Exponential growth regime:  If we start with bare silicon (i.e. silicon with only native oxide) then we find that  the value of t is not zero (or 2 nm) in the model fit. Instead, a value of about 25 nm provides a good fit.  This indicates that the initial growth is very rapid. This growth is approximated by an exponential curve. This is also called exponential growth regime.  Different hypotheses have been proposed to explain this observation. In one hypothesis, the holes diffuse faster and setup an electric field. Due to this, the net diffusivity of the oxidizing species is higher. This space-charge regime is about 15 nm thick for dry oxidation and 0.5 nm thick for wet oxidation. Hence, the effect is observed in dry oxidation but not in wet oxidation. In another hypothesis, even before oxidation starts (i.e. temperature is raised), oxygen diffuses into the silicon. Once the temperature is increased, oxidation occurs simultaneously throughout the depth (of 5 or 10 nm) and hence initial oxide growth rate is high. In any case, the fact is that initial growth rate of oxide , in dry oxidation, is much higher than that predicted by Deal-Grove model.