Orientation‑Driven Polarization Fatigue in Bi₃.₁₅Nd₀.₈₅Ti₂.₉₉Mn₀.₀₁O₁₂ Thin Films Across 200–475 K

Abstract

Bi₃.₁₅Nd₀.₈₅Ti₂.₉₉Mn₀.₀₁O₁₂ (BNTM) thin films were fabricated in three distinct crystallographic orientations—(200), (117) and mixed—using a sol‑gel route. Polarization fatigue was examined from 200 K to 475 K, revealing contrasting temperature dependencies: (200)-oriented films exhibited increasingly severe fatigue with rising temperature, whereas (117)-oriented films improved. Mixed films showed a non‑monotonic trend. Temperature‑dependent impedance spectra uncovered markedly lower activation energies (0.12–0.13 eV) for (200)-oriented films versus 0.17–0.31 eV for the others, indicating faster oxygen‑vacancy diffusion. Piezoresponse force microscopy (PFM) revealed a predominance of neutral head‑to‑tail domain walls in (200) films, while non‑neutral tail‑to‑tail/head‑to‑head configurations were more frequent in (117) and mixed films. These results highlight the crucial role of domain wall character and vacancy dynamics in determining orientation‑dependent fatigue behavior.

Background

Bi₄Ti₃O₁₂ (BIT) and its Nd‑substituted derivatives (BNT) are promising ferroelectric platforms for ferroelectric random‑access memory (FRAM) owing to their high Curie temperatures, large remanent polarizations and intrinsic anti‑fatigue properties. The lattice parameters of BIT (c = 3.284 nm, a = 0.544 nm, b = 0.541 nm at 300 K) support strong anisotropy, with polarizations of ≈ 4 µC/cm² along c and ≈ 50 µC/cm² along a. Several factors—film thickness, precursor concentration, and annealing conditions—strongly influence the preferred orientation of Nd‑substituted BIT. Prior work demonstrated that BNT films annealed at 750 °C show superior tunability compared with 700 °C, yet excessive Bi volatilization can lead to leakage and fatigue. While orientation‑dependent fatigue has been reported, the mechanisms, especially at elevated temperatures relevant for FRAM operation (−40 °C to 125 °C), remain unclear.

Temperature can modulate domain pinning/unpinning and defect migration. Some studies report enhanced fatigue resistance with temperature, attributed to accelerated domain unpinning, whereas others observe the opposite, likely due to increased oxygen‑vacancy diffusion and dead‑layer growth. A comprehensive microscopic understanding—combining impedance spectroscopy, PFM, and first‑principles insights—has yet to be achieved for anisotropic BNTM films.

Methods

High‑purity precursors (Bi(NO₃)₃·5H₂O, Nd(NO₃)₃·6H₂O, Ti(OC₄H₉)₄, Mn(CH₃COO)₂·4H₂O) were dissolved in 2‑methoxyethanol/glacial acetic acid with acetylacetone as chelator. A 10 % Bi excess compensated for volatilization. Solution concentrations of 0.04 M, 0.08 M, and 0.10 M yielded BNTM‑1, ‑2, and ‑3 films. Spin coating was repeated 10 (700 °C, 2.5 min) for BNTM‑1, 4 (700 °C, 5 min) for BNTM‑3, and a 4‑step cycle (650 °C, 2.5 min) followed by a 720 °C, 5 min anneal for BNTM‑2, all in O₂. Pt top electrodes (200 µm diameter) were sputtered by DC. XRD (Cu Kα) assessed orientation; SEM (Hitachi S4800) examined morphology; Agilent B1500A measured dielectric and AC impedance up to 475 K. Ferroelectric loops were recorded with Radiant Technologies; PFM imaging used an MFP‑3D AFM (platinum‑coated Si cantilever, 30 nm lift, 35 kHz).

Results and Discussion

Orientation and Morphology

XRD confirmed (200) orientation for BNTM‑1 (α₂₀₀ = 63.5 %, α₁₁₇ = 29.2 %), (117) for BNTM‑3 (α₂₀₀ = 32.1 %, α₁₁₇ = 60.2 %), and mixed for BNTM‑2 (α₂₀₀ = 43.2 %, α₁₁₇ = 48.5 %). SEM revealed bullet‑shaped grains in BNTM‑1, plate‑like in BNTM‑2, and rod‑like in BNTM‑3, with thicknesses ≈ 470 nm, 454 nm, and 459 nm, respectively. Thicker spin‑coating layers favored (117) orientation due to geometric constraints.

Polarization Switching

Hysteresis loops (Vₘ = 16 V) showed temperature‑dependent coercive voltages (V_c) and remanent polarizations (2P_r). For BNTM‑1, 2P_r increased up to 10 V at lower T then decreased beyond 10 V with T rise. BNTM‑2 and ‑3 displayed a rise from 220 K to 300 K followed by a drop from 300 K to 400 K. These trends correlate with depolarization fields and domain wall density.

Fatigue Behavior

After 10⁹ switching cycles (8–10 V pulses), BNTM‑1 suffered negligible fatigue at 300 K but worsened at 400 K (11.3 % loss). BNTM‑3 improved from 300 K to 400 K (−34.5 % loss). BNTM‑2 displayed a non‑monotonic trend: fatigue increased up to 350 K (51.4 % loss) then improved at 400 K (31.2 % loss). The divergent behaviors are attributed to competing domain‑pinning versus dead‑layer growth, modulated by oxygen‑vacancy diffusion.

Dielectric Response

Dielectric constant (ε_r) rose with temperature for all films, indicating stronger domain unpinning. Post‑fatigue ε_r dropped more markedly in BNTM‑1 and ‑3 at higher T, consistent with dead‑layer expansion. BNTM‑2 showed a weaker correlation, likely due to increased charged domain‑wall density enhancing dielectric response.

Impedance and Activation Energy

AC impedance spectra (1 MHz–1 kHz) revealed grain‑related high‑frequency arcs. Extracted grain resistances (R_g) followed Arrhenius behavior. Activation energies were 0.12–0.13 eV for BNTM‑1, substantially lower than 0.17–0.31 eV for BNTM‑2 and ‑3, indicating faster oxygen‑vacancy migration in (200)-oriented films. The lower E_a supports the observed severe fatigue at elevated temperatures for BNTM‑1.

Domain Wall Analysis via PFM

PFM imaging revealed dominant 180° domains in BNTM‑1, whereas 90° domains (horizontal polarization) prevailed in BNTM‑2 and ‑3. Tail‑to‑tail/head‑to‑head configurations were more frequent in BNTM‑2 and ‑3, promoting domain‑wall pinning. In contrast, BNTM‑1 exhibited mainly neutral head‑to‑tail walls, reducing pinning and explaining its comparatively better fatigue at lower T.

Conclusions

Orientation profoundly influences temperature‑dependent polarization switching and fatigue in BNTM thin films. (200)-oriented films suffer enhanced fatigue with temperature due to rapid oxygen‑vacancy diffusion and dead‑layer growth, while (117)-oriented films benefit from increased domain‑wall unpinning. Mixed orientations show intermediate, non‑monotonic behavior. Lower activation energies in (200) films and a higher prevalence of non‑neutral domain walls in (117) and mixed films underpin these trends. These insights provide a microscopic foundation for engineering orientation‑controlled ferroelectric memories with improved endurance.