Jun Hyun Han

300°C GaN/AlGaN Heterojunction Bipolar Transistor

A GaN/AlGaN heterojunction bipolar transistor has been fabricated using Cl 2 /Ar dry etching for mesa formation. As the hole concentration increases due to more efficient ionization of the Mg acceptors at elevated temperatures (> 250°C), the device shows improved gain. Future efforts should focus on methods for reducing base resistance, which are briefly summarized.

Wax moth, Galleria mellonella, high density lipophorin receptor: alternative splicing, tissue-specific expression, and developmental regulation

A lipophorin (Lp) receptor cDNA from the fat body of Galleria mellonella (Lepidoptera) was cloned and sequenced. This is the first result in this order, Lepidoptera. It showed the pattern of the VLDL receptor belonging to the LDL receptor family. Sequence homology with other Lp receptors in insects, Locusta migratoria and Aedes aegypti, was 70 and 61%, respectively and each domain was highly conserved. Polyclonal anti-Lp receptor antibody prepared against expressed Lp receptor fragment between ligand binding domain and EGF-precursor homology domain (R305-D549 of amino acid residues) specifically detected the Lp receptor. Through immuno-blotting, the Lp receptor of larval fat body has an approximate molecular mass of about 97 and 110 kDa under non-reducing and reducing conditions, respectively. This result was in agreement with that of the ligand-blotting. The variant Lp receptors were expressed in the fat body of G. mellonella; one is an Lp receptor which lacks 84 bp of O-linked sugar domain and the other is a full length form of the Lp receptor. Both forms were detected by the polyclonal anti-Lp receptor antibody. The Lp receptor from the fat body of G. mellonella was differently expressed depending on the tissue and the developmental stage with specific abundance in prepupal stage. A steroid hormone, 20-hydroxyecdysone (20-HE) plays a crucial role in insect development. With regards to this conception, day 1-2 last instar larvae were treated with 20-HE and drastic induction of the Lp receptor was observed 48 h after treatment. It was also observed that cholesterol caused an induction of the Lp receptor.

Effect of deformation histories on texture evolution during equal- and dissimilar-channel angular pressing

Abstract Finite element analyses and full constraint Taylor analyses based on the rate sensitivity model were performed to analyze deformation history and corresponding texture evolution during equal- and dissimilar-channel angular pressing, respectively.

Nanoscale ripples on polymers created by a focused ion beam

We show that focused ion beam irradiation results in the creation of peculiar one- and two-dimensional nanoscale features on the surface of polyimide-a common polymer in electronics, large scale structures, and the automobile industry, as well as in biomedical applications. The role of ion beam incident angle, acceleration voltage, and fluence on the morphology of the structural features is systematically investigated, and insights into the mechanisms of formation of these nanoscale features are provided. Moreover, by using the maskless patterning method of the focused ion beam system, we have developed a robust technique for controlled modification of the polymeric surface. The technique, which is analogous to using a gray glass with varying darkness to control the radiation from the sun, but at a much smaller scale, enables the ion intensity and angle to be controlled at each surface point of the polymer, giving rise to structural surface features with desired shape and morphology.

Influence of chromizing treatment on the corrosion behavior of AISI 316 stainless steel in supercritical water oxidation

SCWO, sometimes referred to as hydrothermal waste processing, uses the solvating traits of water in its supercritical condition to effectively destroy liquid organic wastes. One major problem in the supercritical water oxidation process is corrosion, because all metallic tubes in the process are exposed to high temperature and high pressure as well as severe corrosive species such as Cl−, F−, S2−, and O2−. The presence of Cl− when the pH of a solution is very low and the solution has excess oxygen causes active corrosion and metal loss by metal-chloride and/or oxychloride formation. This study performed a chromizing treatment on 316 stainless steel and immersion tests in supercritical water. Weight change of chromized steels and untreated steels was measured, and the chemical state and composition of oxide films on 316 stainless steel were investigated. On the basis of SCWO tests using distilled water, the oxide layer was found to be very thin and homogeneous and weight gain was observed regardless of testing temperature, while the chromizing treatment slightly reduced weight gain. In the case of SCWO tests using salt water, weight loss was observed regardless of testing temperature and its corrosion mode was pitting by chloride ion, while chromizing treatment greatly decreased the corrosion rate.

Reassessment of the status of Streptomyces setonii and reclassification of Streptomyces fimicarius as a later synonym of Streptomyces setonii and Streptomyces albovinaceus as a later synonym of Streptomyces globisporus based on combined 16S rRNA/gyrB gene sequence analysis.

