Youngji Cho

Abnormal variation of the growth rate under high NH3 injected regime in the growth of GaN by NH3-source MBE

Unusual growth-rate variation during GaN formation using gas-source MBE has been discussed with respect to the chemical reactions occurring in the transition layer. A series of samples were prepared to confirm the assumption by verifying the growth regime and the impacts on the crystal quality of the GaN film. We found that the growth rate can be varied along with the amount of NH3 supply even under NH3-rich condition with a fixed Ga flux. Two growth conditions were investigated for their impact on the transition layer. One was the atomic force microscopy result, which revealed that the adatom migration length is closely related to the transition layer formation. The other one is the photoluminescent spectra, which revealed that the luminescence property of GaN is strongly related to the transition layer.

Characterization of Films Sputtered with the Cu-Ga Target Prepared by the Cold Spray Process

【The microstructural properties and electrical characteristics of sputtering films deposited with a Cu-Ga target are analyzed. The Cu-Ga target is prepared using the cold spray process and shows generally uniform composition distributions, as suggested by secondary ion mass spectrometer (SIMS) data. Characteristics of the sputtered Cu-Ga films are investigated at three positions (top, center and bottom) of the Cu-Ga target by X-ray diffraction (XRD), SIMS, 4-point probe and transmission electron microscopy (TEM) analysis methods. The results show that the Cu-Ga films are composed of hexagonal and unknown phases, and they have similar distributions of composition and resistivity at the top, center, and bottom regions of the Cu-Ga target. It demonstrates that these films have uniform properties regardless of the position on the Cu-Ga target. In conclusion, the cold spray process is expected to be a useful method for preparing sputter targets.】

Morphology and Structure Anlysis of Graphene by Low Voltage TEM

Graphene, a single atomic monolayer of sp 2 -bonded hexagonal carbon with extraordinary mechanical, electronic, and optical properties, has become a subject of great interest in materials science. Since its first isolation in 2004, graphene has been researched by many research groups in the fields of physics, chemistry and material. And it is expected that graphene will be applied in various industry [1]. To analyze properties of graphene, optical and electrical analyses are well used [2, 3]. For observation of graphene morphology, electron microscopy, atomic force microscopy and scanning tunneling microscopy, etc. have been used. But it is very difficult to observe graphene because of its ultra-thin thickness.

Transmission Electron Microscopy Specimen Preparation for Layer-area Graphene by a Direct Transfer Method

Graphene is a monolayer of carbon atoms arranged in a honeycomb lattice and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into zero-dimensional fullerenes, rolled into one-dimensional nanotubes or stacked into three-dimensional graphite. Graphene has unusual electrical, mechanical, and thermal properties, so it presents new opportunities in fundamental research and practical applications (Zhang et al., 2005; Geim & Novoselov, 2007; Geim, 2009). Recently, large-area synthesis methods for graphene have been advanced epitaxial growth on SiC (Brar et al., 2007; Qi et al., 2010) and chemical vapor deposition on metal substrates such as Ni (Dedkov et al., 2008) and Cu (Li et al., 2009). Particularly, Cu is a considerably attractive substrate because it can be produce layer-area graphene. Transmission electron microscopy (TEM) is a direct and re latively fast imaging tool ideally suited for suspended atomically thin membranes, so it has been successfully applied to study adsorbates on graphene and the atomic structure of graphene (Huang et al., 2011). To observe graphene by TEM, the graphene must transfer from metallic Cu substrates to a TEM grid. Standard transfer of layer graphene has been carried out using a polymer coating such as polymethyl methacrylate (PMMA) or polydimethylsiloxane as a temporary support during etching of the metal substrate to prevent tearing of the graphene (Reina et al., 2009). However, the standard transfer method using these polymers can contaminate and mechanically damage the graphene because it includes several wet chemical steps. Thus, a simple and gentle method is necessary for good TEM images of graphene. Direct transfer method doesn’t include polymer coating process. So it is expected simple and effective method to make graphene specimen by avoiding wet chemical steps (Regan, 2010).

High Performance Printed Ultraviolet-Sensors Based on Indium-Tin-Oxide Nanocrystals

A UV sensor was fabricated by screen printing indium–tin-oxide (ITO) nanocrystals on to quart glass. The initial printed ITO layer showed a resistivity too high for sensing applications, but considerable improvements were achieved through annealing under external pressure. The effects of this pressurized annealing were investigated using a commercial ITO film. The annealing aided the development of low-resistivity ITO through the repression of complex defects. The feasibility of the ITO sensor was confirmed through annealing coil-shaped ITO sensors under different conditions. Pressurized annealing greatly enhanced the output signal intensity under similar UV illumination conditions.

Growth of O- and Zn-polar ZnO films by DC magnetron sputtering

Department of Nano-semiconductor Engineering, National Korea Maritime University, Busan 606-791, Korea*Department of Offshore Plant Management, National Korea Maritime University, Busan 606-791, Korea**Division of Electrical and Electronics Engineering, National Korea Maritime University, Busan 606-791, Korea***PAN-Xal Co., Ltd., Suwon 443-380, Korea****Center for Interdisciplinary Research, Tohoku University, Sendai 980-8577, Japan

Formation of ultra-shallow junctions using epitaxial CoSi2 thin film as diffusion sources

Abstract An epitaxial CoSi 2 thin film was grown from a 20 nm Co/5 nm Ti bilayer by rapid thermal annealing (RTA) at 900°C and subjected to BF 2 − implantation followed by a second RTA at 900°C for the formation of the shallow p + —n junction. The B redistribution studied by secondary ion mass spectrometry was dependent upon the as-deposited B profile. For the 30 keV, 10 15 cm −2 implantation, the as-deposited B concentration at the CoSi 2 /Si interface was 10 17 cm −3 and the shallow junction of 35 nm depth was formed after 300 s anneal. For the 50 and 80 keV implantations, the interfacial concentration was higher than 10 18 cm −3 and the shallow junctions of 60 and 100 nm depth were formed after 60 and 10 s anneal, respectively.