Effect of the cluster size in modeling the H2 desorption and dissociative adsorption on Si(001)

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Abstract
Three different clusters, Si 9 H 12 , Si 15 H 16 , and Si 21 H 20 , are used in density-functional theory calculations in conjunction with ab initio pseudopotentials to study how the energetics of H 2 dissociative adsorption on and associative desorption from Si(001) depends on the cluster size. The results are compared to five-layer slab calculations using the same pseudopotentials and high quality plane-wave basis set. Several exchange-correlation functionals are employed. Our analysis suggests that the smaller clusters generally overestimate the activation barriers and reactionenergy. The Si 21 H 20 cluster, however, is found to predict reaction energetics, with E a des =56±3 kcal/mol (2.4±0.1 eV ), reasonably close (though still different) to that obtained from the slab calculations. Differences in the calculated activation energies are discussed in relation to the efficiency of clusters to describe the properties of the clean Si(001)-2×1 surface.