Atomic Layer Deposition of Environmentally Benign Tin-Titanate for Ferroelectric Application

Perovskite-based oxides that possess ferroelectric properties, such as BaTiO₃ and PbTiO₃, have long been a research interest because of their wide applications in devices such as actuators and sensors.¹ Due to the toxicity of lead, emphasis has been put on the replacement of Pb²⁺ by isoelectronic, environment friendly Sn²⁺, in hope of maintaining similar ferroelectric performance.

Theoretical studies using first principle calculations have predicted that perovskite SnTiO₃ (P-SNO) would exhibit excellent ferroelectric properties,^2-6 yet the actual synthesis of P-SNO by conventional methods remains unsuccessful due to the less stable Sn²⁺ at high temperatures.^2,3 No success was achieved in depositing P-SNO thin films, either. Several studies suggest that P-SNO is metastable,^2,7,8 which leads to the proposition of alternative synthetic routes that can stabilize the metastable structure,⁹ such as atomic layer deposition (ALD)³/chemical vapor deposition (CVD), sol-gel, etc.²

Different from the predicted perovskite structure, nonpolar, ilmentile-type thin film of SnTiO₃ has been achieved on sapphire and perovskite substrates using pulsed laser deposition (PLD) techniques from ceramic SnO₂ and TiO₂ targets.¹⁰ This result further motivates a more thorough investigation as to whether or not thin film deposition techniques could stabilize the metastable P-SNO structure which is crucial for high ferroelectric performance.

In this work, we report the result of atomic layer deposition of tin-titanate using tin(II)acetylacetonate (Sn(acac)₂) and tetrakis(diethylamino)titanium (TDEAT). The ALD reaction is carried out on Si substrates using a custom-built ALD reactor (Fig. 1).¹¹ ALD process parameters were optimized for SnO_x and TiO₂ deposition. Linear growth behavior was observed for both the films, indicating self-limited ALD growth. The ALD temperature windows of both ALD processes were found to overlap, which facilitates co-deposition of SnO_x and TiO₂ for tin-titanate deposition. The resulting film is subsequently analyzed by several characterization techniques to determine its stoichiometry, composition, impurities and structural and surface characteristics. The results of this work provide more insights into achieving lead-free, perovskite SnTiO₃ structure to facilitate future research on this subject.

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