Characterization of novel epitaxial materials for advanced tunneling devices

Novel tunneling structures, such as ferromagnet/ barrier/ ferromagnet and superconductor/ barrier/ superconductor, have stimulated tremendous interest because of potential applications for spin tunneling and Josephson junction devices. Cr-doped AlN/GaN has been proposed as the basis for spintronic devices, while MgB2 as well as TiNbN and yttrium-barium-copper oxide (YBCO) have been suggested as key components for superconducting tunneling devices. The research of this dissertation involved characterization of these materials using advanced electron microscopy methods, and correlation of the microstructure with electrical, magnetic and superconducting properties.; Cr-doped Al(Ga)N thin films grown on 6H-SiC and sapphire substrates by molecular beam epitaxy (MBE) have been characterized. Magnetic measurements showed ferromagnetic response from 4K up to 900K. Observations showed that the ferromagnetism was determined by several factors including growth temperature, Cr concentration, film quality and fraction of Cr substitution. Cr segregated to form one-dimensional columns for Al(Cr)N thin films, whereas more uniform Cr distribution was obtained for Ga(Cr)N thin films. Small clustering of CrN ranging from 2nm to 10nm was observed for Cr-doped GaN films grown at elevated temperature. There was no significant amount of oxygen present in these films. Any known magnetic impurity phase was considered unlikely to be responsible for the measured ferromagnetism.; Thin films of MgB2 grown on various substrates with different techniques have been characterized. Electrical measurements indicated that MgB2 thin films grown by MBE had lower superconducting transition temperatures and higher resistivity compared with those grown by in situ reactive evaporation. Cubic MgO phase was detected for specific substrates and growth circumstances with reactive evaporation growth. Both deposition temperature and substrate type were critical factors for MgB 2 thin film epitaxial growth.; Further superconducting materials including Nb-based superconductors and YBCO high-temperature superconductors were characterized. A TiNbN/ TaN/ TiNbN/ Nb device structure was examined, and TaN barrier layer quality was a key factor for the superconducting device performance. A 90° grain boundary was revealed for YBCO thin film grown on MgO. Surface defects were observed for YBCO films grown on sapphire with CeO2 buffer layer, and copper oxide and yttrium oxide phases were detected underneath the surface pits.