Raman studies of reactive DC-magnetron sputtered thin films of YBaCuO on MgO
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I. INTRODUCTION The recent discovery of the high Tc metal oxide superconductors1'2 has generated a great deal of excitement because of the many potential applications of these materials. For both scientific and technological reasons, a large effort has been devoted to produce these materials in a thin-film form, especially the YBa 2 Cu 3 0 7 _^ (1-2-3) compound, which has a transition temperature above the boiling point of liquid nitrogen (77 K). Various deposition techniques such as magnetron sputtering, e-beam evaporation, and laser ablation have been employed or developed to make epitaxial films on various substrates.3"6 However, the role of many film processing conditions (target types, substrates, deposition sequence, the annealing temperaturetime profile, etc.) and deposition parameters (substrate temperature, the O2 partial pressure, deposition rate, the thickness of each sublayer, etc.) still are not well understood. Therefore, systematic studies are necessary in order to produce films with reproducible high quality. Two of the most important parameters are the film stoichiometry and the post-deposition oxygen heat treatment. According to the ternary phase diagram of Y2O3, CuO, and BaO in the bulk Y - B a - C u - O system,7"9 YBa 2 Cu 3 0 7 _ x is surrounded with other phases: CuO, BaCuO2, Y2BaCu05, and YBa3Cu20^, which are either insulating or semiconducting. In order to produce high quality films of this compound, the stoichiometry needs to be properly maintained (so as to obtain high Jc and Tc as well as to avoid the formation of a second phase), and the interdiffusion between the substrate and the deposited film needs to be minimized. Heat treatment is an important factor in obtaining the correct phase with the desired oxygen stoichiometry and also in reducing the interdiffusion reaction between the film and the substrate. Raman spectroscopy is an important tool for studying the chemical and physical properties of materials. The pho1312
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J. Mater. Res., Vol. 4, No. 6, Nov/Dec 1989
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non spectrum of the high Tc superconducting YBa2Cu307_^ could yield important information about oxygen content in the annealed film10"12 and on the underlying physics such as the isotope effect.13"15 Another important application of Raman spectroscopy lies in its sensitivity to characterize the various chemical phases.16"18 In this paper, we report systematic Raman studies of superconducting thin films with TC(R = 0) in the range 25 to 90 K. Each film was studied at different locations on the free surface (air/film) and the interface (MgO/film). The correlation between the Raman spectra and the effects of stoichiometry, heat treatments, film crystallinity, homogeneity, and impurities will be presented. II. EXPERIMENTAL Films were deposited by DC-magnetron sputtering from separate metal targets: Y, Cu, and Ba 05 Cu 03 . By moving the substrate platform over the three targets sequentially, multilayer deposition on MgO was carried out at ambient temperature in an atmosphere
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