Magnesium Diboride (MgB2) and Ca-doped YBa2Cu3O7-? : Thin Films and Devices. Fabrication and Characterization

Doktorsavhandling, 2005

Magnesium diboride (MgB2) is a superconductor with an exceptionally high transition temperature, Tc, of 39 K. This work describes deposition of high quality and superconducting films of MgB2 for electronic applications. Experiments on Ca-doped YBa2Cu3O7-? films and fabrication of bicrystal Josephson junctions are also described. Pulsed laser deposition (PLD) was used to prepare in-situ superconducting MgB2 films on SrTiO3(100) substrates with an onset to superconductivity at 30 K and a transition to the superconducting state at 20 K. ?-2? x-ray diffraction spectra showed no film peaks indicating that the films consisted of small grain sizes. A small value of the root-mean-square (RMS) roughness and a maximum peak-to-valley distance of 3 nm were indicative to the films smooth surface morphology. A two-step method was used to grow superconducting thin films of MgB2 on SrTiO3(001) and Al2O3(1-102) substrates by first preparing precursors of Mg-B bilayers at room temperature. The precursor films were subsequently annealed in Mg vapour to obtain the superconducting MgB2 phase, with Tc ranging from 22 K to 30 K. The films grown on SrTiO3(001) substrates showed a growth in the (101) direction while those on Al2O3(1-102) were c-axis oriented. The films exhibited a fairly smooth surface with an RMS roughness of about 8.5 nm and a peak-to-valley distance between 20 and 27 nm. As-grown MgB2 films with Tc in the range of 32 K to 34 K were deposited by co-evaporation technique on Al2O3(0001), MgO(111) and SiC(0001) substrates. The films were highly oriented in the c-axis and exhibited a smooth surface morphology. Through optimization of the growth parameters, films with Tc ~35 K and an RMS roughness of 2 nm were obtained. The films were used to fabricate microbridges, weak link junctions and proximity effect Josephson junctions by using conventional photolithography, Ar-ion beam and focused ion beam (FIB) milling. The current-voltage (I-V) characteristics of the weak links and proximity effect Josephson junctions showed the dc and ac Josephson effect. MgB2 films grown by a customer-designed molecular-beam epitaxy (MBE) system (at NTT-Basic Research Laboratories, Japan) were used to fabricate nanobridges. Electron beam lithography and Ar-ion beam milling by utilizing amorphous carbon as etching mask were used to realize the nanobridges. The nanobridges showed the dc Josephson effect. Tri-layer devices (MgB2/Pd/Nb and MgB2/Al-AlOx/Nb) and multilayers involving normal metals and superconductors (MgB2/Au/Nb/Al-AlOx/Nb and MgB2/Pd/Fe/Nb) were also fabricated. The films of palladium (Pd) and gold (Au) for the metal interlayers and aluminium (Al) forming the oxide barriers after oxidation as well as the superconducting niobium (Nb) were deposited by RF sputtering. The Pd/Fe bilayer in the MgB2/Pd/Fe/Nb devices was deposited by e–beam evaporation. These devices exhibited both the ac and dc Josephson effect and I-V characteristics close to the resistively shunted junction (RSJ) type. Ca-doped YBa2Cu3O7-δ films were deposited by pulsed laser deposition on asymmetric 30°, 40° and 45° bicrystal SrTiO3(001) substrates. The films were patterned into bicrystal grain boundary Josephson junctions by conventional photolithography and Ar-ion beam milling. An increase in Jc and IcRN product was observed in the 30° and 40° bicrystal junctions when compared with similar junctions of normal YBa2Cu3O7-?.