Sang-Ro Lee

Comparative analysis for the crystalline and ferroelectric properties of Pb(Zr, Ti)O3 thin films deposited on metallic LaNiO3 and Pt electrodes

The crystalline quality and ferroelectricity of the Pb(Zr,Ti)O3 (PZT) films deposited on the metallic LaNiO3 (LNO) and Pt electrodes were comparatively analyzed to investigate the possibility for their application to non-volatile memory devices. LNO thin films were successfully deposited on various substrates by using r.f. magnetron sputtering even at a low temperature ranging from 250 to 500 °C, and the ferroelectric PZT thin films were spin-coated onto the LNO and Pt bottom electrodes. Metallic LNO thin films exhibited [100] orientation irrespective of the substrate species and PZT films coated onto LNO films had highly a- and c-axis orientations, while those with Pt bottom electrode were polycrystalline. PZT films with LNO bottom electrode had smaller grain size and larger dielectric constant compared to those grown on the Pt electrode. The ferroelectric thin films fabricated on LNO bottom electrode displayed an asymmetric D–E hysteresis loop, which was explained by the defect effects formed at the interface. Especially, the LNO/PZT/LNO capacitor was found to significantly improve the polarization fatigue and the effects of the LNO electrodes to the fatigue were discussed.

Synthesis of cubic boron nitride films using a helicon wave plasma and reduction of compressive stress

Abstract A high density helicon wave plasma was used to synthesize a thin film of cubic boron nitride (c-BN) on the Si(100) substrate. Borazine (B 3 N 3 H 6 ) vapor was directly introduced into the reaction chamber where the plasma density was greater than 10 10 –10 11 cm −3 . The plasma density high enough to dissociate borazine effectively resulted in depositing a relatively good stoichiometric BN film without additional nitrogen sources. The bombardment of energetic ions with a high flux and energy is necessary for the growth of c-BN. The BN film synthesized at the condition of plasma density and rf substrate bias of 8.5 × 10 10 cm −3 and −350 V, respectively, was confirmed to be of cubic phase by both the FT-IR and TEM analyses. However, there existed an excessive compressive residual stress of 4–8 GPa in the c-BN film, which resulted in poor adhesion. The adhesion of c-BN film was significantly improved without a phase transformation through the post-heat treatment at 800 °C in N 2 atmosphere to relax the compressive residual stress.

Cytoprotective effect of dieckol on human endothelial progenitor cells (hEPCs) from oxidative stress-induced apoptosis.

Abstract Although endothelial progenitor cells (EPCs) have been used to promote revascularization after peripheral or myocardial ischemia, excess amounts of reactive oxygen species (ROS) are often involved in senescence and apoptosis of EPCs, thereby causing defective neovascularization and reduced or failed recovery. Here, we examined the cytoprotective effect of Ecklonia cava-derived antioxidant dieckol (DK) on oxidative stress-induced apoptosis in EPCs to improve EPC bioactivity for vessel repair. Although H2O2 (10 − 3 M) increased the intracellular ROS level in EPCs, DK (10ug/ml) pretreatment suppressed the H2O2-induced ROS increase and drastically reduced the ratios of apoptotic cells. H2O2-induced ROS increased the phosphorylation of p38 MAPK and JNK; this was inhibited by DK pretreatment. H2O2 treatment increased the phosphorylation of NF-κB, which was blocked by pretreatment with SB 203580, a p38 MAPK inhibitor, or SP 600125, a JNK inhibitor. H2O2 decreased the cellular levels of Bcl-2 and c-IAPs, cellular inhibitors of apoptosis proteins, but increased caspase-3 activation. However, all these effects were inhibited by pretreatment with DK. Injection of DK-mixed EPCs (DK + EPCs) into myocardial ischemic sites in vivo induced cellular proliferation and survival of cells at the ischemic sites and, thereby, enhanced the secretion of angiogenic cytokines at the ischemic sites. These results show that DK + EPC exhibit markedly enhanced anti-apoptotic and antioxidative capabilities, unlike that shown by EPCs alone; thus, they contribute to improved repair of ischemic myocardial injury through cell survival and angiogenic cytokine production.

