Ultrathin SiO2 layer with a low leakage current density formed with ∼ 100% nitric acid vapor

An ultrathin silicon dioxide (SiO2) layer with 0.65-1.5 nm thickness has been formed by ∼ 100% nitric acid (HNO3) vapor oxidation, and its electrical characteristics and physical properties are investigated. The oxidation kinetics follows a parabolic law except for the ultrathin (≤0.8 nm) region, indicating that diffusion of oxidizing species (i.e. oxygen atoms generated by decomposition of ∼ 100% HNO3 vapor) through a growing SiO2 layer is the rate-determining step. The diffusion activation energy for HNO3 vapor oxidation is 0.14 eV, much lower than that of thermal oxidation of 1.24 eV. The leakage current density for the 0.65 nm SiO2 layer formed by HNO3 vapor oxidation is lower by approximately one order of magnitude than that for a thermal oxide layer with the same thickness. The low leakage current density is attributed to (i) the atomically flat SiO2/Si interface and uniform thickness of the ultrathin SiO2 layer, (ii) the low concentration of suboxide species and the low interface state density and (iii) the high atomic density of the SiO2 layer, which leads to a high band discontinuity energy at the SiO2/Si interface. The leakage current density is further decreased by PMA at 250 deg. C in 5 vol% H2 atmosphere.