Scanning nonlinear dielectric microscopy
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In this article, scanning nonlinear dielectric microscopy (SNDM) with atomic resolution is reviewed. First, experimental results on the detection of ferroelectric domains are shown following a presentation about the theory and principle of SNDM. Next, a three-dimensional (3D) type of SNDM for measuring the 3D distribution of ferroelectric polarization and noncontact scanning nonlinear dielectric microscopy (NC-SNDM) are proposed. Using NC-SNDM under ultrahigh vacuum conditions, we clearly resolve the electric dipole moment distribution of Si atoms on a Si (111)7 7 surface. We also succeeded to resolve a fullerene (C60) molecule. Since the technique is applicable not only to semiconductors but also to both polar and non-polar dielectric materials, SrTiO3 and TiO2 surfaces were observed by NC-SNDM. Finally, we characterize an ultrahighdensity ferroelectric data storage system using SNDM as a pickup device and a congruent lithium tantalate single crystal as a ferroelectric recording medium.
I. INTRODUCTION
Recently, ferroelectric materials, particularly those in thinfilm form, have attracted the attention of many researchers. Their large dielectric constants make them suitable as dielectric layers in microcapacitors in microelectronics. They are also investigated for application in nonvolatile memory using the switchable dielectric polarization of ferroelectric material. To characterize such ferroelectric materials, a highresolution method is required for observing the microscopic distribution of the remanent (or spontaneous) polarization of ferroelectric materials. In recent years, we have developed and reported on the use of scanning nonlinear dielectric microscopy (SNDM) for the measurement of the microscopic distribution of dielectric properties on dielectric material surfaces.1–6 Another frequently reported high-resolution method used for observing ferroelectric domains is piezoelectric response imaging using scanning force microscopy.7,8 Compared with this, SNDM is the first successful purely electrical method for observing ferroelectric polarization distribution without the influence of the screening effect from free charges. Because this microscopic technique has quite a high sensitivity to variation in capacitance, on the order of 1022 F, we have also succeeded in visualizing the electron and hole distributions stored in semiconductor flash memory devices using SNDM.9–12 Therefore, this a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.219 J. Mater. Res., Vol. 26, No. 16, Aug 28, 2011
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technique is expected to be useful for developing new semiconductor devices. In addition to above-mentioned SNDM techniques, which detects the polarization components perpendicular to the specimen surface only, a new three-dimensional (3D) type of SNDM (3D-SNDM) for measuring the 3D distribution of ferroelectric polarization has been developed.13–15 Using this 3
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