A new hybrid two-zone/crucible furnace process for the growth of epitaxial Tl 2 Ba 2 CaCu 2 O 8 superconducting films
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A new hybrid two-zone/crucible furnace process for the growth of epitaxial Tl2 Ba2 CaCu2 O8 superconducting films M. P. Siegal, D. L. Overmyer, E. L. Venturini, F. Dominguez, and R. R. Padilla Sandia National Laboratories, Albuquerque, New Mexico 87185-1421 (Received 2 February 1998; accepted 29 June 1998)
˚ thick are grown on LaAlO3 (100) Epitaxial Tl2 Ba2 CaCu2 O8 (Tl-2212) films ,5500 A substrates using a new hybrid two-zone/crucible furnace process enabling precise control of thallination. This method combines the best features of both conventional crucible and two-zone processing for the first time: superb film properties and reduced handling of hazardous Tl-oxide powders. Single-phase, highly c-axis oriented Tl-2212 films are grown with smooth morphology, Meissner transition ,103 K, and critical current density ,1.1 3 107 Aycm2 at 5 K for twenty consecutive runs without having to change or add to the Tl-oxide source.
There are many reports on the growth of Tl–Ba– Ca–Cu–O high temperature superconducting (Tl–HTS) thin films with Tc . 100 K since 1988.1 The volatility of Tl-oxides at the 700–900 ±C growth temperatures typical of these cuprates makes it difficult to control the formation of single-phase Tl-superconductors2 ; hence, ex situ methods are used more extensively to grow Tl-HTS thin films. Nonsuperconducting precursor films, which may or may not contain Tl, can be deposited by a variety of tools, such as sputtering, pulsed-laser deposition, or chemical vapor deposition. Samples are then placed in a thallination furnace for conversion to a superconducting material. This latter stage differentiates between Tl-HTS growth methods. Two thallination methods emerged for the growth of Tl-HTS films. The most commonly used is the crucible method.3,4 Briefly, a precursor film is placed into a crucible with a Tl-oxide source consisting of a pellet or mixed powder of Tlv Baw Cax Cuy Oz (Tl-vwxy). Tl-oxide sublimates from the source at high temperature and, by a vapor diffusion process, thallinates the precursor film to a given composition. This method can grow all four major Tl-HTS phases as high quality thin films.5 The problem with the crucible method is that Tl-oxide from the source is lost during the process and must be replenished for each growth run. This replacement requires many hours, as the furnace must be allowed to cool for access to the material, and then reheated for the next run. It also requires handling hazardous Tl-oxide for each run. Consequently, the crucible method is not optimal for safe and efficient manufacture of Tl-HTS films. The second method used to grow Tl-HTS films is the two-zone furnace process: Tl2 O3 is placed in a lowJ. Mater. Res., Vol. 13, No. 12, Dec 1998
temperature zone and a precursor-coated substrate in a high-temperature zone. In principle, this method allows for independent control of the substrate temperature, p(O2 ) and p(Tl2 O), each of which is a critical thermodynamic variable for the formation of a given Tl-HTS phase.
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