Chun Hong Kang

Carbon nanotube-graphene composite film as transparent conductive electrode for GaN-based light-emitting diodes

Transparent conductive electrodes (TCE) made of carbon nanotube (CNT) and graphene composite for GaN-based light emitting diodes (LED) are presented. The TCE with 533-Ω/□ sheet resistance and 88% transmittance were obtained when chemical-vapor-deposition grown graphene was fused across CNT networks. With an additional 2-nm thin NiOx interlayer between the TCE and top p-GaN layer of the LED, the forward voltage was reduced to 5.12 V at 20-mA injection current. Four-fold improvement in terms of light output power was observed. The improvement can be ascribed to the enhanced lateral current spreading across the hybrid CNT-graphene TCE before injection into the p-GaN layer.

Red to Near-Infrared Emission from InGaN/GaN Quantum-Disks-in-Nanowires LED

The InGaN/GaN quantum-disks-in-nanowire light-emitting diode (LED) with emission centered at ∼830nm, the longest emission wavelength ever reported in the InGaN/GaN system, and spectral linewidth of 290nm, has been fabricated with p-side-down on a Cu substrate.

Unleashing the potential of molecular beam epitaxy grown AlGaN-based ultraviolet-spectrum nanowires devices

Abstract. There have been recent research advances in AlGaN-based self-assembled nanowires (NWs) as building blocks for ultraviolet (UV) optoelectronics grown by plasma-assisted molecular beam epitaxy. We review the basic growth kinetics on various foundry-compatible-metal/silicon-based substrates and the epistructure design for UV devices. We highlight the use of diffusion-barrier-metal thin film on silicon substrate as a solution to enhance device performance. NWs offer the opportunity to mitigate the detrimental quantum-confined Stark effect (QCSE), which lowers the recombination rate thereby reducing the device efficiency. On the other hand, the polarization-induced doping from the graded composition along NWs can be advantageous for eluding the inefficient doping in AlGaN-based UV devices. Sidewall surface states and the associate passivation treatment, as well as the use of ultrafast electron-microscopy characterization, are crucial investigations in shedding light on device performance under the influence of surface dangling bonds. For investigating the electrical performance of individual NWs and NWs light-emitting diode as a single entity, recent reports based on conductive atomic force microscopy measurements provide fast-prototyping in-process pass-fail evaluation and a means of improving growth for high-performance devices. Stress tests of NWs devices, crucial for reliable operation, are also discussed. Beyond applications in LEDs, an AlGaN-based NWs solar-blind photodetector demonstrated leveraging on the dislocation-free active region, reduced QCSE, enhanced light absorption, and tunable-composition features. The review opens pathways and offers insights for practical realization of AlGaN-based axial NWs devices on scalable and low-cost silicon substrates.

Light based underwater wireless communications

The financial support from King Abdulaziz City for Science and Technology (KACST), Grant No. KACST TIC R2-FP-008 is gratefully acknowledged. This work was partially supported by the King Abdullah University of Science and Technology (KAUST) baseline funding, BAS/1/1614-01-01, KAUST funding KCR/1/2081-01-01, and GEN/1/6607-01-01, as well as KAUST-KFUPM Special Initiative (KKI) Program, REP/1/2878-01-01.

DNA/AuNP-graphene back-gated field effect transistor as a biosensor for lead (II) ion detection

Lead is a very toxic substance that causes metabolic disruption by mimicking the chemical profile of other important ions in the human body. The current technique of determining the presence of lead in drinkable water are simply too expensive to be implemented for large-scale real time monitoring. As an alternative, a sensor based on graphene field effect transistor is developed. The graphene layer itself is decorated with gold nanoparticle (AuNP) and it acts as a sensing medium while guanine-rich deoxyribonucleic acid (DNA) that is attached to the AuNP acts as a chemical probe. The guanine-rich DNA forms G-quadruplex when exposed to lead ions and thus changes the overall charge on the surface of the graphene. The changes can be observed via I-V measurement of the sensor. The fabricated sensors are capable of detecting lead ions even at 20 nM concentration. Such a sensor is scalable and can offer a cheap and effective option for monitoring the quality of water in real-time.

Facile Formation of Interconnected Multi-Walled Carbon Nanotube-Graphene Nanocomposite for Nanoelectronics Applications

Novel nanocomposite made of one-dimensional (1-D) multi-walled carbon nanotube (MWCNT) and two-dimensional (2-D) graphene was prepared. MWCNT was spin coated onto copper foil and followed by chemical vapor deposition (CVD) growth of graphene. The MWCNT-Graphene nanocomposite was transferred onto target substrate by using a standard polymer-based transfer technique. HRTEM and Raman spectroscopy showed high crystallinity of fused MWCNT and graphene layer. Low defect-related D-peak was also observed even after the nanocomposite underwent high temperature processing. As compared to pristine graphene, electrical characterization of MWCNT-Graphene nanocomposite also revealed the reduction of sheet resistance by ~71% and almost 2-fold improvement in room-temperature carrier mobility. These improvements are surmised due to additional conducting channels formed by MWCNT in the graphene layer. Hence, higher electrical conductivity can be expected. With the introduction of MWCNT across the graphene layer, highly desirable electrical properties can be achieved and as such leveraging the viability of graphene-based nanoelectronics devices.

Synthesis parameters and transfer techniques of mono-few layer graphene for transparent conductive electrode

Properties of high quality graphene synthesized using chemical vapor deposition (CVD) are reported. Parameters such as the rate and time of gas flow are varied during synthesis. The as-grown graphene is then transferred onto oxidized silicon substrate by employing single and double layer polymer-based techniques. Next, surface morphology, electrical and optical properties of the synthesized mono-few layer graphene are characterized using optical microscope, Raman spectroscopy, UV-VIS spectroscopy, and four-point probe measurement. Results show that by using the appropriate combination of synthesis parameters and transfer technique, high quality graphene with electrical sheet resistance as low as 900 Ω/sq and optical transparencies as high as 94% can be obtained. Such quality graphene is highly desirable as transparent conductive electrodes in applications like light emitting diodes, solar cells and optical sensors.

Graphene-based hybrid thin films as transparent conductive electrode for optoelectronic devices

Graphene-based hybrid thin films are investigated specifically for its optical transmittance and sheet resistance. The hybrid films were made of stacked or fused chemical vapor deposition (CVD)-grown graphene, carbon nanotubes (CNT) and copper nanowires. It was found that the fused graphene/CNT has the highest transmittance nearly 90% in the visible region and the lowest sheet resistance (RS) of ~830 Ω/□. Upon further optimization, it is believed that the latter parameters can be significantly improved and made it feasible to be used as transparent conductive electrode (TCE) for optoelectronic devices.

Mesa-height Dependent Quantum Efficiency Characteristics of InGaN Micro-LEDs

The mechanisms of mesa-height dependent efficiency and efficiency droop of blue InGaN/GaN micro-LED is presented. Device with a large etch-depth (> 1.3 µm) shows significant strain relief with aggravated current crowding.