Characterization of Ground State Neutral and Ion Transport During Laser Ablation of 1:2:3 Superconductors by Transient O
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CHARACTERIZATION OF GROUND STATE NEUTRAL AND ION TRANSPORT DURING LASER ABLATION OF 1:2:3 SUPERCONDUCTORS BY TRANSIENT OPTICAL ABSORPTION SPECTROSCOPY DAVID B. GEOHEGAN AND DOUGLAS N. MASHBURN Solid State Division, Oak Ridge National Laboratory, Oak Ridge TN 37831-6056. ABSTRACT Optical absorption in the plume produced by excimer laser ablation of YlBa2Cu307-x under film growth conditions has been observed for the first time and used to characterize the transport of ground state Y, Ba, and Cu neutrals as well as Y+ and Ba÷ ions. Spatially and temporally resolved absorption measurements (0.6 mm, 20 ns resolution) indicate significant ground state number densities at times following the laser pulse that are up to an order of magnitude longer than the duration of fluorescence from excited states. Time-of-flight absorbance profiles result in velocity distributions that are broadened significantly toward lower velocities and reveal a low velocity component (1016 cm- in the white plasma close to the pellet have been estimated using spectrally broadened emission lines. The effects of oxygen ambient pressures and the detection of YO, BaO, and CuO also are reported. This technique is applicable as an situ monitor of the kinetic energy of ablated species during low temperature deposition of epitaxial 1:2:3 superconducting thin films. INTRODUCTION The emergence of laser ablation as the preferred deposition technique for epitaxial, high-Tc superconductor thin films has sparked a renewed interest in the laser-surface interaction and in subsequent gas-phase processes occurring in the plume of material ejected from a solid target. The composition, kinetic energy, internal energy, collisional processes, and plasma properties of the transported laser plume are of interest, with respect to the understanding and control of film stoichiometry, morphology and crystal structure. The laser energy and power densities utilized during pulsed laser deposition (typically < 10 J cm-2 , < 300 MW cm-2 for excimer lasers) typically produce a white plasma close to the target and a visible plume of fluorescence extending several centimeters from the target. Efforts to monitor the transport of the ablated vapor have concentrated upon spectroscopy of the fluorescence from the electronically excited atoms, ions and molecules in the expanding laser plasma. 1-4 Recently, Zheng, et al.1, 2 measured the time dependence of the fluorescence from excited Y, Ba, Cu and 0 atoms in the plume and showed that the velocity distribution of each species could be modeled by a supersonic expansion mechanism characteristic of a free expansion jet, with typical kinetic energies of - 40 eV measured for the species present in a vacuum. However, monitoring excited states by emission spectroscopy relies upon production processes in the expanding plasma to populate the observed states, which may be more representative of the production processes of the plasma than the density of neutrals in the plume. 4 -6 The bulk of the plume actually is thought to be comprised of ground s
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