PNNL

Abstract

X-ray photoelectron diffraction was used to investigate the interface structure created by deposition of 1 monolayer-equivalent (ML-eq) of Si onto As-stabilized GaAs(001)-c(2x8)/(2x4). The system was grown via molecular-beam epitaxy at a substrate temperature of 450 degrees C. AlKalpha-excited azimuthal angle-resolved Si 2p photoelectron intensities were measured and compared with model angular distributions generated by means of spherical-wave, single-scattering cluster theory. Both the distrubution of 1 ML-eq of Si atoms and the coordinates of atoms in the near-surface region were varied to optimize agreement with experiment. Substantial evidence was found for substitutional Si in the first three atomic layers. The extensive amount of diffraction modulation observed at polar angles up to 90 degrees indicates that Si does not form a segregated ML at the surface, diffuses into the subsurfaces region. The best agreement between theory and experiment was obtained by modeling the ratio of Si fractional occupancies in the first, second, and third layers as 1.0:0.5:0.25. In addition, the (2x1) reconstruction geometry was modeled with a topmost Si atom displacement of 0.5 A from bulk-truncated coordinates along [110] (the dimer bond direction), and a second-layer Si atom displacement of 0.4 A along [110]. Thus, the near-surface region is under considerable strain as Si ations form surface dimers while being "anchored" to a lattice with dimensions that are 4% larger that those of bulk Si. These results provide an explanation for the unusual behavior of the AlAs/Si/GaAs heterojunction band offset.