HRTEM and EELS studies of nanoscale structured electronic materials

This thesis focuses on the growth and structure of a number of nanoscale structured electronic materials characterized using HRTEM and EELS. Both rare earth silicide nanostructures self-assembled on Si(001) and sputtered GMR multilayers have been studied by characterizing their crystal structures, interfacial epitaxy, interfacial chemical nature, and electronic nature, which provide fundamental insights into material behavior at the nanometer scale.; High aspect ratio nanowires and nanosized islands have been observed to self-assemble on Si(001) for both the Gd and Tm silicide systems. HRTEM results show that Gd silicide nanostructures exhibit either the hexagonal GdSi 2-x or the orthorhombic GdSi2 crystal structures, with lattice parameters consistent with the bulk phases. In the case of Tm, the observed nanostructures are likewise either hexagonal or orthorhombic. The hexagonal phase has lattice parameters consistent with the bulk, while the orthorhombic does not. For both systems, bi-phasic silicide structures were observed, which may reflect a mechanism for strain accommodation at the interface with the substrate. In the case of Gd, the phase with lower strain lies at the substrate. For the case of Tm, the relative mismatches of the two phases predicted from bulk silicide lattice parameters disagree with that derived from measured lattice constants, and it is a relaxed orthorhombic phase at the interface that appears to have the lowest mismatch with the substrate. EELS studies were carried out to compare the electronic structures of metallic Gd, thin film Gd silicide, and Gd oxide in bulk phases and Gd silicide nanostructures. The results from the three bulk phases are similar, while the intensity ratio of M5:M4 in the GdSi2 nanostructures varies from the bulk, which may suggest that a slightly different spin state exists in the silicide nanostructures.; As-sputtered and annealed F(Co, or Py)/Al multilayers have been studied using HRTEM and EELS. Although the interfacial intensity profiles from EELS spectrum images suggest some limited intermixing exists in the F/Al interfacial regions, both HRTEM and diffraction studies show no obvious intermediate phase formation. In particular, the annealing treatments do not significantly alter the multilayer structures. In contrast, intermediate phase formation has been observed in both Cu/Al multilayers and Cu/Al regions in spin valves. Tetragonal Al2Cu and bcc AlCu3 are formed in the Cu(8nm)/Al(10nm) multilayers, while Al2Cu and fcc Cu are formed in the Cu(5nm)/Al(3nm) multilayers. For Cu/Al/Cu layers in the spin-valves, evidence of Al2 Cu and AlCu3 phase formation in the annealed spin-valve with the 30nm Al layer was found, while Al2Cu and Cu were observed in the as-sputtered spin-vale with the 10nm Al layer. These results are discussed in terms of the balance between interfacial and plume free energies in order to rationalize the formation of non-equilibrium structures.