Enhanced Permittivity in Wurtzite ScAlN Through Nanoscale Sc Clustering

ScN alloyed AlN (Sc_xAl_{1- x}N, ScAlN, or AlScN) is a wurtzite semiconductor with attractive ferroelectric, dielectric, piezoelectric, and optical properties. Here, using electron ptychography in combination with a suite of experimental and theoretical methods, we show that ScAlN films (with x spanning 0.18 to 0.36) contain nanoscale Sc-rich clusters which maintain the wurtzite crystal structure. While both molecular beam epitaxy (MBE) and sputter deposited Sc_0.3Al_0.7N films show Sc clustering, the degree of clustering is significantly stronger for the MBE-grown film, offering an explanation for some of the discrepancies between MBE-grown and sputtered films reported in the literature. Moreover, the MBE-grown Sc_0.3Al_0.7N film exhibits a dispersive and anomalously large dielectric permittivity, roughly double that of sputtered Sc_0.3Al_0.7N. We attribute this result to the Sc-rich clusters locally reaching x ∼ 0.5 and approaching the predicted ferroelectric-to-paraelectric phase transition, resulting in a giant (local) enhancement in permittivity. The Sc-rich clusters should similarly affect the piezoelectric, optical, and ferroelectric responses, suggesting cluster-engineering as a means to tailor ScAlN's functional properties.