Juro Mita

Crystallization of Sr0.7Bi2.3Ta2O9+α Thin Films by Chemical Liquid Deposition

By chemical liquid deposition in which an alkoxide with a carbon number of 4 or smaller was used as raw material, a Sr0.7Bi2.3Ta2O9+α (SBTO) thin film was fabricated for use as a ferroelectric memory device for the purpose of decreasing the temperature of crystallization and improving surface morphology. The crystallization process was also examined. Crystallization began when the film was heat treated in oxygen at 650° C. When it was heat treated at higher than 700° C, it showed ferroelectric properties, and the squareness (remanent polarization/saturation polarization) of its hysteresis loop was improved at 800° C. A film heat treated at temperature 650° C was a cluster of fine particles, and a film heat treated at 800° C was a cluster of large particles. A film heat treated at 700° C was a mixture of fine particles and large particles. Therefore, it is concluded that the alkoxide with a carbon number of 4 or smaller as raw material enable the lowering of the heat-treatment temperature and improvement of the surface morphology of SBTO thin films.

Preparation of MgO Protective Layer for AC‐Type Plasma Display Panel by Means of Screen‐Printing

We studied how to form an MgO protective layer on an ac-type plasma display panel (PDP) using a screen-printing method. In our study, we focused on high luminous efficiency and low drive voltage. To raise the luminous efficiency, it is necessary to limit the discharge current. The firing voltage (V t ), which is the voltage at which all cells start to discharge, and the sustaining voltage (V s ), which is the minimum voltage required for all cells to sustain discharge, have no clear relationship to luminous efficiency. To attain both a higher luminous efficiency and a lower drive voltage, a thinner more crystalline layer is necessary. We found that we could prepare an MgO layer by screen-printing and sintering fine vapor-deposited MgO powders of different grain sizes with an MgO liquid binder. An MgO protective layer prepared by this method provided both a low drive voltage and superior luminous efficiency 2.8 times higher than that of a sputtered layer. The screen-printing method thus enables larger ac-PDPs to be produced at lower cost

Role of excess Bi in SrBi2Ta2O9 thin film prepared using chemical liquid deposition and sol-gel method

We studied the role of excess Bi, added to improve ferroelectirc properties of SrBi 2 Ta 2 O 9 (SBT) thin film whose Bi-layered structure is fatigue-free characteristics for Nonvolatile memory. The lost of Bi by annealing process was not observed even after annealing at 850°C in chemical liquid deposition (mixed alkoxide solution system). In SBT films composed of fluorite and Bi-layered structure grains, Bi-layered structure grains had a higher Bi content than that in fluorite grains. Excess Bi was added to promote crystallization of fluorite to the Bi-layered structure easy. SBT film close to stoichimometric composition formed by a hydrolyzed, condensed solution (sol-gel method) had superior ferroelectric properties despite its closely stoichiometric Sr 0.9 Bi 2.1 Ta 2 O 9 composition. Closely stoichiometric Sr 0.9 Bi 2.1 Ta 2 O 9 SBT film shows no fatigue even after 3 x 10 12 switching cycles. The improved SBT formation solution that is hydrolyzed, condensed solution makes excess Bi unnecessary.

EFFECT OF ANNEALING METHOD TO CRYSTALIZE ON SR0.9BI2.3TA2O9+ THIN FILM PROPERTIES FORMED FROM ALKOXIDE SOLUTION

Sr0.9Bi2.3Ta2O9+α (SBT) thin films were formed from a mixed alkoxide solution and were heat-treated by various annealing methods. The SBT thin film heat-treated with the standard temperature rising at a rate of 10°C/min (standard thermal annealing: STA) showed higher squareness (effective remanent polarization/effective saturation polarization) and effective remanent polarization values than those of the film heat-treated with a rapid temperature rise at a rate of 125°C/s (rapid thermal annealing: RTA). This difference was large in the region of low applied voltage. The low values of the SBT film with RTA treatment were caused by lower crystal continuity in the direction of film thickness than that of the SBT film with STA treatment. The SBT film with RTA treatment showed a higher leakage current density for the entire applied voltage region than that of the film with STA treatment. The higher leakage current density was caused by fine nonstoichiometric (Bi-rich) particles which exist at grain boundaries of large grains. Therefore, it was concluded that the high squareness ratio and low leakage current density are obtained in SBT film which has only one grain in the film thickness direction and does not have nonstoichiometry fine particles around grain boundaries.

