Rare-earth silicide nanostructure epitaxial growth and crystallography

This thesis focuses on rare-earth metal (RE) silicide nanostructures (nanowires and islands) epitaxially grown on Si(001) substrates. Gd, Dy and Tb are taken as prototypical silicide nanowire forming metals. Combined studies by scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) reveal that RE silicide nanowires have the hexagonal crystal structure, and nano-islands have either orthorhombic or tetragonal structures. In the nanowires, there is a small lattice mismatch between the silicide and the silicon along the long axis, and a large mismatch across their widths. This observation confirms the commonly accepted growth mechanism that RE silicide nanowires are formed due to the anisotropic lattice mismatch between the silicide and the silicon.; Further analysis shows that the lattice parameters of the nanostructures do not significantly deviate from bulk silicide values. Hence, the strain between the silicide and the silicon due to lattice mismatch is accommodated by interfacial dislocations, which are observed. Furthermore, other strain accommodation mechanisms such as edge dislocations and stacking faults within the silicide, and nanostructure lattice tilting, are also observed.; Both the STM and TEM studies suggest the possibility of a phase transformation from the hexagonal to the orthorhombic/tetragonal phases (or nanowires to islands). The transformation appears to nucleate either at nanowire intersections or at the ends of isolated nanowires.; Sc silicide nanowire formation has also been studied. It is expected that Sc silicide nanowire growth will be distinct from the other RE metal silicides, since the anisotropy in the lattice mismatch between the hexagonal Sc silicide and the substrate is arranged in this way: the large lattice mismatch direction is along the a-axis and the small mismatch is along the c-axis, while, in case of other RE silicides, the large mismatch is along the c-axis and small mismatch is along the a-axis. TEM of the Sc nanowires shows that they are composed of two layers, with hexagonal Sc3Si5 at the silicide-silicon interface, and hexagonal SC5Si3 on the top. The stabilization of the metal rich silicide phase is a unique feature of this system.