Strontium aluminum tantalum oxide and strontium aluminum niobium oxide as potential substrates for HTSC thin films
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I. INTRODUCTION The rise of electronic technology utilizing high temperature oxide superconducting materials requires at the very least the mastering of processing of the epitaxial thin films of high Tc superconducting (HTSC) materials on suitable substrates and the understanding of their properties. Superconductor device applications demand that the substrate materials for HTSC meet several requirements: lattice matching (or atomic structure matching)1 to the HTSC materials for ideal epitaxial films, thermal compatibility in terms of thermal expansion matching over the temperature range of film processing and annealing (>900 K) down to the operating temperature (90 K), low dielectric constants for integrated circuit designs (300) arising from the phase transition near the working temperature (~110 K). A12O3 sapphire single crystal substrates, which have low dielectric constant K 18
J. Mater. Res., Vol. 10, No. 1, Jan 1995
http://journals.cambridge.org
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and low loss at microwave frequencies, require a buffer layer to avoid chemical interactions with the YBCO. One of the substrates, probably the one most currently used, LaAlO3, though good in lattice matching and of reasonable dielectric properties, is heavily twinned and goes through a ferroelastic phase transition at —435 °C.2 Table I lists some of the most commonly used substrates for comparison purposes. The objective of our investigation was to screen various complex oxide materials to identify suitable HTSC substrate materials and to test the feasibility of single crystal growth by utilizing a laser-heated pedestal crystal growth (LHPG) technique. The LHPG technique has several unique features that are of special importance for this task. These advantages include containerless crystal growth and, therefore, the capability of growing refractory materials (that have low dielectric losses in general), capability of growing both incongruently and congruently melting compositions, and rapid growth rates. Fiber geometry also provides one-dimensional dielectric material that may by itself meet microwave antenna requirements for some special device applications.5 The compounds Sr(Ali/2Tai/2)O3 (SAT) and Sr(Ali/2Nbi /2 )O 3 (SAN) were first prepared and tested to learn their crystallographic phases and their melting behavior by the group at AT&T Bell Labs.6 Ceramic samples were identified to have double cell cubic perovskite structure with a = 7.795 A and melting temperatures of 1900 °C and 1790 °C for SAT and SAN, respectively. On the basis of our understanding of the crystal chemistry-dielectric property relationships of various © 1995 Materials Research Society
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R. Guo et al.: Strontium aluminum tantalum oxide and strontium aluminum niobium oxide
TABLE I. Most commonly used high Tc superconductor substrates.
Substrates
Symmetry, lattice
Dielectric constant
constants (A)
K (at RT)
YBa 2 Cu 3 O 7 _ s
Thermal expansion 6 a (X 10 - / ° Q aa = 14
Ortho. a = 3.836 b = 3.883 c = 11.68 (Ref. 3)
= 9 ac = 19
Oi
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