Lateral interactions between adsorbed hydrogen atoms on the Si(100)-(2 x 1) surface

We have performed both first-principles density functional pseudopotential planewave calculations and density functional all-electron cluster calculat ions to investigate the interactions between various configurations of adso rbed hydrogen atoms on the Si (100)-(2 x 1) surface. We quantify the streng th for a number of nearest-neighbor interactions. These include interaction s for a paired-hydrogen dimer with an unpaired-hydrogen dimer, an unpaired- hydrogen dimer with an unpaired-hydrogen dimer in cis or trans geometry, an d a paired-hydrogen dimer with another paired-hydrogen dimer. All these int eractions are attractive, and, in particular, the interaction between paire d-hydrogen dimers is approximately 0.08 eV in strength. We also quantified the next-nearest-neighbor interaction between a paired-hydrogen dimer and a nother paired-hydrogen dimer. This interaction is also attractive and is ap proximately equal to 0.08 eV, which is of the same magnitude as the nearest -neighbor interaction strength. This shows that there is an energy decrease of approximately 0.16 eV associated with adding a hydrogen-paired dimer to the end of a chain of hydrogen-paired dimers. The size of the energy decre ase per added paired-hydrogen dimer is in good agreement with the value of 0.17 eV obtained recently by fitting scanning tunneling microscopy results using Monte Carlo simulations of a lattice gas with only nearest-neighbor i nteractions. Our results shed light on the use, in the simulations, of a ne arest-neighbor interaction whose strength depends upon whether the dimer is at the end of a chain or in the middle. Thus, we provide a first-principle s basis for understanding the observed clustering of hydrogenated dimer pai rs on the Si(100)-(2 x 1) surface. (C) 2000 Elsevier Science B.V. All right s reserved.