The influence of monolayer coverages of Sb on the optical anisotropy of vicinal Si(001)

The influence of monolayer coverages of antimony (Sb) on vicinal silicon (001) is studied by reflectance anisotropy spectroscopy (RAS). Adsorption of Sb at room temperature, which is known to break the Si dimer bond and to cause step reconstruction, changes the shape of the RAS spectrum completely, even before annealing. An interesting feature appears in the region of 3.7 eV, which sharpens when the system is annealed to 300 degrees C to form the Si(001)-(1 x 1)-Sb structure. Our results support the view that Si(001)-(1 x 1)-Sb is a distinct phase and we associate the 3.7 eV peak with optical transitions involving Si-Sb backbonds. Further annealing to 550 degrees C forms Si(001)-(2 x 1)-Sb, where the Sb atoms are now dimerised. This heat treatment decreases the vicinal surface domain imbalance by splitting up the double-height steps, and the RAS signal is reduced in size. Annealing to 750 degrees C causes Sb desorption and the formation of the 0.25 ML Si(001)-c(4 x 4)-Sb structure, with an increase in optical anisotropy, which we attribute to double-height steps reforming as the Sb dimer-induced strain in the system is alleviated. Complete desorption of Sb restores the Si(001)-(1 x 2) structure, and the original RAS signal.