Enhanced Piezoelectric and Ferroelectric Properties in the Lead-Free [(BiFeO3)m/(SrTiO3)n]p Multilayers by Varying the Thickness Ratio r = n/m and Periodicity p

Multilayer heterostructures of [(BiFeO3)m/(SrTiO3)n]p were synthesized on ITO-coated quartz substrates via pulsed laser deposition, with varying thickness ratios (r = n/m) and periodicities (p = 1–3). Structural, electrical, and piezoelectric properties were systematically investigated using X-ray diffraction, AFM, and PFM. The BiFeO3 layers crystallized in a distorted rhombohedral phase (R3c), free of secondary phases. Compared to single-layer BiFeO3 films, the multilayers exhibited markedly lower leakage current densities and enhanced piezoelectric response. Electrical conduction transitioned from space-charge-limited current at low fields (E < 100 kV/cm) to Fowler–Nordheim tunneling at high fields (E > 100 kV/cm). Optimal performance was achieved for r = 0.30, p = 1, with minimal leakage (J = 8.64 A/cm2 at E = 400 kV/cm) and a peak piezoelectric coefficient (d33 = 55.55 pm/V). The lowest coercive field (Ec = 238 kV/cm) occurred in the configuration r = 0.45, p = 3. Saturated hysteresis loops confirmed stable ferroelectric domains. These findings demonstrate that manipulating layer geometry in [(BiFeO3)m/(SrTiO3)n]p stacks significantly enhances functional properties, offering a viable path toward efficient, lead-free piezoelectric nanodevices.