Geun-Hyoung Lee

Difference in optical bandgap between zinc-blende and wurtzite ZnO structure formed on sapphire (0001) substrate

Two photoluminescence peaks were observed from ZnO film formed by the oxidation of zinc blende ZnS film on sapphire (0001) substrate. The emission peaks correspond to hexagonal and columnar ZnO nanocrystallites. X-ray diffraction patterns show that the hexagonal and columnar crystals indicate c-axis oriented wurtzite and zinc blende ZnO structure, respectively. The difference in the emission energies was 0.10 eV, which is very close to the calculated difference in energy gap between zinc blende and wurtzite ZnO structure. With increasing thickness of ZnO, only the hexagonal crystallites were observed on the film surface and therefore the intensity of the emission peak of 3.27 eV from wurtzite structure dominates the spectrum.

Very Thin TiO2 Films Prepared by Plasma Enhanced Atomic Layer Deposition (PEALD)

TiO2 thin films were prepared by plasma-enhanced atomic layer deposition (PEALD) with an alternating supply of reactant sources, Ti(N(CH3)2)4; Tetrakis(dimethylamino)titanium and O2 plasma. The uniform and smooth TiO2 thin films were successfully deposited by PEALD and the film thickness per cycle was saturated at approximately 0.36 Å at 200°C. Excellent step coverage of TiO2 thin films was obtained by Ti(N(CH3)2)4 chemisorbed chemical reactions with O2 plasma on the surface. While dielectric constant of TiO2 films slightly decreased with increasing annealing temperature, the leakage current characteristics of TiO2 films were significantly improved with increasing annealing temperature. The leakage current density of a 36 nm-TiO2 film on Si substrate annealed at 600°C was approximately 1 × 10− 6 A/cm2 at 400 kV/cm.

Ga2O3 THIN FILM DEPOSITED BY ATOMIC LAYER DEPOSITION WITH HIGH PLASMA POWER

Abstract Ga2O3 thin films were deposited at 200°C on p-type Si (100) by plasma-enhanced atomic layer deposition technique with an alternating supply of reactant source, [(CH3)2GaNH2]3, and high-power oxygen plasma of 180 W. The as-deposited thin films were annealed at 500, 700, 900°C in oxygen ambient for 10 min in a rapid thermal annealing system, respectively. X-ray diffractometer and atomic force microscope were used to investigate the structural properties and the surface morphologies of the thin films. The as-deposited thin film was amorphous and the thin films annealed at high temperatures were monoclinic β -phase Ga2O3. The electrical properties of Pt/Ga2O3/Si structured thin film were investigated using a semiconductor parameter analyzer. It was found that the as-deposited thin film and the thin film annealed at 500°C were leaky, however, the insulating properties of the thin films annealed at high temperatures were greatly improved. Spectroscopic ellipsometry was also used to derive the refractive indices and the thicknesses of the thin films.

Effect of deposition flux on surface roughness of LiNbO3 thin film grown on sapphire substrate by pulsed laser deposition

Abstract The variation of surface morphology and roughness of LiNbO 3 film on sapphire substrate with deposition flux was investigated. The film deposition was carried out by pulsed laser deposition (PLD) and then flux was controlled with frequency of pulsed laser. With increasing the deposition flux, that is the frequency of laser pulse, the film surface became smoother and thus the surface flatness was improved. In case of the frequency of 15 Hz, the rms roughness of the film surface was constrained below 2.0 nm even at the film thickness of about 300 nm.

Effect of Oxidation Temperature on the Morphology of ZnO Nanowires Fabricated by Directed Melt Oxidation of Al-Zn Alloy

ZnO nanowires were synthesized by directed melt oxidation of Al-Zn alloy at the temperature over 900°C in an air. Straight type nanowires were formed on the surface of the alloy oxidized at 950°C. However for the alloys oxidized over 1000°C, tetrapod-shaped ZnO nanowires with four legs, extended from center, were found in high yield and distributed uniformly on the whole surface. No other shaped crystals and powders were found. X-ray diffraction (XRD) pattern revealed that the nanowires were ZnO with wurtzite structure of hexagonal phase. The diameter of nanowires was controlled to 80 nm and the length to 20 micrometers. Higher temperature produced nanowires with larger diameter and longer length.

