Papers by Author: Hee Joon Kim

Abstract: The characteristics, such as electron temperature and the electron density, of CF4/Ar discharge in 2.45GHz microwave has been investigated by using a Langmuir probe with the microwave power and position. The results showed that the electron temperature and the electron density decrease with increasing distance from the plasma source. Increasing power enhances the dissociation and ionization of gas, and increases the electron densities. The electron temperature was decreased by reducing the mean free path of electrons with increasing microwave power. The electron temperature is 7.63 ~ 2.49 eV, and the electron density is 0.85×1011 ~ 4.3×1011 cm-3. From obtained electron energy distribution function, we known that high energy electron decreased with increasing microwave power and distance from the plasma source. The generated plasma by developed our system has good quality as results of Debye length λD = 35.8 ~ 67.3 μm, and Ln(ND) = 33.4 ~ 35.2.

Abstract: This work examined the mechanical properties of alumina that can directly be enhanced by ratio of nano sized alumina powders additives to micro size alumina powders (n/m ratio). These new materials have mechanical properties that are strongly grain size dependent and often significantly different from those of their coarser grained counter parts. The mechanical characteristics of samples were observed by using the indentation test system. This data shows that the relative density for the sample is increased with increasing Meyer hardness.

Abstract: The decomposition characteristics of CF4 with Argon or oxygen in 2.45GHz microwave has been investigated by using a Langmuir probe with variation of the microwave power and chamber pressure. For CF4/Ar and CF4/O2/Ar discharges, the ion density and the electron density decrease with increasing microwave power. The electron temperature was decreased by reducing the mean free path of electrons with increasing microwave power. Also with increasing pressure, the electron temperature increase, and ion and electron density decrease by increase of inelastic collision frequency and of collision with the walls in the chamber. The electron temperature is 13.6 ~ 5.9 [eV], the electron density is 4.4×1010 ~ 2.2×1010 [cm-3] and ion density is 5.2×1011 ~ 4×1010 [cm-3]. According as add oxygen, ion and electron density increased relatively comparing to CF4/Ar discharge. The electron temperature is 8.5 ~ 6.2 [eV], the electron and ion density is 5.1×1010 ~ 2.1×1010 [cm-3] and 3.7×1011 ~ 7.3×1010 [cm-3], respectively.

Abstract: In this paper, elastic moduli of nanostructured alumina are evaluated by simultaneous measurement of longitudinal and shear wave velocities using mode-converted ultrasound in scanning acoustic microscope (SAM). Mode-converted longitudinal and shear waves inside alumina sample are captured to calculate acoustic wave velocities and determine elastic constants such as Young’s modulus and Bulk modulus. Al2O3 nanostructured alumina samples are formed by compacting micro-sized Al2O3 powder with nano-sized Al2O3 powder from 10wt% to 50wt%, and tested by SAM to investigate elastic moduli. A correlation is found from experiment that the more percentage of nano-particles are added, the higher elastic moduli are obtained. It is also shown that the mode-converted ultrasound is sensitive enough to characterize mechanical modulus of nanostructured alumina quantitatively.

Abstract: Polymer light emitting diodes (PLEDs) with ITO/PEDOT:PSS/MEH-PPV/LiF/Al structures were prepared by spin coating method on the patterned ITO(indium tin oxide)/glass substrates. MEH-PPV [Poly(2-methoxy-5(2-ethylhexoxy)-1,4-phenylenevinyle)] and PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrene sulfolnate)]
polymers were used as the light emitting and hole transport materials. The dependence of the plasma treatment of ITO anode films on the optical and electrical properties of the PLEDs was investigated. The sheet resistances increased with increasing the plasma intensities from 40W to 300W in RF power. In contrast, the surface roughness was improved as the plasma intensity increased. The maximum current density and luminance were found to be about 97.5 mA/
and 55 cd/m2 at 8 V for the PLED sample coated on ITO/glass substrate with plasma treatment of 100W for 30s under 40 mtorr O2 pressure.

Abstract: Thermal diffusivity of nonstoichiometric PrMnO3 and NdMnO3 phases were measured by laser flash method from room temperature to 1100 K, in addition to the data of electrical conductivity, thermal analysis and high temperature X-ray diffractometry to detect the phase transition. The thermal diffusivity curve varied with increasing temperature and showed a clear anomaly with a sudden dip at the phase transition temperature. The transition temperature decreases with oxygen nonstoichiometry in each phase.

Abstract: Optical emission spectra from plasma during deposition of diamond film were investigated by an optic multi-channel spectrometer using a CCD array sensor. The diamond film was deposited by DC plasma enhanced (PE) chemical vapor deposition (CVD) using hydrogen and methane gas mixture, where substrate was located at near the plasma and the discharge was performed by intermittent discharge. When Pg during the deposition was increased from 50 to 250 Torr, the optical emissions of hydrogen (Hα and Hβ) and C2 were increased, and corresponding to these increases, deposition rate of the diamond film was increased and crystalline quality became superior. When Cm was changed from 1 to 3 %, the emission from C2 was increased, and whereas, the emission from hydrogen was decreased. Corresponding to these changes of the emission, the deposition rate of the film was increased and amorphous component in the deposited film was also increased. These results show that the increase of C2 results in the increase of the deposition rate, and increase of hydrogen is effective to eliminate amorphous component, and therefore, monitoring of the optical emission from hydrogen and C2 is useful for the deposition process of the diamond film.

Abstract: In the paper, zinc oxide (ZnO) thin films are deposited by plasma enhanced chemical vapor deposition (PECVD) at different substrate temperatures. The ZnO films are characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The analysis results indicate that highly crystalline films with high orientation can be obtained at a substrate temperature of 300 oC with 50 ml/min flow rate from Diethylzinc (DEZ). Furthermore, the investigation of optical property shows that ZnO films are transparent, and the peak transmittance in the visible region is as high as 85%.

Abstract: In this paper, nano-sized Al2O3 powders are synthesized by a thermal MOCVD (Metal Organic Chemical Vapor Deposition) combined with plasma. The effect of reaction temperature on the characteristics of the synthesized Al2O3 powders is investigated. The experimental results demonstrate that while the temperature is increased from 200oC to 1000oC, the mean diameter of Al2O3 powders reaches from 400nm to 10nm. Hence, the increment of temperature can promote the synthesis of fine Al2O3 particle. Furthermore, the powder morphologies and crystallite size are also examined by the transmission electron microscopy (TEM) and XRD. Based on TEM observation, it is found that the particles are sphere shape. The XRD analysis shows that the particles are typical γ-Al2O3 crystalline phase over 400oC. Lastly, the effect of plasma on the characteristic of Al2O3 synthesized by thermal MOCVD is also considered.

Abstract: In order to fabricate an alumina ceramics with high density at low sintering temperature, nanosized γ–Al2O3 powders with average size of 9.7 nm were added to microsized γ–Al2O3 powders with 2 #m and they were well mixed. Its sintering behavior was studied in the temperature range of 1000oC to 1300oC and in holding time from 1 hour to 10 hours. Compacted samples with a different mixed ratio of nanosized and microsized Al2O3 powders (N/M ratio) were prepared and pressured at 1 GPa in a uniaxial direction. The phase transformation from γ–Al2O3 to α–Al2O3 takes place at 1100oC for 1hour sintering in all compacted samples. This rate is increased with increasing N/M ratio. The relative density varied from 70% to 95% depending on temperature and N/M ratio. With increasing sintering temperature from 1000oC to 1300oC, it was changed from 70% to 93%. Especially, the relative density was enhanced about 9% higher than that of only microsized sample by only 10 wt% addition of nanosized powders.