Ferroelectric thin films have drawn growing interest for their potential in electro-optic modulation, nonlinear optics, and integrated photonics, owing to their intrinsic spontaneous polarization. Among them, BaTiO₃ (BTO) stands out for its strong ferroelectricity, piezoelectricity, and nonlinear optical properties, making it a promising material for high-performance optoelectronic devices. To realize these functionalities, it is essential to achieve high crystallinity and controlled orientation in BTO films—especially along the c-axis, which is critical for polarization-related responses. While techniques such as pulsed laser deposition (PLD) and atomic layer deposition (ALD) have been used for BTO growth, they face challenges in scalability and substrate compatibility. Radio frequency (RF) magnetron sputtering offers a scalable alternative with good control over film uniformity and composition. However, obtaining high-quality epitaxial BTO films via sputtering remains challenging due to the sensitivity of the process to parameters like substrate temperature, gas composition, and RF power. This work systematically investigates the effects of key sputtering parameters on the structural and optical properties of c-axis-oriented BTO thin films. The relationship between deposition conditions, domain structure, and optical performance is further explored using second harmonic generation (SHG), aiming to guide the fabrication of high-quality ferroelectric films for integrated photonic applications.

Epitaxial BaTiO₃ (BTO) thin films were deposited on (001) (La,Sr)(Al,Ta)O₃ (LSAT) and (110) GdScO₃(GSO)single-crystal substrates using RF magnetron sputtering with a 2 in ceramic BTO target. The chamber pressure was maintained at 1.5 Pa, with either pure Ar or Ar/O₂ gas mixtures as the sputtering atmosphere. Substrate temperatures ranged from 25 ℃ to 750 ℃, and RF power was set at 50 W or 75 W (2.47 W/cm² and 3.70 W/cm², respectively). A 10 min pre-sputtering step was performed before each run to stabilize film composition. Structural characterization was conducted using an X-ray diffractometer (SmartLab, Rigaku) with a Cu-Kα₁ source and Ge (220) monochromator. Measurements included θ-2θ scans, rocking curves, X-ray reflectivity (XRR), and reciprocal space mapping (RSM). Surface morphology and roughness were analyzed via atomic force microscopy (AFM, tapping mode), while domain structures were evaluated using second harmonic generation (SHG) with a Ti:sapphire femtosecond laser (800 nm, 35 fs). SHG measurements were performed in reflection geometry at a 45° incidence angle. Film composition was assessed using a Zeiss GeminiSEM 500 with a windowless EDS detector at 20 kV to determine the Ba/Ti ratio. Optical constants, including refractive index (n₀) and extinction coefficient (k₀), were measured by spectroscopic ellipsometry (UVISEL plus, Horiba) and fitted using the Lorentz model. Ellipsometric thickness values were validated by XRR.

The deposition temperature plays a critical role in determining the crystallinity, strain state, morphology, and polarization of BaTiO₃ (BTO) thin films grown on LSAT and GSO substrates. High-resolution X-ray diffraction (HRXRD) results show enhanced c-axis orientation and crystallinity with increasing temperature, peaking at 700 ℃ on LSAT. A further rise to 750 ℃ slightly reduces quality, likely due to defect formation. On GSO, improved Laue oscillations and narrower FWHM at 750 ℃ indicate better lattice matching and epitaxial growth. Scherrer-based estimates reveal that vertical coherence length peaks at 700 ℃ (about 32 nm) on LSAT and increases steadily on GSO, reaching about 60 nm at 750 ℃. This contrast reflects superior lattice compatibility between GSO and BTO. Reciprocal space mapping shows strain evolution with temperature. At 500 ℃, films exhibit strong in-plane tensile strain (c/a ratio < 1), which relaxes at higher temperatures as c/a exceeds 1. Calculated strain confirms this, with in-plane strain decreasing and out-of-plane strain increasing from 500 ℃ to 750 ℃. SHG measurements reveal that low-temperature films (500 ℃) favor in-plane polarization, while higher temperatures induce out-of-plane domains and complex multi-domain structures, indicating temperature-driven polarization reorientation. AFM results show the smoothest surface at 600 ℃ (RMS is 1.175 nm), with roughness slightly increasing at higher temperatures due to strain relaxation and defect formation. These findings align with XRD and XRR trends. Ellipsometry and XRR confirm that film thickness decreases from about 80 nm at 500 ℃ to about 50 nm at 600-650 ℃ due to enhanced lateral growth, then slightly increases at higher temperatures. Clearer XRR fringes at 700 ℃ indicate improved interface quality. Deposition optimization identified the best conditions as 30° angle, pure Ar atmosphere, 13.361 cm target-substrate distance, and 75 W RF power, achieving uniform and high-quality films. Finally, SEM and EDS analyses show denser, smoother films at elevated temperatures. However, Ba/Ti ratios deviated from stoichiometry (0.66-0.97), likely due to off-axis sputtering effects, contributing to strain and preferential c-axis domain formation.

Epitaxial BaTiO₃ (BTO) thin films were successfully deposited on (001)-oriented LSAT and (110)-oriented GSO substrates via radio-frequency (RF) magnetron sputtering. The structural, morphological, compositional, and optical properties of the films were systematically investigated. X-ray diffraction (XRD) analysis confirmed that the films predominantly crystallized in the tetragonal phase with a strong (001) preferred orientation, indicating high-quality epitaxial growth. Particularly, the film deposited on the GSO substrate at 750 ℃ exhibited superior crystallinity, as evidenced by the appearance of distinct Laue oscillations. Deposition parameters such as target-to-substrate distance, deposition angle, O₂/Ar gas volume flow ratio, and RF power were found to significantly influence the film's structure and performance. Optimal results were achieved using a target-to-substrate distance of 13.361 cm, a deposition angle of 30°, and an RF power of 75 W, with pure Ar atmosphere yielding the best film quality and highest deposition rate. The synergistic optimization of these parameters is essential for obtaining high-crystalline-quality BTO thin films and provides a solid foundation for their application in optoelectronic and ferroelectric devices.