Wake-Up in Al_1−xB_xN Ferroelectric Films

The polarization wake-up process is demonstrated here for ferroelectric switching in epitaxial Al_0.93B_0.07N films on W coated c-axis oriented Al₂O₃ (sapphire) substrates. During the wake-up process, the remanent polarization grows from ≈0 to >100 µC cm⁻². As it does so, both the reversible and irreversible Rayleigh coefficients rise substantially, suggesting that the concentration of mobile interfaces that separate regions of opposite dipole orientation is increasing. The irreversible Rayleigh coefficient is very small (≈3.5 × 10⁻⁴ cm kV⁻¹), four to five orders of magnitude below those of perovskite ferroelectric films such as PbZr_0.52Ti_0.48O₃. These small values are consistent with the high coercive fields observed in the nitride ferroelectrics. The temperature dependence of the Rayleigh coefficients suggests that the interface motion is thermally activated. On increasing frequency, the Rayleigh coefficients drop, suggesting time-dependent pinning processes also occur in this family of materials. With information from anisotropic etching experiments upon field-cycling, a self-consistent model that describes a polar domain microstructure evolution process during wake-up is proposed.