The results of a laboratory investigation of silicon—silicon dioxide film produced by high‐temperature thermal oxidation of silicon in water vapor shows that (1) the oxidation rate in steam is dependent on the doping impurity concentration of the crystal, and (2) the oxidation in steam at 970°C for various lengths of time of phosphorus‐diffused wafers with surface concentration of 2×1021cm−3results in an impurity pile‐up region near the silicon surface. On the other hand, boron‐diffused wafers with surface concentration in the order of 5×1018cm−3deplete their impurity concentration near the silicon surface.