Hyun Ryu

Temperature-dependent growth of carbon nanotubes by pyrolysis of ferrocene and acetylene in the range between 700 and 1000 °C

Abstract Aligned carbon nanotubes were grown by pyrolysis of ferrocene and acetylene in the temperature range 700–1000 °C. The average diameter is constantly 20 nm for all growth temperatures. As the temperature increases, the growth rate increases by 60 times. The length reaches up to 3 mm at 1000 °C. These long carbon nanotubes exhibit a cylindrical structure. The relative amount of crystalline graphitic sheets increases significantly with the growth temperature. The Arrhenius plot yields the activation energy 35±3 kcal/mol, which is close to the diffusion energy of carbon in bulk γ-Fe. We suggest that the bulk diffusion of carbons would play an important role in the growth of cylindrical structured carbon nanotubes.

Room‐temperature visible photoluminescence from silicon‐rich oxide layers deposited by an electron cyclotron resonance plasma source

Highly split, visible light emissions at room temperature were observed in the range from 335 to 650 nm in silicon‐rich oxide filmsdeposited in the plasma phase of a mixture of silane and oxygen. The mechanism of the light emissions is classified into two categories. The photoluminescence bands at both 365 and 469 nm are related to the intrinsic defects of the E′ center and the neutral oxygen vacancy, respectively. However, the relatively sharp peaks at 403 and 535 nm are correlated with the development of polycrystalline core of Si‐enriched parts.

Investigation of Pt/Ta diffusion barrier using hybrid conductive oxide (RuO2) for high dielectric applications

The Pt/Ta diffusion barrier using hybrid conductive oxide (RuO2) for dynamic random access memory and ferroelectric random access memory capacitor bottom electrodes is proposed. The thermal stability of Pt+RuO2 (50 nm)/Ta+RuO2 (50 nm)/TiSi2/poly-Si/SiO2/Si contact system is investigated and compared to that of the Pt(50 nm)/Ta(50 nm)/TiSi2/poly-Si/SiO2/Si contact system. The Pt+RuO2/Ta+RuO2/TiSi2/poly-Si/SiO2/Si contact system sustained its structure up to 650 °C, whereas Pt/Ta/TiSi2/poly-Si/SiO2/Si contact system was completely degraded after annealing at 650 °C. In the former case, the addition of ruthenium dioxide (RuO2) into the Pt bottom electrode layer led to retardation of the oxygen indiffusion, preventing the indiffusion of oxygen up to 650 °C. In addition, the Ta+RuO2 diffusion barrier showed an amorphous structure and RuO2 is bound to the Ta matrix, inhibiting the interdiffusion of O, Pt, and Si through grain boundaries which can act as fast diffusion paths up to high temperatures. Therefore, i...

Structural defects in the growth of multiple periods of InAs quantum dots on a GaAs substrate

Microstructural observations on 20 periods of InAs quantum dots on a GaAs substrate grown by molecular beam epitaxy system were carried out by using high resolution transmission electron microscopy. The spherical cap-shaped InAs quantum dots were formed in a self-organized fashion, dot over dot, along the growth direction. However, two types of anomalities were found in the growth of these superlattice structures. One is the stoppage of quantum dot formation after 4 or 5 layers have been deposited. The morphology of the quantum dots was rather flat and faceted and a black and white contrast layer has appeared in the dot structure. The other type was a volcano-like defect which was grown vertically along the growth direction with a size of about 120 nm in diameter and about 400 nm in spacing. Inside the defect, black and white contrast layers have been formed along the [110] direction at the bottom of the epilayer and then changed to the [111] direction as the growth continued to the top layer.

Sintering Behavior and Microwave Dielectric Properties of Tricalcium Phosphate Polymorphs

The phase transformation and sintering behavior of tricalcium phosphate polymorphs [?- and ?-Ca3(PO4)2] are investigated using differential thermal analysis, X-ray powder diffraction, and dilatometry. The low-temperature phase, ?-Ca3(PO4)2, can be sintered at 1125 ?C prior to phase transformation to ?-Ca3(PO4)2. However, it is difficult to obtain dense ?-Ca3(PO4)2 samples owing to volume expansion and subsequent microcrack formation during the transformation of ?-Ca3(PO4)2 to ?-Ca3(PO4)2. Herein, we develop ?-Ca3(PO4)2 with a density higher than 97% by microwave heating ?-Ca3(PO4)2 without any reversible phase transformation. Also, we initially explore the microwave dielectric properties of tricalcium phosphate. Both tricalcium phosphate polymorphs show dielectric constants lower than 10. Of importance is the fact that a dense ?-Ca3(PO4)2 sample has a high quality factor of 22,000 GHz.

