Kie Moon Song

Investigation of the transition between glow and streamer discharges in atmospheric air

Generally, the parameter p ? d (pressure ? gap distance) in dielectric barrier discharge (DBD) controls the electrical breakdown and also the plasma characteristics. We investigated the optimum plasma transition p ? d by controlling the pressure. To find the transition p ? d (p ? dtr) condition, optical emission spectroscopy (OES) was used to measure emission spectra from the DBD. All p ? d data were normalized by the second positive system of nitrogen molecules, the wavelength of which was 337.1?nm. Then we compared the relative intensities of species generated during the discharge by OES analysis. Species selected for comparison were the first negative system (FNS) of nitrogen molecules (391.4?nm) and atomic oxygen spectra (777.1?nm). Experimental results showed that relative intensities were almost constant as p ? d decreased, but at specific p ? d data, the intensity started to increase. The increase in FNS of nitrogen molecules means not only an increase in electron energy but also a change in the plasma mode, streamer to glow transition. In the case of DBD using alumina with 1?mm thickness applied ac power, the plasma transition occurred at the 1?Torr?cm condition.

Growth control of single and multi-walled carbon nanotubes by thin film catalyst

Abstract Single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were synthesized on silicon substrate by the control of catalyst size, hydrocarbon species, and carbon flux through chemical vapor deposition (CVD). Catalysts for SWNTs and MWNTs could be obtained by an agglomeration of sputtered Co–Mo and pure Co thin films, respectively. The addition of Mo in the Co catalyst provides an effective nucleation site for SWNT and the low carbon flux by using methane gas in CVD reaction makes it possible to grow a single-walled structure.

Highly efficient inverted polymer solar cells with reduced graphene-oxide-zinc-oxide nanocomposites buffer layer

In this study, we reported a 36% improvement in the performance of inverted solar cells as a result of increased short-circuit current (JSC) obtained using a composition of zinc oxide (ZnO) and reduced graphene oxide (RGO) as an n-type buffer layer. RGO-ZnO nanocomposites show higher electron conductivity than intrinsic ZnO; moreover, they show reduced contact resistance at the interface between the active layer and n-type buffer layer. These factors prevent carrier loss resulting from defects and recombinations in the device, thereby significantly increasing the JSC value for the device. Thus, an efficiency of 4.15% was achieved for inverted solar cells with a controlled RGO-ZnO nanocomposites layer.

Mechanism of field emission from chemical vapor deposited undoped polycrystalline diamond films

Field emission characteristics of undoped polycrystalline diamond films with different structural properties have been investigated. By introducing positive bias voltage and/or increasing CH4/H2 ratio, the film quality is significantly deteriorated together with the increase of nondiamond carbon component and the surface morphologies of the films lost their unique facet shape. The reason for the increase of nondiamond carbon content is described in terms of both the increase of substrate temperature and the excessive generation of CHn radicals. It is confirmed that an increase in the nondiamond carbon content markedly enhances field emission properties of diamond films. From the spatial distribution of emission sites, it is suggested that the transport path of field-emitted electrons depends on the nondiamond carbon content: for the film with a large amount of nondiamond carbon, electrons transport preferentially through the conducting mediums such as grain boundaries while for the film with a relatively ...

The structural, optical and secondary electron emission properties of MgO and Mg–O–Cs thin films prepared by ion beam assisted deposition

Abstract Energetic oxygen ion bombardment induces densification and enhances the film crystallinity and refractive index of the evaporated MgO thin films. The optimum oxygen ion bombardment energy is 100 eV and its energy shows have the highest crystallinity and optical properties. The oxygen ion bombardment affects the optical properties that are dependent on the structural properties of the MgO thin films. The ion-induced secondary electron emission property is enhanced more by the cesium incorporation than by the ion beam assisted depsition. This phenomenon is a combined effect of densification of the structure and the reduction of the surface work function.

