Scanning tunnelling microscopy and reflection high-energy electron diffraction showed that the intensity of the two-fourths fractional order feature in the reflection high-energy electron diffraction pattern for the GaAs(001)-2 x 4 structure reflects the degree of ordering of the vacancy rows. The scanning tunnelling microscopic images show a high degree of surface ordering only when the reflection high-energy electron diffraction produces nearly the equal intensity sharp streaks for all the fourfold fractional order diffraction. Two possible mechanisms were suggested to explain the observed scanning tunneling microscopy and reflection high-energy electron diffraction correlation. Surface Ordering of the Molecular Beam Epitaxially Grown GaAs(001)-2 x 4-As Reconstruction. J.Zhou, Q.Xue, H.Chaya, T.Hashizume, T.Sakurai: Applied Physics Letters, 1994, 64[5], 583