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

铁电材料具有可逆的自发极化，并表现出D-E电滞回线.理想情况下，磁滞回线应该位于D-E平面的中心.然而，在实际的铁电薄膜电容器中，电滞回线常常会沿电场轴沿着方向发生偏移，即“电压偏移”。当极化方向一致时，电压随时间逐渐变化的现象称为“（静态或热）印记”。静电印记是提高铁电随机存取存储器（FeRAM）可靠性的重要问题之一，因为静电印记在保持数据的过程中不断发展，显著的电压漂移导致存储数据的变化和阻止进一步的极化反转。采用化学溶液沉积（CSD）法在Pt/Ti/SiO2/Si基片上制备PZT薄膜，并在700℃下烧结，在射频磁控溅射沉积Pt电极后，在相同温度下进行后退火。PZT薄膜电容器的压印过程可用Tagantsev等人提出的三个形式相同的方程拟合，而是三组不同的参数这表明，随着时间的推移，引起印迹的空间电荷的传导机制经历了三个阶段的变化。第一种机理的温度依赖性较小，而第二种机理的温度依赖性较大。第三个是否具有温度依赖性尚不清楚，因为数据点太少，它影响了长时间的印记。从这些结果中，我们推测，压印的进展是由电荷注入从电极由于第一Fowler-Nordheim和第二肖特基发射在界面层，最后Poole-Frenkel导电膜体。

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.