Superconducting High T C Thin Films Prepared by Laser Deposition: Comparison of Laser Sources
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SUPERCONDUCTING HIGH TC THIN FILMS PREPARED BY LASER DEPOSITION:
COMPARISON OF LASER SOURCES L. LYNDS, B. R. WEINBERGER, M. P. LINDSAY AND D. M. POTREPKA United Technologies Research Center, E. Hartford, CT 06108 ABSTRACT A comparison of the energetics and stoichiometry of effluents from YBa 2 Cu307 _x targets ablated by Nd:YAG (1064 nm) and excimer (248 nm) lasers is presented. Nd:YAG ablative processes are characterized by specularly directed neutral atom/cluster formation with translational energies in the range of 2 - 20 eV. In sharp contrast, excimer ablation leads mainly to highly forward directed atomic ions with centroid energies between 200 and 400 eV. Analysis of the energetics indicates that different mechanisms are at play but both processes are non-equilibrium in nature.
Laser induced extraction of material from a target has been employed for depositing a wide variety of thin films extending from superconductors to dielectric materials [1]. It is apparent that the nature of deposited thin films is dependent on optical parameters used for evaporating targets such as fluence, far field characteristics of the light source, wavelength or energy of laser radiation and absorption rate of photons by the target solid. Reported, herein, is a comparison of a free-running Nd:YAG (1064 nm) [2,3] and excimer (248 nm) laser with regard to the energetics of effluents ablated from YBa 2 Cu3 07_x targets and the spatial distribution of the stoichiometry of the collected material. Since subsequent reactive collisions with oxidizers such as 02 and NO are required for in-situ formation of HTS thin films [4], it is important to understand the nature of the species that may participate in reactions near or at the collection surface. Time-of-flight mass spectroscopy and retardation potential measurements were used to determine the angular distribution of particle energies. Pulsed laser RETARDATION GRID ablation of YBa Cu3O-, P0 targets was performed in a e cylindrical vacuum chamber " e The described in [2]. relevant experimental geometry is defined in Fig. 1. Radiation was generated
with a Lambda Physik LPX 200 excimer (KrF) laser using either a stable or unstable resonator configuration. Targets were mounted on an externally controlled rotatable stage so that the target angle could be adjusted in the horizontal
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TARGET,
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FIG 1. Experimental Geometry
Mat. Res. Soc. Symp. Proc. Vol. 169. 01990 Materials Research Society
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plane relative to the laser and drift tube entrance. A 90% transmission, voltage-controlled, photo-etched Ni retardation grid apertured the entrance to the drift tube. The mass quadrupole filter was situated on the end of a 200 cm drift tube. An intervening port allowed a Faraday cup to be injected into the flow stream for electron density measurements. The large chamber volume made it possible to assemble arrays 2 of rectangular (1.5 x 3 cm ) quartz collection plates to obtain the spatial distribution of the stoichiometry of the ablated material through subsequent wet chemical analysis usi
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