Abstract
We observed the oxidation of a silicon substrate covered with thermally grown tantalum oxide films during exposure to a high‐temperature oxidizing ambient. It was found that the oxidation rate of a silicon substrate covered with 
greatly depended on the oxidizing ambient. In a wet oxygen ambient, the oxidation rate was equal to that of bare silicon. However, the rate became very small in a dry oxygen ambient. We believe that these results can be attributed to the differences between the species that diffuse through the 
, resulting in a diffusion coefficient over 4000 times larger in wet oxygen than that in dry oxygen at 1000°C. The silicon oxide grown at the interface is stoichiometric silicon dioxide and the transitions between the 
layers are very abrupt.