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We present Auger-electron spectroscopy, low-energy electron diA'raction, and Ap measurements of Cs adsorption on Si(100) and subsequent oxidation of the substrate. Our data provide evidence against complete charge transfer from the alkali metal to Si as proposed recently. The amount of Si02 produced by alkali-metal-promoted low-temperature oxidation of silicon is found to be strictly proportional to the alkali-metal coverage, which, together with measured changes in the work function during oxidation, allows us to question some of the current mechanisms put forward to explain this phenomenom.

We have studied the potassium-and cesium-promoted oxidation of Si(100)2&(1 with photoelectron and Auger-electron spectroscopies in the range of submonolayer alkali-metal-atom coverages. The alkali-metal atoms efficiently promote the oxidation of silicon. Our results demonstrate that there are two oxygen species on the surface during the oxidation reaction: oxygen atoms which are bonded to potassium from the very beginning of the process and oxygen atoms bonded to silicon.These two oxygen species are detected in core level 0 1s and in valence-band photoemission spectra.Oxygen is transferred from K to Si, and this process is thermally activated and its efficiency is increased by heating the substrate. To explain these results we propose a model based on the decrease of local work function produced by potassium, which reduces the activation barrier for oxygen dissociation. Oxygen reacts with potassium, forming a potassium oxide which efficiently transfers oxygen to silicon.

Photoelectron spectroscopy data suggest that the mechanism of enhanced oxidation of Si promoted by multilayers of K deposited on its surface is based on the formation of potassium oxides, identified as K2O2 and KO2, that transfer oxygen efficiently to the Si substrate upon annealing at 900 K.

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