Hyung Soo Ahn

Fibrous and Porous Microstructure Formation in 6H‐SiC by Anodization in HF Solution

The anodization reaction of 6H-SiC using a HF solution was investigated to understand the formation of porous SiC. The anodization reaction proceeds via two stages in which the oxidation of SiC is followed by the removal of SiO by fluorine ions. The microstructure of porous SiC changes from fibrous to dendritic as the anodization current is increased, and also shows other transitional morphology. The photoluminescence of these porous SiC with different microstructures was also investigated, and the photoluminescence band of higher energy was found to become enhanced as the microstructure of porous SiC changed from fibrous to porous dendritic.

Nature of yellow luminescence band in GaN grown on Si substrate

High-optical-quality GaN was grown on a (111)Si substrate by metal–organic vapor phase epitaxy using an AlInN buffer layer and an indium doped AlN nucleation layer. The photoluminescence spectra at room temperature showed a strong and narrow edge-emission peak and weak defect-related emission bands. We found four spectral peaks in the green and yellow luminescence bands at G0: 514.5 nm (2.410 eV), G1: 546.5 nm (2.269 eV), Y1: 553.5 nm (2.240 eV), and Y0: 584.5 nm (2.121 eV), independent of the growth methods/conditions. The results suggest that the emission is associated with an intrinsic defect such as a Ga vacancy.

Mixing/Resonance of Electronic States and Optical Nonlinearity in a GaAs/AlGaAs Asymmetric Triple Quantum Well Structure

The input power dependence on the transmission of a light through a GaAs/AlGaAs asymmetric triple quantum well (ATQW) shows strong nonlinearity at 77 K, the behavior of which depends on the wavelength of the input light. By studying the relation of nonlinearity to the photoluminescence spectra, the resulting phenomenon is attributed to the Stark shift and change of the optical oscillator strength which is due to the triple resonance of the electronic states. The triple resonance is induced by the temporal separation of photo-excited electrons and holes.

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.

Structural Change of InGaN Nanostructures Grown by Mixed-Source Hydride Vapor Phase Epitaxy

We determined the effect of the type of substrate on the growth of InGaN nanostructures by mixed-source hydride vapor phase epitaxy (HVPE). InGaN nanostructures were formed on c-plane, r-plane sapphire, and undoped GaN substrates at various growth temperatures. Also, we looked into the changes in the structural and optical characteristics of InGaN nanostructures when antimony (Sb) is used as a surfactant during the growth of InGaN nanostructures. The samples were characterized by scanning electron microscopy (SEM) and photoluminescence (PL) measurement. The density of the nanostructures on the surface and the indium composition of the InGaN layer varied depending on the type of substrate and growth temperature. The aligning direction of the nanostructures markedly changed and the indium composition increased when Sb was used as the surfactant during the growth of the InGaN nanostructure, compared with the results of the InGaN nanostructures grown without Sb addition. # 2011 The Japan Society of Applied Physics

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.

Growth of AlN layer on patterned sapphire substrate by hydride vapor phase epitaxy

Even though a patterned sapphire substrate (PSS) has been used for the growth of a high-quality epilayer because of its many advantages, it has not been successfully used to grow an AlN epilayer for ultraviolet (UV) light-emitting diodes (LEDs) on a PSS up to now. We report the growth of a high-quality AlN epilayer on a PSS, as a substrate for the manufacture of UV LEDs, by hydride vapor phase epitaxy (HVPE). The X-ray diffraction (XRD) peaks for the AlN epilayer grown on the PSS indicate that crystalline AlN with a wurtzite structure was grown successfully on the PSS. Furthermore, HVPE combining both in situ HVPE technology and liquid-phase epitaxy (LPE) using a mixed source is proposed as a novel method for the growth of a flat AlN epilayer on a PSS.

Crystal Orientation of GaN Nanostructures Grown on Al2O3 and Si(111) with a Zr Buffer Layer

We studied the crystallographic orientation of GaN nanostructures grown on Si(111) and Al2O3 substrates. We evaluated Zr metal as a novel alternative to conventional buffer layers such as AlN and ZnO. One-dimensional structures (nanorods and nanoneedles) were grown by hydride vapor phase epitaxy at 650 and 600 °C, and investigated using field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) analysis. FE-SEM images showed that the GaN nanorods had a uniform diameter along the growth direction. XRD results indicated that the nanostructures had a hexagonal crystal structure, and pole figure measurements revealed that GaN nanostructures grown on the Si(111) substrate had a stronger c-axis crystallographic orientation than those grown on the Al2O3 substrate.