Sam Nyung Yi

Oxygen plasma effects on the electrical conductance of single-walled carbon nanotube bundles

We report the electrical properties of single-walled carbon nanotube bundles aligned between two electrodes with a gap of 8 µm by the alternating current dielectrophoresis method. The resistance increased exponentially as the number of structural defects increased from the oxygen plasma treatment. However, no noticeable change was observed in the carrier concentration and tube–tube interaction. The resistance dependence on the plasma treatment time was explained on the basis of the localization of electron states at defect sites. The aspects of the defects were examined by measuring the effects of ammonia adsorption on the conductance of the device.

Photoluminescence properties of a single GaN nanorod with GaN/AlGaN multilayer quantum disks

Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.

Enhancement of free-carrier screening due to tunneling in coupled asymmetric GaN/AlGaN quantum discs

We present an investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved microphotoluminescence measurements, supported by three-dimensional multiband k∙p computational modeling. We observe that with increasing optical excitation the carrier lifetime decreases and emission energy blueshifts. This originates from the screening of built-in piezo- and pyroelectric fields in the quantum discs by photogenerated free carriers. Due to nonresonant tunneling of carriers from the smaller disk to the larger disk, free-carrier screening is enhanced in the larger disk. Computational modeling was in good agreement with the experimental results.

Enhanced magnetic energy harvesting properties of magneto-mechano-electric generator by tailored geometry

By tailoring the truncated shape of a cantilever structured magneto-mechano-electric (MME) generator that is composed of a piezoelectric single crystal fiber composite and a magnetostrictive Ni plate, a superior output harvesting power density of over 680% was obtained as compared to a typical rectangular shaped generator. The effect of the MME generators shape on the strain distribution induced by magnetostriction and vibration characteristics and harvesting properties were simulated by finite element analysis modeling and confirmed experimentally, respectively. The truncated shape was effective for not only utilizing a more uniform in-plane strain distribution in the active piezoelectric area but also magnifying the flexural vibration amplitude, which in turn can make the generator more powerful under tiny magnetic oscillations.

The growth of GaN on the metallic compound graphite substrate by HVPE

The GaN layer was typical III-V nitride semiconductor and was grown on the sapphire substrate which cheap and convenient. However, sapphire substrate is non-conductivity, low thermal conductivity and has large lattice mismatch with the GaN layer. In this paper, the poly GaN epilayer was grown by HVPE on the metallic compound graphite substrate with good heat dissipation, high thermal and electrical conductivity. We tried to observe the growth mechanism of the GaN epilayer grown on the amorphous metallic compound graphite substrate. The HCl and gas were flowed to grow the GaN epilayer. The temperature of source zone and growth zone in the HVPE system was set at and , respectively. The GaN epilayer grown on the metallic compound graphite substrate was observed by SEM, EDS, XRD measurement.

Development of the Hybrid Conjugated Polymer Solar Cell Based on GaN Quantum Dots

We report the hybrid p–n junction based on GaN quantum dots (QDs) as an electron transport layer and poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole transport layer, which has not been tried for the solar cell until now. The growth of GaN QDs was achieved by the hydride vapor phase epitaxy (HVPE) technique and P3HT film sequentially was coated on the top of QDs. The overall performance of P3HT/GaN QDs hybrid heterojunction was analyzed by current density–voltage (J–V) characteristics and finally exhibited an open-circuit voltage, short-circuit current density, and fill factor of 160 mV, 3.6 mA/cm2, and 0.25, respectively. Also, its efficiency was shown up to 0.14% in an active area of 0.04 cm2 under AM1.5G illumination with an intensity of 100 mW cm-2. In this paper, we discuss the factors which affect the power conversion efficiency for future works.

Characterization of the InGaN/GaN Multi-Quantum-Wells Light-Emitting Diode Grown on Patterned Sapphire Substrate with Wide Electroluminescence Spectrum

We report the characterization of the InGaN/GaN multi-quantum-well (MQW) light-emitting diode (LED) grown on a patterned sapphire substrate by metal organic chemical vapor deposition (MOCVD) using the selective area growth (SAG) method. The SAG patterns were designed to be circular and their diameters were 700 and 200 µm. After the growth, the InGaN/GaN MQW LED of 200 µm diameter had various crystal facets and a shape similar to volcanic craters, which were not observed in the 700-µm-diameter sample. We obtained an active layer with compositional nonuniformity and superior optical properties. We found wide electroluminescence (EL) spectral peaks near 470, 570, and 600 nm. The distribution of the EL spectrum of the sample was similar to that of a conventional phosphor-converted white LED.

Optical Gain in Wurtzite ZnO/ZnMgO Quantum Well Lasers

The optical gain of wurtzite ZnO/ZnMgO quantum well (QW) lasers is investigated using the non-Markovian gain model with many-body effects. The results are compared with those of GaN/AlGaN QW lasers. The ZnO/ZnMgO QW lasers are found to have a much larger optical gain than the GaN/AlGaN QW lasers for a given sheet carrier density. This is mainly attributed to the fact that the average hole effective mass and the transition matrix element of ZnO/ZnMgO QW lasers are smaller and larger than those of GaN/AlGaN QW lasers, respectively. Also, the radiative current density contribution to the threshold current density of ZnO/ZnMgO QW lasers is expected to be markedly reduced compared with that of the GaN/AlGaN QW lasers if a large threshold carrier density (> 3×1019 cm-3) is required for both systems. These results show that ZnO-based QW lasers are promising candidates for optoelectric applications in visible and UV regions.

A theory of cyclotron resonance line widths based on continued fraction representation

Using Moris projection technique, we derive a cyclotron resonance power absorption formula in a continued fraction form, which can be changed into a power series form in the lowest order approximation. It is shown that the expansion coefficient e plays the role of an additional fitting parameter. With appropriate values of e and the deformation potential constant for Ge and Si, the temperature dependence of the width agrees quite well with the existing experimental data.

Mechanism of light emission and manufacturing process of vertical-type light-emitting diode grown by hydride vapor phase epitaxy

We developed a vertical-type light-emitting diode (LED) in which the substrate is removed using a hydride vapor phase epitaxy (HVPE) apparatus consisting of a multi-graphite boat filled with a mixed source and a high-temperature (T ≈ 900 °C) RF heating coil outside the source zone. The new chip-growth process with a significant reduction in the number of production steps is completed in only four steps, namely, photolithography, epitaxial layer growth, sorting, and metallization. We analyze the emission mechanism of these lights from measurement results to validate the characteristics of the light emitted from these vertical-type blue LEDs and white LEDs (WLEDs) without substrates, and propose that this mixed-source HVPE method may be a promising production technique for LEDs.