HYDROGEN DESORPTION FROM SI(100)(2X1)-H INDUCED BY POTASSIUM ADSORPTION

The interaction of K with Si(100)(2 x 1)-H has been studied by using thermal desorption and electron energy loss spectroscopy. We have found that above 360 K potassium adsorption induces concomitant H-2 desorption. This mechanism has been ascribed to a two step process consisting of the reaction of an incoming K atom with surface Sill species to form a surface complex, followed by the decomposition of this complex to give rise to the H-2 desorption. We have found that the isolated reaction intermediate, whose decomposition leads to the H-2 desorption around 450 K, is formed by the deposition of a small amount of K at 90 K. For the deposition of a large amount of K, the reaction intermediates form two types of potassium hydrides upon annealing and give rise to sharp H-2 desorption peaks at 360 and 380 K. Analysis of the K uptake curves above 360 K in terms of the Kisliuk model shows that K adsorption occurs via a precursor-mediated process, which is similar to that on clean Si(100)(2 x 1) but with a much low probability(< 1/10) for the precursor to chemisorb. We suggest that the low probability for chemisorption is due to the activation barrier, estimated to be similar to 3 kcal/mol, for the precursor K atoms to react with surface SiH species.