| Summary: | A thin silicon oxide (SiO<sub>x</sub>) layer (thickness: 1.5–2.0 nm) formed at an Al<sub>2</sub>O<sub>3</sub>/Si interface can enhance the interface properties. However, it is challenging to control the characteristics of thin SiO<sub>x</sub> layers because SiO<sub>x</sub> forms naturally during Al<sub>2</sub>O<sub>3</sub> deposition on Si substrates. In this study, a ~1.5 nm-thick SiO<sub>x</sub> layer was inserted between Al<sub>2</sub>O<sub>3</sub> and Si substrates by wet chemical oxidation to improve the passivation properties. The acidic solutions used for wet chemical oxidation were HCl:H<sub>2</sub>O<sub>2</sub>:H<sub>2</sub>O, H<sub>2</sub>SO<sub>4</sub>:H<sub>2</sub>O<sub>2</sub>:H<sub>2</sub>O, and HNO<sub>3</sub>. The thicknesses of SiO<sub>x</sub> layers formed in the acidic solutions were ~1.48, ~1.32, and ~1.50 nm for SiO<sub>x-HCl</sub>, SiO<sub>x-H2SO4</sub>, and SiO<sub>x-HNO3</sub>, respectively. The leakage current characteristics of SiO<sub>x-HNO3</sub> were better than those of the oxide layers formed in the other acidic solutions. After depositing a ~10 nm-thick Al<sub>2</sub>O<sub>3</sub> on an SiO<sub>x-acidic</sub>/Si structure, we measured the effective carrier lifetime using quasi steady-state photoconductance and examined the interfacial properties of Al<sub>2</sub>O<sub>3</sub>/SiO<sub>x-acidic</sub>/Si using surface carrier lifetime simulation and capacitance–voltage measurement. The effective carrier lifetime of Al<sub>2</sub>O<sub>3</sub>/SiO<sub>x-HNO3</sub>/Si was relatively high (~400 μs), resulting from the low surface defect density (2.35–2.88 × 10<sup>10</sup> cm<sup>−2</sup>eV<sup>−1</sup>). The oxide layer inserted between Al<sub>2</sub>O<sub>3</sub> and Si substrates by wet chemical oxidation helped improve the Al<sub>2</sub>O<sub>3</sub>/Si interface properties.
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