Effect of single crystal substrates on the growth and properties of superconducting Tl₂Ba₂CaCu₂O₈ films

Abstract

Superconducting Tl 2 Ba 2 CaCu2O8 films were prepared by a two-step postdeposition anneal process under identical conditions on a number of single crystal substrates: (100) LaA10 3 , (100) NdGaO 3 , (100) LaGaO 3 , (100) SrTiO 3 , (100) MgO, and (0001) A1 2 O 3 . The properties of the resulting Tl 2 Ba 2 CaCu 2 0 8 films were strongly dependent on the substrate. Films on all substrates were predominantly c-axis oriented. Films prepared on LaA10 3 , NdGaO 3 , and LaGaO 3 also exhibited in-plane epitaxy, while films on MgO, A1 2 O 3 , and SrTiO 3 were not aligned with the substrates. Some a-axis oriented grains of Tl 2 Ba 2 CaCu 2 0 8 formed on NdGaO 3 and both a-axis oriented grains and cracks were present on LaGaO 3 . Large amounts of secondary phases, as indicated by additional peaks in the x-ray diffraction scans, were observed for films on A1 2 O 3 and SrTiO 3 and smaller amounts on each of the other substrates. Depth profiling by sputtered neutral mass spectroscopy (SNMS) confirmed the presence of strong film-substrate reactions on A1 2 O 3 and SrTiO 3 , and also indicated the presence of a reaction on NdGaO 3 . The T c onset of the films as measured both inductively and by ac magnetic susceptibility varied depending on the substrate. The highest quality films were prepared on LaA10 3 . NdGaO 3 may also prove to be an acceptable substrate if the nucleation and growth of a-axis oriented Tl 2 Ba 2 CaCu 2 0 8 grains can be eliminated. The remaining substrates appear to be less suitable due to severe film-substrate interactions, lack of epitaxial growth, or crystallographic phase changes.

Thallium-based HTS thin films, processing, properties and applications

“…An interfacial Ba-Ca-Ti-O layer is also formed as a result of this interaction, but the superconducting film nevertheless has a strong epitaxial relationship with the substrate lattice [109]. Holstein et al observed a similar chemical interaction between Tl(2212) thin films and SrTiO 3 substrates [115]; however, in a crosssectional TEM study of TBCCO films grown on SrTiO 3 substrates, Morosin et al [116] found no evidence of chemical interaction even though thalliation took place in air at 850 • C. There is thus no current consensus on the reactivity of this interface at high processing temperatures. Less surprisingly, no interaction has been observed between Tl(1223) thin films grown by in situ techniques and SrTiO 3 substrates [91,114] and this is attributed to the lower deposition temperatures (between 650 and 720 • C).…”

“…One technologically important substrate material on which it is impossible to deposit TBCCO thin films is sapphire. During thalliation severe film-substrate interactions occur which result in the formation of an interfacial layer of BaAl 2 O 4 and a concomitant degradation in the superconducting properties of the films [115,119]. In addition, the non-orthogonal axes in the substrate have a large lattice mismatch with TBCCO and there is a large difference in the thermal expansion coefficients of the materials, which can lead to cracking of the films.…”

Growth of Tl2Ba2CaCu2O8 superconducting thin films on curved substrates for metamaterials applications

“…This study involved imaging the surface morphology and collecting EBSD data from several small regions (about 20 mm wide) at radial positions every 30 around the surface of the cylinder. The surface morphology everywhere consisted of flat, plate-like grains as observed in films grown on planar MgO [19]. EBSD analysis showed that the Tl-2212 plates grow on MgO cylinders with their c-axes everywhere roughly parallel to the substrate surface normal regardless of the angle made with the MgO [1 0 0] axis (see Fig.…”

“…MgO has a cubic crystal structure with a lattice parameter of 0.42 nm; about 10% misfit with the (a-b) plane of the tetragonal Tl-2212 phase (with a ¼ 0:385 nm). As a result of the large lattice misfit, Tl-2212 films grown on planar MgO substrates tend to have strong c-axis alignment with the surface normal, but no unique in-plane epitaxial relationship [17][18][19] and therefore the films contain high-angle grain boundaries.…”

“…The surface morphology of Tl-2212 films grown on latticematched LaAlO 3 cylinders is very dependent on radial position [19][20][21]. As the y angle (i.e.…”