Microstructural Studies of Tl₂Ba₂Ca₂Cu₃O_x Thin Films on LaAlO₃ and MgO Substrates

Tl₂Ba₂Ca₂Cu₃O_x thin films have been fabricated on (001) LaAlO₃ and (001) MgO substrates. Films grown on LaAlO₃ have T_c=112K and R_S(80K, 10GHz)=0.2mΩ, while films on MgO have T_c=117K and R_s(80K, 10GHz)=0.7mΩ. The grain size and alignment of the films has been investigated using X-ray diffraction, Scanning Electron Microscopy and Electron Backscattered Diffraction. We show evidence for a markedly higher in-plane angular spread for films on MgO and believe that for films grown on this substrate the lowest achievable values of R_s are limited by disorder in the in-plane alignment of the TBCCO film caused by the large lattice mismatch between the materials.