-Chip Mounted Active Matrix Addressable Light Emitting Diode on Silicon (LEDoS) Micro-Displays

In this paper, we describe the design and fabrication of 360 PPI flip-chip mounted active matrix (AM) addressable light emitting diode on silicon (LEDoS) micro-displays. The LEDoS micro-displays are self-emitting devices which have higher light efficiency than liquid crystal based displays (LCDs) and longer lifetime than organic light emitting diodes (OLEDs) based displays . The LEDoS micro-displays were realized by integrating monolithic LED micro-arrays and silicon-based integrated circuit using a flip-chip bonding technique. The active matrix driving scheme was designed on the silicon to provide sufficient driving current and individual controllability of each LED pixel. Red, green, blue and Ultraviolet (UV) LEDoS micro-displays with a pixel size of 50 μm and pixel pitch of 70 μm were demonstrated. With a peripheral driving board, the LEDoS micro-display panels were programmed to show representative images and animations.

High-Performance Green and Yellow LEDs Grown on

High-performance GaN-based green and yellow light-emitting diodes (LEDs) are grown on SiO2 nanorod patterned GaN/Si templates by metalorganic chemical vapor deposition. The high-density SiO2 nanorods are prepared by nonlithographic HCl-treated indium tin oxide and dry etching. The dislocation density of GaN is significantly reduced by nanoscale epitaxial lateral overgrowth. In addition to the much improved green LED (505 and 530 nm) results, the fabricated yellow (565 nm) InGaN/GaN-based multiquantum well (MQW) LEDs on Si substrates are demonstrated for the first time. High-quality GaN buffer and localized states in MQWs are correlated to obtaining high-efficiency long-wavelength emission in our devices.

{\rm SiO}_{2}

Colloidal quantum dots which can emit red, green, and blue colors are incorporated with a micro-LED array to demonstrate a feasible choice for future display technology. The pitch of the micro-LED array is 40 μm, which is sufficient for high-resolution screen applications. The method that was used to spray the quantum dots in such tight space is called Aerosol Jet technology which uses atomizer and gas flow control to obtain uniform and controlled narrow spots. The ultra-violet LEDs are used in the array to excite the red, green and blue quantum dots on the top surface. To increase the utilization of the UV photons, a layer of distributed Bragg reflector was laid down on the device to reflect most of the leaked UV photons back to the quantum dot layers. With this mechanism, the enhanced luminous flux is 194% (blue), 173% (green) and 183% (red) more than that of the samples without the reflector. The luminous efficacy of radiation (LER) was measured under various currents and a value of 165 lm/Watt was recorded.

Nanorod Patterned GaN/Si Templates

Novel gallium nitride-based flip-chip light-emitting diodes (FCLEDs) with high-density distributed n-type point-contacts were designed and fabricated. With high density and uniformly distributed n-type point-contacts, the point-contact (PC) FCLEDs had higher light output power (LOP) by 18% over the reference flip-chip LED with conventional contacts fabricated from the same wafer. The forward voltage of the PC-FCLEDs was 0.16 V lower than the reference FCLED and the wall-plug efficiency was increased by 24% at the same current level. The maximum LOP of the PC-FCLEDs measured at 2.4 A was 43% more than the maximum obtained by the reference LED at 1.8 A. It was also found that the PC-FCLEDs suffered lower efficiency droop. The optical performance improvement of the PC-FCLEDs is attributed to an increase of the light extraction and the uniform carrier distribution, which results from the small and high density deeply etched holes and PCs. The electrical performance was enhanced through a minimized lateral current spreading distance.

Resonant-enhanced full-color emission of quantum-dot-based micro LED display technology

This letter reports the design and fabrication of optimized electrode structures for matrix addressable monolithic light-emitting diode arrays. The variation of forward voltages of LED pixels in the same row are greatly reduced from 2.62 V (81.9%) to 0.02 V (0.6%) in an 8×8 LED array. The LED arrays are designed with 8×8 square pixels, 500×500 μm2 with a 50-μm wide gap in between. A 24×24 large-scale blue LED panel is demonstrated with 0.06 V (1.8%) forward voltage variation by integrating nine LED array modules on a 2.2-cm diagonal silicon-based substrate. With a similar concept, a 30×30 green LED micro-display with scaled pixel pitch shows excellent display uniformity.

