Eou Sik Cho

Nd:YVO4 laser ablation of graphene films on glass and poly(ethylene terephthalate) substrates

In this work, a novel processing technique for patterning of graphene monolayer prepared on various platforms, glass and poly(ethylene terephthalate) (PET) substrates, is suggested and experimented for its validity evaluation. Two process parameters of the Q-switched 1064 µm Nd:YVO4 laser system, scanning speed and repetition rate, were controlled for investigating the geometric features of the graphene line patterns and obtaining an optimum process conditions in the given laser system. The differences among line patterns acquired by controlled process conditions and substrate materials have been investigated and the underlying physics is contemplated. Multiple tools were utilized for evaluating the results, which included an optical microscopy for dimension check, Raman analysis, and G- and 2D-peak mapping technique. Continuous graphene line patterns have been successfully obtained from the most of the schemed process conditions and it would be revealed that the laser pattering is a plausible technique for patterning the graphene monolayers in fabricating various electronic and optical devices.

Development of sacrificial layer wet etch process of TiNi for nano-electro-mechanical device application

We report the wet etching of titanium nickel (TiNi) films for the production of nano-electro-mechanical (NEM) device. SiO 2 and Si 3 N 4 have been selected as sacrificial layers of TiNi metal and etched with polyethylene glycol and hydrofluoric acid (HF) mixed solution. Volume percentage of HF are varied from 10% to 35% and the etch rate of the SiO 2 , Si 3 N 4 and TiNi are reported here. Within the various experiment results, 15% HF mixed polyethylene glycol solution show highest etch ratio between sacrificial layre and TiNi metal. Especially Si 3 N 4 films shows high etch ratio with TiNi films. Wet etching results are measured with SEM inspection. Therefore, this experiment provides a novel method for TiNi in the nano-electro-mechanical device.

Surface Functionalization of Liquid-Phase Exfoliated, Two-Dimensional MoS2 and WS2 Nanosheets with 2-Mercaptoethanol

In this work, the UV-Vis-NIR absorption spectrum of liquid-phase exfoliated two-dimensional (2D) MoS2 nanosheets, revealed two prominent peaks at 608 nm (2.04 eV) and 668 nm (1.86 eV). These peaks were blue-shifted compared to the reported literature values and are attributed to the quantum confinement effect. Interestingly, the WS2 nanosheets exhibited the same characteristic absorption peak at ~624 nm (1.99 eV). Raman spectroscopy analysis revealed that both nanosheets displayed distinctive peaks [377.8 cm-1 and 405.6 cm-1 for MoS2, 348.3 cm-1 and 417.9 cm-1 for WS2] that originate from optical phonon modes (E12g and A1g). These peaks are shifted toward higher wavenumbers (i.e., blue-shift or phonon-stiffening) compared to bulk MoS2 and WS2, probably due to enhanced Stokes Raman scattering. Subsequently, surface functionalization of the nanosheets with 2-Mercaptoethanol was successfully performed and confirmed using optical characterization techniques, including FT-IR spectroscopy. In addition, we determined the spectral broadening after functionalization, which would be attributed to photon confinement of the nano-sized layer structure, or to inhomogeneous broadening.

A Study on the Formation of 2-Dimensional Tungsten Disulfide Thin Films on Sapphire Substrate by Sputtering and High Temperature Rapid Thermal Processing

As direct formation of p-type two-dimensional transition metal dichalcogenides (TMDC) films on substrates, tungsten disulfide (WS2) thin films were deposited onto sapphire glass substrate through shadow mask patterns by radio-frequency (RF) sputtering at different sputtering powers ranging from 60 W to 150 W and annealed by rapid thermal processing (RTP) at various high temperatures ranging from 500 °C to 800 °C. Based on scanning electron microscope (SEM) images and Raman spectra, better surface roughness and mode dominant E12g and A1g peaks were found for WS2 thin films prepared at higher RF sputtering powers. It was also possible to obtain high mobilities and carrier densities for all WS2 thin films based on results of Hall measurements. Process conditions for these WS2 thin films on sapphire substrate were optimized to low RF sputtering power and high temperature annealing.

