Growth and Thermal Stability of Thin Niobium Overlayers on (0001) Sapphire Substrates
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OVERLAYERS ON (0001) SAPPHIRE SUBSTRATES
T. WAGNER*, M. LORENZ**, P. A. LANGJAHR*, and M. RUHLE* *Max-Planck-Institut ffir Metallforschung, Institut fur Werkstoffwissenschaft, 70174 Stuttgart, Germany **Deutsche Forschungsanstalt fuir Luft und Raumfahrt, Institut ftir Technische Thermodynamik, 70569 Stuttgart ABSTRACT Thin Nb films were deposited by MBE on (0001)(x-A1203 in UHV. At a substrate temperature of 800'C, reflection high energy electron diffraction (RHEED) revealed that the film grew with the orientation relationship (planes of similar symmetry are parallel to each other): (0001)a-A1203 II (11 l)Nb; [2110]a-A1203 ii [li0]Nb. Investigations with conventional transmission electron. microscopy (CTEM) revealed that the film was not perfectly epitaxial after deposition. Small Nb grains with different orientations were embedded in the epitaxial Nb film. During annealing at temperatures well above the deposition temperature, secondary grain growth of the small embedded Nb grains occured, leading to a different epitaxial relationship between the Nb film and the sapphire: (0001)(x-A120311 (110)Nb; [0110]ox-A1203 I1[001]Nb. INTRODUCTION The physical and chemical properties of metal/ceramic bonds are strongly influenced by the interface between the two materials [1]. Depending on the processing (e.g. diffusion bonding, internal oxidation, evaporation), interfaces with different structures and chemical compositions may develop. Thin Nb films grown under well-defined conditions by molecular beam epitaxy (MBE) on the basal plane of sapphire typically have the following well-known epitaxial orientation relationship (OR I) [2]: (0001)x-A120311 .(1 ll)Nb; [21i0]a-A1203 11 [110 ]Nb. If a Nb film is grown by MBE, the interfacial energy y'(Nb), surface energies 7S of the Nb and sapphire and the strain energy in the Nb, S-m(Nb), which results from the misfit strain, strongly influence the growth. If instead a Nb/sapphire interface is produced by internal oxidation of a NbAI alloy, a different epitaxial orientation relationship between cc-A1203 precipitates and Nb matrix (OR II) will develop [3]: (0001)(x-A1203 I/(110)Nb; [ 0110] aA1203 11 [001]Nb. In contrast to OR I, in OR l1 the close-packed planes of Nb and sapphire are parallel to each other. During the initial growth stage of this orientation, the interfacial energy of a phase boundary between (x-A1203 precipitates and the Nb matrix is of great importance in determining a precipitate orientation. It is obvious, then, that different driving forces are responsible for evolution of the two interfacial orientations OR I and OR II. Unfortunately, the processing temperatures are very different. MBE Nb films are typically grown at substrate temperatures around 900'C, whereas the internal oxidation has been carried out at 1500'C. Consequently, it is of fundamental interest to investigate why the high temperature orientation relationship develops under MBE conditions. Therefore, Nb thin films were grown by MBE with OR (I) and annealed at a temperature which corresponds to the te
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