Initial reactive sticking coefficient of O-2 on Si(111)-7x7 at elevated temperatures

Kinetics of the initial stage of oxide growth in the reaction of oxygen with Si(111)-7x7 at temperatures from room temperature to T-tr, and pressures from 5x10(-9) to 2x10(-7) Torr are investigated with optical second-harmonic generation, here T-tr is the transition temperature from oxide growth to Si etching without oxide growth. At a fixed pressure, the initial reactive sticking coefficient (S-0), obtained from the rate of oxide growth, decreases with increasing temperature to S-0=0 at T-tr. We have found that the initial reactive sticking coefficient depends on the O-2 pressure. At temperatures above 320 degrees C, the whole temperature dependence of S-0 is situated in the region of higher temperatures for higher O-2 pressures (P-ox). Moreover, an additional bend in the temperature dependence of S-0 is observed for P-ox>1x10(-8) Torr near T-tr. A precursor-mediated adsorption model involving the reaction of volatile SiO formation is considered. The parameters of this model, obtained from the best fits to the experimental data, show that oxide growth rate constant increases and volatile SiO formation rate constant decreases as a function of O-2 pressure. At zero oxide coverage, the pressure dependence of the reaction rate constants is suggested to originate from interaction in the layer of the chemisorbed precursor species, whose coverage depends on the O-2 pressure. The volatile SiO formation is described by a three-step sequential two-channel process through the chemisorbed O-2 precursor species, whereas one of the channels with a larger activation energy is suggested to induce the additional bend in S-0(T) near T-tr at higher O-2, pressures.