Si(111)2 × 1 surface isomers: DFT investigations on stability and doping effects

The relative energy stability between Si(111)2 × 1 positive and negative buckling is still a puzzle. Both isomers possess comparable energies in Density Functional Theory (DFT) calculations and, hence, should occur with the same probability. This is in contrast to experimental findings, which show that the positive buckling is the observed structure at low and room temperature. Here we investigate the role of the exchange-correlation (xc) approximation in DFT functionals. We find that the particular choice of the xc functional does not change the relative stability between the Si(111)2 × 1 isomers. Moreover, we investigate the effect of donor atoms on ground-state and optical properties of both isomers, highlighting the differences with respect to the undoped case for several possible positions of the phosphorus (P) donor atoms. Considering the most stable position of P, relative stability between the isomers is not strongly affected by doping whereas geometric parameters are different with respect to the no doping case. The optical response bears some resemblance with the undoped case but the intensity of reflectance anisotropy peak is decreased for both isomers.