An in-Sitij Tem Study of the Dynamic Behavior of Domain Walls in a Free-Standing Lead Tit an Ate thin Film Under Externa
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ABSTRACT The evolution of domain structure with external stress in a free-standing PbTiO3 ferroelectric thin film of -lOOnm in thickness is observed by in-situ TEM technique. The thin film is composed of granular grains of -100nm in diameter, most of them appear to be singledomained whereas others are multi-domained showing domains of different sizes(5-20nm). For some single-domained grains new domains appear during tension. For multi-domained grains, rearrangement of domain walls and coarsening of domains have been observed during tension. In many cases the domain walls disappear under high stress, i.e., a multi-domained grain changes into a single-domained grain. However, it is also observed that a large portion of single-domained grains appear not to respond to external stress. The dynamic behavior of domain walls in very thin ferroelectric thin films may help to understand the switching of these very thin films. INTRODUCTION In recent years ferroelectric thin films have been intensively studied due to their promising applications in micro-electronic devices [1-2]. It has been established experimentally that abnormal change of electrical properties, such as sharp increase in coercive field [3] and a drop in permitivity [41, occurs when film thickness or grain size is below about 200nm. These changes are of fundamental importance for the application of very thin ferroelectric films. However, the reason has remained unclear up to now.
It is well-known that the properties of thin films are determined by their microstructures. Among many microstructural factors, domain structure and the mobility of domain walls play a central role in determining electrical properties, especially the switching behavior of ferroelectric thin films. It is thus very important to study the domain structure, and especially to study the dynamic behavior of domain walls under external field such as electric or stress field. Optical microscopy has been applied to observe the evolution of domain structure with external electrical or stress field [5,6], but mainly confined to single crystals. Domain evolution in fine-grained ferroelectric thin films cannot be observed by optical microscopy because of the limited resolving power of optical microscope and the difficulty in etching domain walls in very fine grains. With the attached tension stage, transmission electron microscopy (TEM) can provide an in-situ observation of the evolution of microstructures (including domain structures) under external stress (tensile stress). Therefore, it is possible to use this technique to directly study the dynamic behavior of domain walls in ferroelectric thin films under external stress. However, insitu TEM deformation experiment for ferroelectric thin films have been confronted with a serious problem in preparing specimens since such samples are extremely fragile. Due to this reason no successful experiment has been reported on ferroelectric thin films to our knowledge.
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Mat. Res. Soc. Symp. Proc. Vol. 404 01996 Materials Research Society
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