Pulsed Excimer Laser Deposition of YBaCuO and BisrCaCuO Compounds : Material Distribution and Stoichiometry
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PULSED EXCIMER LASER DEPOSITION OF YBaCuO AND BiSrCaCuO COMPOUNDS: MATERIAL DISTRIBUTION AND STOICHIOMETRY CLAUDE FUCHS and ERIC FOGARASSY Centre de Recherches Nucl6aires (IN2P3), Laboratoire PHASE (UPR du CNRS n'292), B.P. 20, F-67037 STRASBOURG CEDEX, France ABSTRACT We investigate, inthis study, the laser evaporation processes for the deposition of YBaCuO and BiSrCaCuO compounds, using a pulsed ArF (193 nm) excimer laser. The deposition parameters, especially those which are relevant to the geometrical configuration and optical arrangement of the experimental set-up and laser irradiation conditions are examined indetails. INTRODUCTION Pulsed laser evaporation from solid targets appears to be a very convenient means to deposit thin films of a wide variety of materials including recently the new high Tc superconductor compounds [1-3]. The nature and the relative concentrations of atomic species and macroscopic size particles leaving the irradiated target surface and their spatial distribution, which are at the origin of the properties and quality of the deposit, depend on the various parameters involved inthe laser-solid interaction. We investigate, inthis study, the laser evaporation processes for the deposition of YBaCuO and BiSrCaCuO compounds, using a pulsed ArF (193 nm) excimer laser. In order to obtain laser fluences required for inducing significant material deposition, the incident beam must be usually focused. We demonstrate inthe first part of this work the strong influence of the beam focalization on the final material distribution. This behaviour has been tentatively correlgted to Phe properties of the plasma formed above the target surface, at high energy densities (>10o W/cm'). Ina second part, the angular variation of the stoichiometry inthe thin films was measured and correlated both on the classical thermal evaporation law and plasma expansion dynamics. Finally, a possible explanation for the ejection of macroscopic particles (=0.1 to 1lam diameter) from the irradiated target is proposed interms of purely mechanical phenomenon which takes place inthe liquid phase. EXPERIMENTAL Bulk superconducting targets of YBa 2Cu307.x and Bi2Sr1.sCa1 2Cu208. , prepared by standard ceramic techniques, were irradiated under vacuum (P= 10- torr), through Tsuprasil quartz window, with a pulsed ArF excimer laser (Lambda Physik EMG 201 MSC) which provides at 193 nm, 300 mJ pulses of 20 ns duration [full width at half maximum (FWHM)] at a maximum repetition rate of 60 Hz. The rectangular excimer laser beam was focused onto the target, under normal incidence or at oblique angle, using different types of cylindrical or spherical quartz lenses inorder to give energy densities ranging between 0.1 and 10 J/cm 2 (power densities comprised between 5 x 106 and 2 5 x 109 W/cm ). In addition, the target is rotated during the laser irradiation inorder to avoid the formation of a crater which is able to modify considerably the angular distribution of evaporated material as already shown ina previous paper [4]. Material was deposi
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