Influence of Deposition Conditions on Ferroelectric, Piezoelectric, and Leakage Characteristics in Cobalt-Doped CaBi₄Ti₄O₁₅ Thin Films Prepared using Pulsed Laser Deposition

Technical Summary: Influence of Deposition Conditions on Ferroelectric, Piezoelectric, and Leakage Characteristics in Cobalt-Doped CaBi₄Ti₄O₁₅ Thin Films

Problem Statement
Lead-free ferroelectric materials are increasingly required for high-temperature piezoelectric devices, non-volatile memories, and energy harvesting due to environmental regulations against lead-based materials like PZT. Among lead-free candidates, the Aurivillius phase material CaBi₄Ti₄O₁₅ (CBT) offers high Curie temperatures (~790 °C) and thermal stability. However, its practical application is hindered by low remanent polarization, high coercive fields, and significant leakage currents. These limitations are often attributed to incomplete crystallization, grain boundary effects, and defect chemistry, particularly oxygen vacancies. While cobalt (Co) doping has been shown to improve grain growth and suppress leakage in bulk ceramics, the specific influence of deposition parameters—specifically substrate temperature and oxygen partial pressure—on the microstructure and functional properties of Co-doped CBT (CBT-Co) thin films remains insufficiently understood.

Methodology
The study employed Pulsed Laser Deposition (PLD) to fabricate 4 mol% Co-doped CaBi₄Ti₄O₁₅ thin films on Pt(111)/Ti/SiO₂/Si substrates. The experimental design systematically varied two key processing parameters:

• Substrate Temperature: 600 °C and 700 °C.
• Oxygen Partial Pressure: 0.01, 0.05, and 0.10 mbar.

The films were characterized using a comprehensive suite of techniques:

• Structural Analysis: X-ray Diffraction (XRD) for phase identification and crystallinity.
• Morphological Analysis: Field Emission Scanning Electron Microscopy (FESEM) for surface and cross-sectional imaging, and Atomic Force Microscopy (AFM) for surface roughness and topography.
• Ferroelectric Characterization: Polarization-Electric field (P-E) hysteresis loops measured at 1 kHz.
• Piezoelectric Characterization: Piezoresponse Force Microscopy (PFM) to analyze local domain switching, phase reversal, and effective piezoelectric coefficients ($d_{33}$).
• Electrical Characterization: Current-Voltage (I-V) measurements to analyze leakage current density and conduction mechanisms.

Key Results

• Structural and Morphological Evolution:

  • All films exhibited a polycrystalline Aurivillius phase with orthorhombic symmetry (Bb21m), dominated by the (119) orientation.
  • Crystallinity: Films deposited at 600 °C showed improved crystallinity as oxygen pressure increased from 0.01 to 0.10 mbar. Conversely, at 700 °C, crystallinity peaked at 0.05 mbar; increasing pressure to 0.10 mbar reduced peak intensity, likely due to reduced adatom kinetic energy from gas collisions.
  • Morphology: The film deposited at 700 °C and 0.05 mbar exhibited the most dense, uniform, and well-developed grain structure. Higher oxygen pressure (0.10 mbar) at 700 °C led to heterogeneous grain growth and increased surface roughness (7.381 nm RMS).
  • Secondary Phases: Minor pyrochlore phases were detected in films deposited at 600 °C/0.05 mbar and 700 °C/0.05–0.10 mbar, attributed to bismuth volatilization, though the Aurivillius phase remained predominant.

• Ferroelectric Properties:

  • Films deposited at 600 °C/0.01 mbar showed no ferroelectric switching due to poor crystallinity.
  • Optimized Performance: The film deposited at 600 °C and 0.10 mbar demonstrated the highest remanent polarization ($P_r$) of 6.17 µC cm⁻² and a coercive field ($E_c$) of 148.8 kV cm⁻¹ at 20 V. This sample also exhibited an "imprinted" hysteresis loop, suggesting an internal bias field likely caused by defect dipoles and oxygen vacancies.
  • Films deposited at 700 °C showed lower $P_r$ (~5.55 µC cm⁻²) but higher $E_c$ (up to 221.7 kV cm⁻¹ at 0.05 mbar), indicating stronger domain pinning associated with microstructural evolution.

• Piezoelectric Response:

  • PFM analysis confirmed reversible polarization switching and 180° phase reversal.
  • The effective piezoelectric coefficient ($d_{33}$) was highest for the film deposited at 600 °C and 0.05 mbar (128 pm/V). The film at 700 °C/0.05 mbar showed a comparable value (125 pm/V), while the 700 °C/0.10 mbar film showed a significantly reduced response (~42 pm/V).

• Leakage Current:

  • Leakage behavior varied non-monotonically with processing conditions. At 600 °C, leakage increased with oxygen pressure. At 700 °C, the film deposited at 0.10 mbar exhibited lower leakage than the 0.05 mbar sample, attributed to enhanced grain growth and reduced defect density at the higher temperature.

Significance and Claims
The authors claim that the study establishes a direct correlation between deposition conditions, defect chemistry, and functional performance in Co-doped CBT thin films. The research highlights that:

1. Processing-Structure-Property Relationship: Careful control of substrate temperature and oxygen partial pressure is critical for tailoring microstructure and minimizing leakage while maximizing ferroelectric and piezoelectric responses.
2. Optimization Strategy: While the 700 °C/0.05 mbar condition yielded the most balanced performance (characterized by the most favorable microstructure, well-saturated hysteresis loops, stable domain switching, and a high effective piezoelectric coefficient of ~125 pm V⁻¹), the 600 °C/0.10 mbar condition provided the highest macroscopic remanent polarization ($P_r$), and the 600 °C/0.05 mbar condition yielded the highest piezoelectric coefficient.
3. Lead-Free Viability: The optimized films demonstrate a promising combination of stable polarization switching, high piezoelectric response, and reduced leakage, validating Co-doped CBT as a viable candidate for lead-free applications in sensors, MEMS, FeRAM, and energy harvesting.

The paper concludes that PLD allows for the precise tuning of oxygen vacancy concentration and domain dynamics, offering an effective route to developing high-performance lead-free ferroelectric thin films.