Gwangseok Yang

Body-Attachable and Stretchable Multisensors Integrated with Wirelessly Rechargeable Energy Storage Devices.

A stretchable multisensor system is successfully demonstrated with an integrated energy-storage device, an array of microsupercapacitors that can be repeatedly charged via a wireless radio-frequency power receiver on the same stretchable polymer substrate. The integrated devices are interconnected by a liquid-metal interconnection and operate stably without noticeable performance degradation under strain due to the skin attachment, and a uniaxial strain up to 50%.

Defect-engineered graphene chemical sensors with ultrahigh sensitivity

We report defect-engineered graphene chemical sensors with ultrahigh sensitivity (e.g., 33% improvement in NO2 sensing and 614% improvement in NH3 sensing). A conventional reactive ion etching system was used to introduce the defects in a controlled manner. The sensitivity of graphene-based chemical sensors increased with increasing defect density until the vacancy-dominant region was reached. In addition, the mechanism of gas sensing was systematically investigated via experiments and density functional theory calculations, which indicated that the vacancy defect is a major contributing factor to the enhanced sensitivity. This study revealed that defect engineering in graphene has significant potential for fabricating ultra-sensitive graphene chemical sensors.

Three-dimensional graphene foam-based transparent conductive electrodes in GaN-based blue light-emitting diodes

We demonstrated three-dimensional (3D) graphene foam-based transparent conductive electrodes in GaN-based blue light-emitting diodes (LEDs). A 3D graphene foam structure grown on 3D Cu foam using a chemical vapor deposition method was transferred onto a p-GaN layer of blue LEDs. Optical and electrical performances were greatly enhanced by employing 3D graphene foam as transparent conductive electrodes in blue LED devices, which were analyzed by electroluminescence measurements, micro-Raman spectroscopy, and light intensity-current-voltage testing. The forward operating voltage and the light output power at an injection current of 100 mA of the GaN-based blue LEDs with a graphene foam-based transparent conductive electrode were improved by ∼26% and ∼14%, respectively. The robustness, high transmittance, and outstanding conductivity of 3D graphene foam show great potentials for advanced transparent conductive electrodes in optoelectronic devices.

GaN-based ultraviolet light-emitting diodes with AuCl 3 -doped graphene electrodes

We demonstrate AuCl3-doped graphene transparent conductive electrodes integrated in GaN-based ultraviolet (UV) light-emitting diodes (LEDs) with an emission peak of 363 nm. AuCl3 doping was accomplished by dipping the graphene electrodes in 5, 10 and 20 mM concentrations of AuCl3 solutions. The effects of AuCl3 doping on graphene electrodes were investigated by current-voltage characteristics, sheet resistance, scanning electron microscope, optical transmittance, micro-Raman scattering and electroluminescence images. The optical transmittance was decreased with increasing the AuCl3 concentrations. However, the forward currents of UV LEDs with p-doped (5, 10 and 20 mM of AuCl3 solutions) graphene transparent conductive electrodes at a forward bias of 8 V were increased by ~48, 63 and 73%, respectively, which can be attributed to the reduction of sheet resistance and the increase of work function of the graphene. The performance of UV LEDs was drastically improved by AuCl3 doping of graphene transparent conductive electrodes.

Influence of High-Energy Proton Irradiation on β-Ga2O3 Nanobelt Field-Effect Transistors

The robust radiation resistance of wide-band gap materials is advantageous for space applications, where the high-energy particle irradiation deteriorates the performance of electronic devices. We report on the effects of proton irradiation of β-Ga2O3 nanobelts, whose energy band gap is ∼4.85 eV at room temperature. Back-gated field-effect transistor (FET) based on exfoliated quasi-two-dimensional β-Ga2O3 nanobelts were exposed to a 10 MeV proton beam. The proton-dose- and time-dependent characteristics of the radiation-damaged FETs were systematically analyzed. A 73% decrease in the field-effect mobility and a positive shift of the threshold voltage were observed after proton irradiation at a fluence of 2 × 1015 cm-2. Greater radiation-induced degradation occurs in the conductive channel of the β-Ga2O3 nanobelt than at the contact between the metal and β-Ga2O3. The on/off ratio of the exfoliated β-Ga2O3 FETs was maintained even after proton doses up to 2 × 1015 cm-2. The radiation-induced damage in the β-Ga2O3-based FETs was significantly recovered after rapid thermal annealing at 500 °C. The outstanding radiation durability of β-Ga2O3 renders it a promising building block for space applications.

