Reduced splashing effect in laser ablated superconducting thin films formed from a melt-quenched nonsuperconducting amor
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Comparison of superconducting thin films of Bi-Sr-Ca-Cu-O deposited by laser ablation from a melt-quenched, amorphous, nonsuperconducting target and a polycrystalline sintered superconducting target shows that they have similar superconducting properties, but the melt-quenched target yields a much smoother film. Additionally, the melt-quenched target is much easier to prepare with target preparation time reduced by a factor of twenty-five.
I. INTRODUCTION
A major technical difficulty in the preparation of thin films by the pulsed laser ablation technique is the so-called splashing effect,1 in which microscopic particles are sputtered from the source target and deposited intact on the film substrate. These particles, typically 0.1 to 10 fim in size, can seriously reduce the quality of the deposited film. Splashing is especially troublesome in the deposition of high-Tc superconducting thin films by laser ablation as the target is customarily a sintered superconducting metal oxide pellet. These invariably have very rough grainy surfaces from which the aforementioned microscopic particles are readily sputtered by the laser. In addition, these targets often have internal voids, as indicated by a density lower than that of the crystalline solids, and the splashing effect will be exacerbated by the rapid expansion of any gas trapped in these voids during laser heating. Splashing can be reduced by reducing the laser power density. However, splashing remains a problem because relatively high laser fluences are required for the production of the essential high-energy ionic and atomic species in the laser plume. Other ways of minimizing splashing suggested to date2"5 mostly involve increasing the complexity of the deposition apparatus. Here, we discuss and present preliminary results on another, much simpler, approach to reducing splashing by reducing the surface roughness and inhomogeneity of the target. This approach involves using a target prepared by melt quenching a stoichiometric mixture of the starting oxides in the Bi-Sr-Ca-Cu-O (BSCCO) superconductors. In addition to ease of preparation, a)Permanent
address: Istituto di Teoria e Struttura Elettronica e Comportamento Elettrochimico dei Composti di CoordinazioneConsiglio Nazionale delle Ricerche (ITSE-CNR), Area della Ricerca di Roma, 00016 Monterotondo Scalo, Italy.
J. Mater. Res., Vol. 5, No. 10, Oct 1990
http://journals.cambridge.org
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these targets are likely to have a number of advantages over targets prepared by the customary sintering process insofar as minimizing splashing is concerned. One of these is that melt-quenching yields BSCCO in an amorphous state whose compositional and microstructural homogeneities are markedly better than in samples obtained by the conventional method of sintering a mixture of the oxides.6"8 Furthermore, melt-quenching is expected to produce a target whose surface is smoother and relatively free of individual grains as compared to the surface of a sintered target. Finally, the density of the melt-quenche
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