A leakage current reduction of alumina thin films was achieved through the oxidation of ultrathin AlN films as compared to conventional ALD-Al₂O₃ films. It was suggested that wider-bandgap alumina, such as θ-Al₂O₃, was formed, resulting in reduced leakage. The mechanism underlying the formation of wider-bandgap alumina is likely caused by the residual stress resulting from the ultrathin film thickness of AlN when its nitrogen was replaced with oxygen during the oxidation process.