Hosuk Lee

Highly Transparent, Low Resistance, and Cost-Efficient Nb:TiO2/Ag/Nb:TiO2 Multilayer Electrode Prepared at Room Temperature Using Black Nb:TiO2 Target

Highly transparent, low resistance, and cost-efficient Nb-doped TiO 2 (NTO)/Ag/NTO multilayer electrodes were fabricated using tilted dual-target sputtering at room temperature without in situ or postannealing processing. Due to the effective antireflection and low resistivity properties of the Ag layer sandwiched between the NTO layers, an NTO/Ag (9 nm)/NTO electrode fabricated at room temperature showed a low sheet resistance of 5.26 Ω/□, a high optical transmittance of 86%, and a figure of merit value of 42.47 × 10- 3 Ω -1 even though the NTO layer had an amorphous structure. The NTO/Ag/NTO multilayer electrode is a promising indium-free and cost-efficient transparent conducting electrode substitute for conventional Sn-doped In 2 O 3 electrodes due to the low temperature process, low cost of required elements, and stability of the NTO layer.

Effect of density of localized states on the ovonic threshold switching characteristics of the amorphous GeSe films

We investigated the effect of nitrogen (N) doping on the threshold voltage of an ovonic threshold switching device using amorphous GeSe. Using the spectroscopic ellipsometry, we found that the addition of N brought about significant changes in electronic structure of GeSe, such as the density of localized states and the band gap energy. Besides, it was observed that the characteristics of OTS devices strongly depended on the doping of N, which could be attributed to those changes in electronic structure suggesting a method to modulate the threshold voltage of the device.

Optical Properties of Ultrathin Copper Thin Films Sandwiched between Nb-Doped TiO2 Films Studied with Spectroscopic Ellipsometry

Cu thin films sandwiched between Nb-doped TiO2 (NTO) thin films were grown on glass substrates using tilted-dual target DC magnetron sputtering deposition. The thicknesses of the top and bottom NTOs were nominally 30 nm, and the thicknesses of the Cu films (t) varied between 1.5 nm and 50 nm. We measured the ellipsometric angles (Ψ,Δ) of the NTO/Cu/NTO/glass by using spectroscopic ellipsometry and estimated the thicknesses and dielectric functions of the Cu films by using a multi-layer model analysis. Transmission electron microscopy measurements showed that the Cu layers evolved from aggregates of Cu nanoparticles to coalesced Cu thin films as the Cu film thickness increased. Sheet resistance data also showed that the t = 8 nm film was near the percolation threshold, suggesting that films thinner than 8 nm were aggregations of Cu nanoparticles that were not well-connected. The films thicker than 8 nm were above the percolation threshold. From the Drude model, we estimated the plasmon frequency (ωp) and the electron relaxation time (τ), which were found to increase with increasing film thickness. By applying standard critical point analysis to the second derivatives of the dielectric function spectra, we found several peaks near 1.5, 2.1, 2.5, 3.5, and 4.3 eV, and attributed to interband transitions. The peak energies (except 1.5 eV) matched to the band structure calculations of bulk Cu found in the literature.

Dielectric Function and Electronic Excitations of Functionalized DNA Thin Films

We measure the dielectric functions of organic-soluble, functionalized DNAs bearing functional moieties in the near-infrared, visible, and ultra-violet spectra by using spectroscopic ellipsometry. Natural double-stranded DNA is dissolved in water and reacted with carbazole-based trimethyl ammonium bromide, cetyltrimethylammonium bromide, and chalcone-terminated trimethyl ammonium bromide. The functional DNA products are all precipitated and filtered for washing and drying. We successfully prepare functionalized DNAs that are insoluble in water but soluble in organic solvents. The thin films are fabricated by using the spin coating technique after preparing solutions in either homogeneous or mixed organic solvents. We measure the ultraviolet–visible absorbance spectra of the films. The absorbance spectra show that the optical energy gaps of the functionalized DNAs change little even though the DNAs are connected to the complex molecules by electrostatic interaction. From the measured ellipsometric angles, we estimate the dielectric functions by using parametric optical constant model and layer model analysis. Depending on the nature of the attached complex molecules, the dielectric functions change, new optical structures develop below and above band gaps arising from the side molecules, and the optical energy gaps of the DNAs are altered slightly by weak coupling to the tethered complex molecules.