Fan-Yi Hsu

Domain structure study of SrBi2Ta2O9 ferroelectric thin films by scanning capacitance microscopy

Scanning capacitance microscopy was used to image the polarization-induced microstructural patterns of sol-gel derivative SrBi2Ta2O9 (SBT) thin films. A sharp image contrast was induced between the nanosized domains owing to the various polarities, so that the domain structure in the SBT thin film was clearly revealed. As a result, the switched and unswitched regions could be unequivocally identified. This investigation also confirms that the reversal polarization process of a ferroelectric domain is much easier inside a large grain than in a small grain.

Variations of differential capacitance in SrBi2Ta2O9 ferroelectric films induced by photoperturbation

In this letter, we demonstrated the impact of illumination on the differential capacitance variation of a strontium bismuth tantalite (SBT) capacitor during scanning capacitance microscopy measurements. It was found that illumination with a stray light of laser in an atomic force microscope could perturb the dC∕dV signals of the samples. We attribute this phenomenon to the generation of free carriers by the photon absorptions via defect traps in the SBT thin film. Therefore, this present work suggests that the effect of laser illumination must be carefully taken into consideration whenever a field-sensitive technique is employed to analyze the properties of a ferroelectric material.

Properties of Pt/SrBi2Ta2O9/BL/Si MFIS structures containing HfO2, SiO2, and Si3N4 buffer layers

The physical and electrical properties of Pt/SrBi 2 Ta 2 O 9 (SBT)/buffer layer (BL)/Si metal/ferroelectric/insulator/semiconductor (MFIS) structures incorporating HfO 2 , SiO 2 , and Si 3 N 4 as buffer layers were investigated. When employing HfO 2 as the buffer layer, an MFIS structure exhibiting a high memory ratio was constructed, presumably because of the SBT characteristics and the high quality of the HfO 2 layer on the Si substrate. This study demonstrates that HfO 2 is one of the best buffer-layer materials for ferroelectric memory applications.

Influence of Ultrathin Tantalum Buffer Layers on Microstructure and Ferroelectric Properties of SrBi2Ta2 O 9 Thin Films

The effects of buffer layers on the crystallization process of strontium bismuth tantalite (SBT) thin films, using a metallorganic decomposition technique, are investigated in this study. Ultrathin tantalum (Ta) buffer layers of various thicknesses were deposited onto Pt/TiO 2 /SiO 2 /Si substrates using magnetron sputtering. The crystallinity, microstructure, and electrical properties of the resulting SBT films on top of the Ta layer were found to be strongly dependent on the thickness of the buffer layer. By optimizing this buffer layer thickness, a homogeneous bismuth-layered structure with uniformly distributed fine grains, it was clearly evident that the SBT film, annealed at 750°C for 1 min, and the corresponding remanent polarization (2P r ), can be as large as 18.8 μC/cm 2 at an applied voltage of 5 V. It is suggested that these buffer layers cause the SBT composition to depart from stoichiometry to an enriched Ta condition. During the crystallization procedure in oxygen ambient, the excess Ta atoms in local regions easily reacted with O atoms to form TaO x centers. They are believed to serve as the nucleation sites that cause the reduction of the activation energy of SBT crystallization.

Contrast Mechanism of Ferroelectric Domains in Scanning Capacitance Microscopy

The contrast mechanism of the scanning capacitance microscopy (SCM) to exhibit the ferroelectric domain structures is demonstrated. An image, developed by the mapping of differential capacitance in a virgin SrBi 2 Ta 2 O 9 (SBT) thin film, was observed and three sharp contrast states of white, black, and gray were revealed. A hysteresis loop depicting the opposing polarities at zero bias was obtained by local measurement and, subsequently, the hysteretic capacitance-voltage curves, as the characteristic of domain switching, were acquired by integrating the calibrated differentiation capacitance. It is concluded that the white and black contrast in SCM images are due to the antiparallel 180° domains whereas the gray contrast is due to the trivial differential capacitance variation. These results strongly suggest that the local capacitance variation can be correlated well with the polarities of polarizations in nanoscale domains.

