ATOMIC-STRUCTURE AND FAULTED BOUNDARIES IN THE GAAS(001) BETA(2X4) SURFACE AS DERIVED FROM X-RAY-DIFFRACTION AND LINE-SHAPE ANALYSIS

The atomic structure of the 2x4 reconstructed GaAs (001) surface, prep ared in optimized molecular beam epitaxy growth conditions, has been f ully determined by in situ grazing incidence x-ray diffraction measure ments. The structure involves only two dimers in the As top layer, in agreement with recent scanning tunnel microscopy experiments by Avery er al. and Hashizume et al. and, in addition, the presence of an As di mer in the third layer as a consequence of a missing Ga row in the sec ond layer is clearly established. Full agreement is obtained for the a tomic displacements down to the fourth atomic layer below the surface with state-of-the-art first-principles total-energy calculations by Sc hmidt and Bechstedt. A quantitative line-shape analysis using diffuse scattering formalism demonstrates the influence on the intensity distr ibution of the most frequent faults in the reconstruction propagation and enables to understand the variations observed in the reflection hi gh-energy electron-diffraction patterns with the surface preparation w ithout need of a new surface structure.