N.-H. Cho

Chemical Structure and Physical Properties of Diamond-Like Amorphous Carbon Films Prepared by Magnetron Sputtering

Thin films of amorphous carbon (a-C) and amorphous hydrogenated carbon (a-C:H) were prepared using magnetron sputtering of a graphite target. The chemical structure of the films were characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The mass density, hardness, residual stress, optical bandgap, and electrical resistivity were determined, and their relation to the films chemical structure are discussed. It was found that the graphitic component increases with increasing sputtering power density. This is accompanied by a decrease in the electrical resistivity, optical bandgap, mass density and hardness. Increasing the hydrogen content in the sputtering gas mixture results in decreasing hardness (14 GPa to 3 GPa) and mass density, and increasing optical band gap and electrical resistivity. The variation in the physical properties and chemical structures of these films can be explained in terms of the changes in the volume of sp{sup 2}-bonded clusters in the a-C films and changes in the termination of the graphitic clusters and sp{sup 3}-bonded networks by hydrogen in the a-C:H films.

Effects of substrate temperature on chemical structure of amorphous carbon films

Amorphous carbon thin films were prepared at 30, 200, and 450 °C by magnetron sputtering of a graphite target. The surface structure and chemical bonding (sp2/sp3) of the carbon films were characterized by scanning tunneling microscopy (STM) and Raman spectroscopy. STM images show that graphite microcrystallites of 20–40 A in size are present at the surfaces of all the films and the number of the microcrystallites increases with increasing substrate temperature. The microcrystallites often contain structural defects. Raman measurements show that increasing the substrate temperature results in an increase in the sp2‐bonded fraction of carbon atoms and a decrease in the microstructural defects. These results indicate that the microstructural changes are correlated with changes in the chemical bonding ratio (sp3/sp3) and no diamond microcrystallites are present in the amorphous carbon. A three‐dimensional atomic structure of the graphite microcrystallites is discussed in terms of turbostratic graphite.

Antiphase boundaries in GaAs

Antiphase boundaries in GaAs have been produced by growing the GaAs on {001} Ge substrates. The GaAs was grown by the technique of organometallic vapor phase epitaxy to a thickness in excess of 1 μm. The antiphase boundaries are shown to be faceted with facets parallel to the {110} planes being particularly common. The rigid‐body translation at the different facet planes is shown to be small for the {110} planes but it can be large for other facet planes.

Morphological and nanostructural features of porous silicon prepared by electrochemical etching.

Porous layers were produced on a p-type (100) Si wafer by electrochemical anodic etching. The morphological, nanostructural and optical features of the porous Si were investigated as functions of the etching conditions. As the wafer resistivity was increased from 0.005 to 15 Ω·cm, the etched region exhibited ‘sponge’, ‘mountain’ and ‘column’-type morphologies. Among them, the sponge-type structured sample showed the largest surface area per unit volume. Silicon nanocrystallites, 2.0 to 5.3 nm in size, were confirmed in the porous layers. The photoluminescence peaks varied in the wavelength range of 615 to 722 nm. These changes in the maximum peak position were related to the size distribution of the Si crystallites in the porous silicon. The doping levels of the wafers significantly affect the size distribution of the Si crystallites as well as the light-emitting behavior of the etched Si, which contains nanoscale Si crystallites.

Effect of the grain size and chemical features on the phase transition and physical characteristics of nano-grained BaTiO3 ceramics

Abstract Nano-grained BaTiO3 ceramics were synthesized by sintering BaTiO3 nano-powders; these powders were coated with Mn-materials. The phase transition and physical features of the BaTiO3 ceramics were investigated as a function of the sintering temperature and the amount of coating materials. With increasing the sintering temperature and/or the amount of the coating materials, the formation of a tetragonal phase was increased; this seems to be due to grain growth and grain boundary chemical features. The relation of the dielectric features of the nano-grained BaTiO3 ceramics with the structural variation is discussed in terms of process parameters.

