Won Mok Kim

Resolution enhancement in surface plasmon resonance sensor based on waveguide coupled mode by combining a bimetallic approach.

In this study, we present and demonstrate a new route to a great enhancement in resolution of surface plasmon resonance sensors. Basically, our approach combines a waveguide coupled plasmonic mode and a kind of Au/Ag bimetallic enhancement concept. Theoretical modeling was carried out by solving Fresnel equations for the multilayer stack of prism/Ag inner-metal layer/dielectric waveguide/Au outer-metal layer. The inner Ag layer couples incident light to a guided wave and makes more fields effectively concentrated on the outer Au surface. A substantial enhancement in resolution was experimentally verified for the model stack using a ZnS-SiO2 waveguide layer.

Effect of fluorine addition on transparent and conducting Al doped ZnO films

Al doped ZnO (AZO) films with varying fluorine content were prepared by radio frequency magnetron sputtering at a room temperature to investigate doping effects of fluorine on the structural, the optical, and the electrical properties. The small amount of fluorine addition to AZO films resulted in beneficial effect on the electrical conductivity by improving the direct current (dc) Hall mobility, and the minimum specific resistivity was as low as 5.9×10−4Ωcm. With increasing fluorine content in AZO films, the optical absorption loss in the visible range decreased regardless of carrier concentration in the films. X-ray diffraction and scanning electron micrograph analyses showed that the crystallinity of AZO films was deteriorated by addition of fluorine. Small amount of fluorine addition to AZO film resulted in decrease of absorption loss as well as increase in Hall mobility, and the beneficial effects of fluorine addition was deduced to be caused by killing in-grain point defects. From the comparison bet...

Fiber-optic waveguide coupled surface plasmon resonance sensor

A novel approach to give an excellent tunability and self-referencing capability was presented by applying a concept of waveguide coupled surface plasmon resonance mode to a fiber-optic sensor. The presence of dielectric waveguide sandwiched between two metal layers made it possible to precisely tune the resonance wavelength in a broad range from visible to infrared region and to generate multiple modes which may be selectively used for suitable applications. Our approach also verified the potential capability of self-referencing based on a remarkable difference in sensitivity between the plasmonic and waveguide modes excited by p- and s-polarized lights, respectively, without using an additional reference channel. Experimental measurement carried out on sucrose solutions with varying concentration demonstrated the feasibility of our approach.

Floating gated silicon-on-insulator nonvolatile memory devices with Au nanoparticles embedded in SiO1.3N insulators by digital sputtering method

Floating gated silicon-on-insulator nonvolatile memory devices with Au nanoparticles embedded in SiO1.3N insulators were fabricated. The tunneling SiO1.3N insulator, Au nanoparticles, and control SiO1.3N insulator were sequentially deposited by digital sputtering method at 300°C. The size of Au nanoparticles was controlled in the range of 1–5nm by adjusting the deposition thickness of Au layer and the density of Au nanoparticles was approximately 1.5×1012cm−2. A significant threshold voltage shift of fabricated floating gate memory devices was obtained due to the charging effects of Au particles and the memory window was larger than 2.5V.Floating gated silicon-on-insulator nonvolatile memory devices with Au nanoparticles embedded in SiO1.3N insulators were fabricated. The tunneling SiO1.3N insulator, Au nanoparticles, and control SiO1.3N insulator were sequentially deposited by digital sputtering method at 300°C. The size of Au nanoparticles was controlled in the range of 1–5nm by adjusting the deposition thickness of Au layer and the density of Au nanoparticles was approximately 1.5×1012cm−2. A significant threshold voltage shift of fabricated floating gate memory devices was obtained due to the charging effects of Au particles and the memory window was larger than 2.5V.

Completely Transparent Conducting Oxide-Free and Flexible Dye-Sensitized Solar Cells Fabricated on Plastic Substrates

To achieve commercialization and widespread application of next-generation photovoltaics, it is important to develop flexible and cost-effective devices. Given this, the elimination of expensive transparent conducting oxides (TCO) and replacement of conventional glass substrates with flexible plastic substrates presents a viable strategy to realize extremely low-cost photovoltaics with a potentially wide applicability. To this end, we report a completely TCO-free and flexible dye-sensitized solar cell (DSSC) fabricated on a plastic substrate using a unique transfer method and back-contact architecture. By adopting unique transfer techniques, the working and counter electrodes were fabricated by transferring high-temperature-annealed TiO2 and Pt/carbon films, respectively, onto flexible plastic substrates without any exfoliation. The fabricated working electrode with the conventional counter electrode exhibited a record efficiency for flexible DSSCs of 8.10%, despite its TCO-free structure. In addition, the completely TCO-free and flexible DSSC exhibited a remarkable efficiency of 7.27%. Furthermore, by using an organic hole-transporting material (spiro-MeOTAD) with the same transfer method, solid-state flexible TCO-free DSSCs were also successfully fabricated, yielding a promising efficiency of 3.36%.

