Microstructural characterization of the initial stages of growth of yttrium barium copper oxide thin films: A comparison of lattice-matched and non-lattice-matched growth.

摘要

Thin films of {dollar}rm YBasb2Cusb3Osb{lcub}7-delta{rcub}{dollar}, a membrane of the family of high-temperature oxide superconductors, display a complex set of microstructural features that must be better explored before such films may reach their full technological potential and before investigations of the superconducting properties of {dollar}rm YBasb2Cusb3Osb{lcub}7-delta{rcub}{dollar} thin films may be fully understood. This dissertation focuses on the characterization of three major areas that determine the microstructures encountered in such films: the initial nucleation and growth of the film on the bare substrate, the twinning transformation that occurs in the {dollar}rm YBasb2Cusb3Osb{lcub}7-delta{rcub}{dollar} after film growth, and the effect on growth of the nominal orientation of the substrate surface with respect to the low-index crystallographic direction of the substrate. Towards these ends, through-thickness series of ultrathin films on lattice-matched LaAlO{dollar}sb3{dollar} and non-lattice-matched MgO substrates were studied by transmission electron microscopy, supplemented by x-ray diffraction. The twinning microstructure as a function of film thickness was also investigated, and a model for the dependence of the twin spacing on film thickness is discussed. Deposition onto vicinally polished MgO substrates was utilized to better understand surface morphology effects on nucleation, growth, and film properties. Finally, implications of the twinning transformation and of the microstructures produced by deposition onto vicinal substrates are discussed with regard to the observations of ultrathin films.