An ultrathin silicon dioxide (SiO2) layer of 1.2–1.4 nm thickness has been formed by immersion of Si wafers in nitric acid (HNO3) aqueous solutions, and its electrical characteristics and physical properties are investigated as a function of the HNO3 concentration. Measurements of transverse optical and longitudinal optical phonons of Si–O–Si asymmetric stretching vibrational mode for SiO2 indicate that the atomic density of the SiO2 layer increases with the HNO3 concentration. X-ray photoelectron spectroscopy measurements show that the valence band discontinuity energy at the SiO2/Si interface also increases and the concentration of suboxide species decreases with the HNO3 concentration. The leakage current density of the <Al/SiO2/Si(100)> metal-oxide-semiconductor (MOS) diodes with the SiO2 layer formed in HNO3 aqueous solutions decreases with the HNO3 concentration and also decreases by postmetallization annealing (PMA) treatment at 250 °C in 5 vol % hydrogen atmosphere. For the MOS diodes with the SiO2 layer formed in 98 wt % HNO3, the leakage current density measured after PMA is much lower than that for thermally - grown SiO2 with the same thickness and even lower than that for silicon oxynitride with the same effective oxide thickness. The decrease in the leakage current density by PMA is attributed to (i) elimination of interface states, (ii) elimination of slow states, and (iii) increase in the band discontinuity energy at the SiO2/Si interface.