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

We have performed both first-principles density functional pseudopotential planewave calculations and density functional all-electron cluster calculations to investigate the interactions between various configurations of adsorbed hydrogen atoms on the Si(100)-(2×1) surface. We quantify the strength for a number of nearest-neighbor interactions. These include interactions for a paired-hydrogen dimer with an unpaired-hydrogen dimer, an unpaired-hydrogen dimer with an unpaired-hydrogen dimer in cis or trans geometry, and a paired-hydrogen dimer with another paired-hydrogen dimer. All these interactions are attractive, and, in particular, the interaction between paired-hydrogen dimers is approximately 0.08 eV in strength. We also quantified the next-nearest-neighbor interaction between a paired-hydrogen dimer and another paired-hydrogen dimer. This interaction is also attractive and is approximately 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 decrease 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 interactions. Our results shed light on the use, in the simulations, of a nearest-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-principles basis for understanding the observed clustering of hydrogenated dimer pairs on the Si(100)-(2×1) surface.