Ho-Seok Nam

Ab initio study on influence of dopants on crystalline and amorphous Ge2Sb2Te5

The pronounced effects of dopants such as Si, N, and O atoms, on material properties of Ge2Sb2Te5 are investigated at the atomic level using ab initio calculations. In the crystalline phase, stable doping sites are determined by characteristic chemical bonds such as Ge–N and Ge–O. The comparison of lattice parameters between theory and experiment supports the existence of dopants at vacant or interstitial positions. The electronic density of states indicate that the localization at the valence top increases with N- or O-doping, explaining the increase of resistivity in experiments. The amorphous structures of doped Ge2Sb2Te5 are obtained by melt-quench simulations and they are well understood by selective bonds between dopants and host atoms. The chemical bonds around dopants are more favorable in the amorphous phase than in the crystalline state, accounting for increased amorphous stability of doped Ge2Sb2Te5. The atomic and electronic structures of amorphous Ge2Sb2Te5 do not support a viewpoint that the...

Effects of atomic size difference and heat of mixing parameters on the local structure of a model metallic glass system

Atomic size differences between constituting elements and the heat of mixing are key factors in designing a metallic glass system. In this study, the effects of atomic size differences and the heat of mixing on the glass-forming ability and the local structure of metallic glasses were studied via molecular dynamic simulations of an ideal system known as the Lennard-Jones embedded-atom method model. The atomic size difference and the heat of mixing of the system were varied by means of the Lennard-Jones parameters. The glass transition behavior was characterized based on the chemical short-range order and by a Voronoi analysis. Our simulations lead to optimized windows of atomic size differences and heat of mixing parameters for metallic glass-forming of the model system. Both a greater negative heat of mixing and a larger atomic size difference are necessary for the enhancement of the glass-forming ability.

Prediction of atomic structure of Pt-based bimetallic nanoalloys by using genetic algorithm

The atom-arrangements in Pt-based bimetallic nanoalloys were predicted by the combined use of genetic algorithm (GA) and molecular dynamics (MD) simulations. The nanoparticles of these nanoalloys were assumed to be a 3.5 nm-diameter truncated octahedron with Pt and noble metals of fixed composition ratio of 1:1. For the GA, a Python code, which concurrently linked with the MD method that uses the embedded atom method inter-atomic potentials, was developed for the prediction of the atom arrangements in these bimetallic nanoalloys. Successfully, the GA calculation predicted the core-shell structures for both Pt-Ag and Pt-Au nanoalloy, but an onion-like multilayered core-shell structure for Pt-Cu nanoalloy. The structural characteristics in the bimetallic nanoalloy were mainly due to the differences in the surface energy and cohesive energy between Pt and the other alloying metal elements and their miscibility gap and so on. Briefly, the prediction performance was analyzed to show the superior searching ability of GA.

Size-dependent transition of the deformation behavior of Au nanowires

Inspired by the controversy over tensile deformation modes of single-crystalline 〈110〉/{111} Au nanowires, we investigated the dependency of the deformation mode on diameters of nanowires using the molecular dynamics technique. A new criterion for assessing the preferred deformation mode—slip or twin propagation—of nanowires as a function of nanowire diameter is presented. The results demonstrate the size-dependent transition, from superplastic deformation mediated by twin propagation to the rupture by localized slips in deformed region as the nanowire diameter decreases. Moreover, the criterion was successfully applied to explain the superplastic deformation of Cu nanowires.

Effects of Solvent on the Formation of the MUA Monolayer on Si and Its Diffusion Barrier Properties for Cu Metallization

We investigated the effects of solvents, such as ethanol and isooctane, on self-assembly of the mercaptoundecanoic acid (MUA) monolayer on Si and its diffusion barrier properties for Cu metallization. The use of isooctane as a solvent produced MUA self-assembled monolayers (SAMs) (∼1.3 nm thick) on Si. These acted as an effective diffusion barrier against Cu diffusion up to 200°C. In contrast, the MUA SAMs produced by ethanol allowed the diffusion of Cu to a MUA-Si interface at 200°C, stimulating the out-diffusion of Si into Cu and thus resulting in the degraded diffusion barrier properties. This was possibly due to the partial formation of interplane hydrogen bonding between the terminal groups of the bound acid and free thiol groups. This provided less dense thiol surface groups, thus leading to poor adhesion of Cu to MUA SAMs. The fabricated Cu/isooctane-assisted MUA source/drain electrode a-Si:H thin film transistors with a channel length of 10 µm exhibited an excellent electron mobility of 0.74 cm2/V-s, threshold voltage of −0.51 V, Ion/Ioff ratio of 3.25 × 106, specific contact resistance of 4.24 Ω-cm2 after annealing at 200°C.

Consecutive and Selective Chemical Vapor Deposition of Pt/Al Bilayer Electrodes for TiO2 Resistive Switching Memory

This paper demonstrated a new method for consecutive and selective chemical vapor deposition (CVD) of Pt and Al on a TiO2 surface in the presence of a highly hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFS) monolayer. The printed PFS monolayers effectively served as a monolayer resist for the consecutive CVD of Al and Pt in the fabrication of the patterned, robust Pt/Al bilayer electrodes for TiO2 resistive switching random access memory (ReRAM) devices. The Al underlayer promoted the nucleation and the growth of the CVD Pt, and thus provided a base for a more uniform and thicker deposition of Pt than bare TiO2 surfaces. In addition, the enhanced adhesion of Pt with the additional Al layer significantly improved the endurance of the resistive switching TiO2 thin film. Thus this bottom-up approach may be well extended in the fabrication of next generation devices.

Effect of Temperature on Coalescence Behavior of Unsupported Gold Nanoparticles

The coalescing behavior of gold nanoparticles was studied by employing molecular dynamics simulations based on a semi-empirical embedded-atom method. Investigations on the coalescing process of the faceted nanoparticles revealed that at relatively low-temperatures, plastic deformation by slip motion was the main mechanism of coalescence, while near the melting point, coalescence was preceded by local fluid motion. Different initial configuration and coalescing temperature have a substantial influence on the coalescing behavior, making different final structures such as twinned face-centered cubic or amorphous nanoparticles.Graphical Abstract