TRANSPARENT TITANIUM DIOXIDE THIN FILM DEPOSITED BY PLASMA-ENHANCED ATOMIC LAYER DEPOSITION

ABSTRACT TiO2 thin films were deposited at 200°C on Si (100) and quartz substrates using plasma enhanced atomic layer deposition (PEALD) with alternating supply of reactant source, Ti[N(CH3)2]4, and oxygen plasma, and then were annealed at various temperatures in oxygen ambient using a rapid thermal annealing (RTA) system. While dielectric constant of TiO2 films slightly decreased with increasing annealing temperature, the leakage current characteristics of TiO2 films were significantly improved with increasing annealing temperature. The leakage current density of a 40 nm-TiO2 film on Si substrate annealed at 900°C was approximately 8× 10−8 A/cm2 at 150 kV/cm. All of the TiO2 thin films showed high transmittances as high as 70∼ 97% in the pertinent wavelength range and annealed TiO2 films exhibited a bandgap energy of about 3.30 eV. Raman spectra also confirmed the formation of anatase phase in TiO2 films annealed by RTA process.

Initial growth behaviour of LiNbO3 films on α-Al2O3 substrates with atomic scale step structure

Abstract Evolving morphology of the LiNbO3 films growth on C(001), A(110) and R(102) α-Al2O3 substrates with atomic scale step structure was investigated during the initial stage of the growth. On C(001) substrate, LiNbO3 nucleated randomly irrespective of the step structure on the substrate, while this material showed preferential nucleation along the step edges on the surface of A(110) and R(102) substrates. This tendency of initial growth of LiNbO3 on A(110) and R(102) substrates enhances the step-flow growth, allowing the fabrication of nanometric wire and smoother films.

Effects of Oxygen Working Pressure on Structural and Optical Properties of TiO2 Films Grown on Glass Substrate

TiO2 thin films were grown on glasses using a pulsed laser deposition (PLD) system. In order to fully clarify the correlation between their properties and the working pressure of oxygen at which they are grown, a wide range of pressures from 10 to 75 mTorr was investigated. The optical gap of TiO2 films gradually increased from 3.3 to 3.41 eV with increasing working pressure up to 25 mTorr but became notably large at 50 mTorr and continued to increase significantly with further pressure increase. X-ray diffraction (XRD) results showed that the films grew preferentially along the (101) orientation. (101) peak intensity increased as working pressure increased up to 25 mTorr. However, the peak intensity began to decrease and eventually disappeared with further increase in working pressure, indicating structural changes from crystalline to disorderlike phases. The TiO2 films showed high crytallinity with large grains at 25 mTorr but showed a columnar structure with small grains at 75 mTorr. The growth mode of TiO2 thin films was observed to change from lateral to islandlike growth in response to the increase in working pressure. The correlation between oxygen pressure and film properties is discussed in terms of structural and compositional changes.

Characterization of ZnO Tetrapods Prepared by a Simple Oxidation of Zn Plate in Air Atmosphere

ZnO tetrapods were synthesized by a simple evaporation and oxidation of Zn plate in air atmosphere. The Zn plate was placed in an alumina crucible and the crucible was simply inserted into a furnace at 930 °C without any additives. The oxidation time was changed in the range of 30 to 120 min to investigate the effect of oxidation time on the morphology and optical property of ZnO tetrapods. The X-ray diffraction (XRD) results showed the ZnO tetrapods had wurtzite structure. The field emission scanning electron microscopy (SEM) images revealed that the shape of the tetrapod legs changed from needle to hexagonal shape, and then to cylindrical shape with the increase in oxidation time. On the other hand, the highest UV/green emission ratio was observed for the ZnO tetrapods which consisted of legs with hexagonal shape. In addition, the dead spots in cathodoluminescence (CL) images were observed at the center of the tetrapods.

Morphological Variation and Luminescence Properties of ZnO Micro/ Nanocrystals Synthesized by Thermal Evaporation Method

ZnO micro/nanocrystals with different morphologies were synthesized by thermal evaporation of various zinc source materials in an air atmosphere. Zinc acetate, zinc carbonate and zinc iodide were used as the source materials. No catalysts or substrates were used in the synthesis of the ZnO crystals. The scanning electron microscope(SEM) image showed that the morphology of ZnO crystals was strongly dependent on the source materials, which suggests that source material is one of the key factors in controlling the morphology of the obtained ZnO crystals. Tetrapods, nanogranular shaped crystals, spherical particles and crayon-shaped crystals were obtained using different source materials. The X-ray diffraction(XRD) pattern revealed that the all the ZnO crystals had hexagonal wurtzite crystalline structures. An ultraviolet emission was observed in the cathodoluminescence spectrum of the ZnO crystals prepared via thermal evaporation of Zn powder. However, a strong green emission centered at around 500 nm was observed in the cathodoluminescence spectra of the ZnO crystals prepared using zinc salts as the source materials.