Formation mechanism of volcano-like structural defects in multiple periods of InAs quantum dots on GaAs

Abstract Studies on detailed formation mechanisms of volcano-like structural defects on 20 periods of InAs quantum dots on a GaAs substrate grown by molecular-beam-epitaxy system were carried out by microstructural examinations with high-resolution transmission electron microscopy. The volcano-like structure has started to be formed through the wave-like modulation of the growth surface mainly caused by the gradients in the surface chemical potential. Considerable retardation of growth has occurred and a deep valley is formed inside the structure and the width of a fully grown structure has reached 100 nm. The extensive growth competition between quantum dot structure and volcano-like structure has also occurred in the area where the changes of curvature are large. A spinodally decomposed structure has formed inside the defect whose wavelength of modulation is initially in the [1 1 1] direction, reflecting the initial inclined surface profile, and then changed to the [1 1 0] direction as growth time goes on from the bottom of the structure. This means that the modulated structure has revealed the structure far from the equilibrium state.

Transmission Electron Microscopy Microstructure of (K0.5Na0.5)NbO3 Ceramics with CuO Addition

The microstructures in Na0.5K0.5NbO3 (NKN) ceramics sintered at 960 °C with CuO additives were investigated with transmission electron microscopy (TEM). As a new microstructural constituent, CuO pockets have been observed at the grain boundaries of NKN and also inside the NKN matrix. The melting of CuO is caused by the element Na from the matrix, which forms Na–Cu–O eutectoid compounds whose melting point is lower than the sintering temperature. The kinetics of melting reaction largely depends on the size of the pocket. The interaction starts at the interfaces between the pocket and the matrix and advances into the interior of the pocket. The smaller pocket would melt earlier during the sintering process, flow into triple junctions between matrix grains, and participate in sintering via liquid phase sintering. In the larger pockets, the melting starts at the interfaces. Thus the outer areas are melted, but the CuO particles in the center remained unmelted. The NKN matrix then grows further into the pocket through liquid phase sintering, leaving low-angle grain boundaries behind that interface with the remaining CuO particles. The unmelted CuO particles in the pocket remain as the second-phase particles.

Characterization of Liquid Phase in (Na0.5K0.5)NbO3 Ceramics with V2O5 Additions

The characterizations of the microstructure and the chemical compositions on (Na0.5K0.5)(Nb0.93V0.07)O3 ceramics system sintered at 900 °C for 8 h were carried out using high resolution transmission electron microscopy (HRTEM), energy filtered (EF) TEM, and energy dispersive spectroscopy (EDS). The liquid phase was found at the triple junction of the grains and abnormal grain growth occurred in the presence of liquid phase. The liquid phase has formed by the melting of V2O5 at the sintering temperature. The presence of V element in liquid phase was confirmed by EDS analyses. The interface between the matrix phase and the liquid phase was atomically flat and consists of a stepped ledge structure of one or two atomic height. The KVO3 phase which may be formed during the cooling of the specimens was observed at these interfaces and have a good orientation relation with the matrix.

Microstructural Observations in (Na0.5K0.5)NbO3 Ceramics with CuO and ZnO Additives

The characterizations of CuO and ZnO pockets which had formed in (Na0.5K0.5)NbO3 (NKN) matrix sintered at 920 °C with CuO of 1.5 mol % and ZnO of 1.5 and 3.0 mol % as the additives were investigated from a microstructural point of view using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Two types of pockets, composed of CuO and ZnO as a dominant component, were observed in the microstructure as new microstructure constituents. The abnormal grain growth has occurred by the liquid phase sintering. The pockets were melted partially or completely by the interactions with element Na in the matrix which has formed a eutectic compound whose melting point is lower than the sintering temperature. The reaction starts at the interfaces between the pocket and matrix and the kinetics depends not only on the size of the pocket but also on the environments where the pockets are located. When the additive content of ZnO was increased to 3.0 mol %, there are interactions between CuO and ZnO and both elements are found in the compound pocket. The sintering kinetics was much enhanced by the presence of both additives.

Microstructural observations on 1: 2 ordered domain boundaries in Ba(Mg1/3Ta2/3)O3

High resolution transmission electron microscopy (HRTEM) has been used to investigate the detailed microstructures of the antiphase boundaries (APBs) of 1:2 B-site ordered domsins in fully ordered Ba(Mg1/3Ta2/3)O3 ceramic compound. The orientation relationship (102)I // (012)II, [010]I // [100]II was found between two domains. The ledged coherent interface has formed with (101) plane as an atomic habit plane. The growth ledge which possesses the kinks on ledge structure on the riser has the height of 1.22 nm which is equivalent to three (101) planes. The coalescence of the growth ledges and the genera~ion of these ledges at the structurally faulted region had been found experimentally. New interface model which was coherent and had a growth ledge was constructed based on the experimentally obtained diffraction pattern and HRTEM images and thereafter the lateral growth by the ledge mechanism in the coarsening of ordered phase boundaries was proposed.