Large Built-in Fields and Tunable Ferroelectricity in Composition-Graded ScAlN Thin Films Deposited by Reactive...

The control of the coercive field and growth of abnormally oriented grains (AOGs) in wurtzite ferroelectric Sc_xAl_1-xN thin films is crucial for piezoelectric and ferroelectric applications. However, elevated Sc concentrations generally result in significant AOG growth and high leakage current, degrading piezoelectric and ferroelectric properties. Here, compositionally graded Sc_xAl_1-xN layered structures grown by sputtering are explored to effectively limit the AOG growth, compared to the 8-10% typically observed in conventional films, without requiring process optimization. Notably, a large built-in electric field of ≈0.45 MV cm^-1, nearly 2.5 times larger than in graded PbZr_xTi_1-xO₃ (PZT) and BaSr_xTi_1-xO₃ (BST) systems, is achieved. This built-in field induces unique features, including horizontal shifts in polarization-electric field hysteresis loops, bi-stable states in capacitance-voltage characteristics, and highly asymmetric electrostrain behavior. Unlike other graded systems, this large built-in field in the graded Sc_xAl_1-xN films arises primarily from polarization and chemical gradients. These findings offer a simplified, scalable and CMOS-compatible strategy to overcome the AOG challenges and tune ferroelectric properties of Sc_xAl_1-xN thin films, providing the path for potential applications in advanced photonic and MEMS devices with improved performance and low-power consumption.