Influence of surface roughness on the electrical properties of Si–SiO2 interfaces and on second-harmonic generation at these interfaces

Si(111) wafers, cut at angles between 0° and 5° in the [112̄] direction, were prepared by a standard RCA cleaning process followed by a rinse in HF/NH4F solutions with different pH values. Si–SiO2 structures were formed on these surfaces by a two-step low-temperature plasma-assisted oxidation/deposition process, and compared with interfaces formed by thermal oxidation, followed by rapid thermal annealing. The Si(111)–SiO2 interface, subjected to predeposition rinse in a 40 wt. % NH4F solution, displayed a midgap interface trap density Dit of ∼4×1010 cm−2 eV−1. The Dit values increased systematically up to ∼2×1011 cm−2 eV−1 as the pH of the final rinse was decreased. Using a second-harmonic generation pumped at 1053 nm, the influence of off-axis orientation and surface structure of Si(111) surfaces was examined. The surface structure was modified by thermal oxidation at 850 °C, annealing at temperatures in the range of 900–1100 °C, and removing the oxide in a HF solution. Changes in the characteristic rotational anisotropy are analyzed using harmonic functions reflecting the onefold and threefold symmetry of vicinal Si(111) surfaces, and are compared with changes in the thermal oxide Dit values on annealing.