Properties of thick SiO2/Si structure formed at 120 °C by use of two-step nitric acid oxidation method

Thick (i.e., ∼10 nm) SiO₂/Si structure has been formed at 121 °C by immersion of Si in relatively low concentration HNO₃ followed by that in 68 wt.% HNO₃ (i.e., two-step nitric acid (HNO₃) oxidation method of Si, NAOS) and spectroscopic properties and electrical characteristics of the NAOS SiO₂ layers are investigated. The SiO₂ thickness strongly depends on the concentration of HNO₃ aqueous solutions employed in the initial oxidation, and it becomes the largest at the HNO₃ concentration of 40 wt.%. The MOS diodes with the ∼9 nm SiO₂ layer formed by the NAOS method possess a relatively low leakage current density (e.g., 10⁻⁸ A/cm² at the forward bias of 1 V) and it is further decreased by more than one order of magnitude by post-metallization annealing (PMA) in hydrogen at 250 °C. The good leakage characteristic is attributable to atomically flat SiO₂/Si interfaces and high atomic density of 2.30-2.32 × 10²² atoms/cm³ of the NAOS SiO₂ layers. High-density interface states are present in as-prepared SiO₂ layers and they are eliminated by PMA in hydrogen.