Taek Sang Hahn

Thickness dependence of room temperature permittivity of polycrystalline BaTiO3 thin films by radio-frequency magnetron sputtering

Polycrystalline BaTiO3 thin films with thickness ranging from 2100 to 20 000 A were prepared on platinum substrates using off-axis radio-frequency magnetron sputtering. The variation in room temperature permittivity of the films was investigated with respect to thickness using x-ray diffraction and transmission electron microscopy. All films were ferroelectric and their room temperature permittivity, which was significantly higher than previously reported values, showed a strong dependence on film thickness. Higher permittivity was attributed primarily to the presence of ferroelectric domains. The room temperature permittivity of the thin films showed large variations with grain size, as in the case of BaTiO3 ceramics. The increase in permittivity with increasing film thickness was attributed to the decrease in defect concentration with grain growth. The 20 000 A film showed an abrupt decrease in permittivity and the presence of an intergranular phase having titanium-excess composition; these phenomena ar...

Electron‐paramagnetic‐resonance study of the Mn2+ luminescence center in ZnS:Mn powder and thin films

The number of isolated Mn2+ ions and Mn2+ clusters in ZnS:Mn powder and thin films has been studied using Mn2+ spectra measured at room temperature with an X‐band electron‐paramagnetic‐resonance spectrometer. While the concentration of the isolated Mn2+ ions decreases with increasing Mn concentration, the concentration of the clusters increases. At low Mn concentration, the Mn2+ ion substitutes for the Zn ion in ZnS:Mn in the cubic phase. At high Mn concentrations, where the ZnS powder has a dominant hexagonal phase, the Mn ion still prefers to substitute for Zn in ZnS:Mn at the cubic site rather than at the hexagonal site.

Effects of a new stacking method on characteristics of multilayered BaTiO3 thin film

A new deposition method used to prepare BaTiO3 thin films resulted in multilayered structure with higher dielectric constant, capacitance per unit area, and breakdown strength than those prepared by a conventional stacking method; the new method continuously decreased the substrate temperature after initial deposition of a polycrystalline BaTiO3 layer. The observed high dielectric constant could be explained only by a multilayered amorphous/microcrystalline/polycrystalline structure, the nature of which was confirmed by scanning electron microscopy and index of refraction measurement. Well‐defined ferroelectric hysteresis loops were observed as well with insignificant leakage current effects.

Decrease of the number of the isolated emission center Mn2+ in an aged ZnS:Mn electroluminescent device

We observed a decrease of the number of effective emission centers Mn2+ in the aged ZnS:Mn electroluminescence (EL) devices compared to the fresh EL devices using the electron‐paramagnetic‐resonance technique. Such phenomena can take place during the operation of the EL device, since the isolated Mn can easily diffuse into another site and forming cluster. Another possible explanation is that Mn2+ changes into Mn1+ or Mn3+ by transferring the electronic charge of the isolated Mn2+ to the neighboring Mn ions via sulfur and/or sulfur vacancy. As a result, luminance is lowered due to the decrease in the number of efficient emission centers of isolated Mn2+.

Ninety Degree Domains in RF-Sputtered BaTiO3 Thin Films on Platinum Substrates

We report 90° domains in (110)-oriented polycrystalline BaTiO3 thin films. The domain and crystal structures of the thin films were investigated using transmission electron microscopy and high temperature X-ray diffraction, respectively. From X-ray diffraction, it was found that the prepared films had a (110)-preferred orientation and a tetragonal structure at room temperature. Observation in the plan-view and cross-sectional modes using transmission electron microscopy clearly indicated the presence of a 90° domain structure. In the cross-sectional mode, a 90° domain structure was observed along the film growth direction and in the plan-view mode, it was also found that a 90° domain boundary was initiated from the (111) twin boundary. The formation of a 90° domain boundary was discussed in terms of planar defects such as the (111) twin and we also proposed two kinds of stress relief mechanisms induced by 90° domain formation.

111) Twins in BaTiO3 Thin Films Deposited by RF Magnetron Sputtering

Polycrystalline BaTiO3 thin films were prepared by rf magnetron sputtering and twin structures in the thin films were investigated using a transmission electron microscope. (111) twin boundaries were found in the thin films with thicknesses of 2100 and 10000 A. Not only single twin boundaries but (111) twin lamellae with a very narrow width of ≈1 nm were observed in thin films of both thicknesses. This result suggested that the twin lamellae would be stable although the interfacial energy appeared to be much higher than single twin boundaries. A correlation between twin lamella width and grain size was also found.

Luminescence and electron paramagnetic resonance studies of white‐light emitting SrS: Pr, F thin film electroluminescent devices

The fluorescence emission and excitation spectra of white‐light emitting SrS: Pr, F thin film electroluminescent devices have been investigated. It was determined from the results obtained that the dominant electroluminescence mechanism was that the ionization of Pr3+ centers occurs first, then subsequently recombination with electrons occurs, and finally Pr3+ center transitions give rise to luminescence. The emission mechanism of SrS: Pr, F seems to be the same as that of a SrS: Pr, K electroluminescent device, except for the appearance of strong peaks around 610–670 nm. The impurity excitation peak in the lower excitation energy, longer‐wavelength region in the FL spectrum may be an important factor for the selection of an effective white‐light emitting EL material. The electron paramagnetic resonance experiment of SrS: Pr, F was performed on powder and thin film specimens. The hyperfine structure of an isolated Mn2+ ion was observed in this SrS: Pr, F thin film. This Mn center which was substituted for...

Effect of an interlayer on the emission characteristics of a white‐light‐emitting electroluminescent device with a Pr and Ce doubly doped ZnS phosphor layer

We have investigated light‐emission characteristics of a white‐light‐emitting electroluminescent device with a doubly doped ZnS:Pr,Ce,F phosphor layer. We found that optimum codoping of Ce enhances the emission characteristics compared to the electroluminescent device with a singly doped ZnS:Pr,F layer. We also found that introducing an additional thin‐insulating SixNy interlayer between the lower insulating layer and the phosphor layer significantly stabilizes the aging characteristics and improves the luminous efficiency.

Origin of (111) Twin Lamellae in BaTiO3 Thin Films on Platinum Substrates.

The formation mechanisms of (111) twin lamellae in BaTiO3 thin films were examined using transmission electron microscopy. The voids in thin films were connected with (111) twin lamellae. These voids were formed due to the incomplete coalescence of islands, and it was found that (111) twin lamellae were produced in order to accommodate the small misfit between the growing islands during contact. With respect to planar defects, the cause of the formation of (111) twin lamellae could be explained by the maintenance of a TiO6 octahedron and the relationship with a hexagonal BaTiO3 structure. It could be understood that the very narrow width of the twin lamellae is due to the contact process of two growing islands as well as the very small grain size of thin films.

Effect of heavy-ion irradiation on transport properties of YBa/sub 2/Cu/sub 3/O/sub x/ films

We have investigated the effect of heavy-ion irradiation on the transport properties of YBa/sub 2/Cu/sub 3/O/sub x/ thin films. Parallel columnar defects to the c axis and crossed columnar defects were introduced into films by heavy-ion irradiation with dose equivalent to 1 or 2 T vortex density. The electrical transport properties including resistivity, critical current density, and the Hall resistivity were measured as a function of temperature, applied magnetic field, and fluence. The irreversibility line defined as an onset of dissipation progressively shifted to higher temperature with increasing dosage and showed its dependence on inclination angle. Critical current also showed a clear enhancement compared to the unirradiated samples. The Hall scaling behavior and the Hall conductivity were modified after heavy-ion irradiation.