Yba 2 Cu 2 O 7-X , Thin Films Prepared by Multilayer Deposition
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YBa 2Cu.,O,, THIN FILMS PREPARED BY MULTILAYER DEPOSITION VALENTIN GARCIA-VAZQUEZ AND CHARLES M. FALCO Department of Physics, Optical Science Center and Arizona Research Laboratories, University of Arizona, Tucson, AZ 85721 INTRODUCTION Various sputtering techniques have been developed for depositing YBa2Cu 3OT- thin films.[1] In this paper we report details of a multilayer deposition technique using a combination of sputtering and evaporation, followed by an ex situ furnace annealing. SAMPLE DEPOSITION We installed[2] a thermal evaporation source containing barium fluoride[3] in a sputtering system with two magnetically-enhanced dc triode sputtering guns[4] containing separate metal targets of yttrium and copper. We operate all three sources simultaneously as the substrate is sequentially rotated 12.7 cm above each source by a servo-controlled motor. The angular velocity of the substrate holder above each source is independently calibrated for each source to adjust the deposition thickness of each layer. All deposition rates (approximately 10 A/sec) are kept constant from run to run. The base pressure prior to each deposition is in the low 10- torr range and the combination of sputtering and evaporation is done in a pure argon environment, without oxygen, at a pressure of 5.6 mtorr. Each deposition consists of passing the substrates over the three sources to form sequential layers of copper, yttrium and barium fluoride of respective thicknesses approximately 16 A, 15 A, and 60 A. This process is then repeated a number of times to form multilayers of total thickness approximately 3000 A. The overall stoichiometry is adjusted by altering the angular velocity ratios of the substrate above the targets, and is measured by Rutherford backscattering spectroscopy (RBS) of multilayers deposited on carbon substrates.[5] RBS analysis always shows a 1:2 ratio for the Ba:F2, suggesting a molecular BaF 2 evaporation. For the above deposition conditions, the desired 1:2:3 stoichiometry for Y:BaF 2:Cu in the final film is obtained when 1:4:1.06 is used for the thickness ratios (assuming bulk atomic densities for the RBS analysis). ANNEALING After deposition, samples are transferred in air to a conventional annealing furnace. For the results reported here, reaction with oxygen was carried out at various annealing temperatures T, between 850 and 900 C, for annealing times t, from 20 to 210 min., followed by a slow cooling down to room temperature. Oxygen was bubbled through TL=60 C water[61 before being introduced to the furnace during the initial 10 to 30 minutes (tý,) of the annealing procedure to promote the complete removal of fluorine [7]. At all other times of the annealing cycle, only dry oxygen was used. After the high temperature portion of the anneal, additional time at Td= 7 50 C and 550 C was used for some samples to increase the oxygenation of the films. RESISTANCE MEASUREMENTS A closed-cycle refrigerator instrumented for transport properties measurements was used to determine the resistance vs. temperature. The standa
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