Insook Yi

Electrical anisotropy in single crystals of Bi-layer structured ferroelectrics

Abstract Single crystals of PbBi 2 Nb 2 O 9 (PBN), PbBi 4 Ti 4 O 15 (PBT) and Pb 2 Bi 4 Ti 5 O 18 (P 2 BT), belonging to the bismuth layer-structured ferroelectric compound, were grown and dielectric, conducting and ferroelectic properties were evaluated separately in directions of a ( b )-axis and c -axis. The relationship between crystal structure (the number of oxygen octahedra in the perovskite block m ) and electrical properties was discussed comprehensively. The dielectric permittivity at T c in the a ( b )-axis direction was more than 20 times larger than for the c -axis direction. The anisotropy depended on m and was larger in the compound with even-number m than that with odd-number m . The DC conductivity increased with increasing m in both a ( b )-axis and c -axis directions. The spontaneous polarization in the c -axis direction was not recognized for PBN and PBT with even-number m but observed for P 2 BT with odd-number m , although the value of remanent polarization was much smaller than for the a ( b )-axis direction.

Study on Ag mesh/conductive oxide hybrid transparent electrode for film heaters

Ag mesh-indium tin oxide (ITO) hybrid transparent conductive films were fabricated and evaluated for use in film heaters. PS monolayer templates were prepared using highly mono-dispersed PS spheres (11.2 μm) obtained by a filtering process with micro-sieves. At first, three Ag meshes with different sheet resistances (20, 100, and 300 Ω sq(-1)) and transmittances (70, 73, and 76%) were evaluated for film heaters in terms of voltage and long-term stability. Subsequently, in an effort to obtain better transmittance, Ag mesh-ITO hybrid heaters were fabricated utilizing finite ITO depositions. At the optimised ITO thickness (15 nm), the sheet resistance and the transmittance were 300 Ω sq(-1) and 88%, respectively, which indicates that this material is a good potential candidate for an efficient defroster in vehicles.

Non-contact atomic force microscopy study of atomic manipulation on an insulator surface by nanoindentation.

Experimental results on vertical manipulation on an insulator surface using non-contact atomic force microscopy are presented. Cleaved ionic KCl(100) single crystal is used as an insulator surface. With the nanoindentation method used, the vertical manipulation of a single atom in an ionic crystal surface is more difficult than in a semiconductor surface. Therefore, in many cases, more than one surface atom is manipulated while, in rare cases, single-atom manipulation is successfully performed. Lateral manipulation of a vacancy has occasionally succeeded on the KCl(100) surface. We have presumed that the lateral manipulation was induced by pulling.

Ferroelectric Properties of Very Thin Pb(Zr0.4Ti0.6)O3 Film Determined by Kelvin Force Microscope.

We attempted to examine scaling issues in terms of PbZr0.4Ti0.6O3 (PZT) film thickness. Several very thin PZT films were obtained by the combination of the wet cleaning method and the etch-back process. The polarizability of PZT film was evaluated by measuring the surface potential utilizing the Kelvin force microscope (KFM) mode. The surface potential decreased as the thickness of the PZT film decreased. In particular, the decreasing rate of the surface potential was accelerated below 60 nm, which indicates a possible limitation of the PZT film thickness in terms of the remnant polarization value. Additionally, the retention property as a function of PZT film thickness was also examined by measuring the surface potential in terms of elapsed time. The result shows that the retentivity of the remnant polarization becomes worse as the film thickness decreases.

Simultaneous force and current mapping of the Si(111)-(7×7) surface by dynamic force microscopy

We simultaneously obtain spatial maps of the interaction force and current between conductive tips and the Si(111)-(7×7) surface. The difference in the topographic profiles between atomic force microscopy and scanning tunneling microscopy is clearly visualized. We observe a current drop in the region where the chemical bonding force between the tip and surface atoms becomes significant. The peak values of the conductance above adatom sites are in good agreement with the value previously obtained by the theoretical simulation.

Application of Scanning Probe Microscope for Novel Characterization of Ferroelectric Capacitor.