Orientation Control of Sr0.7Bi2.3Ta2O9+α Thin Films by Chemical Liquid Deposition

The orientation of Sr 0.7 Bi 2.3 Ta 2 O 9+α (SBT) films, which are layer-type bismuth compounds, was controlled by varying the Sr-source. In this paper, the effect of crystal orientation on film characteristics is described. The crystal orientation of the SBT ferroelectric films did not affect the surface morphology, leakage current or fatigue characteristics, but it did affect the shape of the hysteresis loop (polarization) and the window value of the C-V characteristics when the films were connected to a metal-oxide-semiconductor (MOS) diode. Although a complete c-axis orientation film with a stoichiometry of SrBi 2 Ta 2 O 9 shows no spontaneous polarization in general, the highly c-axis orientated Sr 0.7 Bi 2.3 Ta 2 O 9+α film in this study showed some spontaneous polarization. The polarization values are larger than expected by considering orientation alone. A deviation from stoichiometry resulted in an increase made in the polarization along the c-axis. Therefore, control of the crystal orientation and composition of SBT films is quite an important factor in actual applications.

ZnS:Mn Thin Film Electroluminescent Devices Having Doubly-Stacked Insulating Layers

ZnS:Mn thin film electroluminescent devices having doubly-stacked insulating layers have been developed. The insulating layer consists of electron-beam evaporated Ta2O5, and rf-magnetron sputtered SiO2 stacked layers. It is concluded that the thick semi-insulating Ta2O5 films (1000 nm) deposited on both sides of the ZnS:Mn active layer act as a carrier injctor for electroluminescence and the thin insulating SiO2 films (80 nm) inserted between the electrodes and Ta2O5 films act as a carrier limiter. The breakdown voltage margin defined by (Vbd-Vth)/Vth was improved from 15% to more than 80% by introducing thick semi-insulating Ta2O5 films.

Correlation between composition, microstructure, and ferroelectric properties of SrBi{sub 2}Ta{sub 2}O{sub 9} thin films

SrBi{sub 2}Ta{sub 2}O{sub 9} thin films prepared by a solution-deposition process were formed at various annealing temperatures. P-E hysteresis loops of the films exhibited well-defined shapes, and the leakage current decreased with increasing annealing temperature except for one annealed at 750 C. The considerable amount of metallic Bi on the film surface diffused from the inner part was evaluated by a quantitative x-ray photoelectron spectroscopic analysis. A discontinuity in morphology such as cavities detected by transmission electron microscopic observation at grain boundaries between large single-crystal grains and microcrystallite regions, which was pronounced in the film annealed at 750 C, is suggested as inducing an increase in leakage current by the condensation of metallic Bi at the cavities.

Characterization of ferroelectric SrBi2Ta2O9 thin films prepared from alkoxide solutions

SrBi 2 Ta 2 O 9 thin films prepared by chemical liquid deposition using alkoxide precursor solutions, followed by annealing at various temperatures, were characterized with a focus on the correlation between composition, microstructure, and ferroelectric properties. P-E hysteresis loops of the films exhibited well-defined shapes at temperatures above 700°C, and the leakage current decreased with increasing annealing temperature, except for a film annealed at 750°C. The crystal growth proceeded and ferroelectric properties of the films improved with increasing annealing temperature, whereas a large leakage current was observed for the film annealed at 750°C. Quantitative X-ray photoelectron specroscopic analysis and high-resolution transmission electron microscopic observation indicated the presence of a substantial amount of metallic bismuth in all films, predominantly in microcrystallite regions and on film surfaces, including the inner surface of cavities. The atomic fraction of metallic bismuth increased upon slight argon etching; however, the total bismuth content was significantly reduced. The metallic bismuth was amorphous and detected as vibrating spherical particles measuring approximately 5 nm under electron irradiation. The discontinuity in morphology, such as cavities at the interface between large single-crystal grains and microcrystallite regions, was most remarkable in the film annealed at 750°C. The condensation of metallic bismuth at the cavities is believed to increase the leakage current when the path formed by cavities penetrates through the film.

Difference in Electroluminescent ZnS:Tb, F Thin Films Prepared by Electron-Beam Evaporation and RF Magnetron Sputtering

To clarify the difference in ZnS:Tb,F films fabricated by electron-beam evaporation (EB) and by rf magnetron sputtering (SP), the doping condition of Tb and F ions was investigated by electron probe micro analysis and secondary ion mass spectroscopy. The F/Tb atomic ratio of 3 and EL spectra for EB films are found to be hardly affected by annealing. As the model for luminescent centers, it is proposed that the Tb and F ions are substituted for Zn and three S ion sites, respectively, with two Zn vacancies for satisfying charge compensation. For the SP films, interstitial F ions are released from ZnS film and Tb-F complex centers are formed by annealing.

Effects of Annealing on ZnS:Tb, F Electroluminescent Thin Films Prepared by rf Magnetron Sputtering

The effects of annealing on sputtered ZnS:Tb, F thin films is investigated by electron probe microanalysis, secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. It is shown that the annealing decreases the F/Tb atomic ratio from 4 to 1, due to the release of F atoms. It is concluded that many of the F ions not contributing to the formation of luminescent centers with Tb ions exist in as-sputtered film and that efficient Tb-F complex centers are formed by annealing at over 400°C. Luminance is enhanced by increasing the Tb-F complex centers and decreasing the hot-electron scattering centers of the F ions.