Seong-Je Park

Anisotropic tuning behavior in epitaxial Ba0.5Sr0.5TiO3 thin films

The tuning properties of epitaxial Ba0.5Sr0.5TiO3 (BST) thin films were investigated by a scanning microwave microscope (SMM) and an LCR meter. Although the BST films on LaAlO3 and MgO substrates showed similar tuning behavior when measured by the LCR meter at 1 MHz, remarkably different tuning properties were observed in the planar capacitors measured by SMM. The BST films on LaAlO3 substrates were hardly tuned when measured by SMM, while the BST films on MgO showed significant tuning behavior between the electrodes. We attribute these different tuning properties to the anisotropic tuning caused by the strain in BST films. This will enable the design of much improved tunable devices while minimizing the loss associated with the dielectric.

Photo-induced hybrid nanopatterning of titanium dioxide via direct imprint lithography

A novel ultraviolet (UV)-assisted imprinting procedure that employs photosensitive titanium(IV) di-n-butoxide bis(2-ethylhexanoate) is presented for the fabrication of well-ordered titanium dioxide (TiO2) nanostructures at room temperature. The main novelty of this technique is the use of the photosensitive titanium organic compound, rather than a commonly used UV-curable resin, for direct UV-assisted nanoimprint lithography. Fourier transform infrared and X-ray photoelectron spectroscopy studies suggest that exposure to UV light resulted in the gradual removal of organic groups from films prepared from titanium(IV) di-n-butoxide bis(2-ethylhexanoate) photochemically and successively converted the films to TiO2 at room temperature. This approach allows direct fabrication of TiO2 nanopatterns with lines down to 35 nm in width, hole arrays of 265 nm in diameter, and three-dimensional TiO2 hybrid micro/nano-patterns without observable defects for use in applications where ordered surface nanostructures are required, such as photovoltaics, photonics, and optical waveguides.

An Antireflective Nanostructure Array Fabricated by Nanosilver Colloidal Lithography on a Silicon Substrate

An alternative method is presented for fabricating an antireflective nanostructure array using nanosilver colloidal lithography. Spin coating was used to produce the multilayered silver nanoparticles, which grew by self-assembly and were transformed into randomly distributed nanosilver islands through the thermodynamic action of dewetting and Oswald ripening. The average size and coverage rate of the islands increased with concentration in the range of 50–90 nm and 40–65%, respectively. The nanosilver islands were critically affected by concentration and spin speed. The effects of these two parameters were investigated, after etching and wet removal of nanosilver residues. The reflection nearly disappeared in the ultraviolet wavelength range and was 17% of the reflection of a bare silicon wafer in the visible range.

Nanosilver Colloids-Filled Photonic Crystal Arrays for Photoluminescence Enhancement

For the improved surface plasmon-coupled photoluminescence emission, a more accessible fabrication method of a controlled nanosilver pattern array was developed by effectively filling the predefined hole array with nanosilver colloid in a UV-curable resin via direct nanoimprinting. When applied to a glass substrate for light emittance with an oxide spacer layer on top of the nanosilver pattern, hybrid emission enhancements were produced from both the localized surface plasmon resonance-coupled emission enhancement and the guided light extraction from the photonic crystal array. When CdSe/ZnS nanocrystal quantum dots were deposited as an active emitter, a total photoluminescence intensity improvement of 84% was observed. This was attributed to contributions from both the silver nanoparticle filling and the nanoimprinted photonic crystal array.

Efficient three-dimensional nanostructured photoelectric device by Al-ZnO coating on lithography-free patterned Si nanopillars

An efficient three-dimensional (3D) nanostructure photoelectric device is presented. An Al-doped ZnO (AZO) coating was applied to lithography-free patterned Si nanopillars and spontaneously formed a radial heterojunction (n-AZO/p-Si) photodiode having a quality ideality factor of 1.64. A significantly enhanced photocurrent of 5.45 mA/cm2 was obtained from the 3D nanostructure relative to that of a planar substrate (1.1 mA/cm2). This enhancement is induced by enlargement of the light-active surface area and an anti-reflection effect. Due to the intermediate refractive index of AZO, the reflection was distinctively reduced in the air-Si system. It discusses an effective approach for realizing nanostructured photoelectric device.

Dielectric properties of pulsed-laser deposited SrTiO3 films at microwave frequency ranges

The dielectric constant and loss tangent of SrTiO3 thin films were characterized under the influence of an applied dc voltage at about 3.64 GHz. The measurement was carried out utilizing a gold resonator with a flip-chip capacitor at cryogenic temperatures. The analysis of the experimentally observed capacitance and quality factor served to give a measure of the dielectric constants and the loss tangents of the SrTiO3 film at microwave ranges, respectively. A dielectric constant of 830 and a low loss tangent of 6×10−3 at 3.64 GHz were observed at 90 K and 100 V. The dielectric loss decreases as the bias voltage increases. In addition, the quality of the SrTiO3 film is presented in terms of fractional frequency under the bias voltages and cryogenic temperatures.

Imprinted Pattern Profile-Dependent Optical Properties of Metal Nanostructures

As-imprinted right-edged pillar structures were reconfigured into a tapered sidewall profile by CHF3-based reactive ion etch. The transmittance spectra improved as the sidewall became more tapered for longer etch times. The effect was most distinctive (28.4%) in the transmittance trench zone at wavelengths from 460 to 470 nm owing to the reduced diffraction scattering loss. The transmittance enhancement for silver-coated corrugated nanostructures was even greater (57.0%) for a tapered sidewall structure, in close agreement with the predictions of simulations. The infrared transmittance was notably reduced compared with that in the UV–visible zone, suggesting the possible applications of the structures in heat-insulated windows.

Fabrication of large area nanotemplate through nanosilver colloidal lithography

An alternative colloidal lithography applying nanosilver colloids has been developed for the fabrication of wafer-level large area nano templates. The nanotemplate consists of random arrays of nanopillar structures in the range of 50 to 200 nm. This approach uses nanosilver differently from the conventional colloidal lithographic methods applying silica or polystyrene with much smaller colloidal size in the range of 10 – 30 nm than those. The macroscopic uniformity was therefore much improved when spin coating deposited continuous multi-layered nanosilvers. A random array of isolated nanosilver islands with open spaces between them was created through optimized thermal annealing on a silicon substrate. Dimensional properties of nanosilver islands can be controlled by varying the following parameters: nanosilver concentration, spin-coating speed, and thermal annealing time. Subsequent etching of substrate silicon produced various nanopillar profiles, such as completely anisotropic, isotropic, and inversed diamond shapes with the meaningful controllability through varying well-known etch conditons.