日 時:2008年 7月 25日(金)10:00~11:00
場 所:南7号館 2階 202講義室
講 師:Dr. V. Nagarajan (UNSW, Syndey,豪州)
講演題目:「Size Effects in Epitaxial Ferroelectric Pb(Zr,Ti)O3 Heterostructures」
講演内容:
Decreasing ferroelectric thin film thickness and lateral dimensions known as "scaling or size effects", is pressing issues for ferroelectric devices such as MLCC and FRAM. In this presentation, scaling of structural and ferroelectric properties will be presented and modeled based upon epitaxial ferroelectric heterostructures as a function of reduced film thickness and lateral size. Two different compositions have been investigated in detail: PbZr0.2Ti0.8O3 (PZT 20/80) and PbZr0.52Ti0.48O3 (PZT 52/48). The PZT (20/80) system is closely lattice matched with top and bottom SrRuO3 oxide electrodes and therefore facilitates the investigation of highly coherent interfaces without the extrinsic effect of dislocations. The structure has been investigated by direct lattice high resolution electron microscopy as well as synchrotron radiation. Our studies show that a high tetragonality (c/a~1.06) is maintained down to 40 thick films, suggesting indirectly that ferroelectricity is fully preserved even at 10s of nm thicknesses. However, measurement of the switchable polarization (△P) using a novel pulsed probe setup revealed a systematic drop from ~140 uC/cm2 for a 150 thick film to 11 uC/cm2 for the thinnest film. This apparent contradiction between the structural measurements and the measured switchable polarization can be explained by the increasing of the depolarizing field with decreasing thickness. Below a critical thickness the system is predicted to break down into a pinned 180 degree polydomain state to produce a further screening of
this field, even in the presence of metallic electrodes. These results suggest that the ferroelectric hickness limit for a practical ferroelectric memory device would be much thicker than the fundamental limit for ferroelectricity.
問合せ先:J.S. クロス 内線: 3367,e-mail: [email protected] 篠崎和夫,内線: 2518,e-mail: [email protected]