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

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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.