Densification of ∼5 nm-thick SiO₂ layers by nitric acid oxidation

Highlights: • Leakage current density of the commercial PECVD grown ∼5 nm SiO₂ layer has been decreased about three orders of magnitude by densification. • The densification of SiO₂ layer is achieved by high oxidation ability of O·. • Densities of suboxide, fixed charge (N_f) and defect state (N_d) in SiO₂/Si interface are decreased by NAOS and PMA. • Tunneling barrier height (Φ_t) is increased because of the increase of atomic density in SiO₂ layer. - Abstract: Low-temperature nitric acid (HNO₃) oxidation of Si (NAOS) has been used to improve the interface and electrical properties of ∼5 nm-thick SiO₂/Si layers produced by plasma-enhanced chemical vapor deposition (PECVD). Investigations of the physical properties and electrical characteristics of these thin films revealed that although their thickness is not changed by NAOS, the leakage current density at a gate bias voltage of −1 V decreases by about two orders of magnitude from 1.868 × 10⁻⁵ A/cm². This leakage current density was further reduced by post-metallization annealing (PMA) at 250 °C for 10 min in a 5 vol.% hydrogen atmosphere, eventually reaching a level (5.2 × 10⁻⁸ A/cm²) approximately three orders of magnitude less than the as-grown SiO₂ layer. This improvement is attributed to a decrease in the concentration of suboxide species (Si¹⁺, Si²⁺ and Si³⁺) in the SiO₂/Si interface, as well as a decrease in the equilibrium density of defect sites (N_d) and fixed charge density (N_f). The barrier height (Φ_t) generated by a Poole-Frenkel mechanism also increased from 0.205 to 0.371 eV after NAOS and PMA. The decrease in leakage current density is therefore attributed to a densification of the SiO₂ layer in combination with the removal of OH species and increase in interfacial properties at the SiO₂/Si interface.