The reaction sequence and microstructure evolution of an MgB 2 layer during ex situ annealing of amorphous boron film
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Ho-Nyun Lee LG Electronics Institute of Technology, Seoul 137-724, Korea (Received 12 July 2003; accepted 8 October 2003)
The reaction sequence and microstructure evolution of a crystalline MgB2 layer were examined during ex situ annealing of evaporated amorphous boron (a-B) with Mg vapor. Mg was found to migrate rapidly into the a-B layer in the initial stage of reaction with a uniform concentration of about 12 at.%. A thin layer of crystalline MgO was observed at the interface between a-B and the Al2O3 substrate. It was identified that an MgB2 layer started to form at the surface by the nucleation and growth process in polycrystalline form. It appears that there exists two distinct growth fronts in the MgB2 layer: one lying at the surface and the other lying at the interface between the MgB2 layer and the a-B. The microstructural evolution of this layer showed significant differences depending on the location of these two growth fronts.
Since the discovery of superconductivity in MgB2 by Nagamatsu et al.,1 many researchers have become interested in growing thin films of MgB2 to fabricate Josephson junction devices. Among the various methods used so far, the two-step method of amorphous boron (a-B) deposition and postannealing at a temperature above 800 °C under Mg vapor has produced the best-quality films. Unfortunately, however, the films made by this process usually develop rough surfaces detrimental to the operation of thin film devices. It is believed that the formation of rough surfaces during the two-step process is closely related to the microstructure evolution of the MgB2 layer during postannealing. Paranthaman et al.2 and Zhai et al.3 have reported that MgB2 is formed by nucleation and growth processes that result in a polycrystalline grain structure at the surface. Epitaxial MgB2 layer formation is thus unlikely if the layer is formed by the postannealing of the existing a-B layer. However, Bu et al.4 and Tian et al.5 reported epitaxial MgB2 layer formation on an Al2O3 substrate by the two-step process after annealing the as-deposited a-B layer at 800 °C for 5 h. In these reports, they speculate that the layer grows at the interface with a specific crystallographic orientation with respect to the substrate
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 2, Feb 2004
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[(11-20) MgB2//(10-10) Al2O3]. Thus, the detailed reaction sequence and the microstructural evolution during the two-step process of MgB2 formation is still not clear. Therefore, in this communication we systematically investigate the initial stage of MgB2 layer formation during postannealing of an as-deposited a-B layer under Mg vapor and also investigate the microstructure evolution as a function of annealing time. a-B films were deposited on a 1 × 1 cm Al2O3(0001) substrate by electron-beam evaporation at room temperature with a thickness of about 300 nm. Details about the growth conditions have been reported in another paper.6
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