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Epitaxial stabilisation of RE Nickelates

Updated on February 21, 2015
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Epitaxial stabilisation of RE Nickelates

Student: C. Girardot, N. Ihzaz

Objectives
RE Nickelates are normally not stable phases at atmospheric pressure, but they can be stabilised by epitaxy in a thin film form. This stabilisation is related to the generation of a coherent interface which contributes to lower the energy of formation of these phases.The nature of the substrate and essentially the cell parameter misfit between the substrate and the deposited layer play a primary role in the stabilisation. The strain behaviour at the interface can also be able to modify critically the physical properties of the thin films obtained, mainly the position and shape of the metal/insulator transition which is characteristic of these nickelates. 

Main results
RNiO3 (R=Sm, Nd, Sm1-xNdx) thin films have been synthesised by MOCVD at 680°C. on (100) oriented Si, LaAlO3 and SrTiO3 substrates. On Si substrates, the films obtained are a mixture of the elementary oxides Sm2O3 and NiO, since SmNiO3 is not thermodynamically stable under the low pressure conditions used. On LaAlO3, for films thinner than 2000 Å, the nearly perfect match between both lattice cell parameter (LaAlO3 (cubic parameter : a=3.791 Å) and SmNiO3 (pseudo-cubic parameter : a=3.801 Å)) allow the nickelate phase stabilisation by epitaxy. In function of the film thickness the stress is progressively relaxed and for films thicker than 5000 Å, the substrate influence disappear and the elementary oxides issued from the demixion of SmNiO3 start to appear. The type of stabilising substrate used (varying misfit between substrate and film) affects also the microstructure of the layers. On SrTiO3, films have a rough surface with grains of typically 20nm diameter, on LaAlO3 the morphology is characterised by large and flats domains separated by dark zones in SEM imaging. These dark zones are composed by elementary oxides Sm2O3 and NiO, hence the stress relaxation gives rise to a phase demixion at the grain boundaries. Correlation between structural and physical properties in RNiO3. Depending on the substrate used, the RNiO3 thin films have also different resistive behaviour. On LaAlO3 a sharp transition is seen from semiconducting to metal at TIM=390K. This transition is characteristic for the SmNiO3 compound in a bulk form. If the same films (during the same run) are prepared on SrTiO3 substrates there is no Insulator/Metal transition and the resistivity is 2 order of magnitude higher on SrTiO3 than on LaAlO3. Similar results are obtained on NdNiO3. Complementary studies on these films under varying oxygen partial pressure conditions suggest that the high resistivity for films on SrTiO3 is related to an oxygen non stoichiometry. The coexistence of mechanical and chemical effects occurring simultaneously in the layers gives rise to complex phenomena and modifications in the physical properties of RNiO3 thin films. These structural and physical characterisations of RNiO3 thin films have been completed by Raman Spectroscopy studies in a variety of nickelate films in order to analyse in details the structural changes associated to the MI transition. These complementary results are reported by the SPCS group

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Date of update February 21, 2015

Univ. Grenoble Alpes