Step instabilities on Si(I 1 1) vicinal surface and Si(I 1 1) vicinal surface induced by An adsorption were observed by reflection electron microscopy. On the Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and surface steps are bunching. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they are permeable at higher temperature. On the Si(I 1 1) vicinal surface the permeability of steps depends on the heating current direction. The steps are permeable for step-up current heating and are impermeable for step-down cur-rent heating. Above a critical coverage of Au (approximate to 0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands is formed for the step-down current heating.