Spectroscopic and electrical properties of ultrathin SiO2 layers formed with nitric acid

Spectroscopic and electrical properties of ultrathin silicon dioxide (SiO₂) layers formed with nitric acid have been investigated. The leakage current density of the as-grown SiO₂ 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 SiO₂ 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 SiO₂ layers before POA but they are removed almost completely by POA above 800 °C. The atomic density of the as-grown chemical SiO₂ layers is 4% lower than that of bulk SiO₂ 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 SiO₂. The valence band discontinuity energy at the Si/SiO₂ interface increases from 4.1 to 4.6 eV by POA at 900 °C. The high atomic density enlarges the SiO₂ 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 SiO₂ by the enlargement of the band discontinuity energy, (ii) elimination of trap states in SiO₂, and (iii) elimination of interface states.