Nanometer Scale Domain Measurement of Ferroelectric Thin Films Using Scanning Nonlinear Dielectric Microscopy
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Nanometer Scale Domain Measurement of Ferroelectric Thin Films Using Scanning Nonlinear Dielectric Microscopy Hiroyuki Odagawa, Kaori Matsuura and Yasuo Cho Research Institute of Electrical Communication, Tohoku University, Sendai, 980-8577, Japan ABSTRACT A very high-resolution scanning nonlinear dielectric microscope with nanometer resolution was developed for the observation of ferroelectric polarization. We demonstrate that the resolution of the microscope is of a sub-nanometer order by measurement of domains in PZT and SBT thin films. The experimental result shows that nano-sized 180° c-c ferroelectric domain with the width of 1.5 nm for PZT thin film are observed. The result also shows that the resolution of the microscope is less than 0.5 nm for the PZT thin film.
INTRODUCTION Recently, we have proposed and developed a new purely electrical technique for imaging the state of ferroelectric polarization and local crystal anisotropy of dielectric materials, which involves the measurement of point-to-point variation of the nonlinear dielectric constant of a specimen, and is termed "scanning nonlinear dielectric microscopy" (SNDM) [1-5]. This is the first successful purely electrical method for observing the ferroelectric polarization distribution without the influence of the shielding effect by free charge. To date, its resolution has been improved down to one nanometer. Moreover, SNDM can evaluate both linear and nonlinear dielectric constants quantitatively [6], and also can evaluate the local crystal isotropy, for example, crystal polarity of ZnO thin film deposited on LiNbO3 substrate without the influence of the substrate polarity [7]. In this paper, at first we briefly describe the theory for detecting polarization and the technique for the nonlinear dielectric response. Next, we report the results of the imaging of the ferroelectric domains in PZT and SBT thin films using SNDM with nanometer resolution. Especially in a measurement of PZT thin film, it was confirmed that the resolution was sub-nanometer order. We also describe the theoretical resolution of SNDM.
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NONLINEAR DIELECTRIC IMAGING First, we briefly describe the theory for detecting polarization. Precise descriptions of the principle of the microscope have been reported elsewhere (see refs.3,4). Figure 1 shows the system setup of the SNDM using the LC lumped constant resonator probe [4]. In the figure, Cs(t) denotes the capacitance of the specimen under the center conductor (the needle or the cantilever) of the probe. Cs(t) is a function of time because of the nonlinear dielectric response under an applied alternating electric field Ep3(=Epcosωpt, fp=5kHz). The ratio of the alternating variation of capacitance ∆Cs(t) to the static value of capacitance Cs0 without time dependence is given as [3] ∆C S (t ) ε 333 ε (1) = E P cos ω P t + 3333 E P2 cos 2ω P t CS 0 ε 33 4ε 33 where ε33 is a linear dielectric constant and ε333 and ε3333 are nonlinear dielectric constants. The even rank tensor, including the linear dielectric constant ε33, does
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