Jae-sik Yoon

Bio: Jae-sik Yoon is an academic researcher. The author has contributed to research in topics: Niobium & Particle size. The author has an hindex of 3, co-authored 10 publications receiving 43 citations.

Characteristic Variation of Niobium Powder Produced under Various Reduction Temperature and Amount of Reductant Addition

Abstract: Niobium powder was fabricated by sodiothermic reduction process using K 2 NbF 7 as the raw materials, KCl and KF as the diluents and Na as the reducing agent. The apparatus for the experiment was designed and built specifically for the present study. Varying properties of niobium powder depending on reaction temperature and excess of reducing agent were analyzed. The niobium particle size increased significantly as the reduction temperature increased from 993 to 1093 K. The particle size was fairly uniform at a given reaction temperature, varying from 0.2 μm to 50 nm, depending on the reaction temperature. The yield of niobium powder increased from 58 to 83% with a increasing a reaction temperature. The average particle size of niobium powder is improved from 70 nm to 0.2 μm with a increasing amount of sodium excess. And the yield ratio of Nb powder was 82% in the 5% excess sodium.

Ohmic contacts to N-face p-GaN using Ni/Au for the fabrication of polarization inverted light-emitting diodes.

Abstract: The electrical properties of Ni-based ohmic contacts to N-face p-type GaN were investigated. The specific contact resistance of N-face p-GaN exhibits a linear decrease from 1.01 omega cm2 to 9.05 x 10(-3) omega cm2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing annealing temperature. However, the specific contact resistance could be decreased down to 1.03 x 10(-4) omega cm2 by means of surface treatment using an alcohol-based (NH4)2S solution. The depth profile data measured from the intensity of O1s peak in the X-ray photoemission spectra showed that the alcohol-based (NH4)2S treatment was effective in removing the surface oxide layer of GaN.

Preparation of Niobium Powders for Solid Electrolyte Capacitors through Hunter Process Using Metallothermic Reduction Method

Abstract: The niobium capacitor shows somewhat more unstable characteristics than the commercial tantalum capacitors, but it will be nonetheless considered as an excellent substitute of tantalum capacitors in the future. In this study, niobium powder is fabricated by metallothermic reduction process using K2NbF7 as a raw material, KCl and KF as diluents, and Na as a reducing agent. The niobium particle size greatly decreases from 0.7 to 0.2µm as the amount of diluent increases. However, when a higher surface area of niobium powder is desired, more amounts of diluents are used in the said method. The niobium powder morphology and particle sizes are very sensitive to the amount of sodium excess, thus the particle size of niobium powder increases with increases in the amount of sodium excess. When more diluent and sodium are used, the niobium powder is contaminated further by impurities such as Fe, Cr, Ni and others. [doi:10.2320/matertrans.M2012261]

Handbook of Chemistry and Physics

Abstract: There is a special reason for reviewing this book at this time: it is the 50th edition of a compendium that is known and used frequently in most chemical and physical laboratories in many parts of the world. Surely, a publication that has been published for 56 years, withstanding the vagaries of science in this century, must have had something to offer. There is another reason: while the book is a standard fixture in most chemical and physical laboratories, including those in medical centers, it is not as frequently seen in the laboratories of physician's offices (those either in solo or group practice). I believe that the Handbook can be useful in those laboratories. One of the reasons, among others, is that the various basic items of information it offers may be helpful in new tests, either physical or chemical, which are continuously being published. The basic information may relate

Graphene-GaN Schottky diodes

Abstract: The electrical characteristics of graphene Schottky contacts formed on undoped GaN semiconductors were investigated. Excellent rectifying behavior with a rectification ratio of ∼107 at ±2 V and a low reverse leakage current of 1.0 × 10−8 A/cm2 at −5 V were observed. The Schottky barrier heights, as determined by the thermionic emission model, Richardson plots, and barrier inhomogeneity model, were 0.90, 0.72, and 1.24 ± 0.13 eV, respectively. Despite the predicted low barrier height of ∼0.4 eV at the graphene-GaN interface, the formation of excellent rectifying characteristics with much larger barrier heights is attributed to the presence of a large number of surface states (1.2 × 1013 states/cm2/eV) and the internal spontaneous polarization field of GaN, resulted in a significant upward surface band bending or a bare surface barrier height as high as of 2.9 eV. Using the S parameter of 0.48 (measured from the work function dependence of Schottky barrier height) and the mean barrier height of 1.24 eV, the work function of graphene in the Au/graphene/GaN stack could be approximately estimated to be as low as 3.5 eV. The obtained results indicate that graphene is a promising candidate for use as a Schottky rectifier in GaN semiconductors with n-type conductivity.

A new system for achieving high-quality nonpolar m-plane GaN-based light-emitting diode wafers

Abstract: High-quality nonpolar m-plane GaN-based light-emitting diode (LED) wafers have been deposited on LiGaO2(100) substrates by a combination of pulsed laser deposition and molecular beam epitaxy technologies. The high-resolution X-ray diffraction measurement reveals that high-quality crystalline nonpolar m-plane GaN films have been achieved on LiGaO2(100) substrates. Scanning electron microscopy and atomic force microscopy reveal the very flat surface with a surface root-mean-square roughness of 1.3 nm for p-GaN in the nonpolar m-plane GaN-based LED wafer grown on LiGaO2(100) substrates. A strong photoluminescence emission peak is observed at 446 nm with a full width at half maximum (FWHM) of 21.2 nm. Meanwhile, the electroluminescence spectra of nonpolar m-plane GaN-based LEDs on LiGaO2(100) substrates show a very slight blue shift in wavelength and is kept constant in FWHM with the increase of current from 20 to 150 mA. At an injection current of 20 mA, the light output power for this nonpolar LED is 30.1 mW with the forward voltage of 2.8 V in a chip size of 300 × 300 μm2. Furthermore, the nonpolar m-plane GaN-based LED on LiGaO2(100) exhibits the best external extraction efficiency value of 50.8%. These results indicate the good optoelectronic properties of nonpolar LEDs grown on LiGaO2(100) substrates. This achievement of nonpolar m-plane GaN-based LEDs on LiGaO2(100) substrates opens up a new possibility for achieving highly-efficient LED devices.

A New Method for Producing a Nanosized γ-Al 2 O 3 Powder

Abstract: A new method for producing a nanosized γ-Al2O3 powder was proposed, by which a saturated solution of aluminum oxychloride and sucrose was subjected to sequential heat treatment to 350°C to form a transient species and then to 800°C to form a nanosized γ-Al2O3 powder. The optimal treatment parameters were determined. Stages of the process were identified. The transient species and the nanosized γ-Al2O3 powder were studied.