A new model of imprint mechanism in ferroelectric memory
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A new model of imprint mechanism in ferroelectric memory Hiromu Miyazawa, Yasuaki Hamada, Takamitsu Higuchi, Eiji Natori, Takeshi Kijima, Tatsuya Shimoda, Masato Yoshiya1, and Tamio Oguchi2 TPRC, Seiko Epson Corporation, 281 Fujimi-machi, Nagano-ken 399-0293, Japan 1 Adaptive Machine Systems , Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan 2 ADSM, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima-shi, 739-8530, Japan ABSRACT We have studied imprint mechanism of ferroelectric memory based on the oxygenvacancy screening model. Interface charge density and offset bias for PZT resulted from the imprint were estimated as a function of effective depth for diffusion of oxygen vacancy based on this model. In order to understand microscopic dynamics during the imprint that might affect the interface charge density, molecular dynamics simulation (MD) was carried out. Diffusion length during the real imprint test for PZT was estimated based on the diffusion constant obtained from the MD simulation. By using the diffusion length, interface charge density and, in turn, offset bias was quantitatively estimated. Based on the quantitative analysis, it is found that the oxygenvacancy screening model with microscopic dynamics obtained from the simulation explains well the imprint mechanism for PZT materials family. INTRODUCTION For the development of a large-scale ferroelectric-random-access memory (FeRAM), we need to realize reliable materials and their suitable manufacturing processes which can clear the static imprint tests.[1] For this demand, we have proposed the new material, Pb(ZrTiNb)O3 with Nb 20 mol% at B site (PZTN), and proved that the PZTN has good imprint properties.[2] However, to go further toward Giga-bit memory, imprint mechanism needs to be clarified as to why the PZTN systems have such good imprint properties and conventional Pb(ZrTi)O3 (PZT) systems do not. In other words, we need to study, on the atomistic level, what happens during imprint tests for such a perovskite-type transition-metal oxides. Several models, such as the charge injection model [3,4], have been proposed to explain the degradation mechanism at the imprint tests for conventional PZT.[1,3-5] In this paper, we focused on another model, the oxygen-vacancy screening model, to explain the imprint mechanism in ferroelectric transition-metal-oxides. In the oxygen-vacancy screening model, oxygen vacancy and, in turn, the distribution center of oxygen vacancy in the film moves by unknown distance toward the electrode interface which has negative polarization. Accumulated oxygen vacancies at the electrode interface screen the electric polarization of ferroelectrics. This
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is in contrast to the conventional defect dipole alignment (DDA) model [7], in which positive and negative ion vacancies form dipole pair in the film and their center of mass is fixed at initial state, i.e., only the dipole direction re-aligns along external electric field. This oxygen-vacancy screening model is consistent with our
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