Development of an Ultra-High Vacuum Scanning Nonlinear Dielectric Microscope and Near Atomic-Scale Observation of Ferroe
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Development of an Ultra-High Vacuum Scanning Nonlinear Dielectric Microscope and Near Atomic-Scale Observation of Ferroelectric Material Surfaces Hiroyuki Odagawa and Yasuo Cho Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan ABSTRACT This paper describes a newly developed ultra-high vacuum type scanning nonlinear dielectric microscope (UHV-SNDM) and the results from a ferroelectric LiTaO3 single crystal measured by the UHV-SNDM. In a cleaved (012) surface of LiTaO3 crystal, we can clearly observe a striped pattern with a period of about 0.3 nm and a granular pattern approximately corresponding to the sub-lattice period. From this image, we confirmed that the SNDM can be applied to lattice-level measurement in ferroelectric insulator materials. INTRODUCTION Recently, several researchers have studied and reported on various techniques for determining the direction of ferroelectric polarization and observing the domain structure with high spatial resolution [1-12]. This research is important to clarify the relationship between material properties and the behavior of small ferroelectric domains, especially regarding polarization switching and domain wall movement, which are fundamental properties in ferroelectric device applications. Therefore, this research is not only interesting from an academic standpoint, but also has potential for improving device characteristics. We previously proposed and developed a purely electrical technique called "scanning nonlinear dielectric microscopy" (SNDM) for imaging the state of ferroelectric polarization and the local crystal anisotropy of dielectric materials [7,8]. To date, the spatial resolution of SNDM has been improved down to sub-nanometer levels, exceeding the resolution of piezoelectric scanning force microscopy. SNDM uses an electrical technique instead of force detection, measuring slight variations in capacitance at the top of a probe tip under an applied electric field. Therefore, we expect that SNDM can be applied to an inactive insulator surface, where it is difficult to use other scanning probe microscopy techniques because of the weak interaction between the sample and the probe tip.
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In this paper, we describe a newly developed ultra-high vacuum scanning nonlinear dielectric microscope (UHV-SNDM) that removes the undesirable influence of an absorbed layer on the sample surface. We also describe the results of measuring a cleaved (012) surface of an LiTaO3 single crystal using the UHV-SNDM. We clearly observed a striped pattern with a period of about 0.3 nm and a granular pattern approximately corresponding to the sub-lattice period. We thus confirmed that SNDM could be applied to the lattice-level measurement of physical properties in ferroelectric insulator materials. UHV-SNDM Figure 1 depicts the system setup of the UHV-SNDM. Figure 1(a) shows an SNDM probe head along with the signal-processing components developed in References 7 and 8, and Fig. 1(b) shows the composition of the ultra-high vacuum chamb
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