Woon Jo Cho

Transparent, Low Resistance, and Flexible Amorphous ZnO-Doped In2O3 Anode Grown on a PES Substrate

Transparent and low resistance amorphous ZnO-doped In 2 O 3 (IZO) anode films were grown by radio-frequency (rf) sputtering on an organic passivated polyethersulfone (PES) substrate for use in flexible organic light-emitting diodes (OLEDs). Under optimized growth conditions, a sheet resistance of 15.2 Ω/□, average transmittance above 89% in the green range, and a root mean square roughness of 0.375 nm were obtained, even for the IZO anode film grown in a pure Ar ambient without the addition of oxygen as a reactive gas. All of the IZO anode films had an amorphous structure regardless of the rf power and the working pressure due to the low substrate temperature of 50°C and the structural stability of the amorphous IZO films. In addition, an X-ray photoelectron spectroscopy depth profile obtained for the IZO/PES showed no obvious evidence of interfacial reactions between the IZO anode and the PES substrate, except for some indiffusion of oxygen atoms from the IZO anode. Furthermore, the current-voltage-luminance of the flexible OLEDs fabricated on IZO anode was found to be critically dependent on the sheet resistance of the IZO anode.

Thickness dependence of room temperature permittivity of polycrystalline BaTiO3 thin films by radio-frequency magnetron sputtering

Polycrystalline BaTiO3 thin films with thickness ranging from 2100 to 20u2009000 A were prepared on platinum substrates using off-axis radio-frequency magnetron sputtering. The variation in room temperature permittivity of the films was investigated with respect to thickness using x-ray diffraction and transmission electron microscopy. All films were ferroelectric and their room temperature permittivity, which was significantly higher than previously reported values, showed a strong dependence on film thickness. Higher permittivity was attributed primarily to the presence of ferroelectric domains. The room temperature permittivity of the thin films showed large variations with grain size, as in the case of BaTiO3 ceramics. The increase in permittivity with increasing film thickness was attributed to the decrease in defect concentration with grain growth. The 20u2009000 A film showed an abrupt decrease in permittivity and the presence of an intergranular phase having titanium-excess composition; these phenomena ar...

Polarization-insensitive quantum-well semiconductor optical amplifiers

Theoretical modeling and fabrication of polarization-insensitive semiconductor optical amplifiers that use a multi-quantum-well structure as the gain media are reported. Polarization insensitivity of gain is achieved through the introduction tensile strain into the quantum wells. Gain calculations, using the k/spl middot/p method, were performed to obtain the required amount of tensile strain to obtain polarization insensitivity over a wide energy spectrum. Fabricated amplifiers show a polarization-insensitive (<1 dB) spectral width of 10 nm at 1300 nm in the InGaAsP/InP system, 15 nm at 1300 nm in the AlInGaAs/InP system, and 40 nm at 1550 nm in the AlInGaAs/InP system.

Optical properties of silicon nanoparticles by ultrasound-induced solution method

White-light-emitting silicon nanoparticles, whose surfaces were passivated with butyl, were prepared using a focused ultrasonic energy. The white light was achieved by controlling only the size distribution without adding any fluorescent ions. The white-light-emitting silicon nanoparticles had a wide size distribution of 1–5 nm and an average size of 2.7 nm, which were sufficiently small to indicate the quantum confinement effect for silicon. The photoluminescence spectrum covered a wide range of 320 nm–700 nm with a full width at half maximum of approximately 190 nm.

Polarization-insensitive optical amplifiers in AlInGaAs

We report the theoretical modeling and the fabrication of polarization-insensitive optical amplifiers at 1300 nm in AlInGaAs-InP material system. Gain calculations, using the k.p method, show that the introduction of 0.33% tensile strain into a three-quantum-well structure can achieve gain-matching over a wide energy spectrum. The amplifiers, fabricated and tested, show excellent polarization insensitivity (less than 0.3 dB) at 1280 nm with a gain of 11 dB at 150 mA. Gain-bandwidth needs to be improved by employing antireflection coatings to suppress the facet reflectivity.

Ninety Degree Domains in RF-Sputtered BaTiO3 Thin Films on Platinum Substrates

We report 90° domains in (110)-oriented polycrystalline BaTiO3 thin films. The domain and crystal structures of the thin films were investigated using transmission electron microscopy and high temperature X-ray diffraction, respectively. From X-ray diffraction, it was found that the prepared films had a (110)-preferred orientation and a tetragonal structure at room temperature. Observation in the plan-view and cross-sectional modes using transmission electron microscopy clearly indicated the presence of a 90° domain structure. In the cross-sectional mode, a 90° domain structure was observed along the film growth direction and in the plan-view mode, it was also found that a 90° domain boundary was initiated from the (111) twin boundary. The formation of a 90° domain boundary was discussed in terms of planar defects such as the (111) twin and we also proposed two kinds of stress relief mechanisms induced by 90° domain formation.

Energy transfer from Tm3+ to Er3+ in zirconiumfluoride glasses pumped by a 786 nm laser diode

The upconversion of 786 nm to visible fluorescence was studied for Tm3+ and Er3+ in both Tm3+-Er3+-codoped and Er3+ singly doped ZrF4-based fluoride glasses for several dopant concentrations. It was found that the addition of Tm3+ enhanced the upconversion efficiencies of the green and the red emission corresponding to the transitions from the 4S3/2 and 4F9/2 levels of the Er3+ ion. The increase in the emission of samples containing 2 mol% TmF3 and 1.5 mol% ErF3 amounted to a factor of about 3 for the green emission and a factor of 300 for the red emission. The 1.48 µm emission from the transition of 3F4 to 3H4 for the Tm3+ ion decreased with increasing Er3+ concentration. The mechanism of the energy transfer from Tm3+ to Er3+ was discussed.

Rare earth doped planar waveguides in Zirconia

Zirconia (ZrO2) thin dielectric optical films demonstrate excellent transmission characteristics in the infrared for wavelengths [1] in the 1-5µm range important for optical communications.

Origin of (111) Twin Lamellae in BaTiO3 Thin Films on Platinum Substrates.

The formation mechanisms of (111) twin lamellae in BaTiO3 thin films were examined using transmission electron microscopy. The voids in thin films were connected with (111) twin lamellae. These voids were formed due to the incomplete coalescence of islands, and it was found that (111) twin lamellae were produced in order to accommodate the small misfit between the growing islands during contact. With respect to planar defects, the cause of the formation of (111) twin lamellae could be explained by the maintenance of a TiO6 octahedron and the relationship with a hexagonal BaTiO3 structure. It could be understood that the very narrow width of the twin lamellae is due to the contact process of two growing islands as well as the very small grain size of thin films.

Electrical characteristics of nano-crystal Si particles for nano floating gate memory

Nonvolatile memory device was fabricated by using Nano-Crystal(NC)-Si particles. NC-Si particles had a wide size distribution of 1∼5nm and an average size of 2.7nm, which were sufficiently small to indicate the quantum effect for silicon. The memory window was analyzed by C-V characteristic of NC-Si particles. V d -I d , V g -I d characteristics of the fabricated device were also measured.