Superconducting Tl Ca Ba Cu O thin films prepared by diffusion of Tl into an excimer laser ablated Ca 2 Ba 2 Cu 3 O x film

Abstract

By laser ablation with an excimer laser (308 nm) cuprate films from a Ca2Ba2CuaOz sintered bulk target were deposited onto SrTiO3 (100) single crystal substrates. Thallium was added by a diffusion process by simultaneously annealing of the cuprate films together with a sintered T12Ca2Ba2Cua01o bulk sample both enclosed in a covered metal box; this film production method allowed to reduce the contamination by thallium. Scanning electron micrographs of the film surface show crystalline structures, i.e. a network of needle shaped crystallites laying in the film plane. For comparison, Nd:YAG laser produced films show rough surfaces consisting of clusters of grains.Thin films of T1-Ca-Ba-Cu-O compounds have been produced by several groups using electron beam evaporation [1] and sputtering techniques [2][3][4]. The use of thallium leads, however, to contamination of the film deposition system. This has been partially avoided by a film preparation procedure [5]: First, a Ca-Ba-Cu-O film was prepared by sputtering, then, by a heat treatment, thallium was diffused into the film from a metallic piece of thallium. Films with T~ =° values of 75 K were obtained by this technique [5]. We have recently developed a technique [6] where thallium was brought into the film by a diffusion technique, too; a Nd:YAG laser was used for preparation of the Ca-Ba-Cu-O film. In the work described here we have used an excimer laser, thus reaching a largely improved film quality.The film deposition system [7] consisted of a vacuum chamber with a base pressure of 5 x 10 -8 Torr. The excimer laser beam (308 nm, pulse duration 40 ns, pulse energy 450 m J) was directed by a quartz optic into the vacuum chamber and focused onto a rotating Ca2Ba2CuaOx sintered bulk target. The energy density on the target was about 10 J/cm 2. The SrTiO3 (100) single crystal substrates were heated to 400°C during the deposition procedure. To obtain structurized films a mask could be aligned in front of the substrate. Bridgelike structures could be easily obtained and were used for bolometric applications in optical response measurements.In the next preparation step the film was loaded with thallium by a diffusion procedure: A standard 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland) Tl2Ca2Ba2CuaOao bulk sample was placed upon the film, a stainless steel ring was used as a spacer between film and bulk sample. The whole arrangement was placed into a small covered stainless steel box, and was heated for about 5 min in a preheated furnace to 880°C, then taken off and cooled to room temperature.

Effect of annealing temperature on the structural-microstructural and electrical characteristics of thallium bearing HTSC films prepared by chemical spray pyrolysis technique

“…Because of their high T c (~ 125 K) and high J c , they are readily amenable to operation at liquid nitrogen temperature (~ 77 K). Uptil now, various methods for the preparation of HTSC thin films such as electron beam evaporation (Ramesh et al 1990;Ginley et al 1998a,b;Sugise et al 1998), sputtering (Lengfellner et al 1989;Lin and Wu 1989;Subramanyam et al 1990;Saito et al 1991;Ichikawa et al 1998;Zhou et al 1998), laser ablation (Betz et al 1989;Johs et al 1989), thermal flash evaporation (Barboux et al 1988;Ece and Vook 1989;Verma et al 1989), chemical spray pyrolysis (Kawai et al 1987;Gupta et al 1988;Saxena et al 1988;DeLuca et al 1991;Verma et al 1992) and spin coating (Rice et al 1987) etc have been employed. Out of these techniques, the simple chemical process like spray pyrolysis and spin coating are economically viable.…”

Nernst effect in Tl-Ba-Ca-Cu-O superconducting films due to infrared laser heating

“…The measurements were carried out on polycrystalline T1-BaCa-Cu-O superconducting films of about l#m thickness showing transition temperatures near 100 K. The film preparation procedures and details on the morphology of the films were described elsewhere [6,7]. The films were deposited on 10 • 10 x 1 mm 3 SrTiO3 substrates and had bridge-like patterns.…”

Superconducting Energy Gap from Infrared Response Measurements in Granular Tl 2 Ba 2 CaCu 2 O 8 Films

“…The resulting films were polycrystalline with random orientation of grains. Details of the film preparation and characterization are described elsewhere [14]. The films showed an onset of resistive current flow at about 40 K tor vanishingly small current, decreasing with rising bias current.…”