The 16S rRNA and gyrB genes of 22 Streptomyces strains belonging to the Streptomyces griseus cluster were sequenced, and their taxonomic positions were re-evaluated. For correct analysis, all of the publicly available sequences of the species were collected and compared with those obtained in this study. Species for which no consensus sequence could be identified were excluded from the phylogenetic analysis. The levels of 16S rRNA gene sequence similarity within the cluster ranged from 98.6 to 100% with a mean value of 99.6 ± 0.3%, and those of the gyrB gene ranged from 93.6 to 99.9% with a mean value of 96.3 ± 1.5%. The observed average nucleotide substitution rate of the gyrB gene was ten times higher than that of the 16S rRNA gene, showing a far higher degree of variation. Strains sharing 99.3% or more gyrB sequence similarity (corresponding to an evolutionary distance of 0.0073) always formed monophyletic groups in both trees. Through the combined analysis of the two genes, clear cases of synonymy could be identified and, according to the priority rule, the assertion of the status of Streptomyces setonii as a distinct species and the reclassification of Streptomyces fimicarius as a later synonym of S. setonii and Streptomyces albovinaceus as a later synonym of Streptomyces globisporus are proposed. Emended descriptions of S. setonii and S. globisporus are provided.

Oxygen implant isolation of n-GaN field-effect transistor structures

Multiple-energy (30-325 keV) O{sup +} implantation into GaN field-effect transistor structures (n {approximately} 10{sup 18} cm{sup {minus}3}, 3000 {angstrom} thick) can produce as-implanted sheet resistances of 4 x 10{sup 12} {Omega}/{open_square}, provided care is taken to ensure compensation of the region up to the projected range of the lowest energy implant. The sheet resistance remains above 10{sup 7} {Omega}/{open_square} to annealing temperatures of {approximately} 650 C and displays an activation energy of 0.29 eV. No diffusion of the implanted oxygen was observed for anneals up to 800 C.

GaN Etching in BCl 3 /Cl 2 Plasmas

GaN etching can be affected by a wide variety of parameters including plasma chemistry and plasma density. Chlorine-based plasmas have been the most widely used plasma chemistries to etch GaN due to the high volatility of the GaCl x and NCl x etch products. The source of Cl and the addition of secondary gases can dramatically influence the etch characteristics primarily due to their effect on the concentration of reactive Cl generated in the plasma. In addition, high-density plasma etch systems have yielded high quality etching of GaN due to plasma densities which are 2 to 4 orders of magnitude higher than reactive ion etch (RIE) plasma systems. The high plasma densities enhance the bond breaking efficiency of the GaN, the formation of volatile etch products, and the sputter desorption of the etch products from the surface. In this study, we report GaN etch results for a high-density inductively coupled plasma (ICP) as a function of BCl 3 :Cl 2 flow ratio, dc-bias, chamber pressure, and ICP source power. GaN etch rates ranging from ∼100 A/min to > 8000 A/min were obtained with smooth etch morphology and anisotropic profiles.

Corrosion behavior of magnesium powder fabricated by high-energy ball milling and spark plasma sintering

Microstructural changes and corrosion behavior of pure magnesium for different milling times were investigated. The samples with a finer grain size showed poor corrosion resistance because of unstable or metastable protective film formation after immersion in 0.8 wt% NaCl solution. The corrosion resistance did not improve despite the strong (0002) texture of the sample prepared by spark plasma sintering at 500 °C for 0.3 Ks and milling for 2 h. By studying the microstructural changes and texture development, we concluded that the deformation-dependent grain size is the dominant factor controlling the corrosion properties of mechanically milled magnesium. Increased grain boundary densities lead to an enhancement of the overall surface reactivity and, consequently, the corrosion rate.

Fabrication of AA6061/Al2O3p composites from elemental and alloy powders

The tensile properties and microstructures of AA6061/Al2O3p composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten metal into elemental powders bed as well as alloy powders bed occurred at 700°C for 1 hour under a nitrogen atmosphere, it was possible to fabricate 6061 Al matrix composite reinforced with Al2O3p irrespective of the type of metal powders. Both MgAl2O4 and MgO were formed at interfaces between Al2O3 and the matrix. In addition, MgAl2O4 was formed at within the matrix by in situ reaction during composite fabrication. Fine AlN was formed by in situ reaction in both composites. A significant strengthening in the composites occurred due to the formation ofin situ AlN particle and addition of Al2O3 particles, as compared to the commercial alloy, while tensile properties in the both elemental and alloy powders composites showed similar trend.