Pulsed Laser Deposition of High T c Superconducting Thin Films: Deposition Physics and in-Situ Processing
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PULSED LASER DEPOSITION OF HIGH Tc SUPERCONDUCTING THIN FILMS: DEPOSITION PHYSICS AND IN-SITU PROCESSING
Rajiv K.
Singh and J.
Narayan.
Department of Materials Science and Engineering North Carolina State University Raleigh, NC 27695-7916 ABSTRACT The pulsed laser evaporation (PLE) technique for deposition of thin films is characterized by a number of unique properties. Based on the experimental characteristics, a theoretical model is developed which considers the formation and anisotropic three dimensional expansion of the laser generated plasma. This model explains most of the experimental features observed in PLE. We have also employed the PLE technique for in-situ fabrication of YBa 2 Cu 3 0 7 superconducting thin films on different substrates in 0 the temperature range of 500-650°C. At temperatures below 600 C, a biased interposing ring between the substrate and the target was found to significantly improve the superconducting properties. The minimum ion channeling yields were between 3-3.5 % for films deposited on (100) SrTiO3 and (100) LaA10 3 substrates. The films exhibit very high critical current 2 densities (Jc) with maximum values exceeding 6.5 x 106 amps/cm for silver doped YBa 2 Cu 3 O7 films on (100) LaAIO3 substrates, and the Jc also varies anisotropically with the magnetic field. INTRODUCTION Although the pulsed laser evaporation technique for thin film deposition was first applied more than two decades ago, it has recently gained prominence in the deposition on new high Tc superconductors[l]. The resurgence of this technique has been primarily due to its unique properties which has resulted in the fabrication of YBa 2 Cu 3 0 7 thin films with the best superconducting properties quoted in the literature[2-4]. The dynamics of the PLE technique, and its deposition characteristics are significantly different from a conventional thermal processes[5-8]. To explain the PLE characteristics, we propose a hydrodynamic model which is based on the formation and three dimensional anisotropic expansion of plasma[9,10]. We also present results on in-situ processing of superconducting thin films by the biased laser deposition method in which the placement of a positively biased ring between the substrate and the target has led to reduction of deposition temperatures to 500*C, however, epitaxial quality of films deteriorate sharply below 5500C. PLE
DEPOSITION PHYSICS To develop the theoretical model one must first analyze the salient experimental features of the PLE process in vacuum. This process is characterized by sharply forward peaked nature, with the deposition occurring approximately perpendicular to the substrate. The spatial thickness variations are of the order of (cos 6)8-12, where, 0 is the angle between substrate normal and the radial vector. Under optimized energy density conditions, very small deviation from stoichiometric composition is observed. The laser evaporated species consists of ionized and neutral species of atoms, molecules, and small clusters of higher atomic masses. The plasma temp
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