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，并呈现D-E磁滞回线。理想情况下，磁滞回线应该位于D-E平面的中心。然而，实际的铁电薄膜电容器经常会出现磁滞回线沿电场方向的漂移，即“电压漂移”。当极化对准时，电压随时间逐渐变化的移位称为“(静态或热)印迹”。静态印迹是提高铁电随机存取存储器(FeRAMs)可靠性的重要问题之一，因为静态印迹在保存数据方面有进步，而显著的电压漂移会导致存储数据的变化和防止进一步的极化反转。采用化学溶液沉积法在铂/钛/二氧化硅/硅衬底上制备了PZT薄膜，烧结温度为70 0℃，在射频磁控溅射沉积铂电极后，在相同温度下进行后退火处理。PZT薄膜电容器的压印过程可以用Tagantsev等人提出的形式相同的三个方程来拟合，但三组参数不同。这表明引起印迹的空间电荷传导机制随着时间的推移分三个阶段发生变化。第一种机理对温度的依赖性较小，而第二种机理具有明显的温度依赖性。第三个模型是否具有温度依赖性尚不清楚，因为数据点太少，它影响了一个长期的印记。根据这些结果，我们推测压印过程是由界面层中第一次Fowler-Nordheim和第二次肖特基发射引起的电极电荷注入控制的，最后是薄膜体内的Poole-Frenkel导电。

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.