Effect of single crystal substrates on the growth and properties of superconducting Tl 2 Ba 2 CaCu 2 O 8 films
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Superconducting Tl2Ba2CaCu2O8 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) SrTiO3, (100) MgO, and (0001) A12O3. 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, A12O3, and SrTiO3 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 A12O3 and SrTiO3 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 A12O3 and SrTiO3, and also indicated the presence of a reaction on NdGaO 3 . The Tc 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.
I. INTRODUCTION The superconducting TIBaCaCuO phases are of interest for electronic applications due to their high superconducting transition temperature Tc. High quality thin films of Tl 2 Ba 2 CaCu 2 0 8 are relatively easy to form in comparison with other TIBaCaCuO phases, and with a Tc of up to 110 K in bulk form,1 this phase has attracted the most attention for passive microwave applications. Passive microwave components made from Tl2Ba2CaCu2O8 compare favorably with their counterparts made from YBa 2 Cu 3 0 7 , most notably at liquid nitrogen temperature, where T/Tc is much further removed from unity for Tl 2 Ba 2 CaCu 2 0 8 than for YBa 2 Cu 3 0 7 . However, the highest quality Tl2Ba2CaCu2O8 films have been prepared only on LaAlO 3 substrates,2 and for many passive microwave applications, other substrates possess inherently better dielectric properties. The desire to routinely grow Tl 2 Ba 2 CaCu 2 08 on a wide variety of substrates raises the question of how the substrate affects the growth and properties of this phase. Preparation of Tl 2 Ba 2 CaCu 2 0g films has been reported on several single crystal substrates: MgO,3"5 962
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 5, May 1993
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SrTiO3,5-9-10 CdNdAlO 4 , n NdGaO 3 , 12 9 10 15 All of these studies Y S z wo.i3.i4 a n d A12O3. ' ' have used a postdeposition anneal in the presence of thallium oxide vapor to convert a precursor film into the Tl 2 Ba 2 CaCu
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