Current Research


Monolithic integration of 1.55um QD lasers on Si

Miniaturized laser sources can benefit a wide variety of applications ranging from on-chip optical communications and data processing, to biological sensing. There is a tremendous interest in integrating these lasers with rapidly advancing silicon photonics, aiming to provide the combined strength of the optoelectronic integrated circuits and existing large-volume, low-cost silicon-based manufacturing foundries. Using III-V quantum dots as the active medium has been proven to lower power consumption and improve device temperature stability. Here, for the first time, our group has demonstrated room-temperature InAs/InAlGaAs quantum-dot subwavelength microdisk lasers epitaxially grown on (001) Si, with a lasing wavelength of 1563 nm, an ultralow-threshold of 2.73 μW, and lasing operation beyond 60 °C under pulsed optical pumping. This result unambiguously offers a promising path towards large-scale integration of cost-effective and energy-efficient silicon-based long-wavelength lasers.

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GaAs Quantum Dot Laser on Si

We utilized InAs QDs monolithically grown on pre-patterned on-axis (001) Si substrates to achieve room-temperature continuous-wave (CW) lasing in a micro-disk resonator. The micro-disk cavities allow for rapid feedback of the QD performance and promise high modal gain and dense integration for short-reach communication links.

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Design and Fabrication of High-voltage LED (HVLED) for the LED System-on-Chip (SoC) Application

To improve the brightness of GaN light emitting diode (LED) at high-input power conditions, one approach is to increase LED chip size to alleviate the notorious efficiency droop. Another approach is to connect in series a number of LEDs onto a single board or into a single chip and to operate those LEDs in a high voltage low current mode. Such an operation mode enables the high voltage LED (HVLED) to achieve higher efficiency than the conventional LED at the same input power density for the LED System-on-chip application. The high efficiency of HVLED also translates to easier thermal management..

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Monolithic integration of AlGaN/GaN high electron mobility transistors (HEMTs) and InGaN/GaN light emitting diodes (LEDs) by selective epitaxial ​growth

The past decade has witnessed tremendous progress in the development of III-nitride based light emitting diodes (LEDs) for lighting and high electron mobility transistors (HEMTs) for power applications. Sharing the same GaN-based material system, monolithic integration of HEMTs and LEDs can effectively reduce undesirable parasitics and greatly improve the system stability as well as reliability. However, there are limited reports on the monolithic integration of the two kinds of devices, probably due to the huge difference in material requirements for LEDs and transistors as well as the complexity of device fabrication. In this program, we aim to develop a monolithic integrated HEMT-LED device with comparable performance to stand-alone devices. We found the SER resulted in serious degradation of the underlying LEDs in a HEMT-on-LED structure due to damage of the p-GaN surface. The problem was circumvented using the SEG that avoided plasma etching and minimized device degradation. The integrated HEMT-LEDs by SEG exhibited comparable characteristics as unintegrated devices and emitted modulated blue light by gate biasing​. Besides, to improve the breakdown characteristics of the integrated HEMT-LED devices, carbon doping was introduced in the HEMT buffer by controlling the growth pressure and V/III ratio. The breakdown voltage of the fabricated HEMTs grown on LEDs was enhanced, without degradation of the HEMT DC performance.

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Performance Enhancements of Flip-Chip Light-Emitting Diodes with High-Density n-type Point-Contacts

Novel gallium nitride-based flip-chip light-emitting diodes 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 flip-chip LEDs (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 (WPE) 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 point-contacts. The electrical performance was enhanced through a minimized lateral current spreading distance.

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A Novel Full-Color 3LED Projection System using R-G-B Light Emitting Diodes on Silicon (LEDoS) Micro-displays

Aluminum gallium indium phosphide (AlGaInP)-based and gallium nitride (GaN)-based LEDoS micro-displays were fabricated by flip-chip process for the generation of single color red, green, and blue images. By integration of R-G-B LEDoS micro-displays using a trichroic prism and a projection lens, the world’s first 3LED projector prototype is successfully demonstrated. The color of the images projected on the screen can be adjusted by changing the intensity of the three individually controlled LEDoS micro-displays.

