Atomic Layer Deposition of Tin Titanate - Achieving Ferroelectricity And Beyond
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Ferroelectric perovskites are a type of ternary oxide with ABO3 stoichiometry that possess hysteresis polarization loop under externally applied electric field. They are widely used in actuators, sensors, memory devices, etc. Some most commonly used perovskites, such as PbTiO3, are lead based materials that impose threats to environment wellness and human health. Theoreticians have proposed the replacement of Pb by its isoelectronic brother Sn, thanks to its environmental benignity, abundance and greater calculated polarization effect. Although the synthesis of polar SnTiO3 has been attempted many times – both in bulk and as thin films, there was no success to date. In this thesis, we report the first case of atomic layer deposition (ALD) of SnTiOx, and present our original findings in process tuning, growth behavior, structural evolution, and material properties of the as-deposited and post-deposition treated samples. The ALD conditions were firstly optimized for binary oxide processes; after obtaining a wide overlapping window, co-depositions with alternating precursor sequences were carried out to grow ternary SnTiOx. To study the metastable SnOx, annealing in H2 reducing ambient was applied and the resulting samples were characterized using XPS, X-ray absorption spectroscopy, XRD and high resolution/scanning TEM. It was found that most Sn(II) was formed at annealing temperatures of 450-550C, whereas higher temperatures lead to more metallic Sn and eventually a de-wetting process. To investigate the growth behavior of SnTiOx and achieve correct stoichiometry, a set of sample were deposited with various ALD cycle ratios resulting Sn atomic % ranging from 0 to 100% in the films. Close examination and analysis of the growth rates and compositional change across these films revealed a reduced density of atoms deposited on the other oxide’s surface compared to that on its own, which leads to lower growth rates at higher Sn atomic %. Possible reasons were discussed after probing the films with XPS, XRD and XRR. Lastly, those SnTiOx films with Sn:Ti atomic ratio of 1:1 were annealed in different ambient to form better crystalized structure, and then tested using piezoelectric force microscope. The sample annealed in O2 at 350C showed a hysteresis loop in amplitude, indicating possible ferroelectricity.