Kyung-ho Park

Comparative Study of Amorphous and Crystalline (Ba, Sr)TiO3 Thin Films Deposited by Laser Ablation

(Ba0.5Sr0.5)TiO3 thin films (200-300 nm) were deposited on Pt-coated Si substrates by laser ablation at 500 and 650°C. The leakage currents of crystalline films grown at 650°C were found to be higher than those of amorphous films grown at 500°C. The crystalline thin films showed higher surface roughness than the amorphous films as measured by atomic force microscopy (AFM). A columnar grain structure was observed for crystalline films with a grain size of 20-30 nm by transmission electron microscope (TEM) analysis. These factors may be responsible for high leakage currents of crystalline films. Constant current injection measurements for Au/(Ba0.5Sr0.5)TiO3/Pt capacitors showed that electron trapping states near the top electrode interface were higher in number than at bottom electrode interface. This may be due to the presence of reactive sites on the surface of deposited films as observed by X-ray photoelectron spectroscopy (XPS) measurements.

Control of grain structure of laser-deposited (Ba, Sr)TiO3 films to reduce leakage current

A new technique to realise a low leakage current of (Ba, Sr)TiO 3 (BSTO) thin films by controlling the grain structure of BSTO has been proposed. This includes the deposition of amorphous BSTO at 500 o C and annealing of the amorphous films at 650 o C in O 2 for crystallization. The leakage current of the film producted from the amorphous one was significantly lower than that of BSTO films deposited directly at the substrate temperature of 650 o C. The crystalline structure of the film produced from the amorphous one was found to be circular. In contrast, as-grown crystalline film showed columnar grain structures. The reduction of the leakage current may be attributed to the differences in the grain structures

Planar to columnar transformation of PtSi in the epitaxial growth process of Si/PtSi/Si(111) double heterostructures

Epitaxial growth of Si on PtSi(010)/Si(111) structure, which was fabricated by coevaporation of Pt and Si with the stoichiometric ratio (Pt/Si=1/1), was carried out in a molecular beam epitaxy system. At the substrate temperature of 400 °C, Si grew epitaxially on the PtSi layer and Si(111)/PtSi(010)/Si(111) double heterostructure was obtained. On the other hand, at the substrate temperature of 600 °C, the PtSi layer transformed into epitaxial columns and/or walls in the process of Si deposition and evaporated Si filled the space among the PtSi columns and/or walls epitaxially.

Infrared spectroscopy study of the Si–SiO2 interface

The thermally oxidized vicinal Si(001)– SiO2 interface was studied by using polarized beam Fourier transform infrared spectroscopy and cross‐sectional transmission electron microscopy. We observed an anisotropic absorption of the Si–O–Si stretching mode at the vicinal interface, which is explained with a proposed Si–SiO2 interface model; a single‐layer/double‐layer stepped structure. We also show that the interface of the Si–SiO2 formed at 1000 °C consists mostly of a double‐layer stepped structure with a single domain terrace, while that formed at 850 °C has a single‐layer stepped structure.

Enhancement of Critical Current Density on Bi2Sr2CaCu2Oy by 120 MeV O7+ Ion Irradiation

Irradiation effects of 120 MeV oxygen-ion beam on Bi2Sr2CaCu2Oy (BSCCO) single crystal were investigated with a fluence of 3×1014cm−2. Critical current density (J c) calculated from magnetization loops was enhanced in the temperature range of 20 to 77 K, with a typical value of 200 in enhancement ratio at 40 K and 0.1 T. In comparison with those after electron, neutron and heavy-ion irradiations previously reported, the magnetic field dependence of J c after the oxygen-ion irradiation was the most similar to that after the neutron irradiation.

Tem Studies of Silicon-Silicon Oxide Interface Roughness

Interfaces between two kind of substrate, a bulk silicon wafer and a laser-recrystallized Silicon-On-Insulator (SOI), and its thermally grown oxide have been studied. High resolution transmission electron microscopy (HRTEM) of cross sectional specimen shows that the roughness at the interface is atomically flat and nearly uniform for the bulk single crystal silicon and silicon oxide, while being nonuniform and rough as much as 20 nm height for the recrystallized silicon and silicon oxide interface. Consideration of interface between recrystallized silicon and silicon oxide, and the oxide surface above, the observed roughness at the interface is due to original grain boundaries of polycrystalline silicon which was used as the material for the laser recrystallized silicon formation. It is also discussed HRTEM of the interface between polycrystalline silicon and silicon oxide.

Planar to columnar structure transition of MBE grown Si/PtSi/Si(111) double heterostructure

Pt and Si were codeposited on Si(111) substrates to form epitaxial PtSi layer in a molecular beam epitaxy (MBE) system. Si was also deposited on the epitaxial PtSi layer to fabricate a double heterostructure. At substrate temperatures lower than 500°C, Si grew epitaxially on the PtSi layer and a Si(111)/PtSi(010)/Si(111) double heterostructure was obtained. On the other hand, at substrate temperatures above 500°C, PtSi layers transformed into epitaxial columns and/or walls in the process of Si deposition and these columns were surrounded by epitaxial Si. This planar to columnar transition of Si/PtSi structure depends on the growth rate of Si on the PtSi, and was suppressed by increasing the growth rate.

Microstructure Analysis of Agglomerated Epitaxial Columns of Si/PtSi/Si(111) Double Heterostructure

Micro structures and interface structures of epitaxially grown PtSi and over-capping Si films on Si(111) substrates prepared by MBE were studied by RHEED, HREM, SEM and XRD. An epitaxially grown Si layer on the PtSi layer, which was fabricated by coevaporation of Pt and Si with the stoichiometric ratio (Pt/Si=l/1), was obtained as a Si(111)/PtSi(010)/Si(111) double heterostructure at the substrate temperature of 400°C. On the other hand, it was found that the PtSi layer transformed into epitaxial columns and/or walls when a Si over-capping layer was grown on the PtSi layer at a substrate temperature of 600°C or higher. These columns and/or walls were surrounded by a Si matrix which showed epitaxial relations to the Si substrate with stacking faults.