An improved model for strain tuning of negative capacitance in ferroelectric PbZr0.5Ti0.5O3 thin films

The negative capacitance (NC) effect in ferroelectric (FE) materials offers promising potential for low-power electronics and energy storage. This study investigates the strain-engineered NC behaviour of PbZr0.5Ti0.5O3 (PZT) thin films using Landau-Khalatnikov theory coupled with mechanical boundary conditions. Unlike conventional models that assume a constant Curie temperature, our framework incorporates strain-dependent ferroelectric transition temperatures, which is more accurate for strain tuning of NC. Static and dynamic simulations reveal that compressive and tensile strains significantly enhance the NC effect by amplifying energy barriers, negative curvature in free energy profiles, and transient voltage drop magnitudes. Compressive strains can exhibit superior performance, increasing polarization-electric field (P-E) hysteresis slopes and transient NC responses. While tensile strains show non-classical NC enhancement, which is different from the reported results. The model predicts prolonged voltage drop durations (Δ t ≈ 6.5 μs) and amplitudes (Δ V ≈ 1.5 V) under compressive strain (ε = -0.4%), aligning with experimental results. The NC Field-Effect Transistors (NC-FETs) under compressive strain demonstrate a 4.7× enhancement in current while simultaneously achieving a sub-thermal limit subthreshold swing (SS) of 22.6 mV/dec at Vgs = 0.57 V, surpassing the tensile strain efficacy. The improved model exhibits current elevation and SS reduction compared to the traditional model predictions. These results highlight the necessity of intrinsic strain characterization during fabrication and validate strain-polarity asymmetry in NC modulation. The work reveals the significance of addressing the strain-dependent FE phase transition temperature, offering a robust framework for optimizing strain-engineered NC devices.