MoN-TiN Superconducting Artificial Superstructure Film
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MoN-TiN SUPERCONDUCTING ARTIFICIAL SUPERSTRUCTURE FILM KENJI KAWAGUCHI AND SHIGEMITSU SHIN National Chemical Laboratory for Industry, Tsukuba,
Ibaraki 305, Japan
ABSTRACT We succeeded in synthesizing MoN-TiN superconducting artificial superstructure film (SASF). The structure of MoN layers is estimated to be r-Mo2 N. The behavior of Tc in SASFs is recognized by a proximity effect using adjustable parameters of bulk Tc. Calculated parameters are, however, different from Tc of single layered films. 1. Introduction Many combinations of SASFs have been investigated already. However, very few studies of SASFs composed of ceramic compounds are reported yet. It is important to develop the synthetic and analytic technique of ceramic SASF. Probably the present work is the first example of synthesizing of a ceramic SASF by an alternative reactive deposition method. The first reason why TiN-MoN combination was chosen is that stoichiometric BI-MoN is difficult to synthesize in spite of its predicted high-Tc. Multilayering is one of the effective methods to stabilize such unstable material as BI-MoN. Both MoN and TiN have the same Bi-structure with a small lattice mismatch (TiN=O.423 nm, MoN=O.425 mi [1]). This condition is favorable for epitaxial growth. Secondly, Bl-mono-nitrides and mono-carbides are among the simplest compounds and are suitable for an initial study. As many of them are superconductors with various lattice constants, we can modulate their superconducting properties and crystal structures by changing the constituent without destroying the epitaxial growing condition. In this paper, we report structural properties of MoN-TiN SASFs and preliminary analysis of their superconductivity. 2. Sample preparation and structural properties Samples were prepared by an alternative reactive deposition method. The deposition chamber was first evacuated to 10-9 Torr. Several cleaved MgO used for substrates and preheated at 500'C in single crystal plates were ultrahigh vacuum for two hours. Typical size of substrates is 5x1O mm and the total film thickness is around 100 nm. Source metals, Ti and Mo, were evaporated in an ammonia atmosphere. The pressure of the atmosphere was about Torr. Very slow deposition rate (typically 0.02 nm/s) was chosen to 1xlOD compensate the low atmospheric pressure to some degree. The substrate was maintained at 300'C during the deposition. The optimum substrate temperature to fully nitride the specimen and to minimize the interdiffusion at the interfaces has not been found yet. Further investigation is in progress. Structural properties of MoN-TiN SASFs were studied mainly by X-ray diffraction using a standard diffractometer. Higher-order Bragg peaks at the lowangle and around the MgO(200) peak, as shown in Fig. 1, imply the formation of a well regulated artificial superstructure. Several peaks were also observed around the MgO(400) peak for some samples. Even in the case of the SASF composed of monatomic MoN layers (0.2 nm), higher order reflections at the low-angle and some satellite peaks at
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