High-Level Silicon Surface Passivation by Anodically Grown Silicon Dioxide and Silicon Nitride Stacks

We investigate the surface passivation attained by a stack consisting of anodically grown silicon dioxide and silicon nitride. A very low surface saturation current density J_os of 3 fA/cm₂ is attained after the silicon wafers are submersed in nitric acid under a constant bias for >30 min, followed by a silicon nitride deposition and subsequent annealing in forming gas (FG) at 400 °C. We examine J_os as a function of the anodic SiO₂ thickness (d_ox) and show that for d_ox between 7 and 36 nm, J_os decreases monotonically from 32 to 3 fA/cm², respectively. From capacitance-voltage (CV) and conductance measurements, we show that this reduction in J_os with an increase in oxide thickness primarily results from a reduction in both the interface defect density (D_it) and the hole capture cross section (σ_p). For the lowest J_os of 3 fA/cm²,alow D_it of ~ 2.0 × 10¹⁰ cm^-2·eV^-1 and a low σ_p of ~ 1 × 10^-16 cm^-2 are determined. When the anodic SiO₂ films are capped by a silicon nitride film, negligible surface passivation degradation occurs, in comparison with uncapped anodic SiO₂ films. Finally, we demonstrate that thermally induced bulk silicon defects can be eliminated by replacing high temperature (1000 °C) oxidations with a room-temperature anodic oxidation technique.