Papers by Author: Junsin Yi

Abstract: Nonvolatile memory (NVM) devices with nitride-nitride-oxynitride (NNO) stack structure using Si-rich silicon nitride (SiN_x) as charge trapping layer on glass substrate were fabricated. Amorphous silicon clusters existing in the Si-rich SiN_x layer enhance the charge storage capacity of the devices. Low temperature poly-silicon (LTPS) technology, plasma-assisted oxidation/nitridation method to form a uniform ultra-thin tunneling layer, and an optimal Si-rich SiN_x charge trapping layer were used to fabricate NNO NVM devices with different tunneling thickness 2.3, 2.6 and 2.9 nm. The increase memory window, lower voltage operation but little scarifying in retention characteristics of nitride trap NVM devices had been accomplished by reducing the tunnel oxide thickness. The fabricated NVM devices with 2.9 nm tunneling thickness shows excellent electrical properties, such as a low threshold voltage, a high ON/OFF current ratio, a low operating voltage of less than ±9 V and a large memory window of 2.7 V, which remained greater than 72% over a period of 10 years.

Abstract: Nanostructured vanadium pentoxide films have been synthesized by using a sol–gel technology from V2O5 powder and hydrogen peroxide. The V2O5 powder was dissolved in hydrogen peroxide solution, agitated and heated up to 65oC to form gel by the dissociation of the peroxide complexes. The obtained gel was deposited by dip coating technique and dried in air at room temperature. Structural, morphological and compositional analyses were carried out on the prepared samples using X-Ray Diffractometer (XRD), Raman spectrometer and scanning electron microscopy (SEM). The as-prepared films show an amorphous nature, while those annealed at 400oC exhibit orthorhombic structures. The films seem to have grain like structures on annealing which are expected to help the gas sensing properties of the V2O5 films. The annealed films were connected with copper electrodes and used as sensing element. The change in the resistance of the sensing element with respect to the test gas concentration was measured by noting down the resistance at each concentration. Sensitivity of the material linearly increased with different concentrations of ethanol and ammonia. It is clearly seen that the material has more sensing response for ethanol when compared to that of ammonia.

Abstract: Multiform morphologies of hydroxyapatite nanostructures were synthesized by different methods. Some novel methods were adopted for preparing different morphologies. The morphological analysis confirms that the preparation method greatly influences the morphological characterization. The morphological analysis further confirmed by the TEM analysis. The nanostructure clearly depicts the growth stages of the HAp. The interplanar distances measured in segments (fringes) of the HRTEM micrograph were ~0.35 nm, corresponding to the interplanar spacing of the [002] plane of the hexagonal hydroxyapatite. X-ray Diffraction (XRD) measurements indicate the formation of crystalline hexagonal hydroxyapatite. The morphological dependent properties were analyzed in detail.

Abstract: Cerium-iron oxide composite nanorods were synthesized by a surfactant free precipitation method. The synthesized products were characterized by XRD, FESEM, BET and TEM. Increase in the mole concentration causes the morphology of the cerium oxide to change from nanorods into nanocubes. X-ray diffraction pattern shows a diffraction peak at 28.5° corresponding to (111) reflection plane normal to c axis of a cubic fluorite structure of CeO2 and also hexagonal phase of Fe2O3 and orthorhombic phase of FeO (OH). EDAX analysis on individual nanorod conform the presences of Iron, which supports the XRD spectra. From transmission electron microscopy (TEM), the length and width of the nanorods were estimated in the range of ~100-350 nm and ~20-40 nm respectively. The Brunauer Emmett Teller (BET) analysis showed the increase in surface area for the obtained nanorods with respect to the increase in Fe concentration which in turn enhanced the formation and growth of nanorods.

Abstract: We investigated the structural and electrical properties of the 0.5% Ce-doped Ba(ZrxTi1-x)O3 (BZT) thin films with a mole fraction of x=0.2 and a thickness of 150 nm for the MLCC (Multilayer Ceramic Capacitor) application. Ce-doped BZT films were prepared on Pt/Ti/SiO2/Si substrates by a RF magnetron sputtering system as a function of Ar/O2 ratio and substrate temperature. X-ray diffraction patterns were recorded for the samples deposited with three different substrate temperatures. The thickness and the surface roughness of the films deposited with different Ar/O2 ratios were measured. The oxygen gas, which was introduced during the film deposition, had an influence on the growth rate and the roughness of the film. The surface roughness and dielectric constant of the Ce-doped BZT film varied with Ar to O2 ratios (5:1, 2:1, and 1:1) from 1.21 nm to 2.33 nm and 84 to 149, respectively. The Ce-doped BZT film deposited at lower temperature has small leakage current and higher breakdown voltage.