Work function increase of indium-tin-oxide surfaces by atmospheric air plasma treatment with steady-state airflow

Atmospheric air-plasma treatment of indium–tin–oxide (ITO) surfaces has been investigated as an alternative to a conventional oxygen (O2) vacuum plasma process. For this study, we devised an atmospheric air barrier plasma system having a dimension of 1000×600mm2 and successfully verified a possibility to ignite and maintain an atmospheric pressure discharge only in the ambient air. In particular, we used the steady-state airflow to generate more atomic oxygen radicals as oxygen gas during the vacuum plasma process and to prevent redeposition of the removed or transformed impurities onto the indium–tin–oxide substrate. The x-ray photoemission spectroscopy examination indicated that the adoption of the atmospheric-air plasma treatment reduced the surface content of carbon from 22.1% down to 8.5% and increased that of oxygen from 43% up to 57%. According to the photoelectron spectrometer (AC-1, RIKKEN) result, we obtained a work function of 5.11eV for the treated ITO surfaces after 1min treatment time, which...

Temperature and thermal characteristics of InGaN/GaN vertical light-emitting diodes on electroplated copper

We report the InGaN/GaN multiple quantum well vertical light-emitting diodes (VLEDs) operating at λ ~ 450 nm by the use of laser lift-off and copper electroplating processes. The thermal characteristics of fabricated VLEDs are measured and analyzed in terms of the junction temperature (Tj) using the forward voltage method, which allows us to estimate the thermal resistance (Rth). Between 298 and 378 K, the characteristic temperature is measured to be about 903 K at 350 mA. The far-field patterns of the VLED have a uniform and good near-Lambertian emission. The Tj and Rth values are also confirmed by the emission peak wavelength shift method. The use of electroplated copper with a high thermal conductivity instead of a sapphire substrate provides much better heat dissipation capability. For a 1 × 1 mm2 VLED, the low Tj value of 305.8 K is obtained with an output power of 191 mW at an injection current of 350 mA at 298 K, exhibiting Rth = 7.98 K W−1.

Structural and Electrical Properties of Solution-Processed Gallium-Doped Indium Oxide Thin-Film Transistors

We fabricated solution-processed gallium-doped indium oxide (GIO) thin-film transistors (TFTs). The electrical property, crystallinity, and transmittance were investigated as a function of gallium content. Varying the gallium/indium ratio is found to have a significant effect on structural and electrical properties of thin films. The shrinkage of the lattice of a GIO film originates from substitution of Ga on In sites in the In2O3 lattice, which was verified by X-ray diffraction (XRD) analysis. By increasing the gallium ratio of the channel material, the GIO film shows an amorphous phase. The optimized GIO film (Ga/In= 0.35) has an electron mobility of 3.59 cm2 V-1 s-1, a threshold voltage of 0.1 V, an on/off current ratio of 8.2×107, and a subthreshold slope of 0.9 V/decade, and is highly transparent (~92%) in the visible region.

Improvement of plasma uniformity using ZnO-coated dielectric barrier discharge in open air

The discharge behavior in atmospheric pressure dielectric barrier discharge using ZnO-coated dielectric layer is examined. ZnO thin film on alumina using rf magnetron sputter causes about a factor of a million higher surface conductivity than bare alumina surface. Experimental result shows that discharge uniformity is improved definitely in the case of ZnO-coated dielectric barrier discharge. Increase of surface conductivity stimulates charges to spread over the dielectric surface widely. These charge spreading enhances the uniformity and the stability of atmospheric pressure discharge in open air by initiation of consecutive streamers.

Nucleation and growth control of carbon nanotubes in CVD process

Abstract A catalyst for single-wall carbon nanotube could be achieved by an agglomeration process of co-sputtered Co–Mo thin film on flat silicon wafers. Through the control of catalyst size and carbon flux, SWNTs and double-wall carbon nanotubes were synthesized. The addition of Mo in the Co catalyst makes an effective nucleation site for SWNT formation and the feeding of CH 4 gas in chemical vapor deposition reaction keeps the suitable carbon concentration to grow a single-wall structure.