Pt Nanocrystalline Interfacial Layer in an SBT/Pt/Ti Ferroelectric Capacitor

The microstructural feature of platinum (Pt) bottom electrode in a strontium-bismuth-tantalate (SBT)/Pt/titanium (Ti) stack was investigated by field-emission transmission electron microscopy. The appearance of a thin nanocrystalline Pt (nano-Pt) interfacial layer was unexpectedly observed in the SBT/Pt interface region after crystallization treatment. The energy-dispersive spectrometer analyses indicated a great amount of TiO x formed along the grain boundaries of this nanocrystalline layer. Such a phenomenon is strongly related to the grain-boundary diffusions of O and Ti atoms during the annealing process in oxygen ambient. It is speculated that the structural reconstruction of the Pt electrode was likely triggered by the compressive stress due to the large volume expansion during TiO x growth. Under such compressive stress and thermal conditions, Pt may be plastically deformed and inclined to recover and recrystallize into the nanocrystalline structure. Because the emergence of TiO x in nano-Pt/columnar-Pt interfacial regions would significantly deteriorate the continuity and the ductility of Pt electrode, it possibly causes the peeling of SBT layer and poses a serious problem to the integration of the ferroelectric capacitors.

Seeding Effect by Off-Stoichiometric SrBi2Ta2x O9 Thin Buffer Layer on the Properties of SrBi2Ta2O9 Thin Films

The seeding effect on the crystallization behaviors of the SrBi 2 Ta 2 O 9 strontium bismuth tantalite (SBT) phase on top of a thin Aurivillius buffer layer, SrBi 2 Ta 2x O 9 , was studied. A uniform microstructure and a preferred polar a-axis orientation within the SrBi 2 Ta 1.8 O 9 buffered SrBi 2 Ta 2 O 9 thin film were promoted to achieve a high remanent polarization (2P r ) value of about 19.7 μC/cm 2 . Furthermore, the SBT/buffer heterostructure constructed by the two-step process showed a significantly improved leakage property compared with the nonbuffered SBT. The off-stoichiometric thin buffer layer demonstrates a simple method to improve the microstructure, polar a-axis orientation, and electric characteristics and to introduce less defects in the superposed SrBi 2 Ta 2 O 9 thin films.

Seeding effect of SrBi 2 Ta 2x O 9 thin buffer layer on crystallization and electric properties of SrBi 2 Ta 2 O 9 thin films

The seeding effects of the pre-crystallized (RTP 750degC 30 s) thin buffer layer, SrBi₂Ta_2xO₉ (SBT, x=0.9, 1.0, and 1.1), regarding the structure, the morphology and the ferroelectric properties of the SrBi₂Ta₂O₉ ferroelectric thin films on top of buffer layer, have been investigated. X-ray diffraction patterns revealed that the crystallinity of SrBi₂Ta₂O₉ thin films were significant influenced by the buffer layers with various Ta contents. By adding a Ta deficient buffer layer, the preferred polar alpha-axis orientation and the uniformity of grain size within the SBT thin films were promoted. Meanwhile, the optimized ferroelectric properties were achieved from the SrBi₂Ta_1.8O₉ buffered SrBi₂Ta₂O₉ thin film, with maximum 2Pr values about 19.7 mu C/cm², which was almost 93% greater than that of specimen with the stoichiometric buffered one.

Properties of MOD-derived SrBi 2 Ta 2 O 9 thin films crystallized in an electric field

SrBi2Ta2O9 (SBT) films with thickness of about 200 nm were prepared on Pt/Ta/SiO2/Si substrates by metal-organic decomposition (MOD) method. A high electric field (250 kV/cm) was applied to SBT film during 750degC furnace annealing in atmosphere for 40 min. This applied electric field does some impacts on the microstructures and electrical properties of SBT regardless of the states of electric field. Under an electric field, the films exhibited stronger a/b-axis preferential orientation in Bismuth-layered structure (BLS) phase and a little higher ratio of second phase. In addition, parts of the granular grains grew into rod-like grains. As a result, the electrical properties of SBT thin films were improved by the application of electric field.

Physical and electrical characteristics of SBT ferroelectric thin films with different thickness and varied stresses during annealing

In present study, we applied either a tensile or compressive mechanical stress during annealing on SrBi2Ta2O9 (SBT) ferroelectric thin films with different thickness to investigate the effects of both thickness and stress on the physical and electric properties. The application of external stress during annealing led to the variety of both microstructures and constituted phases (ratio of perovskite vs, pyrochlore) in the films. These variations of structures became more significant in the relatively thinner films. Since the electrical properties of ferroelectrics were strongly depended on their microstructure, domain structure, and constituted phases, our ferroelectric samples demonstrated the distinct ferroelectric behaviors with the condition of various stressing and thickness.