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Structural Characterization of Nb on Sapphire as A Buffer Layer for MBE Growth

Niobium films grown by molecular beam epitaxy on sapphire substrates are among the highest quality ones that thin-film metal science has yet produced. This system is in intense use as a buffer layer for epitaxial growth of other metal thin films, magnetic films and superlattices as well. We studied films of Nb [110] deposited by MBE on Al2O3 [11View the MathML source0] substrates using high-precision X-ray diffraction. Rocking curves of the out-of-plane Nb (110) peak reveal a two-component line shape. The sharper component implies a mosaic distribution an order of magnitude sharper than bulk single crystal Nb and a transverse structural coherence length exceeding 103 nm. The atomic planes associated with the sharp component are exactly aligned with the sapphire (11View the MathML source0) planes, while those associated with the broad component are slightly misaligned. Upon loading the Nb film with a small amount of hydrogen, we find a further, dramatic increase of the lateral coherence length. The resulting mosaic distribution of the sharp component appears to be limited only by that of the sapphire substrate. We will discuss the results in terms of strain relief by hydrogen-induced dislocation motion.

Epitaxially ordered zinc aluminate domains with sub-micron dimensions are formed on bare c-sapphire substrates using a vapour phase method (with vapour generated by carbothermal reduction of ZnO) at various temperatures and growth durations. A zinc aluminate (ZnAl2O4) layer is formed by reaction of the source materials (Zn and O) with the substrate. We observe crystallites with a well-defined epitaxial relationship on the sapphire substrate in addition to polycrystalline material. The epitaxially oriented deposit displays the form of characteristically twinned (singly or multiply) grains of sub-micron dimensions with three variants, consistent with the c-sapphire substrate symmetry. Scanning electron microscopy and transmission electron microscopy studies show that the formation of these grains is associated with the presence of extended defects in the sapphire substrate. Epitaxially ordered grains formed at higher temperatures show a change in the nature of the twin boundaries and epitaxial relations as a function of growth time, attributed to the effects of annealing during growth.


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