Post-growth CdCl 2 treatment on CdTe thin films grown on graphene layers using a close-spaced sublimation method

We investigated the morphological, structural and optical properties of CdCl₂-treated cadmium telluride (CdTe) thin films deposited on defective graphene using a close-spaced sublimation (CSS) system. Heat treatment in the presence of CdCl₂ caused recrystallization of CSS-grown CdTe over the as-deposited structures. The preferential (111) orientation of as-deposited CdTe films was randomized after post-growth CdCl₂ treatment. New small grains (bumps) on the surface of CdCl₂-treated CdTe films were ascribed to nucleation of the CdTe grains during the CdCl₂ treatment. The properties of as-deposited and CdCl₂-treated CdTe films were characterized by room temperature micro-photoluminescence, micro-Raman spectroscopy, scanning electron microscopy, and X-ray diffraction analysis. Our results are useful to demonstrate a substrate configuration CdTe thin film solar cells.

CdTe microwire-based ultraviolet photodetectors aligned by a non-uniform electric field

We report on ultraviolet (UV) photodetectors fabricated by positioning Cadmium Telluride (CdTe) microwires (μWs) precisely by dielectrophoretic (DEP) force, where CdTe μWs were grown using an Au-catalyst-assisted closed-space-sublimation (CSS) method. The optical properties of CSS-grown CdTe μWs were characterized by micro-photoluminescence and micro-Raman spectroscopies. Optoelectronic characteristics were obtained after CdTe μWs were aligned on a pre-patterned SiO2/Si substrate by a non-uniform electric field. Photocurrents were increased with increasing the light intensities. Fast and reliable photoresponse and recovery were observed when CdTe μWs were exposed to UV illuminations. We demonstrated that high quality CdTe μWs grown by the CSS method have significant potentials as optoelectronic devices.

1.5 MeV electron irradiation damage in β-Ga2O3 vertical rectifiers

Vertical rectifiers fabricated on epi Ga2O3 on bulk β-Ga2O3 were subject to 1.5 MeV electron irradiation at fluences from 1.79 × 1015 to 1.43 × 1016 cm−2 at a fixed beam current of 10−3 A. The electron irradiation caused a reduction in carrier concentration in the epi Ga2O3, with a carrier removal rate of 4.9 cm−1. The 2 kT region of the forward current–voltage characteristics increased due to electron-induced damage, with an increase in diode ideality factor of ∼8% at the highest fluence and a more than 2 order of magnitude increase in on-state resistance. There was a significant reduction in reverse bias current, which scaled with electron fluence. The on/off ratio at −10 V reverse bias voltage was severely degraded by electron irradiation, decreasing from ∼107 in the reference diodes to ∼2 × 104 for the 1.43 × 1016 cm−2 fluence. The reverse recovery characteristics showed little change even at the highest fluence, with values in the range of 21–25 ns for all rectifiers.

Effect of 5 MeV proton irradiation damage on performance of β-Ga2O3 photodetectors

Planar thin film β-Ga2O3 photodetectors were irradiated with 5 MeV protons at doses from 1013 to 1015 cm−2, and the resulting effects on photocurrent, responsivity, quantum efficiency, and photo-to-dark current ratio at 254 nm wavelength were measured at both 25 and 150 °C. The photocurrent increased with dose due to the introduction of damage from nonionizing energy loss by the protons. The total calculated vacancy concentration increased from 5 × 1015 to 5 × 1017 cm−3 over the dose range investigated. The dark current increased in proportion with the implant dose, leading to a decrease in the ratio of photocurrent to dark current. The photocurrent induced by 254 nm illumination increased with dose, from ∼0.3 × 10−7 A at 25 °C for a dose of 1013 cm−2 to ∼10−6 A at a dose of 1015 cm−2 at a fixed light intensity of 760 μW/cm2. The photo-to-dark current ratio decreased from ∼60 in the control samples to ∼9 after proton doses of 1015 cm−2, with corresponding external quantum efficiencies of ∼103% in control ...

Energy and dose dependence of proton-irradiation damage in graphene

Monolayer graphenes were irradiated with 5–15 MeV high-energy protons at various doses from 1 × 1016 to 3 × 1016 cm−2, and their characteristics were systematically investigated using micro-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As the energy and dose of the proton irradiation increased, the defects induced in the graphene layers also increased gradually. The average defect distances of 10 MeV proton-irradiated graphene decreased to 29 ± 5 nm at a dose of 3 × 1016 cm−2. The defect formation energies for various types of defects were compared by using density functional theory calculation. After proton irradiation, the results of micro-Raman scattering and XPS indicated p-doping effects due to adsorption of environmental molecules on the damaged graphene. Our results show a direct relationship between the defect formation of the graphene layers and the energy/dose of the proton irradiation.