STM Study Of Erbium Silicide Nanostructures On Si(001)

Low-dimensional nanostructures have been attracting considerable interests for many years due to their unusual physical properties. Two main factors have driven the increasing attention received by them in the last decade: first, they are attractive from a scientific point of view, since they provide a means to create artificial potentials for carriers, electrons, and holes in materials, at length scales comparable to or smaller than the de Broglie wavelength. Many concepts that previously existed merely as simplifying theoretical models can now be practically realized in these nanostructures. The second important factor is that quantum mechanics becomes applicable not only in systems of academic interest, but also to systems of practical impact. Using quantum confinement effects, new device concepts become feasible, which receive additional degrees of freedom in design. In this dissertation we focus on the growth and evolution of erbium silicide nanostructures. The main work and innovation are listed below:1 Well-aligned Erbium silicide nanowires and nanoisland of rectangular and square shape were obtained on the vicinal Si(001) surface. Different amount of Er is deposited onto the vicinal Si(001) surface at room temperature and subsequently annealed at different temperatures for different duration time. The growth behaviors and evolutions of erbium silicide nanostructures are investigated in details using scanning tunneling microscopy analysis. Corresponding to different postannealing temperatures: 600℃-650℃ and 730℃-750℃, nanowires and square islands form on the surface respectively. They are correspondent to different structure, i.e. hexagonal AlB₂ to nanowires and tetragonal ThSi₂ to square islands. At 600℃-650℃ nanowires form on the surface at low coverage (0.07 ML-1.14 ML) but rectangular islands form at high coverage (2.00 ML-2.86 ML). High coverage leads to high density of nuclei and dislocation in nanowires, which makes the nanowires turn shorter and wider. At late growthstage the long and robust nanowires continue to grow and the short nanowires shrink and disappear. The ripening process of nanowires are influenced by coalesce and strain.2 Based on experimental results on the vicinal Si(001), the certain relation between the direction of nanowires and the type of terrace on Si(001) surface is obtained. For the first time a circumstantial model is proposed to describe the ErSi₂/Si(001) interfacial structures. This model not only explains the spatial relationship of the erbium silicide nanowires and the Si dimer rows but also is in good agreement with Er-induced (2×3) surface reconstruction at the initial stage of Er growth on the Si(001) surface. The Si atoms taking part in the chemical reaction to form Er silicide nanowires mainly come from the surface step edges. However, for the Er silicide nanoislands, the Si atoms on the substrate terraces have also been consumed.3 The square and rectangular erbium silicide nanoislands with uniform height on Si(001) are obtained by conventional post-annealing approach. The square nanoislands are obtained on Si(001) with 2.3 ML Er coverage post-annealed at 750℃ for 10 min. When the annealing time was prolonged to 30 min all square nanoislands evolved into rectangular nanoislands. The transition happened at critical size s=32nm where the nanoislands minimized the energy through shape transformation. The Tersoff theory that is modified to fit the Volmer-Weber growth mode of erbium silicide islands can explain the shape transition very well and agrees with experimental data.