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The electrical characteristics and reliability of the aluminum oxide (Al2O3) metal-oxide-semiconductor (MOS) capacitors were investigated under low-temperature process consideration. The simple cost-effective technique in preparing the Al2O3/SiO2 bilayer structure as the high-k gate dielectrics was demonstrated in this paper. SiO2 was prepared by room-temperature anodic oxidation, and Al2O3 was fabricated by room-temperature in situ natural oxidation during the dc sputtering of aluminum in Ar/O2 ambient. Compared to the Al2O3 MOS capacitors without nitric acid (HNO3) compensation, significant improvements in electrical characteristics, reliability, and uniformity were achieved by utilizing HNO3 to moderately oxidize the existing Al2O3 layer. In addition, the charge trapping behaviors of our samples were also studied by time-dependent dielectric breakdown under the 1000-s constant voltage stress and constant current stress tests. It was found that the electron trapping is dominant under a low negative bias stress. However, under a high negative bias stress, the Al2O3 MOS capacitors show hole trapping due to the impact ionization near the SiO2/Si interface. The in situ oxidation in sputtering with HNO3 compensation is suitable for future low-temperature dielectric applications.