Alternative Structures for the Si(111) 7 × 7 Surface: Evidence for a Novel Class of Distorted Honeycomb Reconstructions

A variety of new atomic models for the Si(111) 7 × 7 surface are created and optimized using density functional theory (DFT) calculations and evaluated against a diversity of experimental measures to understand the paradoxes found for the widely accepted dimer‐adatom‐stacking‐fault (DAS) reconstruction model. These include registry‐shifted adlayers, different termination layers, additional atoms, missing dimers or dumbbell atoms within distorted honeycomb (HC) or silicene‐like structures. Their geometric and electronic structures are evaluated with and without spin polarization using DFT and the approximate quasiparticle DFT‐1/2 scheme and show that several new allotropes agree with various experimental features better than DAS. Calculated formation energies are used to construct a phase diagram as a function of silicon's chemical potential, which favors DAS, but only for bulk‐like atom reservoirs, while other geometries are favored for modified substrate chemical potentials. Surface band gaps are found for magnetic phases of most and/or a few nonmagnetic allotropes. The overall best qualitative agreement with experiment is found for a magnetic phase that unlike DAS has a significant laterally distorted HC with high‐ and low‐buckled adatoms on the two sides of the unit cell. The nature of this structure and interactions that drive this new class of reconstruction are discussed.