Recently, we have been studying ferroelectric ultrahigh-density data storage and previously reported the successful formation of nano-sized inverted domain dot arrays at a data density of 10.1 Tbit/in2 in a congruent LiTaO3 (CLT) single-crystal thin plate. In addition, we have achieved real data recording at an area density of 1 Tbit/in2 with a bit error rate of only 1.8×10-2. [1]
However, there are a lot of points to be studied, for example, concerning the mechanism of polarization reversal, the cross-sectional shapes of nano-domain dots formed just below the probe tip and the effects of stress in the domain boundary vicinity. It is very important to elucidate these points experimentally. However, we have been able to obtain information on the sample surface only by conventional scanning nonlinear dielectric microscopy (SNDM) due to the high concentration of the electric field just below the tip.
Thus, in the present study, we measured the cross-sectional shape of nano-domain dots formed in both congruent and stoichiometric lithium tantalate single crystal by using SNDM, aiming to observe the depth penetration behavior of nano-domain dots.
At first, we confirmed that, as expected, reversal nano-domain dots clearly penetrated the entire sample. Next, we evaluated the domain wall thickness from each measurement result. It was found that thinner samples have a tendency to have thinner domain walls. Moreover, we also measured the thickness of domain wall as a function of depth. The results confirmed that the domain walls at the specimen surface were thicker than those at the bottom of the specimen. The minimum domain wall thickness in the vicinity of the bottom electrode was 1.1 nm. This means that a higher stress accumulates at the front surface of the sample than at the bottom of the sample. We propose that this is due to degradation of the crystal quality, caused by the dry etching process, as the sample had been fabricated by dry etching from the surface to reduce the thickness of specimen. This information will be applied to discuss the stability of inverted domain dots.
[1]Y. Cho, S. Hashimoto, N. Odagawa, K. Tanaka and Y. Hiranaga: Appl. Phys. Lett. 87, (2005) 232907
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