Effective Charge Modification Between SiO₂ and Silicon

A positive flatband voltage shift, ΔV_FB≃ +0.4V, with respect to unimplanted portions of the same wafer was obtained when calcium (1 × 10¹³/cm²) was implanted into 87 nm of thermally grown oxide on an n-type <100> substrate and annealed. It was further determined that calcium acts as a low efficiency n-type dopant (<0.1% activated); this eliminates the possibility of calcium interactions in the n-type substrate causing the flatband behavior. 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. The calculations, moreover, predict that aluminum and strontium will behave in a similar fashion to calcium. The prediction with regard to aluminum appears to be verified by recent experimental work.