Yoshihiro Sawada

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.

Control of crystal orientation of ferroelectric SrBi2Ta2O9 thin films with multi-seeding layers

A Bi-Ta multi-seeding layer system is proposed to improve the ferroelectric properties of SrBi2Ta2O9 (SBT) thin films prepared by a sol-gel method for applications to nonvolatile memories. For seeding layers composed of different ratios of Bi to Ta, three different structures were observed after heat treatment at 800°C: γ-Bi2O3, Bi3TaO7 and Bi1.4Ta2O6.75. γ-Bi2O3 with a sillenite structure exhibited a plate-like surface morphology. Bi3TaO7 with a fluorite structure exhibited a flat surface morphology. Bi1.4Ta2O6.75 with a pyrochlore structure exhibited a rough surface morphology. Because a flat surface morphology with a fluorite structure is desirable for a seeding layer, the Bi–Ta seeding layers, with a Bi to Ta ratio equal to 3 to 1, were used as multi-seeding layers. When the seeding layers were sandwiched between SBT thin films, multi-seeding layers provided, after heat treatment at 800°C, a higher degree of crystal orientation in the a- or b-axis direction than a simple SBT thin film without seeding layers. Orientation in the a- or b-axis direction is desirable for SBT thin films to obtain improved ferroelectric properties, especially high remanent polarization values. The SBT thin film with Bi–Ta multi-seeding layers was found to have higher saturation characteristics than that without seeding layers, which suggests that the former type of SBT thin film has a good potential for low-voltage operation. The multi-seeding method is thus effective for improving ferroelectric properties of SBT thin films for nonvolatile memories.

Analysis of the degradation mechanism of Pt/SrBi2(Ta/Nb)2O9/Pt capacitors during reductive annealing

Abstract Degradation mechanism of the Pt/SrBi2Ta2xO9/Pt and Pt/SrBi2(Ta1−x/Nbx)2O9/Pt capacitors exposed to the hydrogen attack has been analyzed. The capacitors were fabricated from thin-films consisting of SrBi2Ta2xO9(1 ≦ × ≦ 1.1) and SrBi2(Ta1−x/Nbx)2O9(0 ≦ × ≦ 1) (SBTN) that were formed using a sol-gel process. In most capacitors, SBTN thin-films were short-circuited while annealing in the reducing atmosphere. In contrast, excessive Ta addition could hold back the short circuit. Analysis of the degradation mechanism were carried out by means of the XRD, TEM and XPS, which revealed that the most significant alteration took place in the vicinity of crystal grain boundaries. It was also found that the film with excessive Ta concentration contains the pyrochlore phase in the grain boundary layers. These findings suggest that the stabilization of grain boundary layers is effective to realize capacitors resistant to process conditions.

Study on low-temperature crystallization of SrBi 2Ta 2O 9 thin films prepared by sol-gel method using steam curing process

Abstract We have been developing our original hydrolyzed sol-gel coating solutions for the SrBi2Ta2O9 (SBT) thin film. The molecular structure of the solution can be controlled by hydrolysis. We studied the SBT thin film forming process using these coating solutions with the steam curing process used together and observed lower crystallization temperature and improvement of crystal orientation.