First-principles study of the electronic structure of Pb(ZrTiNb)O 3 (PZTN) systems
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First-principles study of the electronic structure of Pb(ZrTiNb)O3 (PZTN) systems Hiromu Miyazawa1, Takamitsu Higuchi1, Taku Aoyama1, Takeshi Kijima1, Eiji Natori1, Tatsuya Shimoda1 and Tamio Oguchi2 1 Technology Platform Research Center, SEIKO EPSON Corporation 281 Fujimi, Fujimi-machi, Nagano-ken 399-0293, Japan 2 Graduate School of Advanced Sciences of Matter, Hiroshima University 1-3-1 Kagamiyama, Higashi-Hiroshima-shi, Hiroshima 739-8530, Japan ABSTRACT Using first principles calculations, we have investigated the electronic structure of Pb(ZrTiNb)O3 (PZTN), a system with a low leakage current and high reliability in thin films. We proposed that in PZTN, the oxygen vacancy is suppressed due to the addition of a Nb atom at the B site and that this change prevents a bandgap narrowing which would enhance the leakage current. The oxygen vacancy in the perovskite structure reduces the bandgap because it lowers the d orbital energy of the nearest-neighbor transition metal through the Madelung potential; this bandgap narrowing induces the leakage current in conventional Pb(ZrTi)O3 (PZT) systems with the Shottky type Pb-O deficit. In contrast, the PZTN systems, which also have the Pb deficit but lack the oxygen vacancy, can maintain the bandgap, and attain a low leakage current. INTRODUCTION Perovskite-type oxides, represented by the chemical composition ABO3, are widely used as piezoelectric and ferroelectric thin-film devices. In manufacturing oxide thin-films, the most important factor for reliability is the suppression of leakage current. Recently, Kijima et al [1] introduced a new material which achieves a very low leakage current, a factor of 10-4 lower than those of conventional Pb(ZrTi)O3 (PZT) systems, and designated these systems as Pb(ZrTiNb)O3 (PZTN) systems. In their work, they substitute 20% to 25% Nb for the B site atom and use a few mol percent of Si in the sintering process. Their leakage mode is of the Shottky type and the Fowler-Nordheim (F-N) tunneling mode is well suppressed.[1] Since this new material also appears highly reliable in fatigue and imprint tests, Kijima et. al. suggest that the low leakage current in their PZTN systems is related to this high reliability. They also suppose that the effect of doped Si is to lower the annealing temperature and thus to enable the substitution of 20% to 25% Nb for the B site atom. The idea of Nb substitution at B site for thin film has already been realized by Sadashivan et. al. to compensate the oxygen vacancy[2]. Sadashivan succeeded in substituting 4% Nb for the B site atom. In their paper they qualitatively discussed the relationship between the carrier emission mode and the leakage current at electrode interface. In this paper we will investigate the electronic structure of PZTN and compare it with that of a conventional PZT by substituting Nb for the B-site atom and by subtracting from the A site Pb and/or oxygen. Considering these electronic structures, we will discuss the mechanism of the low leakage current in PZTN systems. Finally, we w
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