Polarity Inversions in AlN Films on Sapphire Exploiting Silicon and Oxygen Diffusion during High-Temperature Annealing

The polarity of III-nitride materials critically influences their physical properties and behavior. In this study, we identify closed polarity inversion domains (IDs) in epitaxial AlN layers grown on sapphire substrates and analyze the mechanism underlying their formation following high-temperature annealing. These IDs are defined by two distinct inversion domain boundaries (IDBs): a lower flat IDB in the c-plane (growth plane), where the polarity transitions from Al-polar to N-polar, and an upper convex IDB, which spans multiple layers and gradually restores Al-polarity. The IDs are uniformly distributed in the entire AlN epilayer thickness, and their lateral dimension varies over a wide range that goes from 50 to 300 nm, while their vertical size remains below 10 nm. A comprehensive characterization of the atomic structure and chemical composition shows that codoping with oxygen (O) and silicon (Si) is essential for the formation and stabilization of the IDs, with O promoting vacancy-impurity clusters and Si facilitating vacancy creation and impurity precipitation during cooling. Our findings highlight the critical role of impurities in polarity inversion within AlN layers, providing fundamental insights for the design of III-nitride-based electronic and optoelectronic devices, where polarity manipulation is critical.