Growth of TlBa₂Ca₂Cu₃O_y superconducting thin film on CeO₂ buffered sapphire substrate

Abstract

We report the growth of TlBa₂Ca₂Cu₃O_y superconducting thin film on CeO₂ buffered r-cut sapphire substrate by amorphous phase epitaxy method, wherein an amorphous phase of a composition, TlBa₂Ca₃Cu₄O_y was deposited by rf magnetron sputtering and subsequently crystallized by annealing at high temperatures in a closed system. There exists an optimum thickness of about 200 Å of CeO₂ buffer layer that gives a smooth buffer layer surface. At the temperature around 840 °C we get only Tl-2212 phase and for a longer annealing time it results in Tl-1212 phase. At higher temperatures Ba reacts with Ce to form BaCeO₃. To avoid this reaction process we introduced another amorphous buffer layer, TlSr₂CaCu₂O_y onto CeO₂ layer. By this method we could prepare a good quality Tl-1223 thin film which showed a T_c of 104 K and a J_c=0.3 MA/cm² at 77 K and 0.1 T field.

Introduction

The high-T_c superconductors, CuBa₂Ca_n−1Cu_nO_y (n=3,4) show high transition temperature, T_c>115 K and high J_c>2 MA/cm² at 77 K and 0 T [1]. Therefore these materials can have technological importance. Considering the high J_c value, it is expected that these materials may have a low microwave surface resistance. Unfortunately, these materials could only be synthesized under high pressure and at high temperatures. Therefore, it is has not been possible to fabricate this superconductor as thin films by normal preparation techniques. However, by alloying them with the corresponding members of TlBa₂Ca_n−1Cu_nO_y (n=3,4) and following a two step thin film fabrication process, namely amorphous phase epitaxy (APE) method, it may be possible to prepare thin films of such materials with a still better superconducting properties with a sustainable T_c in the overdoped state [2]. For applications such as microwave band-pass filters and resonator, etc. the substrate materials should have a low dielectric constant and low tanδ at operating frequencies (>10 GHz). Further, in order to fabricate a good quality superconducting thin film the substrate should have close lattice matching as well as close thermal expansion coefficient with the superconductor and there should not be any chemical reaction between them. The substrate should be free from twinning and available at large areas and low cost. Considering all these requirements, the two substrates MgO and sapphire seem to be the best candidates out of all known materials and they have been widely used for this purpose. Since there is a big lattice mismatch between the sapphire and high-T_c superconductors, a (1 0 0) CeO₂ buffer layer is used to improve the lattice matching and to avoid any inter-diffusion of aluminum [3].

Although our attempt was to grow Cu,Tl-1223 thin films on CeO₂ buffered r-cut sapphire substrate we obtained Tl-1223 phase by following the preparation method described below. We found that at temperatures around 840 °C we get only Tl-2212 phase and for longer time we get Tl-1212 phase. At higher temperatures, Ba reacts with Ce to form BaCeO₃ and hinders the formation of Tl-1223. We succeed to prepare Tl-1223 thin film by using another amorphous buffer layer, TlSr₂CaCu₂O_y on top of CeO₂.