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• Reversed hysteresis is observed for Al 0.5 Si 0.5 O on GaN, possibly due to the presence of K+ and Na+ impurities. • Above 750 °C in PDA, Al 0.5 Si 0.5 O is deteriorated with inhomogeneous Si compositions and a beginning of crystallization. • The interface is impacted with GaO X growth for a PDA above 750 °C. • Hydroxyl groups reduces until 850 °C, while interfacial gallium oxide is reduced at 750 °C. • The optimal PDA temperature for Al 0.5 Si 0.5 O is 750 °C. In this work, we investigate the impact of high-temperature Post-Deposition Annealing (PDA) on Al 0.5 Si 0.5 O x deposited by Atomic Layer Deposition (ALD). Reversed hysteresis is observed and explained by mobile charges originating from K+ and Na+ impurities. The high-temperature annealing does not cure the presence of these mobile charges. We also report the onset of film and interface degradation after annealing above 750 °C under N 2 , with both inhomogeneous aluminium and silicon composition, signs of AlSiO crystallization and interfacial gallium oxide growth. [ABSTRACT FROM AUTHOR]