Interfacial charge modification between SiO₂ and silicon

A positive flatband voltage shift, ΔV_fb ≅+0.4 V, with respect to unimplanted portions of the same wafer, was obtained when calcium (1×10¹³ cm^-2) was implanted into 87 nm of thermally grown oxide on an n-type 〈100〉 substrate and annealed. Calcium acts as a low-efficiency n-type dopant in silicon (<0.1% activated) which eliminates the possibility of calcium interactions in the substrate causing the flatband behavior. Calcium profiles after a 1100°C anneal show considerable loss from the oxide but also indicate occurrence of stable sites in the SiO₂ region near the oxide-silicon interface. Theoretical calculations on a model SiO₂ structure predict an effective negative charge at the Si/SiO₂ interface due to calcium incorporation in agreement with the general behavior observed experimentally; moreover, the calculations predict that aluminum and strontium will behave in a similar fashion to calcium when implanted into SiO₂ while boron will not.