In-situ Deposition of Superconducting MgB 2 Films
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In-situ Deposition of Superconducting MgB2 Films N. Stelmashenko, K.A. Yates, V.N. Tsaneva, M. Kambara, D.A. Cardwell and M.G. Blamire IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE ABSTRACT The discovery of superconductivity in MgB2 has been followed by many papers reporting attractive thin film properties. In most cases these have involved the deposition of precursor films followed by in-situ or ex-situ post annealing in a Mg-rich atmosphere. Although simple device structures have been fabricated from such films, it is desirable for a number of reasons that a heterostructure device technology be developed. Heterostructure growth is likely to require in-situ growth, preferably without post-annealing. To achieve this, low oxygen and high Mg background pressures are required in the vicinity of the sample. By using a novel heater geometry we have been able to grow superconducting MgB2 films from Mg-rich targets at temperatures below 500 °C. This paper reports the growth method, and structural and electrical characterization of the films. INTRODUCTION The recent discovery of the superconducting properties of MgB2 [1], has stimulated an intensive research effort into producing high quality thin films [2-6]. The transition temperature (Tc) of 39K [1] coupled with the high connectivity of the grain boundaries and the long coherence length compared with the cuprate superconductors [7] makes thin film MgB2 a potential material for devices including Josephson junctions [8] and SQUIDs. Pulsed laser deposition (PLD), molecular beam epitaxy (MBE), and magnetron sputtering have all been used to grow thin superconducting films of MgB2. However, to date the films with the highest Tc have been grown by the annealing of a pre-deposited boron film in a Mg vapor [2,3,6]. The high volatility of Mg during annealing at temperatures above 200 °C [9] makes it difficult to grow directly high-quality stoichiometric films. Successful development of heterostructure device applications will require a technique to be developed whereby smooth, homogeneous films with a high Tc are produced in-situ. By using a large Mg:MgB2 flux ratio, Ueda et al. [4], were able to grow untextured MgB2 films in-situ with Tc~36 K by MBE. However, it would be desirable to produce smooth, homogeneous films with high Tc using a technique such as PLD or magnetron sputtering [5]. This paper reports our attempts to reduce the Mg escape rate from the film surface during growth and annealing using pulsed laser deposition from a B/Mg target. We investigated novel heater geometries that reduced the Mg escape rate and hence created a higher Mg vapor pressure at the sample surface. In the first approach we designed a modified substrate heater whereby the geometry constrained the Mg escape from the film during the deposition (Figure 1(a)). The substrate was enclosed inside a baffled heater, with only a narrow aperture for the incoming laser plume potentially generating a higher Mg partial pressure above the sample. The second scheme involved
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