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Optoelectronics Last Revised: 2020-08-03
Introduction

The research in optoelectronics mainly focuses on growth/fabrication of novel devices with III-V compound semiconductors, organic materials, and liquid crystals. Particular interests include: Sb-based high-speed/ low-power electronics, nitride-based optical/power devices, organic transistors, and polymer-dispersed liquid crystals.


Faculty Involved
Name Position Education Research Interests
Cheng, Ko-Ting Distinguished Professor
(Chair)

Ph.D. in Physics, National Cheng Kung University, Taiwan

Physics and electro-optics of LCs, bistable/multistable LC devices, light scattering, alignment and photo-alignment of LCs, flexible electronics, LC display technology, LC simulations, color vision, color gamut, some functional materials (monomers / polymers / azobenzenes / micro-particles / nano-particles / ionic LCs), and LC devices (light shutters / lenses / gratings / apertures / polarization rotator / fast switching /antenna /smart windows)

Lai, Kun-Yu Professor (Associate Chair)

Ph.D., Electrical Engineering, North Carolina State University, USA

Growth and fabrication of III-nitride optoelectronic devices: LEDs, biosensors, etc.

Chang, Jui-Fen Associate Professor

PhD in Physics, University of Cambridge, UK

Organic thin-film transistor, Organic light-emitting device

Chyi, Jen-Inn Chair Professor

Ph.D. degree in Electrical Engineering from the University of Illinois

Semiconductor Materials and Devices, Optoelectronic Devices, High-Speed Devices


Research Summary in DOP

Chyi, Jen-Inn

Prof. Chyi’s research interests are in the areas of MBE and MOVPE growth of III-V semiconductors and their heterostructures for high-speed electronic and optoelectronic devices. His current research projects include MBE growth of InP-based heterojunction bipolar transisters, quantum dot photonic devices, MOVPE growth of GaN-based materials for ultraviolet, blue, green emitters and high power devices.


Lai, Kun-Yu

Prof. Lai’s research is focused on development of III-nitride optoelectronic devices, including LEDs and solar cells. Enhanced efficiency/cost ratios of these devices are achieved through novel growth and fabrication techniques. Additional research interest is directed toward theoretical investigations on the mechanisms responsible for emission/absorption rates in III-nitride semiconductors.

Sb-based High-speed/ Low-power Electronics

Antimonide (Sb) compound semiconductors are the promising alternatives to Si for low power electronics. The density and computing power of integrated circuits are now limited primarily by power-dissipation concerns. Sb-based electronic devices such as heterojunction bipolar transistors (HBTs) offer high speed, low power consumption and good breakdown voltages. High electron mobility InAs/AlSb or InSb/AlSb and high hole mobility InGaSb/AlSb quantum well heterostructure field effect transistors (HFETs) have also been widely pursued for THz amplifiers and high speed complementary logic circuits.

Figure 1. (a) Device structures of the InAs/AlSb quantum well heterojunction field effect transistor (HFET); (b) (top) drain–source I–V characteristics of a 0.2 mm InAs/AlSb nchannel HFET, and (bottom) cut-off frequency (fT) and maximum oscillation frequency (fMAX) characteristics of the 0.2 mm gate-length InAs/AlSb n-channel HFET.

Nitride-based Devices

In the field of nitride-based devices, research activities in this department are driven by the unequalled advantages of III-nitride compounds in solid state lighting and power electronics. InGaN-based LEDs have reached the luminescence efficiencies beyond 150 lm/W, well above most of the records achieved by other light sources. GaN also exhibits great potential for high-speed and high-power electronics because of her high saturation velocity, high breakdown field and high thermal conductivity. The growth of nitrides' commercial success hinges on high quality-price ratios of relevant products, which is pursued with advanced techniques in structure design, crystal growth and device fabrication.


Figure 2. GaN nanostructure with semipolar (11-22) facets grown on Si substrate. The nanostructure is the potential material for next-generation LEDs with high efficiency/cost ratio.

Figure 3. (a) Schematic cross-section and SEM photo of a lateral AlGaN/GaN Schottky barrier diode. (b) The reverse I–V characteristics of the AlGaN/GaN Schottky barrier diode with different gate-source lengths.


Chang, Jui-Fen

Prof. Chang’s research is involved in the development of various high-efficient organic devices. Current projects include enhancement of conductivity in organic field-effect transistors, investigation of charge transport mechanisms in organic semiconductors, investigation of organic light-emitting devices and microcavity organic lasers combined with optical simulations, and development of high efficiency organic solar cells.

Organic Transistors

Organic semiconductors have been the subject of active research for the last two decades. Applications in light-emitting displays and printable electronic circuits are emerging nowadays. Special research efforts in this field are made in detecting n-type field-effect behaviour of organic semiconductors, Hall effect measurements of charge carrier delocalization in solution-processed, crystalline molecular semiconductor, and the investigation of polaron localization at interfaces in high-mobility microcrystalline conjugated polymers.


Figure 4. Organic field-effect transistors (FETs) typically show p-type, but not n-type. However, it is found that the use of an appropriate hydroxyl-free gate dielectric—such as a divinyl- tetramethylsiloxane-bis(benzocyclobutene) derivative (BCB)—can yield n-channel FET (n-FET) conduction in most conjugated polymers. The figure shows the diagram of an n-FET and the chemical structure of the crosslinked BCB dielectric.


Cheng, Ko-Ting

Prof. Cheng’s research interests cover the electro-optical properties of liquid crystals (LCs) and their physics and applications. The used materials include nematic LCs, cholesteric LCs, blue phase LCs, ferroelectric LCs, azobenzenes (chiral dopants, polymers, dyes, etc.), polymers, nanoparticles, and others. Recently, we are studying their properties and developing some applications based on the above materials, including LC alignment, LC photoalignment, LC lens, polymer-dispersed LCs (PDLCs), nanoparticle-doped LCs, blue phase LCs (BPLCs), controlling the reflection band of cholesteric LCs (CLCs), Flexible LC electronics, holographic gratings, polarization convertors, LC displays (LCDs), multistable LCDs, and so on.

Liquid Crystal Electro-optical Devices

The research of liquid crystal electro-optical devices includes scattering mode liquid crystal shutters, the development of liquid crystal display techniques, liquid crystal light modulators, liquid crystal lenses, liquid crystal gratings, multi-viewing-angle liquid crystal devices, liquid crystal flexible electronics, and others. The scattering mode liquid crystal shutters are based on the effects of thermal and optics onto the employed polymer, poly(n-vinyl carbazole) (PVK). A novel phase separation method, thermally induced phase separation (TIPS), is proposed, which involves a combination of dissolution of PVK into liquid crystals and thermally induced phase separation.

Figure 5. Developments of electrically controllable liquid crystal display devices based on the mechanism of particular TIPS. (a) Variations in stable transmission in relation to the temperature during heating (black dots) and cooling (red squares) of the LC sample fabricated from two non-rubbed PVK-coated glass substrates. The LC sample at 25 °C; Inset (a) before (transparent) and, inset (b) after (scattering) thermal treatment via the particular TIPS; (b) Measured transmission of the fabricated scattering mode LC light shutter as a function of an applied AC (1 KHz) voltage. Insets show photographs of the LC light shutter at 25 °C with the applied AC voltages of inset (a) 0 and inset (b) 18 V.



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