Mechanical and adhesion properties of Al/AlN multilayered thin films

Abstract An investigation was conducted on the mechanical properties of compositionally modulated Al/AlN thin films deposited by r.f. magnetron sputtering on Si(100) substrates. The films were made to have modulation periods ranging from 40 to 200 nm. The volume fraction of Al was varied from 0.125 to 0.625 for films with the minimum modulation period of 40 nm, but it was set to 0.5 otherwise. Hardness and adhesion of the deposited films were examined by nanoindentation and scratch test method, respectively. Residual stress of monolithic Al and AlN films with varying thickness and multilayered films was also measured by a conventional beam-bending technique. As compared with a monolithic AlN film, films of various modulation periods with the Al volume fraction of 0.5 were found to have lower hardness. On the other hand, high hardness comparable to and 12% higher than that of monolithic AlN film was registered for films that have the modulation period of 40 nm and the Al volume fraction 0.125 and 0.25, respectively. All the modulated films were found to have critical loads almost twice as high as that of a single AlN film, except the one with the lowest Al volume fraction. From the scratch test, a cohesive failure was observed for films with alternating layers in residual stress state of opposite signs whereas an adhesive failure was noticed otherwise. These observations indicate that a load-carrying capacity of a modulated film depends not only on the modulation period and individual layer thickness but on the residual stress states of alternating layers.

Enhanced charge collection efficiency by thin-TiO2-film deposition on FTO-coated ITO conductive oxide in dye-sensitized solar cells

We have investigated effect of thin TiO2 layers deposited on an indium tin oxide (ITO)/fluorine-doped tin oxide (FTO) double-layered transparent conductive oxide on the performance of dye-sensitized solar cell. FTO is deposited on an ITO-coated glass, followed by TiO2 deposition using a radio frequency magnetron sputtering technique. The thicknesses of the sputtered-TiO2 layers are varied from 10 nm to 20 nm, while the ITO and FTO are fixed to be 150 nm and 70 nm, respectively. Atomic force microscopy (AFM) shows that the surface roughness is similar but the surface morphology is altered by thin TiO2 layer deposition. The sheet resistance of the ITO/FTO conductive glass is hardly changed by thin TiO2 layer deposition. Photovoltaic performance is significantly enhanced after introduction of thin TiO2 underlayer. The 15 nm thick TiO2 underlayer leads to the increases of photocurrent density from 9 mA cm−2 to 10.3 mA cm−2 and fill factor from 0.715 to 0.747, as a result, the overall conversion efficiency is improved from 5.28% to 6.37%, corresponding to 20.6% increase. Photovoltage, however, remains almost unchanged. Photocurrent is improved over the entire wavelength. The increased transmittance at wavelength ranging from 300 to 600 nm contributes in part to increase in photocurrent. Improvement of charge collection efficiency from ∼90% to ∼97% is also attributed to the increased photocurrent, where the increased transport rate is responsible for the improved charge collection, indicating that the thin TiO2 underlayer has influence on opto-electronic property in the dye-adsorbed bulk TiO2 film.

Optical properties of Au nanocluster embedded dielectric films

Abstract Composite films of Au and dielectric material (TiO2 and/or mixed ZnS–SiO2) with a wide range of Au fractions were fabricated through a co-sputtering method utilizing a multi-target sputtering system. The optical absorption spectra of these composite films demonstrated an absorption peak due to surface plasmon resonance of embedded Au nanoclusters. It was found that, depending on the dielectric matrix, Au concentration, and post-deposition annealing, the wavelength of the surface plasmon resonance of nanoclusters shifted by 130 nm: from 550 to 680 nm. The third-order non-linear susceptibility χ(3) of these composite films, which also originates from field enhancement due to the surface plasmon resonance, demonstrated a corresponding variation.

Thin film alloy mixtures for high speed phase change optical storage: A study on (Ge1Sb2Te4)1−x(Sn1Bi2Te4)x

An approach is proposed to develop recording materials for high speed phase change optical data storage. It utilizes a thin film alloy mixture between a stoichiometric GeSbTe alloy and an additive ternary telluride alloy. Selection rules for an additive alloy are suggested. For a test, (Ge1Sb2Te4)1−x(Sn1Bi2Te4)x thin films are deposited by co-sputtering and their structural and thermal properties are studied. Ge1Sb2Te4 and Sn1Bi2Te4 are found to form a completely soluble pseudo-binary system, whose crystalline lattice parameters obey Vegard’s rule over the entire range of x (0<x<1). Furthermore, the alloy mixtures display an increasing tendency for crystallization with Sn1Bi2Te4 content. Dynamic tests of disk samples are made to show the effectiveness of the approach for high speed erasure.

A Study on the Failure Mechanism of a Phase-Change Memory in Write/Erase Cycling

Using a phase-change memory (PCM) device composed of Ge2Sb2e5 (GST), we studied the mechanism of the SET-stuck failure (SSF), a constantly low-resistance state during write/erase (W/E) cycling. The SSF state was characterized with increased RESET current and decreased threshold voltage, which were thought to be due to depletion of Ge and enrichment of Sb inside the active volume of GST. Moreover, we found that device characteristics of an SSF-PCM could be recovered by reversing bias polarity and the repaired device could endure many W/E cycles, implying that field-induced ion migration was the major cause of the SSF of a PCM.