High Resolution Electron Microscopy Studies of Interfaces Between Al 2 0 3 Substrates and MBE Grown NB Films
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HIGH RESOLUTION ELECTRON MICROSCOPY STUDIES OF INTERFACES BETWEEN AL20 3 SUBSTRATES AND MBE GROWN NB FILMS J. MAYER,- J. DURA,** C.P. FLYNN** AND M. RU0HLE* *Max-Planck-Institut ffir Metallforschung, Institut fur Werkstoffwissenschaft, 7000 Stuttgart, FRG "**University of Illinois at Urbana-Champaign, Materials Research Laboratory, Urbana, IL 61801, USA ABSTRACT Single crystal niobium films were grown by Molecular Beam Epitaxy (MBE) on (0001)s sapphire substrates. Cross-sectional specimens with thickness of < 20 nm were prepared so that the Nb/A12 0 3 interface could be investigated by high resolution electron microscopy (HREM). The orientation relationship between the metal film and the ceramic substrate was verified by selected area diffraction: (111)Nb 11(0001)S and [11 0 ]Nb 11[211 0 ]S. The atomistic structure of the interface was identified by HREM. INTRODUCTION Metal/ceramic interfaces exert an important, and sometimes controlling, influence on the performance of engineering materials in many different applications in materials science [1]. A knowledge of the atomistic structure of such interfaces is a prerequisite for an understanding of their properties. Three methods are capable of generating well defined metal/ceramic interfaces: bonding at moderate temperatures and pressures [2], internal oxidation of metallic alloys [3], and evaporation of metals onto clean ceramic surfaces [4]. While with all three methods well defined interfaces could be obtained, the latter method allows control over both substrate material/orientation and overlayer composition. Nb/A12 0 3 serves as an excellent "model" system since Nb and A12 0 3 possess nearly the same thermal expansion coefficients and most thermodynamic quantities are well established for both components. Several HREM studies have been reported concerning the atomistic structure of Nb/A12 0 3 interfaces formed after diffusion bonding [5]. The orientation relationship (OR) between Nb and A12 0 3 is determined by the manufacturing route [6]. While the OR is preset for interfaces prepared by diffusion-bonding, topotaxial or epitaxial OR develops during internal oxidation and epitaxial growth, respectively. During internal oxidation a topotaxial relationship forms between Nb and A12 0 3 [3] so that closepacked planes of both systems are parallel to each other, i.e.: (110 )Nb II(0001)S and [0 11 ]Nb
Ii[01101S (S=sapphire)
(1)
Epitaxial growth of very high quality single-crystalline overlayers of Nb on sapphire has been a subject of recent experiments [4,6]. An OR was determined with a (111)Nb plane parallel to the basal plane of the sapphire [4]:
(111)Nb II(0001)S and [il0]Nb II[21101s
Mat. Res. Soc. Symp. Proc. Vol. 209. @1991 Materials Research Society
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674
The aim of the present study was to characterize by HREM the atomistic structures of Nb/A12 0 3 interfaces formed by MBE growth of Nb layers on a (0001)S sapphire substrate. Quantitative HREM resulted in a determination of the translational state of the two crystals close to the interface and of the
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