Performance Enhancements of Flip-Chip Light-Emitting Diodes With High-Density n-Type Point-Contacts

In this paper, the first full-color active matrix programmable monolithic Light Emitting Diodes on Silicon (LEDoS) displays are fabricated using flip-chip technology. The forward voltage uniformity of the LED pixels was greatly improved by a double-side ground structure. A basic LED micro-array module was fabricated and the AM substrate has scaling-up ability. By integrating a certain number of LED micro-array modules onto a large-scale AM substrate, a large-scale display is obtained. By this scaling-up method, the utilization rate of the LED wafers is increased significantly. The yield of the LED pixels is improved simultaneously. Red, green and blue phosphors were excited by UV light to realize a full-color display.

Investigation of Forward Voltage Uniformity in Monolithic Light-Emitting Diode Arrays

In this paper, we investigated leakage current generated by light emission of micro-light emitting diode (LED) pixels in active matrix (AM) micro-LED displays. Customdesigned structures of metal cover with different coverage lengths were designed and fabricated to completely eliminate the photongenerated leakage current, thus enhancing the performance of AM driving circuits. Working principle of two transistors and one capacitor AM pixel and requirement of its driving transistors were also systematically discussed for liquid crystal displays, organic LED displays, and LED microdisplays. Transistors with the optimized metal-cover design have an ON/OFF-current ratio of 108, which is two orders of magnitude higher than those without metal cover. This concept was successfully implemented in a high-resolution and fine-pitch AM micro-LED display with a resolution of 400 × 240 and a pixel pitch of 30 μm. Vivid images and movies were presented to show the great potential for wearable applications.

P-34: Active Matrix Programmable Monolithic Light Emitting Diodes on Silicon (LEDoS) Displays

This paper presents the design and characterization of the first active matrix light-emitting diode (AMLED) microdisplay with an embedded visible light communication (VLC) transmitter, enabling LED digital signage for location-based applications such as information broadcasters and indoor positioning beacons. The driver system-on-a-chip (SoC) integrates four identical macro-cells, each containing a pixel driver array, a row driver, a column driver, and a first-in first-out memory, to drive a wide quarter-VGA (WQVGA) display featuring 400 × 240 blue micro-LED (μLED) pixels fabricated on a single gallium nitride (GaN) substrate. The size of each μLED pixel is 30 × 30 μm2. At the system level, pulse-width modulation (PWM) superimposed with on-off keying modulation is proposed to accomplish grayscale control for display and simultaneously transmit VLC signal by modulating the μLED array. At the circuit level, a pixel driver cell composed of three transistors and one capacitor (3T1C) with a novel VLC function is employed to implement the control scheme. Flip-Chip bonding is adopted to establish connections between the WQVGA microdisplay and the AMLED driver SoC. Implemented in a 0.5-μm 2P3M CMOS process, the driver SoC enables a high-resolution microdisplay module to achieve 4-bit grayscale at a 100-Hz frame rate, while supporting 1.25-Mb/s VLC for a bit error rate <;10-5 up to 25-cm distance without a lens. When using optical lenses, the VLC distance is extended to >500 cm.

Investigation of Photon-Generated Leakage Current for High-Performance Active Matrix Micro-LED Displays

We report the first 1700 pixels per inch (PPI) passive-matrix blue light-emitting diodes on silicon (LEDoS) micro-displays. By flip-chip bonding a micro-LED array onto an ASIC display driver, we successfully fabricated a 0.19-inch display with a resolution of 256 x 192, the highest ever reported in LED-based micro-display. In addition, the LEDoS micro-display can deliver brightness as high as 1300 mcd/m2 and render images in 6-bit grayscale. The remarkable performance suggests the tremendous potential of LEDoS micro-displays for portable display applications which require high performance, small size and low power consumption.