High exposure hydrogen chemisorption on the surface: a semiempirical cluster study

Abstract

The various equilibrium structures of the hydrogen chemisorbed surface are investigated using the semiempirical molecular orbital Austin 1 calculational method (AM1). Up to five hydrogen atoms are allowed to adsorb near the corner adatom, centre adatom and restatom adsorption sites of the surface. The results obtained from minimizing the energy of the system show very little difference between the chemisorption processes occurring at the two different adatom sites. In both cases, as progressively more hydrogen atoms are chemisorbed, the hydrogen bonded adatom is found to move from its original threefold (T4) site (one hydrogen atom), to an adjacent bridge site (two or three hydrogen atoms) and then on top of a neighbouring first-layer silicon atom (more than three hydrogen atoms). The lowest calculated adatom desorption energies are 1.59 and 1.62 eV, and correspond to the desorption of at a corner adatom and centre adatom site, respectively. In contrast to an adatom, a hydrogen bonded restatom is found to remain close to its original threefold equilibrium position. The smallest desorption energy for this site is 0.75 eV and also corresponds to an surface complex.

STM study of site selective hydrogen adsorption on Si(111)7×7

“…1 Over the years there have been several investigations of this type on the Si͑111͒ 7 ϫ 7 surface but they have mainly dealt with larger exposures resulting in 7 ϫ 1 and 1 ϫ 1 surfaces. [2][3][4][5][6] Most of the studies have focused on adsorption where the adatoms are replaced by hydrogen atoms. The scanning tunneling microscopy ͑STM͒ studies by Mortensen et al 7 and Boland 8 of the room-temperature exposure of the Si͑111͒ 7 ϫ 7 surface showed that the adatom layer can be rearranged revealing the layer below the adatoms.…”

Adsorption kinetics of H on Si(111)7×7 by means of surface differential reflectivity

“…Actually, E a and E b are the activation-barrier energies for the adsorption of H on the active sites and are not directly related to the binding energies which are of the order of a few eV. [7][8][9] On the contrary, to our knowledge, no calculation has been performed for determining the activation energies for H adsorption. Totalenergy calculations for a variety of positions of a given H atom, far from the surface up to the adsorption sites, should make it possible to determine the actual path of the H atom and, as a consequence, the energy barriers the H atom has to overcome, either adsorbing on an adatom dangling bond, or breaking a back bond of a reacted or unreacted Si adatom.…”

“…[5][6][7][8][9] The 7ϫ7 surface reconstruction, well described by the dimer-adatom-stacking fault ͑DAS͒ model, 10 involving building blocks ͑adatoms, rest atoms, dimers...͒ frequently found at the reconstructed surfaces of semiconductors, can be considered, indeed, a model system for the investigation of fundamental aspects of gas chemisorption on semiconductors. The interaction of H with the Si͑111͒7ϫ7 surface involves different processes which depend on the experimental conditions.…”

“…Although several theoretical calculations concerning the binding energy of the Si͑111͒7ϫ7 have been performed, [6][7][8][9] their results cannot be easily compared with our experimen- …”