Inhomogeneities of surface potential in the thermally grown Si-SiO2 interface

The standard deviation σs of the surface potential fluctuation on the Si-SiO2 interface was measured by means of the conductance technique of Nicollian and Goetzberger on a number of wafers with oxide charge densities Qox in the range (1–15) ×1010 cm−2. We could not find a proportionality between σ2s and Qox as was derived in theoretical considerations by Brews but rather we have found a correlation between σ2s and the surface state density Nss. We show that the reason for this is that only charges in the immediate vicinity of the interface contribute to σs. In a second experiment, mobile ions were drifted through the oxide by performing a temperature-bias stress. The resulting change in σ2s is directly proportional to the accompanying change in Qox, but the ratio of Δσ2s to ΔQox is three times higher than would be expected from Brews’s model. A reason for this behavior might be that the interface charge is concentrated in some small patches rather than distributed homogeneously. The assumption of negative surface charge in our samples, however, is not necessary.