Kyu Chang Park

Growth of carbon nanotubes with resist-assisted patterning process

The authors developed a growth method for carbon nanotubes (CNTs) by using a resist-assisted patterning process. The CNTs can be grown directly on the patterned catalyst surface without a diffusion barrier. The growth-site patterns were fabricated on a nickel/silicon (Ni∕Si) substrate by a conventional lithography method using a photopatternable resist. The growth mechanism of the CNTs without diffusion barrier was confirmed by Raman spectroscopy and transmission-electron microscope measurement. The carbon-network formation during forming the process is a key parameter for CNT growth. The technique will be applicable to a low-cost fabrication process of electron-emitter arrays.

Deposition of hydrogen‐free diamond‐like carbon film by plasma enhanced chemical vapor deposition

Hydrogen‐free diamond‐like carbon (DLC) films were deposited by the layer‐by‐layer technique using plasma enhanced chemical vapor deposition (PECVD), i.e., the alternative deposition of thin DLC layer and subsequent CF4 plasma exposure on its surface. The hydrogen‐free DLC could be grown on the Si wafer by repeated deposition of the 5 nm DLC layer and subsequent 200 s CF4 plasma exposure on its surface. On the other hand, the conventional DLC deposited by PECVD contains 25 at. % hydrogen inside. The CF4 plasma exposure on the thin DLC layer appears to etch weak C–C bonds and break hydrogen bonds, resulting in a widening optical band gap and increasing conductivity activation energy.

Effect of H2 dilution on the growth of low temperature as‐deposited poly‐Si films using SiF4/SiH4/H2 plasma

We have studied the effect of H2 dilution on the growth of polycrystalline silicon (poly‐Si) using SiF4/SiH4/H2 by a remote plasma chemical vapor deposition. With an increase of H2 dilution ratio the grain size decreases and the polycrystalline fraction increases, probably due to enhancement of the nucleation rate with H2 flow rate. We have deposited polycrystalline silicon films with a crystalline fraction of 89% at a substrate temperature of 330 °C. The peak of the deconvoluted Raman spectrum contributed from the microcrystalline (or amorphous) Si portion in the deposited poly‐Si film increases and the full width at half maximum decreases with increasing H2 flow rate.

Deposition of n-type diamondlike carbon by using the layer-by-layer technique and its electron emission properties

We have studied the electron emission behavior of the diamondlike carbon (DLC) films by plasma enhanced chemical vapor deposition using a layer-by-layer deposition, in which the deposition of a thin layer of DLC and a CF4 plasma exposure on its surface were carried out alternatively. The electron emission current increases with CF4 plasma exposure time. The increase in emission current appears to be due to the n-type behavior of the DLC.

Cell Gap-Dependent Transmittance Characteristic in a Fringe Field-Driven Homogeneously Aligned Liquid Crystal Cell with Positive Dielectric Anisotropy

Transmittance characteristic in a homogeneously aligned liquid crystal (LC) cell driven by a fringe-electric field is investigated as a function of cell gap using the LC with positive dielectric anisotropy. In this device, the fringe-electric field drives the LCs to rotate so that the dielectric torque is electrode-positional dependent, which results in electrode-position dependency in the LCs rotating angle. As the cell gap decreases to 2 ?m, more LCs are affected by surface anchoring, and the LCs above the center of electrodes, in which the LCs are twisted by elastic force between neighboring molecules, are less twisted compared to the 4 ?m cell. Consequently, when the cell gap decreases from 4 to 2 ?m, the transmittance also decreases even though the cell retardation value remains the same.

Optical emission spectroscopy study for optimization of carbon nanotubes growth by a triode plasma chemical vapor deposition

We carried out the in situ analysis of chemical species for the growth of carbon nanotubes (CNTs), deposited by a triode plasma enhanced chemical vapor deposition with a C2H2 and NH3 mixture, using optical emission spectroscopy (OES). A positive mesh bias enhances the radical density, thus increasing the growth rate. The vertically aligned CNTs were grown at a 50% C2H2 flow rate ratio to NH3 and mesh bias voltage of +300V, resulting from the increased CH radical density and the decreased H and CN radical density through the OES analysis.

Effect of electrical aging on field emission from carbon nanotube field emitter arrays

We studied the effect of bias aging on the field emission properties of selectively patterned carbon nanotube field emitter arrays (CNT) grown using the resist-assisted patterning process. After electrical aging using an electric field of 6.87V∕μm for 40h, it was observed that the electron emission properties and uniformity were remarkably improved. X-ray photoelectron spectroscopy spectra show a shift of 0.2eV in the O1s peak and the concentration of oxygen is reduced. Also, transmission electron microscopy measurements showed that Ni catalyst was removed from CNT tips after aging. Therefore, after electrical aging, we obtained enhanced and stable electron emission over a wide current range.

Carbon Nanotube Electron Emitter for X-ray Imaging

The carbon nanotube field emitter array was grown on silicon substrate through a resist-assisted patterning (RAP) process. The shape of the carbon nanotube array is elliptical with 2.0 × 0.5 mm2 for an isotropic focal spot size at anode target. The field emission properties with triode electrodes show a gate turn-on field of 3 V/µm at an anode emission current of 0.1 mA. The author demonstrated the X-ray source with triode electrode structure utilizing the carbon nanotube emitter, and the transmitted X-ray image was of high resolution.

STABILITY OF ELECTRON EMISSION CURRENT IN HYDROGEN-FREE DIAMOND-LIKE CARBON DEPOSITED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

We have studied the electron emission characteristics of the hydrogen-free diamondlike carbon (DLC) and conventional DLC films. The electron emission current of conventional DLC increases at first and then decreases continuously with bias stress time. The emission current of the hydrogen-free DLC, deposited by the layer-by-layer technique, increases at first and then stabilizes with increasing stress time. The resistivity of the hydrogen-free DLC decreases after long bias stress, and that appears to be due to the changes in the density of states in the gap of the hydrogen-free DLC.

Stabilized electron emission from silicon coated carbon nanotubes for a high-performance electron source

The authors show that carbon nanotubes (CNTs) coated with an amorphous silicon layer around their periphery show enhanced and stable electron emission. The CNT-field emitter array was grown on silicon substrate through a resist-assisted patterning process. The CNTs become coated with silicon from the substrate, which is etched and redeposited onto the CNTs. The authors obtained enhanced and stabilized electron emission from the silicon coated CNTs with a turn-on field of 2 V/μm at an emission current density of 1 μA/cm2. The structure and electron emission properties of the functionalized emitters are discussed.