BaTiO₃ Thin Films from Atomic Layer Deposition: A Superlattice Approach

Abstract

A superlattice approach for the atomic layer deposition of polycrystalline BaTiO 3 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 TiO 2 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 TiO 2 layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO 3 above 500 °C, releasing H 2 O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spectroscopy reveal the presence of hexagonal BaTiO 3 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 BaTiO 3 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.

Synthesis of Doped, Ternary, and Quaternary Materials by Atomic Layer Deposition: A Review

“…Finally, in ternary ALD, the construction of the ALD supercycle plays an important role in phase formation. Two approaches that have been used in constructing the supercycle are (i) minimizing bilayer periods to obtain mixed as-deposited films (with bilayer thicknesses of approximately <1–2 nm) and (ii) depositing nanolaminate films that are composed of alternating sublayers (with sublayer thicknesses of typically >2 nm) known as superlattices which are then annealed to enable intermixing and crystallization. − Both approaches are capable of producing high quality crystalline films. The first approach, which is more commonly employed, may produce crystalline films without an annealing step, but in most cases is insufficient (see Table ).…”

“…The second approach addresses the two major drawbacks of the first approach: it enables fine-tuning of composition, while minimizing nucleation effects (see Section ) due to the use of larger bilayer periods. BaTiO 3 , SrTiO 3 , and BiFeO 3 have been grown with the superlattice approach resulting in single phase films. However, a potential drawback of superlattices is that it may result in incomplete mixing and undesired phases. , An example of these unfavorable effects as a result of large bilayers can be seen in a study of ZnSn x O y (ZTO) ALD by Hägglund et al where the effect of the bilayer period on the crystallographic phase after annealing at 800 °C was investigated .…”

Material strategies to enhance the performance of piezoelectric energy harvesters based on lead-free materials

“…The (100) orientation of BaTiO 3 presents the stress and strain FoMs by several orders smaller than those of the (001) orientation due to extremely high permittivity along a-axis ( ε T 11 ε0 = 3600). BaTiO 3 thin film growth by different physical and chemical methods (metalorganic chemical vapor deposition (MOCVD) [68,69], pulsed laser deposition (PLD) [56,58,70] magnetron sputtering [56,71], molecular bean epitaxy (MBE) [56], atomic layer deposition (ALD) [72,73], sol-gel [74], activated reactive evaporation [75], aqueous chemical solution deposition [76], etc) was reported in the literature. The structural quality of the films grown by ALD was very low as the films consisted of a mixture of polycrystalline and amorphous parts [73].…”

“…BaTiO 3 thin film growth by different physical and chemical methods (metalorganic chemical vapor deposition (MOCVD) [68,69], pulsed laser deposition (PLD) [56,58,70] magnetron sputtering [56,71], molecular bean epitaxy (MBE) [56], atomic layer deposition (ALD) [72,73], sol-gel [74], activated reactive evaporation [75], aqueous chemical solution deposition [76], etc) was reported in the literature. The structural quality of the films grown by ALD was very low as the films consisted of a mixture of polycrystalline and amorphous parts [73]. At RT, the tetragonality of BaTiO 3 is very low (a/c ration is 1.01), therefore the film orientation is highly dependent on the state of the strain induced by a substrate.…”

“…The single-crystal substrates used for the epitaxial BaTiO 3 are not viable in terms of size and price for the real EH applications. The films grown on the (111) textured Pt on SiO 2 /Si or Si based substrates, compatible with piezoelectric transduction applications, presented polycrystalline microstructure independently on the deposition method used [73,76,78,79] Such polycrystalline films were poled and transferred on the plastic substrates by wet etching of the Si substrate for the EH applications (the ability to generate the PD of 7 mW cm −3 was reported) [80]. The textured LaNiO 3 buffer layer was introduced to ameliorate the orientational quality of the films and highly a-axis oriented films were obtained [81].…”