Geometric, electronic, and optical properties of the Si(111)2×1 surface: Positive and negative buckling

The Si(111)2×1 is among the most investigated surfaces. Nonetheless, several issues are still not understood. Its reconstruction is well explained in terms of the Pandey model with a slight buckling (tilting) of the topmost atoms; two different isomers of the surface, conventionally named positive and negative buckling, exist. Usually, scanning tunneling microscopy (STM) experiments identify the positive buckling isomer as the stable reconstruction at room temperature. However, at low temperatures and for high n doping of the substrate, recent scanning tunneling spectroscopy (STS) measurements found the coexistence of positive and negative buckling on the Si(111)2×1 surface. In this work, state-of-the-art ab initio methods, based on density functional theory and on many-body perturbation theory, have been used to obtain structural, electronic, and optical properties of Si(111)2×1 positive and negative buckling. The theoretical reflectance anisotropy spectra (RAS), with the inclusion of the excitonic effects, can provide a way to deepen the understanding of the coexistence of the isomers.