Thin Film Superconducting MgB 2 Grown by MBE without Post-Anneal
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Thin Film Superconducting MgB2 Grown by MBE without Post-Anneal William Jo, Jeong-Uk Huh, Tsuyoshi Ohnishi, Ann F. Marshall, Malcolm R. Beasley, and Robert H. Hammond Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 943054045 ABSTRACT We report the synthesis of superconducting MgB2 thin films grown in-situ by molecular beam epitaxy (MBE). Mg-rich fluxes are deposited with B-flux by electron beam evaporation onto c- and r-plane sapphire substrates. Deposition temperature is varied between 260 ~ 320 oC. Base pressure of the MBE chamber is at low 10-10 Torr, rising to 10-8 Torr during deposition due mostly to the presence of hydrogen and nitrogen. Asgrown MgB2 films show superconducting transition at ~ 34 K with ∆Tc < 1 K. The films on c-plane sapphire substrates exhibit c-axis oriented peaks of MgB2, and full-width at half maximum of 3 degree in their rocking curves. Azimuthal phi-scan of the MgB2(101) peak shows 12-fold symmetric peaks, which is confirmed by selected area diffraction pattern in transmission electron microscopy (TEM). Plan-view TEM shows hexagonal-shaped grain growth with grain size of about 400 Å. INTRODUCTION The discovery of superconductivity in MgB2 at ~ 39 K has attracted much interest in science and technology, since it shows the highest transition temperature among intermetallic compounds.[1] Synthesis of high quality thin films is a critical step towards electronic applications using this new material. Several groups already reported fair quality thin-film preparation of MgB2, which however requires post deposition annealing to produce superconducting MgB2.[2-7] The temperature for this post-treatment is, relatively high, usually between 600 ~ 850 oC. However, as-grown thin films are preferable for Josephson technology because of the need for multilevel structures. Ueda and Naito recently reported the first successful as-grown MgB2 superconducting films.[8] We have essentially confirmed their results, and add new information. Their article reviews the importance of such a development. The method which we and Ueda-Naito use requires deposition temperature only 300 oC or so. This could be an important advantage in device fabrication. EXPERIMENTAL DETAILS We use electron beam evaporation of the pure metals of Mg and B. We monitor the deposition rates using quartz crystal monitor (QCM). The base pressure of the MBE chamber was 2 x 10-10 Torr, although during deposition hydrogen and nitrogen from the
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source and crucible materials increases the pressure into the 10-8 Torr range. Nitrogen was not detected in the samples using XPS. We are guided in our choices of temperature and rates by the article of Liu, Schlom, Li, and Xi.[9] However, we do find some differences, perhaps due to the kinetics of the deposition at ~ 2 Å/sec. Referring to their Fig. 3, we do not find the convenient “window” of Mg-Gas + MgB2. Based on XPS profiles, it is difficult to obtain exactly the MgB2 composition. Too much Mg flux results in MgB2 + Mg solid, and too little results in MgB2 wi
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