From kinetic measurements by optical second harmonic generation of ${\mathrm{O}}_2$ interaction with $\mathrm{Si}\left(111\right)-(7\times 7)$ at temperatures between 610 and 735 °C and pressures from 1$0^{–9}$ to 1$0^{–6}$ torr, we determine the boundaries for oxide nucleation and for quasiequilibrium between surface phases of oxide and silicon. The distinction between these boundaries reflects the influence of the kinetic parameters of the surface reactions on the critical conditions. At $T<\mathrm{}700\mathrm{}{}^{\circ }\mathrm{C}$, the oxide nucleation requires oxygen pressure well above that required for oxide growth.

• Received 28 October 1994

DOI:https://doi.org/10.1103/PhysRevLett.75.272