Phase-selective synthesis of Tl-Ba-Ca-Cu-O thin films and multilayer structures by metal-organic chemical vapor deposition. Process optimization, phase evolution, electrical characterizations, and microstructural developments.

摘要

Metal-organic chemical vapor deposition (MOCVD), utilizing volatile metalorganic sources Ba(hfa)2·mep, Ca(hfa)2·tet, and Cu(dpm)2 (hfa hexafluoroacetylacetonate, mep = methylethylpentaglyme, tet = tetraglyme, dpm dipivaloylmethanate) characterized by thermogravimetric analysis (TGA), is employed to grow BaCaCuO(F) thin films on crystalline (110) LaAlO3. An ex situ, bulk pellet equilibrium anneal incorporates thallium to form epitaxial, superconducting Tl2Ba 2CaCu2O8 (Tl-2212) and TlBa2Ca 2Cu3O9+x (Tl-1223) thin films. The film electrical properties are determined by transport and magnetic measurements, and the film microstructures are characterized by x-ray diffraction, electron microscopy, and profilometry. The Tl-2212 films, formed via a Tl2O3/Tl 2O anneal at temperatures of 720--890°C in flowing O2 /Ar (0--100%), exhibit Tc ∼ 105 K, Jc = 1.2 x 105 A/cm2 (77 K), and microwave surface resistance, Rs , as low as 400 muO (40 K; 10 GHz). These Tl-2212 films are also incorporated into an MOCVD process utilizing Mg(dPM)2 to form Tl-2212/MgO/Tl-2212 trilayer structures. However, rough surface morphology is found to affect the quality of the resulting trilayer structures by allowing pinhole defects that permit interlayer shorting. The Tl-1223 films are formed via a novel TlF annealing process that affords phase-selective nucleation under a wide range of conditions that produce multiple-phase samples when Tl2O3/Tl2O is employed as the thallium source. The microstructure and electrical properties of these TIF-annealed films are fully characterized, with typical values of Tc ∼ 103 K and Jc ∼ 2 x 105 A/cm2 (5 K). Calculated flux pinning activation energies of MOCVD-derived, TlF-annealed Bi- and Sr-substituted Tl-1223 analogs are found to agree with literature values. On unsubstituted TlF-annealed Tl-1223 thin films, a quenching study is undertaken to elucidate the function of fluoride in the TlF annealing process. X-ray diffraction and electron microscopy of thin films quenched at various stages in an optimized annealing schedule reveal the phase evolution and microstructural development of the resulting Tl-1223 thin films. It is proposed that TIF reacts with BaO in the bulk pellet and forms Tl2O early in the annealing process to thereby afford a higher Tl2O partial pressure during initial stages in superconducting phase formation.