Species Temporal and Spatial Distributions in Laser Ablation Plumes
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SPECIES TEMPORAL AND SPATIAL DISTRIBUTIONS IN LASER ABLATION PLUMES John W. Hastie, David W. Bonnell, Albert J. Paul and Peter K. Schenck National Institute of Standards and Technology, Gaithersburg, MD 20899, USA ABSTRACT The intermediate species present in laser ablation plumes, particularly those formed during pulsed laser deposition (PLD) of thin films, have been identified for a variety of advanced materials systems. Optical multichannel analysis spectroscopy has been used to monitor the atomic and ionic species present, via their spectral emissions. Molecular beam-sampling mass spectrometry has been used to monitor the molecular species present, in addition to atoms and ions. With both monitoring approaches, temporal and spatial species distribution information has been obtained. Velocity distributions, obtained from the time-dependent mass spectral studies, show the effects of isentropic expansion to be predominant when compared with gasdynamic models of the plume evolution process. Also, the plume structure was found to be particularly sensitive to target elemental distribution. Examples of systems studied include high temperature superconductors, refractory compounds, ferroelectrics and nanostructured magnetic films. INTRODUCTION Vapor plumes produced by the direct interaction of high energy laser radiation with target surfaces have practical application as a material transport medium for the deposition of thin films and, particularly, complex materials [1-4]. During PLD, various physical and chemical interactions can occur in the target, in the vapor or plasma phase, and at the deposited filmvapor and film-substrate interfaces. In the vapor/plasma phase, time- and spatially-dependent phenomena are especially important as they influence the properties of the resulting films. However, such detailed experimental information for individual species has been relatively sparse. Under normal film producing conditions, the local pressures generated are relatively high and expansion into the reduced pressure surrounding region can lead to gasdynamic effects similar to those present in free jet expansion. Because of the complexity of the species distributions, these effects can also be significantly species dependent. In order to obtain species-specific time- and spatially-dependent data on plumes formed in the PLD process, we have exploited two techniques, optical multichannel analysis of emission spectra and time-resolved molecular beam mass spectrometry. We have applied these techniques to a range of laser ablation plumes, including those for high T, ceramic superconductors Ba 2 YCu3 0x and Bi 2Sr 2 CaCu 2Ox; refractory targets of graphite, BN, SiC, TaC, HfO2 , MgO, A12 0 3 , and Y20 3 -stabilized ZrO 2; electronic materials Si, SiO2 , BaTiO3 , and Pb 2ZrTiO 6 (PZT); and for magnetic nanocomposites of Fe3 0 4 + Ag. Plumes from targets of pure components, e.g., Cu, Pb, etc., and the simpler compounds present in these systems, have also been studied to aid in the analysis of the complex plume emission spectra. EXPE
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