BaTiO₃ Thin Films from Atomic Layer Deposition: A Superlattice Approach

A superlattice approach for the atomic layer deposition of polycrystalline BaTiO₃ thin films is presented as an example for an effective route to produce high-quality complex oxide films with excellent thickness and compositional control. This method effectively mitigates any undesirable reactions between the different precursors and allows an individual optimization of the reaction conditions for the Ba–O and the Ti–O subcycles. By growth of nanometer thick alternating Ba­(OH)₂ and TiO₂ layers, the advantages of binary oxide atomic layer deposition are transferred into the synthesis of ternary compounds, permitting extremely high control of the cation ratio and superior uniformity. Whereas the Ba­(OH)₂ layers are partially crystalline after the deposition, the TiO₂ layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO₃ above 500 °C, releasing H₂O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spectroscopy reveal the presence of hexagonal BaTiO₃ in addition to the perovskite phase in the annealed films. The microstructure with relatively small grains of ∼70 Å and different phases is a direct consequence of the abrupt formation reaction. The electrical properties transition from the initially highly insulating dielectric semiamorphous superlattice into a polycrystalline BaTiO₃ thin film with a dielectric constant of 117 and a dielectric loss of 0.001 at 1 MHz after annealing at 600 °C in air, which, together with the suppression of ferroelectricity at room temperature, are very appealing properties for voltage tunable devices.