MAGNESIUM DIBORIDE THIN FILMS AND DEVICES

Open Access
Author:
Cui, Yi
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 24, 2007
Committee Members:
  • Xiaoxing Xi, Committee Chair
  • Julian Decatur Maynard Jr., Committee Member
  • Peter E Schiffer, Committee Member
  • Suzanne E Mohney, Committee Member
Keywords:
  • MgB2
  • thin film
  • device
Abstract:
Magnesium diboride (MgB2) is a binary compound superconductor with a superconducting transition temperature Tc of ~ 40 K. MgB2 has two conduction bands: a two-dimensional ¦Ò band and a three-dimensional ¦Ð band with weak interband scattering. The two gap superconductivity in MgB2 gives rise to many interesting physical properties not possible in other superconductors. The relatively high Tc combined with phonon mediated superconductivity and relatively long coherence length makes MgB2 promising for electronics applications like rapid single flux quantum (RSFQ) logics and superconducting quantum interference devices (SQUID). The high current density and record-high upper critical field in pure or alloyed MgB2 are also attractive to a variety of high field applications including cryogen-free Magnetic Resonance Imaging (MRI) systems. MgB2 may also be used in RF cavity coatings due to its low surface resistance and in photo detection due to its fast photoresponse coupled with relatively high Tc. High quality epitaxial thin films are produced by the hybrid physical-chemical vapor deposition (HPCVD) technique. The HPCVD MgB2 thin films have the highest Tc and lowest resistivity with sharp transition of all MgB2 materials reported. The HPCVD MgB2 material is free of dendritic flux jumps due to its low resistivity. The root-mean-square (RMS) surface roughness of HPCVD MgB2 films can be ~1 nm when ~1% of nitrogen is added to the hydrogen carrier gas during the growth. The stability of MgB2 films in water is studied; it is found that degradation can be prevented by a thin (10 nm) MgO layer deposited on the film surface. The Tc is enhanced by tensile strain due to the Volmer-Weber growth mode and the mismatches between MgB2 and the substrate in the lattice constants and the coefficients of thermal expansion. High quality superconductor-insulator-superconductor Josephson tunnel junctions were made with various barrier formation techniques. The junction critical current densities and IcRn products are high with clear gap characteristics and low subgap currents. The Fraunhofer-pattern of Josephson supercurrent modulation in magnetic fields demonstrates excellent junction uniformity. The barrier thickness and height were estimated, and the barrier composition was studied by X-ray Photoelectron Spectroscopy (XPS). Josephson tunnel junctions with non-c-axis-oriented MgB2 were made which clearly exhibit tunneling spectra from both MgB2 superconducting gaps. The two-band superconductivity and its effect on vortices were studied by tunneling spectroscopy in magnetic fields. Planar all-MgB2 Josephson junctions were made by creating a weak-link through a TiB2 underlayer or an ion damaged MgB2. Junctions exhibited Josephson critical current and RSJ-like characteristics and Shapiro steps under microwave radiation. Uniform ion damage MgB2 Josephson junction array was also demonstrated.