The Use of BaF 2 Buffer Layers for the Sputter-Deposition of Ticabacuo Thin-Film Superconductors
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THE USE OF BaF2 BUFFER LAYERS FOR THE SPUTTER-DEPOSITION OF TlCaBaCuO THINFILM SUPERCONDUCTORS K.M. Hubbarda, P.N. Arendta, D.R. Brownb, D.W. Cookec, N.E. Elliotta, J.D. Farrd, M. Nastasia, K.C. Otte, E.J. Petersone, and G.A, Reevesa Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT Thin films of the TI-based superconductors often have relatively poor properties because of film/substrate interdiffusion which occurs during the anneal. We have therefore investigated the use of BaF2 as a diffusion barrier. T1CaBaCuO thin films were deposited by dc magnetron sputtering onto MgO substrates, both with and without an evaporation-deposited BaF2 buffer layer, and post-annealed in a TI over-pressure. Electrical properties of the films were determined by four-point probe analysis, and compositions were measured by ion-backscattering spectroscopy. Structural analysis was performed by X-ray diffraction and scanning electron microscopy. The BaF2 buffer layers were found to significantly improve the properties of the TlCaBaCuO thin films. Since the discovery of superconductivity in the TlCaBaCuO system, high quality thin films of this compound have been deposited by a number of techniques [1,2,3]. However, the volatility of TI complicates the growth procedure, and an in-situ process has yet to be demonstrated. Although the required oxygen post-anneals are generally short, film/substrate interdiffusion has been found to deteriorate the superconducting properties of thin films deposited on MgO and other substrates [4]. We have investigated the use of BaF2 buffer layers as a diffusion barrier for TlCaBaCuO thin films sputter-deposited onto MgO. The deposition and annealing procedures have been described in detail elsewhere [5]. Briefly, the deposition was performed using a planar dc magnetron sputtering gun and a metal alloy target of composition 1223 (TICaBaCu). The sputtering gas was 10 mTorr Ar, with a target discharge power density of 2 4 W/cm . The deposition rate was 120nm/min, and the total film thickness was 4000 A. The substrates were at ambient temperature during the deposition. The substrate material was single-crystal MgO . Some of the substrates were coated with a 2000 A thick layer of BaF 2 , deposited by resistive-heating evaporation. Although the substrates were not intentionally heated, the asdeposited BaF2 was found to be highly oriented. Rutherford backscattering analysis of the buffer layers verified that the BaF2 was evaporated stoichiometrically. The films were annealed in a tightly covered alumina crucible, placed in flowing 02 within a box oven. A TI over-pressure was maintained by adding TI powder to the crucible. The sample temperature was ramped from ambient to 800'C at a rate of -20'C/min, held for 20 min, and then decreased at a rate of -40'C/min. The sample designations are 800M(800MB) for the films deposited on MgO(BaF2/MgO). A film was also deposited on SrTiO3 , and annealed at 850*C for 18 min, as a benchmark for the deposition procedure. The resistance vs. temperature properties of the a
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