Characterization of interface of c-BN film deposited on silicon(100) substrate

Abstract The interfacial microstructure of cubic boron nitride (c-BN) film deposited on single silicon substrate using magnetically enhanced active reaction evaporation (ME-ARE) has been investigated through thinning methods, in which X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) absorption spectroscopy were used for compositional and microstructure analysis. c-BN film was etched at size 4×4mm 2 by argon ion in XPS equipment to obtain depth concentration profile of the BN film and B1s XPS spectra at different etching depth. FTIR was alternately used to determine the microstructure of the BN film at different etching depth. The results show that a thin layer of hexagonal boron nitride (h-BN) phase exists at the interface between c-BN layer and substrate. In addition, transmission electron microscopy and selected area electron diffraction further confirm above the conclusion.

Effect of nitrogen ion implantation on the microstructural transformation of boron film

Abstract The structural transformation by nitrogen ion implantation into boron film as a function of film thickness, implant voltage and nitrogen ion dose was systematically investigated. Boron films were deposited on single crystal Si substrates using the electron beam evaporation of pure metallic boron, the thickness being 200, 500, 1000 and 2000 A, respectively. The films were then nitrogen ion-implanted at 25-, 50- and 75-kV implant voltages applied to substrate, respectively, for a fixed nitrogen ion dose of 3×1017 cm−2, and at 1×1017, 2×1017 and 3×1017 cm−2 nitrogen ion doses, respectively, for a fixed implant voltage of 50 kV. The nitrogen ion-implanted boron films were characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS results showed that the nitrogen ion-implanted boron films were nitrogen-deficient. Boron exists in the form of both metallic boron and boron nitride as indicated by XPS B1s bands. FT-IR spectra revealed that amorphous boron nitride (a-BN) or hexagonal boron nitride (h-BN) formed during nitrogen ion implantation into boron films. As implant voltage and nitrogen ion dose increased, amorphous BN was initially formed, while h-BN occurred at greater implant voltage and nitrogen ion dose. The thinner film led to the formation of h-BN at a relatively low implant voltage and nitrogen ion dose.

Interface Characterization and Delamination Mechanism of c-BN Film

Cubic boron nitride films were prepared by helicon wave plasma CVD process on (100) Si. The growth and delamination mechanism of c-BN film was investigated with FT-IR spectroscopy and transmission electron microscopy. The film deposited under the intense impact of energetic ions is usually delaminated from the substrate after deposition. It is found that moisture in the air, surface roughness of the film and substrate, as well as severe compressive stresses in the film are the primary contributors to film delamination. An aqueous oxidation was verified by EDXS analysis, which generate local stress by volume expansion at the crack region in the c-BN layer. From the experimental results and ??? observation a model for the delamination mechanism of c-BN film is suggested. Based on the delamination mechanism, several kinds of remedies such as post annealing and post N 2 plasma treatment were carried out for improving the adhesion.

Synthesis and characterization of cubic boron nitride thin films using the ME-ARE method

Cubic boron nitride (cBN) films were deposited by a magnetically enhanced activated reactive evaporation (ME-ARE) technique. Pulsed DC instead of r.f. power was used to bias the substrate. The effect of deposition parameters such as substrate bias voltage, plasma discharge current and gas flow ratio on the formation of cBN was investigated. BN films were characterized by FTIR and TEM. CBN films with a high content of cubic phase were successfully synthesized. It was found that formation of cBN film requires the bombardment of ions with both high flux and high energy. TEM observation showed that the cBN film had grain sizes of 15-50 nm and that a non-cubic phase BN, 10-15 nm was initially grown.

Effect of post-N+ implantation on the microstructure of the interfacial non-cubic BN layers

Plasma source ion implantation has been applied as a post treatment in order to modify the interfacial sp2 bonded layer of the cubic BN (cBN) films. The effect of ion irradiation on the microstructure of the noncubic BN layer was investigated. From HRTEM observation, the thickness of the BN layer of the representative sample was about 100 nm including 15 nm of initially grown non-cubic layer at the interface between a substrate and the cubic layer. At optimal plasma source implantation conditions, an acceleration voltage of 50 keV and an ion dose of 2×1016 ions/cm2, the microstructure of the interfacial non-cubic BN layer was changed. It was noticed that the ion irradiation caused the recrystallization of the amorphous phase and transformed the small crystallites into another phase. The micro-hardness of the complex consisting of a hard and soft layer increased as a result of phase transformations. This was caused by atomic displacements in the initially grown amorphous and hexagonal layers. The atomic displacements calculated by the TAMIX code were in the range of 0.31–0.52 nm per atom within 70–100 nm in depth.