Process optimization for the fabrication of Tl 2 Ba 2 Ca 2 Cu 3 O 10 thin films

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Process optimization for the fabrication of Tl2 Ba2 Ca2 Cu3 O10 thin films W. L. Holstein and L. A. Parisia) DuPont Central Research and Development, Experimental Station, P.O. Box 80356, Wilmington, Delaware 19880-0356 (Received 22 June 1995; accepted 20 January 1996)

A process has been developed for the fabrication of nearly single phase superconducting Tl2 Ba2 Ca2 Cu3 O10 thin films on (100) LaAlO3 substrates with a superconducting transition temperature Tc of 120 K and low microwave surface resistance at temperatures up to 110 K. Amorphous BaCaCuO precursor films were first deposited by rf magnetron sputtering and then thallinated at elevated temperatures. The double TlO layer phases (Tl2 Ba2 Ca2 Cu3 O10 1 Tl2 Ba2 CaCu2 O8 ) formed preferentially over the single TlO layer phases (TlBa2 Ca2 Cu3 O9 1 TlBa2 CaCu2 O7 ) at high Tl2 O partial pressures. Thin films containing Tl2 Ba2 Ca2 Cu3 O10 and a small amount of CuO were prepared from Cu-rich precursor films (CuyBa . 1.7), while lower Cu content led to the formation of Tl2 Ba2 CaCu2 O8 as a secondary phase. Tl2 Ba2 Ca2 Cu3 O10 film epitaxy was enhanced by carrying out the thallination in reduced oxygen partial pressures of 0.01–0.05 atm. Following the thallination step, the Tl2 Ba2 Ca2 Cu3 O10 thin films had a superconducting transition temperature Tc of only 106 6 4 K. An additional 62 h anneal at 800 ±C or an 8 h anneal at 850 ±C in a Tl2 OyO2 atmosphere increased the Tc to 120 K. The increase in Tc was accompanied by a decrease in the c-axis lattice constant, an enhancement in the long-range order in the c-direction, and the formation of a small amount of Tl2 Ba2 CaCu2 O8 as a secondary phase. Minimization of surface resistance at high temperature (95–110 K) requires that the fraction of Tl2 Ba2 CaCu2 O8 secondary phase in the films be kept low. Process routes are also described for the formation of nearly single phase TlBa2 Ca2 Cu3 O9 and TlBa2 CaCu2 O7 thin films and the formation of a new ordered intergrowth phase, Tl4 Ba4 Ca3 Cu5 O18 , which consists of alternating Tl2 Ba2 CaCu2 O8 and Tl2 Ba2 Ca2 Cu3 O10 layers.

I. INTRODUCTION

The discovery of superconductivity above 100 K in TlBaCaCuO compounds by Sheng and Hermann,1 followed by the discovery of superconductivity in (Tl, Pb)SrCaCuO compounds at similar temperatures by Subramanian et al.,2 initiated widespread efforts to fabricate these materials in useful forms. The TlBaCaCuO compounds form two homologous series of phases, the single TlO layer family with ideal stoichiometry TlBa2 Can21 Cun O2n13 (n ­ 1–5) and the double TlO layer family with ideal stoichiometry Tl2 Ba2 Can21 Cun O2n14 (n ­ 1,4).3,4 Of these phases, Tl2 Ba2 Ca2 Cu3 O10 exhibits the highest superconducting transition temperature Tc . Values for Tc up to 127 K have been reported for small quantities of specially processed powders.5–8 While other TlBaCaCuO phases are being developed for applications, little progress has been made a)

Present address: DuPont Photomask, Poughkeepsie, New York.

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