Thin films for superconducting electronics: Precursor performance issues, deposition mechanisms, and superconducting pha
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Thin films for superconducting electronics: Precursor performance issues, deposition mechanisms, and superconducting phase formation-processing strategies in the growth of Tl2 Ba2 CaCu2 O8 films by metal-organic chemical vapor deposition Bruce J. Hinds, Richard J. McNeely, Daniel B. Studebaker, and Tobin J. Marks Department of Chemistry and the Science and Technology Center for Superconductivity, Northwestern University, Evanston, Illinois 60208-3113
Timothy P. Hogan, Jon L. Schindler, and Carl R. Kannewurf Department of Electrical Engineering and Computer Science and the Science and Technology Center for Superconductivity, Northwestern University, Evanston, Illinois 60208-3113
Xiao Feng Zhang and Dean J. Miller Materials Science Division and the Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, Illinois 60439 (Received 22 February 1996; accepted 17 December 1996)
Epitaxial Tl2 Ba2 CaCu2 O8 thin films with excellent electrical transport characteristics are grown in a two-step process involving metal-organic chemical vapor deposition (MOCVD) of a BaCaCuO(F) thin film followed by a postanneal in the presence of Tl2 O vapor. Vapor pressure characteristics of the recently developed liquid metal-organic precursors Ba(hfa)2 • mep (hfa hexafluoroacetylacetonate, mep methylethylpentaglyme), Ca(hfa)2 • tet (tet tetraglyme), and the solid precursor Cu(dpm)2 (dpm dipivaloylmethanate) are characterized by low pressure thermogravimetric analysis. Under typical film growth conditions, transport is shown to be diffusion limited. The transport rate of Ba(hfa)2 • mep is demonstrated to be stable for over 85 h at typical MOCVD temperatures (120 ±C). In contrast, the vapor pressure stability of the commonly used Ba precursor, Ba(dpm)2 , deteriorates rapidly at typical growth temperatures, and the decrease in vapor pressure is approximately exponential with a half-life of ,9.4 h. These precursors are employed in a low pressure (5 Torr) horizontal, hot-wall, film growth reactor for growth of BaCaCuO(F) thin films on (110) LaAlO3 substrates. From the dependence of film deposition rate on substrate temperature and precursor partial pressure, the kinetics of deposition are shown to be mass-transport limited over the temperature range 350–650 ±C at a 20 nmymin deposition rate. A ligand exchange process which yields volatile Cu(hfa)2 and Cu(hfa) (dpm) is also observed under film growth conditions. The MOCVD-derived BaCaCuO(F) films are postannealed in the presence of bulk Tl2 Ba2 CaCu2 O8 at temperatures of 720–890 ±C in flowing atmospheres ranging from 0–100% O2 . The resulting Tl2 Ba2 CaCu2 O8 films are shown to be epitaxial by x-ray diffraction and transmission electron microscopic (TEM) analysis with the c-axis normal to the substrate surface, with in-plane alignment, and with abrupt film-substrate interfaces. The best films exhibit a Tc 105 K, transport-measured Jc 1.2 3 105 Aycm2 at 77 K, and surface resistances as low as 0.4 mV (40 K,
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