Preparation of Ferroelectric PZT Thin Films by Photoenhanced CVD and an Application to Memory Devices

在PZT的光MOCVD中，研究了用O_3代替O_2作为氧化气体和光辐照对生长的影响。没有观察到生长温度的降低和PZT薄膜的结晶度的改善。然而，当在生长过程中使用O_3和O_3+UV照射时，观察到击穿电压的改善。在高Zr含量区，生长出ZrO_2和烧绿石相薄膜，生长菱方晶系PZT薄膜需要较高的生长温度。为了在高Zr含量区域获得钙钛矿菱面体PZT薄膜，提出了两步生长工艺。在该方法中，首先沉积PbTiO_3缓冲层作为起始层，然后生长PZT薄膜。在高Zr含量区和较低衬底温度下，成功地制备了PZT铁电薄膜，并研究了MOCVD生长PZT铁电薄膜的脉冲开关动力学。随着铁电薄膜电容器面积的减小，开关时间成比例缩短。另一方面，开关时间和开关的电荷密度的激活场依赖于脉冲电压。实验结果表明，PZT铁电薄膜的脉冲开关速度取决于反畴的成核速率，因为测量系统的有效时间常数较大，降低了反畴的成核速率。

In photo-MOCVD of PZT,effects of a use of O_3 as an oxidizing gas instead of O_2 and a photoirradiation on the growth were investigated. A decrease in the growth temperature and an improvement in the crystallinity of the PZT thin films were not observed. However, when O_3 and O_3+UV irradiation during the growth run were used, an improvement in the breakdown voltage was observed. This improvement may have been caused by microscopic change in film structure.At high Zr content region, ZrO_2 and pyrochlore phase films were grown and it required high growth temperature to grow rhombohedral PZT thin films. In order to obtain perovskite rhombohedral PZT thin films at high Zr content regions, a two step growth process was proposed. In this method, at first stage, PbTiO_3 buffer layr as an intial layr were deposited and in succeeding process PZT film were grown. Rhombohedral PZT thin films were successfully obtained at higher Zr content regions and at lower substrate temperatures than when this two step growth process was not used.The pulse switching kinetics of ferroelectric PZT thin films grown by MOCVD was also investigated. The switching time was proportionally shortened as the area of ferroelectric thin film capacitor became smaller. On the other hand, both the switching time and the switched charge density had an activation-field dependence on the pulse voltage. It was proven that the pulse switching speed of ferroelectric PZT films is determined by the nucleation rate of reversed domains because the nucleation rate is lowered by the large effective time constant of the measurement system.