Formation, faceting, and interaction behaviors of antiphase boundaries in GaAs thin films

Antiphase boundaries occur in GaAs epilayers grown on (001) Ge substrates by organometallic vapor-phase epitaxy methods. The formation and structural characteristics of these boundaries were investigated by transmission electron microscopy (TEM). Steps with particular heights at the surface of substrates nucleate antiphase boundaries. The observed faceting behavior of these boundaries indicates that energy associated with the presence of antiphase boundaries is strongly related with the boundary planes, and preservation of the stoichiometry of GaAs appears to play an important role in achieving a lower energy state at antiphase boundaries.

Microstructure and magnetic anisotropy of ultrathin Co/Pt multilayers grown on GaAs (1̄1̄1̄) by molecular‐beam epitaxy

Multilayers of (Co3 A, Pt15 A)x, x=15 or 30 repeats, with or without a 200 A silver buffer layer, were grown on GaAs (111) substrates by molecular‐beam epitaxy. Vibrating sample magnetometry measurements confirmed that the samples with the Ag buffer layer show strong uniaxial magnetic anisotropy perpendicular to the surface. The perpendicular anisotropy exhibited by these metallic superlattices is discussed in terms of the microstructure of the overall multilayer stack, as well as the structural characteristics of the Co interface layer. Samples grown on the Ag buffer layer show strong (111) texture with 30–40‐nm‐size twin‐related grains. These grains, correspond to the two possible (111) stacking sequence for an fcc lattice, i.e., double positioning. However, direct growth on GaAs (111) results in randomly oriented 10–20 nm grains. All samples exhibit a repeat period of 1.83 nm in both low‐angle reflectivity and high‐angle Θ–2Θ x‐ray scattering measurements. In addition, transverse scans through the low‐...

Determination of the lattice translation across antiphase boundaries

Abstract This paper describes a method for quantitative comparisons of electron microscope bright-field images with computer-simulated images. The applicability of the technique is demonstrated by preliminary investigations of the rigid-body lattice translation across antiphase boundaries in GaAs and β-SiC.

Phase formation, sintering behavior, and electrical characteristics of NASICON compounds

The dependence of phase formation, sintering behavior, and electrical characteristics of Sodium Superionic Conductor (NASICON) compounds on sintering temperature, time, and cooling process was investigated. In the von Alpen-type composition Na3.2Zr1.3Si2.2P0.8O10.5, ZrO2 second phase is in thermal equilibrium with crystalline NASICON and liquid phase above 1320°C, and when cooled through 1260–1320°C, the crystalline NASICON was formed by reaction between the ZrO2 second phase and the liquid phase. Maximum relative densities of 96 and 91% were obtained for compositions Na3Zr2Si2PO12 and Na3.2Zr1.3Si2.2P0.8O10.5, respectively. For these compositions, the maximum ionic conductivity and the minimum migration barrier height were 0.45 ohm−1 cm−1 and 0.07 eV, respectively. The migration barrier height of the high temperature form (space group: R3c) is about 30–40% of that of the low temperature form (space group: C2/c). Ionic conductivity increases with increasing sinterability, and a considerably large amount of glass phase in Na3.2Zr1.3Si2.2P0.8O10.5 ceramics significantly lowers ionic conductivity above the transition temperature.

Nanostructural and photoluminescence features of nanoporous silicon prepared by anodic etching

The nanostructural and photoluminescence (PL) features of nanoporous Si (NPS) were investigated in terms of various process parameters such as current density, etching time and oxidation conditions. The NPS was prepared by electrochemical anodic etching of p-type (0 0 1) Si wafers of 4 Ω cm resistivity in HF solution. The pores are of polygon-type columns with 5, 6 and 7 side walls. The average diameter of the column-shaped pores is critically determined by the current density, while the etching time plays an important role on the pore depth; in particular, when the current densities of 30 and 100 mA/cm 2 were applied, the pore diameters were ∼9 nm and 3.3 μm, respectively. The variation in the PL characteristics of the NPS with oxidation condition and etching current density was measured and then related with their structural changes. The aging and thermal treatments produce oxidation and lattice distortion in the NPS. The degree of deviation from the as-prepared NPS during aging or thermal treatment seems to depend on the nanostructure as well as morphology of the NPS. It is found in this study that etching current density plays an important role on such structural features of the NPS.