Spectroscopic and electrical properties of ultrathin silicon dioxide (SiO 2) layers formed with nitric acid have been investigated. The leakage current density of the as-grown SiO 2 layers of 1.3 nm thickness is high. The leakage current density is greatly decreased by post-oxidation annealing (POA) treatment at 900 °C in nitrogen, and consequently it becomes lower than those for thermally grown SiO 2 layers with the same thickness. X-ray photoelectron spectroscopy measurements show that high density suboxide species are present before POA and they are markedly decreased by POA. Fourier transformed infrared absorption measurements show that water and silanol group are present in the SiO 2 layers before POA but they are removed almost completely by POA above 800 °C. The atomic density of the as-grown chemical SiO 2 layers is 4% lower than that of bulk SiO 2 layers, while it becomes 12% higher after POA. It is concluded that the high atomic density results from the desorption of water and OH species, and oxidation of the suboxide species, both resulting in the formation of SiO 2. The valence band discontinuity energy at the Si/SiO 2 interface increases from 4.1 to 4.6 eV by POA at 900 °C. The high atomic density enlarges the SiO 2 band-gap energy, resulting in the increase in the band discontinuity energy. The decrease in the leakage current density by POA is attributed to (i) a reduction in the tunneling probability of charge carriers through SiO 2 by the enlargement of the band discontinuity energy, (ii) elimination of trap states in SiO 2, and (iii) elimination of interface states.