(2x4) GaP(001) surface: Atomic structure and optical anisotropy

We have investigated the microscopic structure and optical anisotropy of (2 x 4) reconstructed GaP(001) surfaces. Optical and electron spectroscopy fr om GaP(001) surfaces prepared in ultrahigh-vacuum conditions were combined with first-principles calculations of the energetics and reflectance anisot ropy. Symmetry, composition and surface optical anisotropy were characteriz ed by low-energy electron diffraction, Auger electron spectroscopy, photoem ission spectroscopy and reflectance anisotropy spectroscopy. In contrast to most earlier reports, we find that the stable Ga-rich surface corresponds to a (2 x 4) reconstruction. No (4 x 2) reconstruction could be observed, i ndependent of the preparation method. Depending on the Ga coverage, however , two distinct line shapes in the reflection anisotropy spectra occur, indi cating the existence of at least two different surface phases with (2 x 4) periodicity. This agrees with our total-energy calculations: Four (2 x 3) s tructural models may be stable depending on the chemical potentials of the surface constituents. All considered (4 x 2) structures, however, are unsta ble. Based on the comparison between calculated reflectance anisotropy spec tra and measured data we suggest mixed Ga-P dimers on top of the Ga-termina ted substrate as groundstate geometry for the cation-rich phase of GaP(001) (2 x 4). Our results indicate the formation of P dimers at the surface for the more anion-rich phase of GaP(001)(2 x 4). [S0163-1829(99)02828-3].