Mechanisms of Temperature-Dependent Hysteresis in Freestanding BaTiO₃/MoS₂ Heterostructures

Freestanding ferroelectric oxides integrated with 2D semiconductors offer a platform for reconfigurable electronic functionalities beyond conventional dielectric gating. However, once released from epitaxial constraint and transferred directly onto metallic gates, the BaTiO₃ (BTO) membranes under study preferentially adopt an in-plane polarization configuration, suppressing out-of-plane ferroelectric coupling at room temperature. Here, we demonstrate that single-layer MoS₂ field-effect transistors gated by 25 nm freestanding BTO exhibit excellent electrostatic control at room temperature, with subthreshold swings down to 85 mV dec^{- 1} and high on/off ratios, yet negligible hysteresis, highlighting the effectiveness of depolarized BTO as an ultrahigh-κ dielectric. Upon cooling, robust counter-clockwise ferroelectric hysteresis emerges, with memory windows of ∼0.4 V, stable over 500 cycles and retention times exceeding 10⁵ s. Temperature-dependent x-ray diffraction reveals structural signatures consistent with the bulk phase transitions sequence of BTO and confirms a predominant in-plane lattice orientation near room temperature. The combined electrical and structural analysis indicates that phase-dependent polarization anisotropy, together with domain-wall dynamics and interfacial screening processes, stabilizes an out-of-plane polarization component at low temperature in the rhombohedral and orthorhombic phases.