Observations on the Growth of YBa 2 Cu 3 O 7 Thin Films at Very High Laser Fluences
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RAND R. BIGGERS.* M. GRANT NORTON**, I. MAARTENSE*, T.L. PETERSON*, E. K. MOSER*, D. DEMPSEY*, AND JEFF L. BROWN*** *U. S. Air Force Wright Laboratory, WL/MLPO, Wright-Patterson AFB, OH 45433 of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164 ***U. S Air Force Wright Laboratory, WL/ELDM, Wright-Patterson AFB, OH 45433
"**Department
ABSTRACT The pulsed-laser deposition (PLD) technique utilizes one of the most energetic beams available to form thin films of the superconducting oxide YBa2Cu3O7 (YBCO). In this study we examine the growth of YBCO at very high 2 2 laser fluences (25 to 40 J/cm ); a more typical fluence for PLD would be nearer to 3 J/cm . The use of high fluences leads to unique film microstructures which, in some cases, appear to be related to the correspondingly higher moveabilities of the adatoms. Films grown on vicinal substrates, using high laser fluences, exhibited welldefined elongated granular morphologies (with excellent transition temperature, Tc, and critical current density, Jc). Films grown on vicinal substrates using off-axis magnetron sputtering, plasma-enhanced metal organic chemical vapor deposition (PE-MOCVD), or PLD at more typical laser fluences showed some similar morphologies, but less well-defined. Under certain growth conditions, using high laser fluences with (001) oriented substrates, the YBCO films can exhibit a mixture of a- and c-axis growth where both crystallographic orientations nucleate on the substrate surface at the same time, and grow in concert. The ratio of a-axis oriented (aol) to c-axis oriented (ci) grains is strongly affected by the pulse repetition rate of the laser.
INTRODUCTION
There is a number of very different techniques for producing thin-film high temperature superconductors (HTS). These techniques, which cover quite a large range of deposition conditions, entail significant differences in deposition times, deposition areas, growth rates, growth mechanisms, and film qualities. In pulsed-laser deposition (PLD). we have chosen to utilize one of the most energetic, versatile, and quickest production methods for quality 2 HTS thin filns. The laser provides a highly energetic pulse (generally from 108 to 109 W/cm ) which is used to ablate material froin a stoichiometric target and this material is directed towards a heated substrate. The energy density. footprint, repetition rate, duration, spatial and temporal uniformity, and wavelength of the pulse are the variable laser parameters. These laser parameters coupled with a number of non-laser parameters such as chamber oxygen pressure, substrate temperature, substrate material and orientation, and target material strongly influence the properties of HTS thin films. An understanding of the deposition process coupled with proper adjustment and control of these deposition parameters should allow growth of thin film HTS with film properties and microstructures more closely matched to the intended film applications.
EXPERIMENTAL
The PLD process used in our laboratory has bee
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