Low-energy photoelectron diffraction structure determination of GaSe-bilayer-passivated Si(111)

The atomic structure of GaSe-bilayer-passivated Si(111) was determined using low-energy photoelectron diffraction (PED). Scanned-energy and scanned-angle PED measurements were combined with multiple-scattering calculations to investigate the specific bonding configuration and to test the applicability of these techniques in an electron energy range where multiple and back scattering are important. Using tunable synchrotron radiation to vary the electron wavelength for emission along the Ga-Si and Ga-Se bonds, we determined the Ga site to be directly atop a surface Si atom with a bond length of 2.35±0.02 . We find the Ga-Se bond lies 65^∘ from the surface normal towards [112], with length 2.44±0.01 . Diffraction stereograms [variable-angle scattering at constant kinetic energy (KE)] for Ga $3d$ emission (KE=230 eV) show threefold symmetric, forward-focusing peaks along Ga-Se bonds, indicating a single-domained bilayer. Se $3d$ stereograms (KE=196 eV) show a sixfold ripple pattern due to scattering from the six in-plane Se next-nearest neighbors. Multiple scattering analysis is required to explain the full diffraction pattern for this two-layer thick film. Polar-angle scans at constant azimuth and kinetic energy were measured using conventional laboratory x-ray sources for Ga $2p$ (KE=131 eV) and Se $2p$ (KE=48 eV). The results, subjected to multiple-scattering analysis, are consistent with the structure determined using synchrotron radiation. These results demonstrate low-energy PED as an effective tool in structural determination.