Role of electronic correlation in the Si(lOO) reconstruction: A quantum Monte Carlo study

Abstract

Abstract The 100 surface of a silicon crystal is believed to reconstruct to form rows of silicon dimers in either a “symmetric” pattern ( 11 I 111 I I) or a somewhat deformed “buckled” pattern (/\/\/\/\) in which the dimers are twisted alternately within each row. At room temperature the dimers may flip back and forth between configurations and they appear symmetric in scanning tunneling microscopy. Below 120 K the buckled configuration appears, but below 20 K the symmetric configuration has been indicated but might be due to flipping, and arguments in favor of each of the configurations have been made. Theoretical work is equivocal. Density functional (DFT) calculations favor the buckled configuration. Multiconfiguration self-consistent field (MCSCF) and configuration interaction (CI) calculations favor the symmetric. The indicated differences in energies for the two structures are small, and electron correlation (note the title) is thus expected to be important in determining the minimum-energy structure. Diffusion QMC provides the required accurate description of correlation for large systems. As in the earlier studies with other methods the authors treated the model systems Si9H12, Sii5H16, and Si21H20 - clusters which have one, two, and three dimers in configurations like those on the Si(lO0) surface. The calculations were made using silicon pseudopotentials tested in calculations for Si2. For the intermediate cluster the energies of the symmetric and the buckled configurations were nearly identical. For the larger cluster, with three dimers, the buckled configuration was favored by 0.34(6) eV or 0.11(2) eV per dimer. This energy difference is similar to that for DFT calculations (LDA to B3LYP). The agreement suggests that the dynamic correlation effects, described better in DQMC and DFT than in MCSCF and CI, are important. The buckled configuration is thus to be favored, but many questions in both theory and experiment remain unresolved for this system.