Effect of the cluster size in modeling the H-2 desorption and dissociativeadsorption on Si(001)

Three different clusters, Si9H12, Si15H16, and Si21H20, are used in density -functional theory calculations in conjunction with ab initio pseudopotenti als to study how the energetics of H-2 dissociative adsorption on and assoc iative desorption from Si(001) depends on the cluster size. The results are compared to five-layer slab calculations using the same pseudopotentials a nd high quality plane-wave basis set. Several exchange-correlation function als are employed. Our analysis suggests that the smaller clusters generally overestimate the activation barriers and reaction energy. The Si21H20 clus ter, 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 act ivation energies are discussed in relation to the efficiency of clusters to describe the properties of the clean Si(001)-2X1 surface. (C) 1999 America n Institute of Physics. [S0021-9606(99)70307-7].