A density functional theory study of the nonlocal effects of NH3 adsorption and dissociation on Si(100)-(2 x 1)

In order to investigate the reliability of cluster approximations in describing nonlocal effects in surface reactions, detailed density functional theory (DFT) calculations are performed for the reaction of NH3 on the Si(100)-(2 x 1) surface. We use the Si9H12 one-dimer, Si21H20 three-dimer, Si33H28 five-dimer and two Si23H24 trench clusters to determine the nonlocal effects both along the dimer row and across the trench. The nonlocal effects along the dimer row are found to be significant while nonlocal effects across the trench are relatively small. The nonlocal character of NH3 adsorption originates from charge transfer to nearest neighbor dimers along the row, which increases the adsorption energy by 26%, Consequently, the Si21H20 three-dimer cluster is necessary and adequate to describe NH3 adsorption. The three-dimer cluster also accurately describes the strain effects that result from unbuckling of the covalently terminated Si dimer of the dissociated state. This results in a decrease of the desorption energy by 3 kcal mol(-1). Incorporating the nonlocal effects yields reaction energies and activation barriers that agree with the experimental results. We also find that the three-dimer cluster reproduces the results of periodic slab calculations and gives a more accurate activation energy for the dissociation reaction. (C) 2000 Elsevier Science B.V. All rights reserved.