We investigated the influence of post-deposition annealing (PDA) temperature on the characteristics of Pt/Al2O3/n-β-Ga2O3 MOS capacitors. Electrical properties such as the flatband voltage (Vfb) shift, Vfb hysteresis, fixed charge (QIL), interface state density (Dit) and leakage current could be divided into two groups such as a low PDA temperature range below 600 °C and a high temperature range above 700 °C. The capacitors with a low PDA temperature exhibited superior electrical properties compared to the capacitors with a high PDA temperature, excluding leakage current property. We found that 700 °C is a critical PDA temperature because Al2O3 film starts to crystallize and solid-solution reaction of Al2O3 gate insulator and β-Ga2O3 substrate also occurs, resulting in Ga diffusion into the Al2O3 film and large roughness at Al2O3/β-Ga2O3 interface. These lead to a large Dit and positive Vfb shift at 700 °C. Even in a low PDA temperature range, it is clear that capacitor at 300 °C exhibited superior electrical properties of Vfb shift of −0.23 V, QIL of 4.1 × 1011 cm−2, and Dit (1.6 × 1012 eV−1 cm−2 at Ec-E = 0.4 eV). Based on these experimental data, lower PDA temperature of 300 °C is suitable.
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