Chemisorption of NH3 on Si(100)-(2x1): A study by first-principles ab initio and density functional theory

The energetics involved in the chemisorption of NH3 on Si(100)-(2 x 1) have been examined using ab initio and nonlocal density functional theory. One- and two-dimer cluster models were employed to model the Si(100)-(2 x 1) su rface. By using various exchange-correlation functionals and Gaussian split -valence basis sets, we have obtained the geometries of the molecularly ads orbed and dissociatively chemisorbed states, as well as the hitherto unrepo rted geometry of the transition state which exists between the two states. The geometries of the various states have been rationalized based on either electrostatic or orbital interactions. In addition, calculations were also performed on models which contain a second-adlayer ammonia molecule to yie ld several possible geometries for the extrinsic precursor state. The extri nsic precursor ammonia binding energies for the various geometries found ar e in the range of 3.93-8.80 kcal/mol. The energetics of the chemisorption p rocess and the binding energies of the extrinsic precursor ammonia are in g ood agreement with available experimental data. (C) 2000 American Institute of Physics. [S0021-9606(00)30603-1].