Epitaxial Stabilization in Thin Films of Oxides
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Epitaxial Stabilization in Thin Films of Oxides A.Kaul1, O.Gorbenko1, I.Graboy1, M.Novojilov2, A.Bosak2, A.Kamenev2, S.Antonov1, I.Nikulin2, A.Mikhaylov1, M. Kartavtzeva2 1 Chemistry Department, 2Department of Materials Science, Moscow State University, 119899 Moscow, Russia
ABSTRACT This paper is a survey of recent author’s results obtained by the authors. It shows that many oxides can be obtained as thin epitaxial films in spite of their thermodynamic instability in bulk state at the film deposition conditions. This new synthetic approach, named epitaxial stabilization, is based on free energy gain due to structural coherence at the film/substrate interface. The epitaxial stabilization can be effectively used to enlarge the spectrum of new functional materials.
INTRODUCTION
Disagreement between phase composition and the predictions of equilibrium phase diagrams is not a rare phenomenon in thin film growth, its origin being of kinetic or thermodynamic nature. The formation of some phases (primary or secondary ones) often can occur in thermodynamic conditions (pressure, T, composition) which are far beyond the stability range of the corresponding bulk phase (polycrystalline or single crystal). In our earlier work on MOCVD of high Tc superconducting RBaCu3O7-γ films we reported on the crystallization of orthorhombic BaCu3O4, Ba2CuO3, tetragonal R2CuO4 or cubic R2O3 as secondary phases incorporated in or overlaying the RBaCu3O7-γ matrix at the overall cation composition of the film demonstrating deviation from the strict 1:2:3 stoichiometry (1). These phases could coexist with R123 epitaxial matrix without any chemical interaction during long high temperature annealing runs, which does not correspond to the equilibrium phase diagrams of respective R2O3 – BaO – CuO ceramic systems. Also, at MOCVD of CMR material Nd1-xMnO3 the NdMn7O12 phase, earlier known only as the high pressure phase, was detected in the films strongly deficient in Nd2O3 (2). A review of similar phenomena, namely the changes of phase relations in thin films of binary and more complex oxides, the formation of unexpected polymorphs, or unusual oxidation states, has been recently published (3). In our recent research of epitaxial stabilization we used the phenomenon for the directed synthesis of the unstable compounds in the state of epitaxial films. The latter can be used as an alternative to the lacking single crystal samples for the study of fundamental physical properties or as practical materials. A good example of such ones is bismuth – iron garnet Bi3Fe5O12, otherwise unstable, but stabilized on the (001 or 111)Gd3(Sc,Ga)5O12 substrates (4,5). Ferromagnetic metallic chromium oxide CrO2 suitable for magnetic recording is unstable in respect of oxygen dissociation but can be easily obtained by CVD technique (6) in epitaxial state on isomorphous rutile TiO2 single crystal substrates of various surface orientations or even on fine particle rutile seeds by hydrothermal synthesis (7).
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The anatase, unstable polymorphous form of T
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