Growth of (111)-textured SrTiO3 thin films on Pt(111)/Al2O3(1-102) substrates by rf magnetron sputter deposition

Technical Summary: Growth of (111)-textured SrTiO₃ Thin Films on Pt(111)/Al₂O₃(1-102) Substrates

Problem Statement
Strontium titanate (SrTiO₃ or STO) is a critical material for high-temperature capacitor applications and as a growth template for complex oxides, owing to its high dielectric constant and stable paraelectric phase up to ~700°C. However, the reliable application of STO films with thicknesses below 50 nm has been limited by surface roughness and structural artifacts when deposited on platinum (Pt) electrodes. Previous studies on textured STO films grown directly on Pt/Al₂O₃ substrates often exhibited root-mean-square (RMS) roughness values between 4 nm and 8 nm, with roughness increasing at higher growth temperatures due to grain faceting. While atomically smooth surfaces are achievable on single-crystal STO substrates, achieving similar quality on Pt templates remains challenging due to the difficulty in distinguishing STO from the underlying Pt layer and optimizing the template's structural properties.

Methodology
This study investigated the growth of stoichiometric STO thin films (20–30 nm) via radio frequency (RF) magnetron sputtering on Pt(111) templates deposited on r-plane sapphire (r-Al₂O₃) substrates. The research focused on correlating the structural properties of the underlying Pt template with the quality of the subsequent STO film.

• Substrate Preparation: r-Al₂O₃ substrates underwent wet chemical cleaning and plasma pre-treatment.
• Pt Template Growth: 100 nm Pt films were deposited using an AJA International system at 600°C. Two deposition powers were compared: 90 W and 180 W.
• STO Growth: STO films were grown in a custom-built RF sputtering chamber using a stoichiometric target. Growth occurred at 300°C with a gas mixture of 3 mTorr Ar and 7 mTorr O₂. Deposition powers ranged from 20 W to 40 W, with substrate-target distances of 12 cm and 14 cm. Growth time was fixed at 5 hours.
• Post-Growth Treatment: Samples were in-situ annealed in 300+ mTorr O₂ at 550°C for 1 hour to enhance crystallinity.
• Characterization:
  • Composition: X-ray Photoelectron Spectroscopy (XPS) verified stoichiometry (Sr, Ti, O ratios).
  • Crystal Structure: X-ray Diffraction (XRD) θ-2θ scans and rocking curves (RC) assessed orientation and crystalline quality. Reflection High-Energy Electron Diffraction (RHEED) analyzed surface ordering.
  • Morphology: Atomic Force Microscopy (AFM) measured surface roughness (RMS) over 3 × 3 µm² areas.

Key Results

1. Template Dependence: The quality of the STO film was directly dependent on the Pt template. Pt films grown at 90 W exhibited significantly higher crystalline quality and smoother surfaces compared to those grown at 180 W.
2. Crystallographic Orientation: XRD confirmed that STO films possessed a highly textured (111) out-of-plane orientation, with weaker (222) reflections. Due to the close lattice match between Pt (3.92 Å) and STO (3.905 Å), the diffraction peaks overlapped, indicating that the STO film adopted the crystallographic orientation of the underlying Pt layer.
3. Crystalline Quality: Rocking curve measurements of the STO films grown on the 90 W Pt template achieved a Full Width at Half Maximum (FWHM) of 0.2°, a substantial improvement over the 3.0° FWHM observed for Pt films grown at 180 W. This narrow FWHM indicates high crystalline quality and reduced mosaic spread.
4. Surface Morphology: AFM analysis revealed that the Pt film grown at 90 W had an RMS roughness of approximately 93.2 pm (0.093 nm), free of microcracks or pinholes. The STO film grown on this template exhibited an RMS roughness of 110.5 pm (0.11 nm). This represents an atomically smooth surface, significantly lower than the 4–8 nm roughness reported in previous textured STO studies.
5. Growth Mode: RHEED patterns displayed streaky, weakly spot-modulated features characteristic of STO(111) orientation, suggesting a quasi-two-dimensional growth mode and high in-plane ordering.
6. Composition: XPS confirmed the presence of Sr, Ti, and O with binding energies consistent with bulk STO, verifying stoichiometric composition.

Significance and Claims
The paper demonstrates that optimizing the growth parameters of the underlying Pt template—specifically utilizing lower sputtering power (90 W) to achieve high crystalline quality and atomic smoothness—enables the growth of near-epitaxial (111) STO films with thicknesses below 30 nm. The study establishes that when the Pt surface is sufficiently smooth and ordered, the resulting STO films inherit these qualities, achieving RMS roughness values less than 1 nm (specifically ~0.11 nm).

The authors conclude that these high-quality, atomically smooth STO films are suitable for use as dielectric materials in capacitor structures, potentially offering high capacitance values (estimated at 8.85–13.28 μF/cm²). The work addresses the historical limitation of surface roughness in thin STO films on Pt, providing a pathway for reliable integration of STO in high-temperature electronic devices and as templates for subsequent complex oxide growth.