Temperature Controlled Scanning Nonlinear Dielectric Microscopy
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Temperature Controlled Scanning Nonlinear Dielectric Microscopy K. Ohara and Y. Cho Research Institute of Electrical Communication, Tohoku Univ. 2-1-1 Katahira, Aoba-ku, Sendai, Japan ABSTRACT Scanning Nonlinear Dielectric Microscopy (SNDM) can obtain the linear and nonlinear dielectric constant distribution of dielectric and ferroelectric materials with sub-nanometer resolution. In this paper, we present the new type of SNDM which can control the sample temperature (80K-730K) in vacuum (less than 5×10-7 Torr), and its application to measure the surface polarization state on LiTaO3 single crystal. INTRODUCTION Recently, ferroelectric materials have been widely studied for application to nonvolatile semiconductor memory systems such as FeRAM[1] and a next-generation recording medium.[2] It is very important to reveal the characteristic, especially the surface characteristic, of ferroelectric materials not only for physical concerns but also for the devices application. Therefore, we have developed a new, purely electrical technique for imaging the state of ferroelectric polarization and the local crystal anisotropy of dielectric materials.[3-5] This technique involves measurement of point-to-point variations of the nonlinear dielectric constant of a specimen and is called scanning nonlinear dielectric microscopy (SNDM). This is the first successful purely electrical method for observing ferroelectric polarization distributions free from free-charge shielding. The resolution of the technique has reached to the sub-nanometer scale.[6] Moreover, this SNDM technique is very useful to obtain the surface polarization state of ferroelectric materials. By changing the order of the detected nonlinear dielectric constant, SNDM can observe the surface polarization state in a different analyzing depth and we find that LiNbO3 single crystal has the surface non-ferroelectric layer and this layer grows up even after it is removed.[7] However, in order to understand the surface state of the ferroelectric materials more precisely, the absorbed layer (especially water adsorbed layer) must be removed because this absorbed layer with large dielectric constant affects to the SNDM technique. In this paper, we present the new type of SNDM which can control the sample temperature in vacuum (less than 5×10-7 Torr). This temperature controlled SNDM can cover the temperature range between 80K and 730K and obtain the linear and nonlinear dielectric constant distribution of ferroelectric materials. Using this temperature controlled SNDM, we remove the water absorbed layer in vacuum and measure the surface polarization state of LiTaO3 single crystal.
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PRINCIPLE OF SNDM First, we present the principle of SNDM, briefly.[3] Equation (1) shows a polynomial expansion of the electric displacement D as a function of electric field E. 1 1 1 D = PS + ε (2)E + ε (3)E 2 + ε (4)E 3 + ε (5)E 4 + K 2 6 24
(1)
where, ε(2), ε(3), ε(4) and ε(5) show linear and nonlinear dielectric constants and are 2nd, 3rd, 4th and 5th rank tensors, respecti
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