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)建立高频极化-电场滞后曲线的测量方法。这对于实现铁电薄膜存储器（FeRAMs）是非常重要的。首先，我们提出了（Pb,Sr）TiO_3作为铁电材料，并通过溅射法制备了该材料的薄膜。首先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，薄膜中的铅元素有所减少。在沉积过程中，在衬底上施加直流偏压场。负电场显著降低了铅组分的沉积速率。通过优化沉积条件，使PST的结晶温度低至430℃。Bi的加入使剩余极化率提高到20 μC/cm^2。另一方面，为了第二个目的，我们在测量系统中使用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，然后积分为计算的极化。PZT薄膜的矫顽力场E_c与测量频率密切相关，而剩余极化几乎与测量频率无关。PZT薄膜的畴切换动力学可以用核控模型来解释。提出了制造高运行速度feram的指导方针。

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.