Tailoring Preferential Orientation in BaTiO₃‐based Thin Films from Aqueous Chemical Solution Deposition

Abstract

Ferroelectric properties of thin films can be enhanced by crystallographic texture. In this work, we report on how heat treatment of films can be designed to tailor the degree of preferential orientation in BaTiO3‐based thin films from aqueous chemical solution deposition. In situ synchrotron X‐ray diffraction in combination with Rietveld refinements was used to study the crystallization process of films from a single deposition and to give an in‐depth characterization of the crystallographic texture of the films. Transmission electron microscopy was employed to evaluate the microstructure and degree of preferred orientation in thicker films from multiple depositions. Texture was induced in the multilayer films by a repeated annealing process. Cube‐on‐cube growth was demonstrated to occur in both single and multi‐layered films provided the heating program was designed to give limited nucleation and growth below the threshold for where cube‐on‐cube growth is favoured, resulting in a very high degree of preferred orientation. The cube‐on‐cube grown films were relaxed with respect to the lattice unit cell mismatch between the film and the substrate, where the relaxation of stress depends on the film thickness. Texture and cube‐on‐cube growth were demonstrated on several types of single‐crystal oxide substrates. Calcium and zirconium substitution did not alter the crystallization process, but zirconium slowed down the texture formation kinetics. The ferroelectric response was strongest in the films with a high degree of preferred orientation.

On the Evolution of Stress and Microstructure in Radio Frequency-Sputtered Lead-Free (Ba,Ca)(Zr,Ti)O3 Thin Films

“…Nevertheless, thick defect-free BCZT films with large electrode areas well integrated with Si remain to be repeatedly demonstrated. Although promising results have been demonstrated using, e.g., sol-gel [42] or sacrificial layers [43], having Si dominate the elastic properties of the overall film-substrate system during processing indeed imposes considerable challenges for any direct integration of technologically applicable BCZT thin films.…”

In situ characterisation for studying nucleation and growth of nanostructured materials and thin films during liquid-based synthesis

“…Ammonia solution was used prior to mixing to adjust the solutions to a neutral pH. More details about the exact procedures have been reported previously [32,38]. Single crystal (100) SrTiO 3 substrates were cleaned with oxygen plasma for 2 min to enhance wetting.…”

“…Pole figures were prepared from the refined texture in the MTEX [40] package for MATLAB. More detailed descriptions of the experimental setup and data processing can be found in our previous work [32,38].…”

“…Our previous work on BaTiO 3 -based thin films from aqueous CSD showed that nucleation of the perovskite occurred both through direct nucleation and through a topotaxial transformation from the intermediate nanosized calcite-like BaCO 3 and BaTi 4 O 9 phases [30][31][32]38]. It was shown that the processing conditions during formation and decomposition of the intermediates were just as important as those for the nucleation of the perovskite phase.…”

“…In our previous work, we designed a setup for in situ synchrotron XRD measurements on oxide thin films from aqueous CSD with a heating plate capable of heating rates up to 20 °C/s and temperatures of 1000 °C, mimicking the annealing in a conventional RTP unit [29]. BaTiO 3based [30][31][32], K 0.5 Na 0.5 NbO 3 [33] and Sr 1-x Ba x Nb 2 O 6 [34] thin films were monitored during crystallisation and texture formation was studied. Combined with in situ IR spectroscopy and in situ total scattering experiments on powders from the same precursor solutions the whole annealing process could be analysed and generalised mechanisms for tailoring thin films from CSD were developed.…”