The growth kinetics of ultrathin SiO2films on silicon in a nitrous oxide (N2O) ambient have been investigated as a function of oxidation temperature and time. The results show that the overall growth follows the linear‐parabolic law proposed by Deal and Grove [J. Appl. Phys.36, 3770 (1965)]. The data analysis indicates that although the oxidation proceeds by surface‐limited reaction in the initial stage, it rapidly changes into a diffusion‐controlled reaction. This behavior is evidenced from the fact that the reaction of the N2O molecule with the silicon surface produces an interfacial nitrogen‐rich layer which acts as a barrier to the oxidant passing through the SiO2/Si interface. From the Arrhenius equation for N2O oxidation, the activation energies for the linear rate constantB/Aand for the parabolic rate constantBare determined to be 1.5 and 2.3 eV, respectively. ©1996 American Institute of Physics.