Sangsu Kim

Electron paramagnetic resonance studies of Co2+ ions in congruent and nearly stoichiometric LiNbO3 single crystals

Congruent and nearly stoichiometric LiNbO3 single crystals doped with Co 2+ ions have been investigated by the electron paramagnetic resonance technique. The spin-Hamiltonian parameters, gk D 2:671.2/, g? D 5:052.2/, Ak D 40.2/ 10 4 cm 1 , and A? D 154.1/ 10 4 cm 1 , are obtained without including the nuclear Zeeman and quadrupole interactions. The factor,k, of reduction due to the covalency is estimated as 0:926k6 1 using the aboveg-values. On the basis of the temperature dependence of the linewidth, the spin-lattice relaxation and the spin-spin relaxation mechanisms are investigated. The former is a sum of a weak direct process with a phonon bottleneck .T 2:4/ and an Orbach process with 1 156 cm 1 (or 225 K), and the latter is a sum of a typical temperature-independent process and an anomalous process with a temperature dependence/T 3:6 . This anomaly is considered as a shortening of T2 due to the increase of the correlation length of the spin-spin interaction as the temperature decreases. The integrated signal intensity also showing an anomaly is a consequence of the linewidth anomaly.

X-ray-induced reduction of Sm3+-doped SrB6O10 and its room temperature optical hole burning

We prepared two kinds of Sm-doped strontium borates. When the samples were prepared in air environment, while the Sm3+ ions doped in strontium tetraborate (STB) were well reduced, the trivalent Sm ions were partially reduced in strontium hexaborate (SHB). The SHB was successfully reduced by subsequent x-ray irradiation (SHBX) and showed a strong Sm2+ emission. The photoluminescence, excitation spectrum and spectral hole burning experiments were conducted for the samples. The crystallographic sites of Sm2+ ions were found to be different in the STB and SHBX. Spectral holes were observed at room temperature in both samples STB and SHBX. The measured hole depths were 24% and 63% in STB and SHBX, respectively. As a result, we could obtain effective and novel hole burning material based on the Sm-doped strontium borates with x-ray-induced reduction.

The influence of metal aluminium on the reduction of the Sm3+ doped in aluminosilicate glass films

Sm3+-doped glass sol was prepared by a sol–gel method and coated on a bare Si substrate and an Al coated Si substrate (Al/Si). The coated glass films were heat treated in a hydrogen atmosphere or air to reduce the Sm3+ to Sm2+ and then the optical properties were examined through a photoluminescence (PL) experiment. While the glass films coated on a bare Si substrate or an Al/Si one were well reduced in hydrogen atmosphere, only the glass films coated on the Al/Si substrate were reduced in air. We thus suggest two possible reducing mechanisms related to hydrogen and the metal aluminium and we found that the metal aluminium coated on a Si substrate plays an important role in the reducing process.

Photo-luminescence properties of CuCl quantum dots and the dependence of biexciton formation rates on quantum dot sizes

We fabricated CuCl quantum dots (QDs) in a glass matrix. The blue shift of the exciton spectrum caused by quantum confinement effects was observed in the linear absorption and the photo-luminescence (PL) spectrum. Inhomogeneous line broadening was mainly induced by the size distribution of CuCl QDs and we found that the size distribution has Gaussian profiles. By solving the steady state rate equation, we also found that the intensities of biexciton PL were proportional to the square of exciton PL intensities, and the results were well accorded with the experimental results. The formation rates of biexciton were proportional to the volumes of CuCl QDs.

Electrode effect on microwave properties of ferroelectric (BaxSr1-x)TiO3 thin films

Microwave properties of coplanar waveguide (CPW) transmission lines fabricated on high dielectric materials, such as ferroelectric Ba 1−x Sr x TiO 3 films, are highly sensitive on the dimension and shape of electrodes. A small change in device dimension affects the total electrical length of the CPW, which may mislead the effective dielectric constant of the dielectric layer. Furthermore, extracting dielectric constant of high-k thin films from the measured microwave properties, such as S-parameters, is very difficult. The well known a modified conformal mapping method frequently exhibits an inconsistent dielectric constant for CPW on high-k materials. CPW transmission lines were fabricated on high-k thin films, ferroelectric Ba 0.6 Sr 0.4 TiO 3 , which were deposited by the pulsed laser deposition with partial oxygen backgrounds. A large phase shift angle of 100° at 10 GHz was observed from the CPW (gap = 4 μm, length = 3 mm) with a 40 V of dc bias, which supports that the idea of the tunable microwave device application using ferroelectrics films. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameters by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film calculated by a modified conformal mapping exhibits a gap dependency; dielectric constant (990 ∼ 830) decreases with increasing gap size (4 ∼ 19 μm, respectively). For comparison, dielectric properties have been extracted by extensive EM-simulation using a HFSS™ (Ansoft) with observed dimensions of CPW devices. Total phase, which is closely related with the dielectric constant of the film, is strongly affected by gap size, film thickness, and slanted angle of CPW.

Ionic Doping Effect in Bi-layered Perovskite SrBi 2 Nb 2 O 9 Ferroelectrics

Doping effect of various ions in Bi-layered ferroelectric (SBN) ceramics was studied. Undoped SBN ceramic and SBN ceramics doped with , , , , , , , and ions were made by a solid state reaction. Dielectric constants were measured with temperature. Ferroelectric transition temperature decreased with , , doping, but the transition temperature increased with , , , , or ionic doping. These results show that the ion size plays an important role in the ferroelectricity of SBN ceramic.

Microstructures and Electrical Properties of Niobium-doped Bi 4 Ti 3 O 12 Thin Films Fabricated by a Sol-gel Route

Bismuth layered structure ferroelectric thin films, / (BTO) and Nb-doped BTO (BTN) were prepared on the Pt(111)/Ti//Si(100) substrates by a sol-gel route. We investigated the Nb-doping effect on the grain orientation and ferroelectric properties. ion substitution for ion in perovskite layers of BTO decreased the degree of c-axis orientation and increased the remanent polarization (2Pr). The fatigue resistance of Nb-doped BTO thin film was shown to be superior to that of BTO, and the leakage current of Nb-doped BTO thin film was decreased about 1 order of magnitude compared with BTO. The improvement of ferroelectric properties with doping in BTO could be attributed to the changes in space charge densities and grain orientation in the thin film.