Novel efforts have been made to analyze the electrical properties of ferroelectric devices by utilizing scanning probe microscopy (SPM). In this study, an attempt was made to measure the hysteresis loop of submicron ferroelectric capacitors using SPM as a probe tip. Conductivity mapping images were useful for understanding the inferior process. Additionally, the surface potential measurements using the Kelvin force microscopy (KFM) mode were made for various ferroelectric films. The polarizability of Pb(Zr,Ti)O3(PZT) film can be evaluated by measuring the surface potential, and seems to be a promising aspect particularly for analyzing various ferroelectric films including hydrogen-damaged films.

Atomic structure of Ge clusters on Si(111)-(7 × 7) by non-contact AFM

We present non-contact (NC) AFM results of Ge clusters on a Si(111)-(7 × 7) reconstructed surface. The low temperature NC-AFM allows us to directly observe the atomic structure of the Ge clusters on the Si(111). The Ge clusters reside in the middle of a half unit cell in the (7 × 7) reconstruction surface and they are ~1.4 A higher than the Si adatoms. By direct NC-AFM observation, the features of the Ge clusters on the Si(111) became clear. First, the Ge atoms reside in spaces between Si adatoms and on the Si/Ge atoms on the Si(111). Second, the Si adatoms shift from their original position through Ge adsorption, and interact with the Ge atoms accompanied by surface relaxation and a change in their spatial heights. In addition, the interatomic distance between the Ge atoms inside the clusters is approximately 4.0 A, which is larger than that between the Ge atoms in the bulk (2.4 A). Our NC-AFM results of Ge clusters provide valuable information for the basic study of clusters on semiconductor surfaces and may be useful for the manipulation and assembly of clusters for the realization of diverse nanostructures at the atomic level.

Lateral Manipulation of Single Defect on Insulating Surface Using Noncontact Atomic Force Microscope

A single atomic defect on an ionic crystal surface was manipulated at the atomic level using an atomic force microscope. During raster scanning, the defect was manipulated at near the tip–sample distance and at room temperature. The defect, which is imaged as a sharp half-moon shape, moves continuously if the tip is close to the sample surface. The defect randomly moves along a zigzag or straight path or jumps long distances. The movement of the defect on an insulator is complex in contrast to that on semiconductors. The defect moves suddenly owing to the tip approaching the surface when the distance feedback is overshot under the condition of a large difference in frequency shift between the feedback set point and a weak interaction on the defect.

Functions of NC-AFM on Atomic Scale

Using the noncontact atomic force microscope (NC-AFM), we investigated functions of NC-AFM. As a result, we found that the NC-AFM works not only the atomic resolution microscope but also novel atomic tools based on a mechanical method such as a three-dimensional mapping tool of atomic force between the tip and sample atoms, a discrimination tool of atomic force mechanisms between the tip and sample atoms, a discrimination tool of atom species on the sample surface, a control tool of atomic force between the tip and sample atoms, a control tool of atom position on the sample surface, and an atom manipulation tool.

Study on the Surface Potential Behavior of Ultra Thin PZT Utilizing SPM (Scanning Probe Microscope)

The polarizability of PZT film was evaluated by measuring the surface potential utilizing the Kelvin force microscope (KFM) mode. The hysteretic and time dependent behaviors of PZT films were monitored in terms of the thickness of the PZT film. The surface charge versus writing voltage shows a hysteretic behavior. This approach would provide a new possibility to examine ferroelectric properties, in particular, for scalability of ultra thin ferroelectric films. The surface potential decreases as the thickness of PZT film decreases. The decreasing rate in the surface potential was accelerated below 60 nm, which indicates a possible limitation of the PZT film to yield the sufficient remnant polarization value. Additionally, the time dependent property as a function of PZT film thickness was also examined by measuring the surface potential in terms of elapsed time. The time dependent behavior based on surface potential shows an exponential decay from the original value. The time dependent behavior of the surface potential appears to be worse as the film thickness decreases.