Characterization of films prepared by laser ablation of precursor targets andex situthalliation

Abstract

Thin films of Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) were grown on LaAlO 3 (001) single crystals by excimer laser ablation using Tl-free Ba 2 CaCu 2 O x targets and subsequent ex situ thalliation. The thalliation process of the film precursor (Ba 2 CaCu 2 O x ) took place in an Al 2 O 3 crucible containing the precursor and Tl-2212 powder. The procedure was carried out at a temperature T a = 820-870 • C and under a reduced oxygen pressure p(O 2 ) = 20-400 Torr. The resulting Tl-2212 films were characterized by XRD, AES, EDAX, ECP, Raman spectroscopy and a.c. susceptibility measurements. The films exhibited epitaxial, c-axis-oriented growth with critical temperatures T c = 98-106.5 K, transition widths T c = 0.5-1 K, and critical current densities at 77 K of j c = (1.2-3.5) × 10 6 A cm −2 . EDAX measurements showed that the films were as a rule somewhat Tl and Ba deficient but Ca rich (Tl 1.7 Ba 1.9 Ca 1.1 Cu 2 O 8 ). AES depth profiling revealed a noticeable depth dependence of the Ca and Ba distribution, though the combined concentration of these elements (Ba + Ca) was fairly constant throughout the whole film thickness. It is postulated that cation disorder (Ba Ca substitution) is responsible, at least in part, for the high critical current density observed in the Tl films. These point defects would contribute to the strong pinning of the flux lines in the films by creating a dense network of pinning centres.

A defective structure for Tl-deficient Tl2-xBa2CaCu2Oysuperconductors

“…A similar phenomenon was also reported by other groups (Hewat et al 1988, Sequeira et al 1988, Morosin et al 1991a, Chrzanowski et al 1996. According to Aselage et al (1992Aselage et al ( , 1994) the T1-2212 phase can be formed at 850°C by annealing Ba2CaCu20, precursors in T120 vapour in a wide range of pressures.…”

“…However, defects always occur in the T1-2212 structure as have been reported. For example, T1 deficiency has been reported for bulk T1-2212 crystals (Hewat et al 1988, Sequeira et al 1988, Morosin et al 1991a, Aselage et al 1992) and for Ti-2212 thin films (Chrzanowski et al 1996, Zhang et al 1997. Some of the T1 ions could be substituted by Ca (Hiraga et al 1988, Kikuchi et al 1989, Morosin et al 1991a or Cu ions (Sasaki et al 1992).…”

Thallium-based HTS thin films, processing, properties and applications

“…Since P (Tl 2 O) is much higher in equilibrium with Tl For a given Tl HTS phase, P (O 2 ), P (Tl 2 O) and thalliation time there is an optimum thalliation temperature. There is considerable evidence suggesting that reducing the oxygen partial pressure encourages the Tl(2223) phase to form at lower temperatures [46,53,54,63,[66][67][68]. Since reduced P (O 2 ) promotes melting, the reduction in annealing temperature is thought to result from a decrease in the temperature required to produce the liquid phase necessary for Tl(2223) formation [44,53].…”

“…Yan et al [73,74] have fabricated Tl(2212) on LaAlO 3 substrates at temperatures between 720 and 760 • C by thalliating in pure Ar. Ex situ thalliation anneals in Ar atmospheres have also been used by O'Connor et al [75] to fabricate Tl(2212) thin films on LaAlO 3 substrates at temperatures of between 720 and 740 • C. Chrzanowski et al [66], in their study of 2212 film growth, found that films thalliated using P (O 2 ) values in the centre of the Tl(2212) stability region had the highest values of T c but that, for the highest values of J c , conditions near the Tl(2212)-Tl(2223) boundary were optimum. They studied the correlation between the superconducting properties of the films and the degree of cation disorder using Auger electron spectroscopy.…”

“…When the crucible is closed, but not sealed, some Tl 2 O vapour will leak from the crucible during thalliation and thus precise control over P (Tl 2 O) and P (O 2 ) is difficult to achieve. This is the most commonly used method of fabricating thin films of the Tl-based HTS materials [6,46,53,66,70,[79][80][81][83][84][85] and is ideal for the fabrication of films on both sides of largearea substrates. Films with high values of T c and J c and very low values of R s [62,86] have been fabricated using this technique.…”