Lattice distortion effects on electrical switching in epitaxial thin film NdNiO 3
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(Received 22 May 1995; accepted 12 August 1995) Crystalline thin films of NdNi03 have been epitaxially grown on the (100) face of single-crystal LaA103 substrates. These films exhibit the characteristic reversible change in electrical conductivity with temperature previously observed in bulk polycrystalline material. The temperature of the electrical transition in the epitaxial thin films was lower than reported for the bulk polycrystalline ceramics. This effect is attributed to lattice strains associated with the film processing and interfacial lattice matching constraints.
A number of crystalline oxide materials exhibit sharply nonlinear electrical and optical characteristics with temperature. Examples of these include the phase transition oxides such as VO2 and V203, as well as the copper oxidebased high-ir,. superconductors. Interest in these materials stems not only from their relevance to electrical and optical device applications, but also from their contribution to a fundamental understanding of the band structure and conduction mechanisms of crystalline ceramic oxides. Recently, electrical switching has been demonstrated in a class of rare-earth nickel oxides of the composition ReNiO' (Re = Nd, Pr, Sm, E u ) . ' ~Bulk polycrystalline ceramics of the NdNi03 compound, for example, have shown excursions in electrical resistivity in excess of three orders of magnitude upon ~ w i t c h i n g . ~These .~ materials form in a perovskite structure of the type GdFeO3, characterized by a near-cubic array of corner-sharing NiOh octahedra with the rare-earth cation occupying an interstitial position in the lattice. The comparatively small size of the rare-earth cation introduces an orthorhombic distortion into the structure, as the NiOh octahedra tilt and rotate to accommodate the interstitial atom. One notable feature of the ReNi03 ceramics is the correlation between the lattice distortion and the electrical transition temperature, with the larger cation species exhibiting lower transition temperatures.' This ability to tune the electrical switching temperature is a highly desirable feature in the implementation of nonlinear device structures of these materials. The mechanism of the metal-to-insulator transition in the ReNiO3 materials is attributed to the collapse of a charge transfer gap, and while there is a slight volume change upon switching, there is no alteration in structural symmetry.'-"'-'" This contrasts fundamentally from transition metal oxides, such as V203, for which the band gap is associated with a Coulomb correlation energy,' and whose switching is accompanied by a change in crystallographic symmetry. Fabrication of the ReNi03 materials typically requires processing at high temperatures and high overpressures of 2992
J. Mater. Res., Vol. 10, No. 12, Dec 1995
http://journals.cambridge.org
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oxygen. These conditions maintain the Ni cation in the correct +3 valence state at annealing temperatures sufficient to provide grain growth and full oxygen stoichiometry throughout the b
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