GaP(001) and InP(001): Reflectance anisotropy and surface geometry

We have investigated the optical anisotropy of GaP(001) and InP(001) surfaces. The samples were prepared by homoepitaxial metalorganic vapor phase epitaxy growth and either directly transferred into ultrahigh vacuum (UHV) or in situ capped and, after transfer, decapped in UHV by thermal desorption of a P/As capping layer. Symmetry, composition, and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, and reflectance anisotropy spectroscopy. We observe (2 X 1)/(2 X 2)-like reconstructions for the very P-rich and (2 X 4) reconstructions for the more cation-rich surfaces. No (4 X 2) reconstruction could be prepared, independent of the preparation method. A comparison of the reflectance anisotropy between GaP(001) and InP(001) surfaces shows similar line shapes for the very cation-rich (2 X 4) surfaces. For less cation-rich surfaces, however, we observe distinct differences between the spectra of the two systems. In both cases, different line shapes in the reflection anisotropy spectra occur for the (2 X 4) periodicity, suggesting the existence of different (2 X 4) geometries. The experimental results are discussed on the background of atomic structures, total energies and reflectance anisotropy spectra obtained ab initio from density-functional theory local-density approximation calculations. (C) 1999 American Vacuum Society. [S0734-211X(99)03504-0].