Transmission electron microscopy studies of nucleation, growth and interfaces in ceramic oxide heterojunctions.

摘要

Transmission electron microscopy studies on the formation and interface structures of two ceramic oxide heterojunctions are reported.; The growth of {dollar}alpha{dollar}-Fe{dollar}sb2{dollar}O{dollar}sb3{dollar} (hematite) on {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} (sapphire) is studied using a novel technique involving low-pressure chemical vapor deposition of hematite directly onto electron-transparent substrates. The technique allows direct observations to be made on the relationship between substrate surface morphology and the early stages of film growth. Epitactic growth of highly faceted hematite islands is reported for four substrate orientations--(0001), {dollar}{lcub}{dollar}1120{dollar}{rcub}{dollar}, {dollar}{lcub}{dollar}1010{dollar}{rcub}{dollar}, and {dollar}{lcub}{dollar}1102{dollar}{rcub}{dollar}-- and discussed in terms of anisotropic surface energies. Preferential nucleation and growth of islands at surface steps is observed for some surfaces.; The structures of the Fe{dollar}sb2{dollar}O{dollar}sb3{dollar}/(0001)Al{dollar}sb2{dollar}O{dollar}sb3{dollar} and Fe{dollar}sb2{dollar}O{dollar}sb3{dollar}/{dollar}{lcub}{dollar}1102{dollar}{rcub}{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} interfaces are characterized using weak-beam imaging. Two types of misfit dislocation networks in the (0001) interface are associated with normal and twin orientations of the film. Lateral-twin boundaries and stacking faults which emanate from surface steps are identified as common defects in the hematite. Films grown on bulk (0001) sapphire substrates give similar, although not identical, results. Thermal stress is shown to be particularly important in thicker films and can lead to cracking. The structure of the {dollar}{lcub}{dollar}1102{dollar}{rcub}{dollar} interface is much more complicated than the (0001) interface as it consists of several types of misfit dislocation networks some of which are associated with tilting of the film. A notation system for rhombohedral structures is introduced which allows analogies to be drawn between these interface and fcc-type heterojunctions.; Studies on heterojunctions and grain boundaries in thin films of the high T{dollar}sb{lcub}rm c{rcub}{dollar} superconductor, YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}rm 7-x{rcub}{dollar}, are presented. YBCO films grown on (001)MgO and (001)ZrO{dollar}sb2{dollar} substrate exhibit significant areas of epitactic growth, the orientation relationship being determined by matching of the film and substrate oxygen sublattices across the interface. Clean YBCO/(001)MgO interfaces are reported whereas the YBCO/(001)ZrO{dollar}sb2{dollar} interface contains a reaction layer. YBCO is shown to have a tendency to form CSL-type grain boundaries in an oriented polycrystalline thin film. Observations of near-{dollar}Sigma{dollar} = 1, 5, 13, 17, and 29 boundaries are reported.