Hitoshi Tabata

Low‐temperature formation of multilayered Bi(Pb)‐Sr‐Ca‐Cu‐O thin films by successive deposition using laser ablation

Crystallized as‐grown Bi(Pb)‐Sr‐Ca‐Cu‐O thin films are formed by the laser ablation method at a substrate temperature as low as 480 °C under N2O gas flow. Moreover, the as‐grown thin film, which has a crystal structure consisting of four or five CuO2 layers between adjacent Bi2O2 layers, can be formed by the combination of N2O gas flow and the successive deposition method with excimer laser pulses.

C-AXIS PREFERRED ORIENTATION OF LASER ABLATED EPITAXIAL PBTIO3 FILMS AND THEIR ELECTRICAL PROPERTIES

Ferroelectric PbTiO3 epitaxial thin films have been formed on a base electrode of Pt(100)/MgO(100) using ArF excimer laser ablation. The x‐ray diffraction patterns of these films show c‐axis orientation with a rocking angle of 0.5 when the film is cooled faster than 75 °C/min after the deposition. The films exhibit clear and large ferroelectric hysteresis loops. The dielectric constant of the films is 130 and remnant polarization is 80 μC/cm2. The values are almost the same as those theoretically predicted. The pyroelectric coefficient of these films is 1000–1500 V/W at 10 Hz, even without poling treatment.

Formation of the High-Tc Phase of the Superconducting Bi-Pb-Sr-Ca-Cu-O Thin Film by the Laser Ablation Method

Films of the Bi-Sr-Ca-Cu-O and the Bi-Pb-Sr-Ca-Cu-O superconductors have been synthesized by a laser ablation method. Pb doping decreases the melting temperature, making the formation of the high-Tc phase much easier. The properties and structures of these thin films are mainly controlled by the thermal treatment and the composition of the targets for ablation. By controlling these conditions, as much as 40% of the total volume of the high-Tc phase is obtained. A precursor state of the 80 K phase can be obtained even at 400°C by this laser ablation method.

Tailored thin films of a superconducting Bi‐Sr‐Ca‐Cu oxide prepared by incorporation of exotic atoms—control of the distance between CuO2 layers

A layer‐by‐layer successive deposition method utilizing laser ablation has been applied to the site‐selective incorporation of +1, +2, and +3 ions at the Ca and Sr site of Bi2Sr2Ca1Cu2O8 superconducting films. The replacement of Ca by larger ions increases the lattice parameter c and Tc simultaneously, while the replacement of Sr does not. These changes are explained by structure models of Bi2Sr2Ca1Cu2O8, and the correlation between Tc and the lattice parameter c is explained on the basis of the distance between CuO2 layers, indicating the importance of the interaction among CuO2 layers.

Tailored Thin Films of Superconducting Bi-Sr-Ca-Cu Oxide Prepared by Excimer Laser Ablation Technique

Site-selective substitution of Ba for Sr or Ca in a layered (Bi-Pb)2Sr2Ca1Cu2Oy structure has been performed by a successive deposition method using pulsed excimer laser ablation. The introduction of Ba to the Sr site diminishes the superconductivity, whereas that to the Ca site improves the superconducting properties, resulting in a higher Tc value. The extent of this improvement is observed in the order of Ba>Sr>Ca, being consistent with the expansion of the lattice parameter of the c axis.

Nanoimprint and lift-off process using poly vinyl alcohol

We developed a lift-off process for a nanoimprint lithography (NIL) using poly(vinyl alcohol) (PVA) as the replicated material. PVA could easily be dissolved in water. A conventional lift-off process using poly(metyl methacrylate) (PMMA) uses acetone as a solvent, while the lift-off process using PVA uses water as a solvent, which is an ecologically friendly process. We demonstrated Au patterns with sub-µm dimensions using a lift-off process with a PVA single layer. In addition, an Hydrogen silsesquioxane (HSQ)/PVA bilayer structure was used for the lift-off process. This bilayer structure could be fabricated by room-temperature NIL and dry etching. Au patterns were easily obtained using the bilayer structure having an inverse tapered shape. In the lift-off process without using HSQ/PVA bilayer, Au wiring with sub-µm linewidth could be obtained, however, 100-nm-linewidth patterns did not remained. Line-and-spacing gratings of 100 nm in the Au patterns were demonstrated using the water lift-off process with the HSQ/PVA bilayer structure.

Dominant Factors for Formation of Perovskite PbTiO3 Films Using Excimer Laser Ablation

Ferroelectric PbTiO3 films have been formed on SrTiO3(100) substrates using ArF excimer laser ablation as a function of substrate temperature with oxygen ( O2+8%O3) pressure. Perovskite PbTiO3 is formed around the substrate temperature of 450° C and oxygen pressure of 10~40 mTorr. Pyrochlore Pb2Ti2O7-x {111}, on the other hand, is formed at higher temperature and lower oxygen pressure than the perovskite phase. The {100}-oriented pyrochlore is formed at lower temperature (around 400° C) and higher oxygen pressure. In addition, PbO films are obtained under similar formation conditions as those of the perovskite phase. This result suggests that PbO formation plays an important role in the construction of the PbTiO3 structure.

Phase control of superconducting Bi-Sr-Ca-Cu-O thin films prepared by laser ablation method

Three phases of Bi2Sr2Can-1CunO2n+4 superconductors, i.e., Bi2Sr2Cu1O6(2201), Bi2Sr2Ca1Cu2O8(2212) and Bi2Sr2Ca2Cu3O10(2223), have been controlled by a laser ablation method. The 2201, 2212 and 2223 phases are selectively formed by controlling the substrate temperature, the starting composition of targets and the ambient pressure. The oxygen partial pressure plays an important role in these three parameters. Under a pressure higher than 6×10-2 Torr, the 2201 phase is dominantly formed in the wide range of substrate temperature and composition. Under a lower oxygen partial pressure (from 3×10-3 to 6×10-2 Torr), on the other hand, the control of the three phases, 2201, 2212 and 2223, is possible for as-deposited films only by changing the target compositions. For the 2212 thin film, Cu-rich samples show good superconducting behavior, while particles are observed on the film surface. The surface morphology of the thin film becomes better for the Cu-poor and Bi-rich samples.

Formation of Bi(Pb)-Sr-Ca-Cu-O Thin Films at 500°C by a Successive Deposition Method Using Excimer Laser

Thin films of high Tc superconductors including La-Sr-Cu-O, Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O have been prepared by various methods. For a device application, as-grown superconducting films prepared with low temperature process are needed. In the present study we have prepared thin films of Bi-Sr-Ca-Cu-O (BSCCO) and Bi(Pb)-Sr-Ca-Cu-O(BPSCCO) by a laser ablation method, and have succeeded in the formation of a crystallized as-grown thin film at the substrate temperature as low as 500°C.

Tailored Thin Films of Superconducting Bi-Sr-Ca-Cu Oxide Prepared by the Incorporation of Exotic Atoms: Superconductivity and the Distance Between CuO2 Layers

The layer-by-layer successive deposition method utilizing laser ablation has been applied to the controlling the structures and the compositions of Bi-Sr-Ca-Cu-O superconducting films. With this method, we achieve the formation of thin films having good morphology by controlling the deposition conditions such as energy density of laser ablation, substrate temperature, and atmosphere. Furthermore the structures and the superconductivity can be controlled by the substitution of various ions for Ca or Sr in the atomic scale.