(2×4)GaP(001) surface: Atomic structure and optical anisotropy

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We have investigated the microscopic structure and optical anisotropy of $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructed GaP(001) surfaces. Optical and electron spectroscopy from GaP(001) surfaces prepared in ultrahigh-vacuum conditions were combined with first-principles calculations of the energetics and reflectance anisotropy. Symmetry, composition and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, photoemission spectroscopy and reflectance anisotropy spectroscopy. In contrast to most earlier reports, we find that the stable Ga-rich surface corresponds to a $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction. No $(4\ifmmode\times\else\texttimes\fi{}2)$ reconstruction could be observed, independent of the preparation method. Depending on the Ga coverage, however, two distinct line shapes in the reflection anisotropy spectra occur, indicating the existence of at least two different surface phases with $(2\ifmmode\times\else\texttimes\fi{}4)$ periodicity. This agrees with our total-energy calculations: Four $(2\ifmmode\times\else\texttimes\fi{}4)$ structural models may be stable depending on the chemical potentials of the surface constituents. All considered $(4\ifmmode\times\else\texttimes\fi{}2)$ structures, however, are unstable. Based on the comparison between calculated reflectance anisotropy spectra and measured data we suggest mixed Ga-P dimers on top of the Ga-terminated substrate as ground-state geometry for the cation-rich phase of GaP(001)$(2\ifmmode\times\else\texttimes\fi{}4)$. Our results indicate the formation of P dimers at the surface for the more anion-rich phase of GaP(001)$(2\ifmmode\times\else\texttimes\fi{}4)$.