Microstructure and ferroelectric behavior of nano-domains in ultra-thin BaTiO 3 films
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C3.18.1
Microstructure and ferroelectric behavior of nano-domains in ultra-thin BaTiO3 films Y. Drezner and S. Berger Department of Materials Engineering, Technion, Haifa Israel 32000. Abstract Microstructure studies of ultra-thin BaTiO3 thin films (2-10nm thick) show nano-domains having a width as small as one unit cell. Only 1800 nano-domains and 900 domain-boundaries are formed in multi-domains structures. The domain-boundaries are formed at {111} twin boundaries. Most of the domains are oriented in parallel to the film plane but out-of-plane domains are also observed. The films exhibit ferroelectric behavior characterized by a polarization hysteresis loop. A remanent polarization of 3nC/cm2, and coercive field of 0.7MV/cm were measured in vertical to the film plane. Temperature-dependent polarization measurements show two peaks of the dielectric constant at about 700C and 1100C. These peaks are attributed to two Curie temperatures associated with the out-of plane and in-plane domain’s orientation, respectively. The switching response of the nano-domains is relatively fast in the range of few nano seconds. The switching time decreases with increasing the applied electric field according to a power law dependence. Introduction The concept of a critical grain-size in ferroelectric properties is well established in theoretical [1,2] and experimental [e.g. 3-8] studies. The first critical grain size refers to the transition from a paraelectric to a ferroelectric behavior when the strain energy relaxation is sufficient to form a single domain [9]. The second critical grain size is associated with the formation of multiple domain boundaries as a mean of additional strain relaxation in the grain [9]. The first critical grain size occurs in the nanometer size range (e.g. 28m for BaTiO3 particles [10]) and the second one occurs in the sub-micron range size (e.g. 100nm for BaTiO3 thin films [4]). This paper shows nanometer ferroelectric BaTiO3 domains having a size smaller than the reported critical grain size. The observation of one unit cell width domains raises doubt with regard to the concept of a critical size in ferroelectric materials. 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 rf-magnetron 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 2-10nm while the LNO electrodes to a thickness of 3–480nm. The microstructure of the BTO domains was characterized using High Resolution Transmission Electron Microscopy (HRTEM), operated at 300kV. The ferroelectric behavior of the BTO films was characterized by measuring the polarization vs. applied electric field (using the modified Sawyer-Tower circuit [11], under applied voltage up to 5Vac at 1kHz and temperatures between 200C and 1500C). Switching time measurements were based on the rise-time technique [11].
C3.18.2
Results and Discussion A typical verti
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