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1700 pixels per inch (PPI) Passive-Matrix Micro-LED Display Powered by ASIC

The first 1700 pixels per inch (PPI) blue passive-matrix light-emitting diodes on silicon (LEDoS) micro-displays powered by ASIC with 6-bit grayscale is realized by flip-chip bonding of a micro-LED array onto a CMOS-based ASIC display driver. The LEDoS micro-display consists of 256 x 192 pixels within a display area of 0.19 inch in diagonal. In our design, all passive-matrix interconnects are implemented on the LED side. With this special architecture, all the solder bumps can be relocated to the peripheral areas of the micro-LED array where the bumps can be bigger and more spread out. At the same time, only 448 bumps are needed for our display with almost 50000 pixels. The huge reduction in bump density significantly improves the bonding reliability. This novel passive-matrix display design and bump arrangement make high resolution and high-yield LEDoS micro-display achievable for a variety of applications.

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Gate-last Self-aligned Technology for In-situ SiNx/AlN/GaN MISHEMTs

GaN-based HEMTs have great potential for radio frequency (RF) /millimeter wave power applications, due to their unique combination of high electron velocity, large sheet carrier density and high breakdown field. In recent years, remarkable progress in GaN HEMTs has been made, with reported fT and fmax exceeding 300 GHz. These results were accomplished through innovative device scaling technologies such as the advanced T-gate fabrication process, heavily doped S/D ohmic contact regrowth, a thin AlN or InAlN barrier or a self-aligned structure.

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MOCVD grown In-situ SiNx gate dielectric for thin barrier AlN/GaN MISHEMTs

Due to the relatively large bandgap and strong polarization effects of AlN, AlN/GaN heterostructures can result in a high two-dimensional electron gas (2DEG) concentration and good carrier confinement with thin barrier layers. AlN/GaN HEMTs are very attractive for high frequency and high power applications. In addition, ultrathin AlN barriers allow good gate control capability and a high aspect ratio (gate length and gate to channel distance) to mitigate the short channel effects. However, low quality thin AlN and poor interfaces can lead to problems such as large leakage current and surface sensitivity, limiting the device performance and reliability. Several dielectrics such as, Al2O3, SiNx, HfO2 and Ta2O5, have been explored as gate insulators in AlN/GaN HEMTs. Most of these insulators are deposited ex-situ, which may introduce additional growth- and process-related defects on the devices.

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Nanometer-scale InGaAs Quantum-well MOSFETs on Si Substrates with Source/drain Regrowth

After half of a century of explosive development driven by Moore’s Law, the downscaling of Si based metal-oxide-semiconductor field effect transistors (MOSFETs) has led to remarkable improvement in transistor density, switching speed and signal processing capability. It is generally recognized that further shrinking transistor dimensions is running into many technological challenges. Integrating III-V high mobility semiconductors, like InGaAs and InAs, on large-diameter Si substrates to replace Si in nMOSFETs has emerged as one of the most promising options for CMOS beyond 10 nm technology node. The high effective carrier velocities of these compound semiconductors can enable energy efficient nano-scale field effect transistors by allowing the transistor supply voltage to be aggressively reduced.

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Selective Area MOCVD Growth of GaAs on Patterned Si (001) Substrates Using the Aspect Ratio Trapping Technique

A unique approach for epitaxial integration of III-V semiconductors on Si is the “aspect ratio trapping” technique. By this approach, III-V materials are selectively grown in high aspect ratio holes or trenches formed by a patterned dielectric on Si. Dislocations generated at the III-V/Si interface are guided to the dielectric sidewalls and terminated there, leaving a low-dislocation density region at the top of the trenches/holes where the devices can be built. The aspect ratio trapping technique opens opportunities for integrating III-V nanoelectronics and nanophotonics on Si. In collaboration with SEMATECH, we developed selective area MOCVD growth of GaAs in trench-patterned Si (001) substrates using the aspect ratio trapping technique.

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InP-on-Si (IOS) Technology Toward Electronics and Photonics Convergence on a Silicon Platform

InP is one of the most essential photonic and electronic materials. Over the last few decades, InP and associated heterostructures have enabled long wavelength lasers, photodetectors, high-frequency and high-speed devices such as heterojunction bipolar transistors, high-electron mobility transistors and metal-oxide-semiconductor field effect transistors. To obtain the best device performance, most of these state-of-the-art devices are grown and fabricated on lattice-matched, but relatively costly and fragile InP substrates. Tremendous benefits could be attained by growing high crystalline quality InP on Si substrates, which are available with large diameter (up to 300 mm), lower cost, good thermal conductivity and mechanical property.

