Structures and optical properties of reconstructed Si and Ge surfaces

Abstract

An approach combining reflection anisotropy spectroscopy (RAS) measurement and hybrid density functional theory (DFT) was taken to test and model various reconstructed surfaces of group-IV semiconductors (Si and Ge) in this project. The results obtained demonstrate that hybrid DFT offers a computationally efficient approach to calculating the optical response of surface structures. Using hybrid DFT functionals, structural, electronic and optical properties were calculated for Si(111)-(2 X 1) superstructures with modified Pandey (MP) chains. The (2x1) surface with asymmetric π chains has two variants, which are ±MP chain structures. The +MP chain structure was found to be more stable than the -MP chain structure by a very small total energy difference (2 meV). Bond lengths between atoms in surface and the next layer are reported and compared with previous experimental data and calculated values. Electronic structures for both the MP structures were calculated which revealed two surface states (occupied and unoccupied). The occupied and unoccupied surface states were found to be mainly localised on the outermost and innermost Si atoms of the asymmetric π chain, respectively. Direct band gaps are reported here to be 0.29 and 0.40 eV, with optical gaps of 0.29 and 0.40 eV for + and -MP structures, respectively. The calculated imaginary parts of dielectric functions for ±MP structures show large peaks below 1 eV with the electric field aligned in the π chain direction. The large positive peaks in RA spectra of + and -MP surfaces are found to be at 0.43 and 0.57 eV, respectively, with the experimental value at 0.46 eV. Transitions responsible for those peaks are identified as surface state transitions.