Bismuth ferrite (BiFeO₃, BFO) is a well-known ferroelectric material with excellent piezoelectric and ferroelectric properties at room and high temperatures, attributed to its high Curie temperature (1103 K). Its rich topological domain structures, such as vortex domains and skyrmions, show potential for high-density data storage due to their small size and stability. Recent studies show that ferroelectric materials near the morphotropic phase boundary (MPB) exhibit enhanced piezoelectric properties. Methods like applying strain, changing substrate orientation, and doping can create MPB structures in BFO films, improving their performance. Ferroelectric topological domains can be induced by engineering film boundaries, offering new opportunities for high-density information storage and logic devices. Pulsed Laser Deposition (PLD) was used to fabricate BFO films. By adjusting parameters, T-phase, R/T mixed-phase, and R-phase films were prepared. The effects of thickness and laser energy on crystal structure, growth mode, and domain structure were investigated. Finite element simulation and thermodynamic analysis revealed the influence of strain relaxation and deposition rate on film properties.

BFO films were fabricated on (001)-oriented LaAlO₃ (LAO) substrates using PLD with a KrF excimer laser (248 nm wavelength, 3 Hz frequency). By adjusting the number of pulses (2700, 3600, 5400) and laser energy density (0.90–1.50 J/cm²), various BFO films (T-phase, R/T mixed-phase, R-phase) were prepared. A Bi-rich B_1.1FeO₃ target was used to compensate for Bi evaporation. Growth conditions were 700 ℃ substrate temperature and 13 Pa oxygen pressure, followed by annealing at 650 ℃ in 20000 Pa oxygen for 30 min and cooling to room temperature at 5 ℃/min. The epitaxial strain on BFO from LAO was –4.5%. Characterization was performed using X-ray Diffraction (XRD) with a Rigaku Ultima Ⅳ, Atomic Force Microscopy (AFM) with a Bruker Dimension XR, and Piezoresponse Force Microscopy (PFM) with a Pt/Ir-coated Si cantilever. Finite element simulation was conducted to study strain distribution in BFO films on LAO, assuming a –4.5% strain boundary condition. The simulation, performed at 300 K with a 1 nm grid size, provided theoretical support for the experimental results.

In thin film systems, strain relaxation occurs as film thickness increases. Changing deposition cycles to increase thickness significantly impacts the crystal and domain structures of epitaxial films. With 2700 deposition cycles, the film exhibits a layer-by-layer step-flow growth mode with distinct in-plane polarization components in the T-phase BFO, forming regular stripe-like domains. Increasing cycles to 3600 results in an interwoven R/T mixed-phase stripe pattern and crescent-shaped domains, indicating a transition from T-phase to R/T mixed-phase structure. Further increasing cycles to 5400 leads to significant surface roughness, transitioning the film to R-phase BFO as the epitaxial relationship is disrupted. This suggests that excessive thickness causes strain relaxation, enhancing in-plane polarization signals in PFM images and increasing domain size.

Laser energy is another crucial factor affecting film growth. Higher laser energy increases the amount of evaporated target material and the plume size. At low energy (0.90 J/cm²), the film maintains layer-by-layer growth with impurity particles, showing in-plane polarizations in different orientations. Increasing energy to 1.35 J/cm² induces a transition to layer-island mixed growth, forming interconnected nanodots with ferroelectric characteristics. Further increasing energy to 1.50 J/cm² results in higher-density, smaller nanodots in an island growth mode, with fragmented domain structures.

Finite element simulation was used to analyze the effect of film thickness on strain relaxation in BFO/LAO films. Films with thicknesses of 10 nm and 50 nm showed minimal strain relaxation, maintaining –4.5% strain (T-phase). However, at 80 nm, strain relaxation began (–4.41%), and at 130 nm, it became more pronounced (–3.87%). This indicates that thicker films experience more significant strain relaxation, consistent with experimental observations. The concept of "evaporation depth" helps explain the impact of laser energy on growth modes. At low laser energy, fewer BFO cells are evaporated, allowing them to diffuse and form layer-by-layer structures. Conversely, high laser energy increases the number of evaporated cells and nucleation sites, promoting island growth. This study provides valuable insights into the effects of deposition cycles and laser energy on BFO film growth, offering guidance for optimizing film properties.

This study explores how epitaxial strain and growth mode affect the crystal structure and ferroelectric domains in BFO films. As film thickness increases, epitaxial strain relaxes, causing significant changes in high-strain BFO films. Films transition from T-phase to R/T mixed-phase and then R-phase BFO with increasing deposition cycles (2700 to 5400), highlighting thickness’s role in strain relaxation and structure. Adjusting laser energy density (0.90 J/cm² to 1.50 J/cm²) shifts growth modes from T-phase to nanodot films, altering microstructure and domain distribution. Finite element simulations show that strain relaxation accelerates when film thickness exceeds 80 nm, with surface strain relaxing from –4.5% to –4.41%. Calculations of evaporated material per pulse reveal that low laser energy promotes layer-by-layer growth, while high energy induces island growth. These findings provide crucial insights into laser energy’s role in controlling film growth and offer valuable guidance for developing high-performance piezoelectric materials and information storage devices.