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High-Speed InGaAs Photodetectors by Selective-Area MOCVD Toward Optoelectronic Integrated Circuits

Selective-area growth of high-crystalline-quality InGaAs-based photodetectors (PDs) with optimized InP/GaAs buffers on patterned (100)-oriented silicon-on-insulator (SOI) substrates by metal-organic chemical vapor deposition (MOCVD). The composite GaAs and InP buffer was grown using a two-temperature method. The island morphology of the low-temperature GaAs nucleation layer inside the growth well of the SOI substrate was optimized. A medium temperature (MT) GaAs layer was inserted prior to the typical high-temperature GaAs to further decrease the dislocation densities and antiphase boundaries (APBs). Both normal-incidence photodetectors (NIPDs) and butt-coupled waveguide photodetectors (WGPDs) were fabricated on the same substrate and showed a low dark current and high-speed performance. This result demonstrates a good potential of integrating photonic and electronic devices on the same Si substrate by direct epitaxial growth.

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First Yellow LEDs from InGaN/GaN MQWs on Si

High performance GaN-based green and yellow light-emitting diodes (LEDs) were grown on SiO2 nanorod patterned GaN/Si templates by metal organic chemical vapor deposition (MOCVD). The high density SiO2 nanorods were prepared by non-lithographic HCl- treated indium tin oxide (ITO) and dry etching. The dislocation density of GaN was significantly reduced by nanoscale epitaxial lateral overgrowth (NELO). In addition to much improved green LED (505 nm and 530 nm) results, fabricated yellow (565 nm) InGaN/GaN-based multi quantum well (MQW) LEDs on Si substrates were demonstrated for the first time.

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Growth and Characterization of Horizontal GaN Wires on Silicon (In collaboration with University of Wisconsin-Madison)

In-plane GaN wires were grown on silicon by metalorganic chemical vapor deposition (MOCVD). Triangular-shaped GaN microwires with semi-polar sidewalls are observed to grow on top of a GaN/Si template patterned with nano-porous SiO2. With a length-to-thickness ratio ~200, the GaN wires are well aligned along the three equivalent directions. Stacking faults (SFs) were found to be the only defect type in the GaN wire by cross-sectional transmission electron microscopy. With proper heterostructure and doping design, these highly-aligned GaN wires are promising for photonic and electronic applications monolithically integrated on silicon. To the best of our knowledge, this is the first demonstration of catalyst-free growth of horizontal GaN wires on Si.

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Low leakage Current AlGaN/GaN HEMTs on Si substrates

AlGaN/GaN high electron mobility transistors (HEMTs) were grown on silicon (111) substrates by Metal Organic Chemical Vapor Deposition (MOCVD). In the HEMT structure, a GaN buffer layer with partically doped with Mg was used to increase the buffer resistivity and minimize the leakage current.

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III-nitride Based Light Emitting Diodes on Silicon (LEDoS) Displays

With ITF support through NAMI, researchers in the ECE Photonic Technology Center (PTC) led by Prof. Kei May Lau of the ECE Department invented the LED on Silicon (LEDoS, pronounced led-dos) technology. LEDoS consists of high-resolution and high-brightness individually addressable emissive elements on a silicon-based active-matrix substrate with the use of integrated circuit technologies.

LEDoS arrays have many potential applications such as micro-display in mobile electronics, e-books, portable micro-projectors, bio-sensor arrays and programmable lighting sources.

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LED arrays on Silicon Substrates by Flip-chip Technology

Currently, different color LED chips have been housed in one package for color mixing in display applications. Monolithic integration of LEDs is essentially non-existent commercially. In this program, we propose to design and fabricate monolithic LED two-dimensional arrays using flip-chip technologies.

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GaAs transistors Grown on Si Substrates by MOCVD for III-V-IV Integration

In this program, we propose to integrate III-V devices on a Si platform. Using recently developed metamorphic techniques for III-V materials, high quality devices can be fabricated on Si substrates that will allow continued use of the traditional and ever improving manufacturing technologies for Si.

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Metamorphic InP-based devices on GaAs substrates by MOCVD

Although InP-based devices have been shown to have superior performance in higher frequency and power applications, the high cost of InP substrates and chip manufacturing makes InP chips less competitive in the market place. A lower cost can be achieved by growing metamorphic InP-based devices on cheaper GaAs substrates. In this project, we focus on optimizing the metamorphic technology, combined with novel device structures, by metalorganic chemical vapor deposition (MOCVD).

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III-Nitride LEDs Grown on Si Substrates by MOCVD

One way to lower the manufacturing cost of light-emitting compounds for commerical use is to utilize advanced Si-based technology as much as possible. In this project, we target the light emitting application of III-nitrides grown directly on Si substrates by metalorganic chemical vapor deposition (MOCVD). 

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