Para to Ortho transition of metallic dimers on Si(001)

TL;DR

This study uses electronic structure calculations to analyze the transition from para to ortho dimer configurations of metallic atoms on Si(001), explaining experimental data and revealing coverage-dependent structural changes.

Contribution

It provides a comprehensive theoretical explanation for the para to ortho transition of metal dimers on Si(001) across different coverages, integrating previous findings with new calculations.

Findings

Para dimers are stable at low coverage.

Ortho dimers become energetically favorable beyond 0.5 ML.

Mixed dimer configurations are stable at intermediate coverages.

Abstract

Extensive electronic structure calculations are performed to obtain the stable geometries of metals like Al, Ga and In on the Si(001) surface at 0.5 ML and 1 ML coverages. Our results coupled with previous theoretical findings explain the recent experimental data in a comprehensive fashion. At low coverages, as shown by previous works, `Para' dimers give the lowest energy structure. With increasing coverage beyond 0.5 ML, `Ortho' dimers become part of low energy configurations leading toward a `Para' to `Ortho' transition at 1 ML coverage. For In mixed staggered dimers (`Ortho' and `Para') give the lowest energy configuration. For Ga, mixed dimers are non-staggered, while for Al `Para' to `Ortho' transition of dimers is complete. Thus at intermediate coverages between 0.5 and 1 ML, the `Ortho' and `Para' dimers may coexist on the surface. Consequently, this may be an explanation of the fact that the experimental observations can be successfully interpreted using either orientation. A supported zigzag structure at 0.5 ML, which resembles ${\rm (CH)_x}$, does not undergo a dimerization transition, and hence stays semi-metallic. Also, unlike ${\rm (CH)_x}$ the soliton formation is ruled out for this structure.