Advancement of Ferroelectric Al1−xScxN thin films

Ferroelectricity in wurtzite-type (w-)Al1−xScxN has enabled new opportunities for integrating ferroelectric thin films into memory and neuromorphic devices. As the first known wurtzite-type ferroelectric, AlScN is notable for its compatibility with CMOS and GaN technologies. This thesis investigates the growth and ferroelectric properties of w-Al1−xScxN thin films, emphasizing temperature stability, epitaxy, and doping. Structural properties were studied through X-ray diffraction (XRD), atomic force microscopy, and scanning transmission electron microscopy (S/TEM). In-situ XRD and permittivity measurements showed that the wurtzite phase in Al1−xScxN remains stable up to 1100 °C. Sputter deposition of w-Al1-xScxN on doped n-GaN yielded high-quality epitaxial films with significantly improved (0002) texture. Lattice-matched w-Al0.89Sc0.11N grown by sputter epitaxy achieved a rocking-curve FWHM of ~252 arcsec. High-resolution STEM confirmed the polarity of these films in the bulk. Growing w-Al0.73Sc0.27N on epi-Pt/GaN rather than n-GaN reduced hysteresis asymmetry, leakage, and coercive field values, despite some loss of texture. Further improvements on epitaxial films were achieved by reducing the electrode thickness from 100 nm to 10 nm. Apart from that, ferroelectricity was also demonstrated in ultra-thin (~5 nm) w-Al1−xScxN layers with switching voltages near 1 V, satisfying low-voltage requirements for memory devices. Unlike conventional oxide ferroelectrics, scaling w-Al0.73Sc0.27N thickness caused no clear degradation of polarization or coercive field. Additionally, oxygen doping during sputtering reduced overall leakage without compromising structural quality. Overall, this work advances w-Al1−xScxN toward device-level implementation, particularly in terms of thermal stability, epitaxy, thickness scaling, and doping.