| Research Abstract |
We have developed a new method of decreasing a leakage current density flowing through silicon dioxide (SiO_2) layers. In this method, a platinum (Pt) layer of 〜 3 nm thickness is deposited on a SiO_2 layer, followed by heat treatment at 〜 300 ℃ in oxygen, and after the removal of the Pt layer, an alminum electrode is deposited, resulting in the <Al/SiO_2/Si(100)> MOS structure. Dissociated oxygen ions (O) are injected into the SiO_2 layer, and the ions react with defect states such as suboxide species in SiO_2 and Si dangling bond interface states at the Si/SiO_2 interface. The reaction results in the elimination of the defect states which work as a current path. Since the migration of O ions is enhanced in thin parts of the SiO_2 layer because of a high electrical field, oxidation selectively occurs in the thin part, leading to the improvement of the SiO_2 thickness uniformity and hence decreasing the leakage current density. When a positive bias voltage is applied to Si with respect to the Pt layer, the interface state density becomes lower.
We have found that SiO_2 layers of 〜 1.4 nm thickness formed by immersion of Si in an azeotropic solution of nitric acid possess a leakage current density as low as or slightly lower than those for thermal SiO_2 layers grown at high temperatures. Consequently, we have succeeded in the observation of a capacitance-voltage curve for ultrathin chemical SiO_2 layers for the first time. When an Al layers deposited and the specimen is heated at 200℃ in hydrogen, the leakage current density is greatly decreased to 1/20 〜 1/4 of those for thermal SiO_2 layers of the same thickness. It is concluded that the decrease in the leakage current density results from 1) elimination of interface states, 2) elimination of SiO_2 gap-states, and 3) widening of the band-gap energy of the SiO_2 layer.
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