Development of Low-Temperature MOCVD Technology for Ferroelectric Thin Films

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The objective of this research project is to develop a low-temperature MOCVD (metalorganic chemical vapor deposition) technology for ferroelectric thin films which will be used for future nonvolatile memory applications. In this research project, we introduced RF plasma in the growth of ferroelectric SrBi_2Ta_2O_9 (SBT) films by liquid-delivery MOCVD to reduce the crystallization temperature. We first used Sr[Ta(OC_2H_5)_6]_2 as a Sr-Ta source and showed that low-temperature (350℃) deposition of the film followed by high-temperature (750℃) crystallization annealing is necessary to obtain ferroelectric SrBi_2Ta_2O_9 (SBT) films. This is because Sr[Ta(OC_2H_5)_6]_2 can be easily decomposed at high temperatures, which makes the composition control difficult. Hence, we next used a novel Sr-Ta source precursor, Sr[Ta(OC_2H_5)_5(OC_2H_4OCH_3)]_2, which is more thermally stable than the conventional source, to overcome this problem. Using the Sr[Ta(OC_2H_5)_5(OC_2H_4OCH_3)]_2, we grew good ferroelectric SBT films with a remanent polarization of 6.2 μC/cm^2 and a coercive field of 62 kV/cm at 750℃ without RF plasma. By applying the RF plasma during the growth, we succeeded in reducing the crystallization temperatures to as low as 615℃. This temperature is about 150℃ lower than the temperature required in the conventional techniques. Remanent polarization of the ferroelectric SBT film grown by the plasma-assisted MOCVD technique at 615℃ with an RF power of 20W was 5.8 μC/cm^2, which is comparable to the remanent polarization obtained by the conventional techniques around 750℃.