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 oxyge n 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 f rom oxide growth to Si etching without oxide growth. At a fixed pressu re, 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 highe r temperatures for higher O-2 pressures (P-ox). Moreover, an additiona l 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 th e 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 c overage, the pressure dependence of the reaction rate constants is sug gested to originate from interaction in the layer of the chemisorbed p recursor species, whose coverage depends on the O-2 pressure. The vola tile 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 t he additional bend in S-0(T) near T-tr at higher O-2, pressures.