Polarity inversion in AlN induced by oxygen incorporation via surface oxidation treatments

Aluminum nitride (AlN) is promising for deep-ultraviolet devices, and control of its crystalline polarity is essential for device fabrication. We have previously demonstrated vertically stacked polarity-inverted structures in AlN prepared by combining sputtering with high-temperature face-to-face annealing (FFA Sp-AlN). However, natural oxidation in the previous process required air exposure for 1 week, limiting repeated polarity inversion in multilayer structures. Here, polarity-inverted AlN bilayers were fabricated by applying deionized-water treatment for 5 min, oxygen-plasma treatment for 10 min, or thermal oxidation for 10 min to the N-polar AlN surface before deposition of the Al-polar layer. The structures were compared with a naturally oxidized reference sample. Polarity inversion was achieved under all four conditions. Notably, deionized-water treatment reduced the oxidation time from 1 week to 5 min while preserving the step-and-terrace morphology and producing no observable voids. Atomic-resolution scanning transmission electron microscopy revealed interface structures similar to previously reported Al x O y N z -containing inversion domain boundaries (IDBs). Secondary ion mass spectrometry showed oxygen and silicon concentration peaks near the IDBs, whereas carbon peaks appeared at different depths. In the oxygen-plasma-treated sample, the void-containing region approximately coincided with the carbon peak, and the carbon peak depths in all samples were broadly consistent with the second-layer thicknesses, supporting their association with the sputtered interfaces. These results demonstrate that intentional surface oxidation, particularly 5-min deionized-water treatment, provides a rapid and practical route to polarity inversion in FFA Sp-AlN