Proposal of new material for ferroelectric thin film memories and method to evaluate material properties.

本研究的目的是1)开发一种可在450℃以下结晶的新型铁电材料；2)建立测量高频极化-电场(P-E)滞后曲线的方法。这对实现铁电薄膜存储器(FeRAMs)具有重要意义。对于第一个目的，我们提出了(Pb，Sr)TiO_3作为铁电材料，并用溅射方法制备了这种材料的薄膜。首次研究了衬底温度与薄膜化学成分的关系，证实了随着衬底温度的升高，薄膜中的铅含量降低。在沉积过程中，对衬底施加了直流偏置磁场。负场显著降低了铅组分的沉积速率。通过优化沉积条件，PST的晶化温度降低到430℃。铋的加入使剩余极化提高到20μC/cm2。另一方面，我们在测量系统中使用了高速运算放大器作为电流-电压转换器，将通过PZT薄膜的位移电流转换为电压信号，然后积分计算极化。PZT薄膜的矫顽场(E_C)强烈依赖于测量频率，而其剩余极化几乎与测量频率无关。PZT薄膜的磁畴转变动力学可以用成核控制模型来解释。提出了制造高运行速度的铁氧体的指导思想。

The purpose of the present research was 1) to develop a new ferroelectric material which could be crystallized below 450 ℃ and 2) to establish the method to measure the polarization vs. electric-field (P-E) hysteresis curves at high frequencies. These are considerably important to realize the ferroelectric thin film memories(FeRAMs). For the first purpose, we have proposed (Pb,Sr)TiO_3 as a ferroelectric material and thin films of this material were prepared by the sputtering process. The relation between the substrate temperature and the chemical composition of the films was first studied and it was confirmed that the lead component in the films was reduced with increasing substrate temperature. DC-bias field was applied to the substrate during the deposition. Negative field markedly reduced the deposition rate of lead component. By optimizing the deposition conditions, the crystallization temperature of PST was decreased as low as 430 ℃. The addition of Bi improved the remanent polarization up to 20 μC/cm_2. On the other hand, for the second purpose, we have employed a high-speed operation amplifier as a current-voltage converter in the measuring system and the displacive current through the PZT thin film was converted to the voltage signal followed by integrating to calculated polarization. The coercive field (E_c) of the PZT thin films strongly depended on the measuring frequency, nevertheless their remanent polarization was almost independent of it. The domain switching kinetics of PZT thin films could be explained by the nucleation-controlled model. A guideline to make FeRAMs with a high operating speed was proposed.