Direct Ablation by Laser of Single Graphene Monolayer and Graphene/Photopolymer Double Layer.

A diode-pumped Q-switched neodymium-doped yttrium vanadate (Nd:YVO4, λ = 1064 nm) laser was applied to obtain graphene patterns on a photopolymer layer by direct ablation. In the transfer process of the graphene layer, the photopolymer was employed as a graphene supporting layer and it was not removed for the simplification of the process. The laser ablation was carried out on graphene/photopolymer double layers for various beam conditions. The results showed that the laser-ablated widths on the graphene/photopolymer double layer were much greater than those on the graphene monolayer, especially at lower scanning speeds and at higher repetition rates. The photopolymer layer was not removed by the laser ablation, and the thermal energy was considered to have been dissipated in the lateral direction of graphene instead of being conducted vertically to the glass substrate. The Raman spectrum results showed that the graphene layer was clearly removed on the laser-ablated region of interest.

Laser Direct Etching on Transparent Conductive Oxide Films Sputtered on Polycarbonate Substrates

As a method of simple patterning of transparent conductive oxide (TCO) films deposited on flexible substrates, laser direct etching was carried out on TCO films sputtered on polycarbonate (PC) substrates. As a result of different binding energies in TCO films, indium tin oxide (ITO) and indium gallium zinc oxide (IGZO) were more easily etched than zinc oxide with different Nd:YVO4 laser beam conditions.

Low Frequency Noise Characteristics on Al/Nb2O5/p-type Schottky Diode Fabricated by Pulsed DC Magnetron Sputtering

Nb2O5films were deposited with pulsed DC magnetron sputter which is versatile and provides the ability to deposit thin film of oxide compounds with high deposition rate. Current measurement data from the metal-insulator-semiconductor (MIS) structure followed Schottky emission mechanism andreverse current characteristics were analyzed with oxygen flow rate variation. Low frequency noise measurements have been carried out with MIS samples at the forward conduction region. The experimental noise data have been successfully explained by the random walk model. The effect of O2flow rate of deposition process on low frequency noise has also been investigated and analyzed.

Effects of Post Annealing Temperatures on Sputtered Indium Tin Oxide Films for the Application to Resistive Touch Panel

Indium tin oxide (ITO) films were deposited on glass substrates by in-line DC sputtering and were annealed at different temperatures in the main chamber of sputter. The post-annealing effects on the optical and electrical properties of the ITO films were investigated and analyzed by various measurement systems. The results show that a higher annealing temperature makes an improvement on the electro-optical properties of the ITO films. The resistivity of the ITO films was reduced from 8.90×10−4Ω·cm to 2.91×10−4Ω·cm and the transmittance was increased from about 76.91% to 82.15% after post annealing at 300 °C in vacuum. The post annealed ITO glass was used in the fabrication of resistive touch panel and it was possible to obtain the better linearity characteristics than those from the touch panel fabricated with non-annealed ITO glass.

A Study on the Characteristics of NbO x Thin Film at Various Frequencies of Pulsed DC Sputtering by In-Line Sputter System

Niobium oxide(Nb2O5) films were deposited on p-type Si wafers at room temperature using in-line pulsed-DC magnetron sputtering system with various frequencies. The different duty ratios were obtained by varying the frequency of pulsed DC power from 100 to 300 kHz at the fixed reverse time of 1.5 μs. From the thickness of the sputtered NbOx films, it was possible to obtain much higher deposition rate in case of pulsed-DC sputtering than RF sputtering. However, the similar leakage currents and structural characteristics were obtained from the metal-insulator-semiconductor(MIS) structure fabricated with the NbOx films and the x-ray photoelectron spectroscopy(XPS) results in spite of the different deposition rates. From the experimental results, the NbOx films sputtered by pulsed-DC sputtering are expected to be used in the fabrication process instead of RF sputtering.