Microstructure and Ferroelectric Characteristics of Ultra-Thin BaTiO 3 Films
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Microstructure and Ferroelectric Characteristics of Ultra-Thin BaTiO3 Films Y. Drezner and S. Berger Department of Materials Engineering, Technion, Haifa 32000, Israel Abstract Microstructure studies of ultra-thin BaTiO3 thin films (2-10 nm thick) show nanodomains having a width as small as one unit cell. Only 180o nano-domains are formed in multidomains structures. The domain-boundaries are formed at {001} twin boundaries. Most of the domains are oriented in parallel to the film plane but out-of-plane orientations are also observed. The films exhibit ferroelectric behavior characterized by a polarization hysteresis loop and a relatively fast switching time. A remnant polarization of about 0.5 µC/cm2 and coercive field of 2.7 V/cm were measured in parallel to the film plane. Temperature-dependent measurements show two peaks of the dielectric constant at about 60oC and 115oC. These peaks are attributed to two transition temperatures associated with the orientation of the nano-domains relative to the film plane and stress. Introduction The subject of ferroelectric behavior in the nanometer-size thickness of thin films is of great scientific and technological interest. In recent years, there has been an intensive research work on the formation and characterization of nanometer size ferroelectric domains. Nanodomains having the size of about 4nm were reported for Pb(Zr0.25Ti0.75)O3 thin films [1,2], 1.24nm for thin PbTiO3 films [3,4], and 2-10 nm BaTiO3 [5]. In addition, a ferroelectric behavior was observed in a monolayer of PVDF polymer film [6]. These experimental reports raise the question of the validity of the concept of a critical size in ferroelectricity, which is well established in theoretical models [6-9]. It seems that under specific conditions, such as high stress, nano-domains can be formed with the smallest width of one unit cell [5]. Once the nanodomains are formed, there is high interest of studying their ferroelectric properties and suitability for devices. Most of the reports on nano-domains concentrate on their formation, microstructure and surface morphology [1-6], but data on their ferroelectric properties are scarce. In this paper we report the ferroelectric properties of nano-domains formed in ultra-thin BaTiO3 films having a thickness of 2-10 nm. The films exhibit ferroelectric behavior characterized by polarization hysteresis loop, Curie temperatures, and switching time. Experimental The studied samples consist of three thin films made of LaNiO3/BaTiO3/LaNiO3 (LNO/BTO/LNO) on a Si (100) substrate. The films were deposited at 4500C using a RFmagnetron sputtering system, followed by a one hour post deposition heat treatment between 570-7000C in air. The BTO films were deposited to a thickness of 2-10 nm and the LNO electrodes to a thickness of 3–480 nm. The microstructure of the BTO domains was characterized using High Resolution Transmission Electron Microscopy (HRTEM, Jeol 3010), operated at 300 kV. The ferroelectric behavior of the BTO films was characterized by measuring the polarizat
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