Size Effects of Ferrelectric Thin Films and Influences of the Size on the Thin Film Memory Device Properties

包括厚度和晶粒尺寸在内的尺寸效应对于实现存储器件的低电压操作和超高集成度变得越来越重要。为了研究金属有机化学气相沉积法（MOCVD）制备的Pb（Zr，Ti）0_3（PZT）铁电薄膜的尺寸效应，进行了两组实验：第一组实验研究了PZT薄膜的电学性能与薄膜厚度的关系;通过改变沉积时间可以控制薄膜的厚度在70 ~ 600 nm之间，随着薄膜厚度的减小，薄膜的介电常数、折射率、晶粒尺寸减小，矫顽场增大。这一实验结果表明，在MOCVD制备薄膜时，厚度效应中包含了增益尺寸效应，为了区分加林尺寸效应和厚度效应，研究了薄膜的晶粒尺寸依赖性。通过改变下I ~* 电极厚度和PZT薄膜生长速率成功地控制了PZT薄膜的晶粒尺寸，当晶粒尺寸从240 μ m减小到120 μ m时，薄膜的介电常数和剩余极化率减小，电流密度和矫顽场增大。电性能对晶粒尺寸的依赖性与陶瓷的一致。极化转换耐久性能也受到加林尺寸的影响。晶粒尺寸为190和240 nm的PZT薄膜在10 '的开关周期内没有出现开关疲劳。另一方面，晶粒尺寸为120 nm的PZT薄膜表现出疲劳现象，我们的实验结果也意味着薄膜的晶界在决定电学性能方面起着非常重要的作用。

The size effects, including thickness and grain size, have become more and more iimportant for the realization of low voltage operation and ultra high integration in memory devices. In order to investigate the size effect of ferroelectric Pb(Zr, Ti)0_3 (PZT) thin films prepared by MOCVD (metalorganic chemical vapor deposition), two types of experiments were performed.In the first set of experiments, dependence of electrical properties of PZT thin films on the film thicknesswas investigated. Thickness was controlled from 70 to 600nm by changing the deposition time.The dielectric constants, remanent polarizations and grain size decreased and the coercive fields increased as film thickness decreased. This experimental result means that the gain size effect was included in the thickness effect when films were prepared by MOCVD.In order to separate the garin size effect from the thickness effect, grain size dependence of films was investigated. Grain size of PZT films was successfully controlled by changing the thickness of bottom I* electrode and by changing the growth rate of PZT films.The dielectric constants and remant polarizations decreased, and current densities and coercive fields of films with a film thickness of 200nm incresed as grain size decreased from 240 to l2Onm. This dependence of electrical properties on the grain size coincided with that of ceramics. Polarization switching endurance properties were also influenced by the garin size. PZT films with grain size of 190 and 240nm showed no switching fatigue up to a switching cycle of 10'. On the other hand, PZT film with a grain size of l2Onm showed the fatigue.Our experimental results also means that grain boundaries of thin films play very important roles in determining electrical properties.