Study on the Change in the Switching Properties of Ferroelectrics Thin Films with the Passage of Time

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Ferroelectric materials have reversible spontaneous polarization and show D -E hysteresis loops. Ideally, the hysteresis loops should be located in the center of a D -E plane. However, actual ferroelectric thin-film capacitors frequently exhibit some shift of the hysteresis loops along electric field axis, that is, "voltage shift." The voltage shift which gradually progresses with time when polarization is aligned is called as "(static or thermal) imprint." This static imprint is one of most serious issues for improving the reliability of ferroelectric random access memories (FeRAMs) because the static imprint progresses in keeping data, and remarkable voltage shifts lead to the change in stored date and the prevention of further polarization reversal.We have carefully investigated the imprint behavior of Pb (Zr,Ti)O_3 (PZT) thin-f-film capacitors. The PZT films were formed on Pt/Ti/SiO2/Si substrates by chemical solution deposition (CSD) with sintering at 700℃, and a post-annealing was carried out at the same temperature after the deposition of top Pt electrodes by rf-magnetron sputtering. The imprint progresses of the PZT thin-film capacitors could be fitted by three equations with the same form which was proposed by Tagantsev et.al., but three different sets of parameters. This indicates that the conduction mechanisms of space charges which caused imprint changed by three steps with time progress. The first mechanism had less temperature dependence while the second one had remarkable temperature dependence. Whether the third one had temperature dependence or not was not clear because of data points were too few, it impacted a longtime imprint. From these results, we speculated that the imprint progresses were controlled by charge injection from electrodes due to first Fowler-Nordheim and second Schottky-emission in interfacial layers, and finally Poole-Frenkel conduction in film bodies.