AZIMUTHAL VARIATION OF THE STEP DISTRIBUTION ON VICINAL SI(001) SURFACES

The step structure and terrace distribution of Si(001) vicinal surface s has been studied by profile analysis of the (00) spot and by measuri ng superstructure spot intensities using high-resolution LEED on a len s-shaped sample. In the [110] azimuth, the transition from single laye r (S-) steps at low miscut angle gamma to double layer (D-) steps occu rs continuously between about 2-degrees and 6-degrees. Beyond 11-degre es, (115) facets are admixed. While the surface in the range 5-degrees less-than-or-equal-to gamma less-than-or-equal-to 7.5-degrees in all azimuths is built up by (001) terraces separated by steps, an increasi ng disorder of the edge pattern is deduced when leaving the [110] azim uth. In the range where the surface in the [110] azimuth is not yet si ngle-domain (gamma = 5-degrees), this decrease starts at a deviation o f about +/-10-degrees from the [110] azimuth, for gamma = 6-degrees an d 7.5-degrees, the intensity decreases immediately. In the [100] azimu th, the existence of long-range ordered S-steps with broad terrace wid th distribution is deduced from the data.