In this work we have focused on the study of LaMnO3±δ(LMO) as switching sandwiched material for new oxide memristors. This perovskite oxide, in which both oxygen and cation vacancies can exist, presents a rich phase diagram. A change from orthorhombic to rhombohedral structure can be achieved by increasing the temperature or varying the oxygen stoichiometry (from δ<0.06 to δ>0.09) . Furthermore, the resistivity and the insulator-to-metal transition temperature can be tuned by the oxygen, making this manganite a very interesting candidate for resistive switching devices.
Epitaxial and polycrystalline LaMnO3+δ films of different thickness were prepared by pulsed injection metal-organic chemical vapor deposition (PI-MOCVD) and integrated in thermomechanically stable metal-insulator-metal (MIM) structures. The LMO oxygen content is adjusted either ex situ or in situ, by thermal anneals under Ar or O2atmospheres in the PI-MOCVD reactor. By controlling the substrate, deposition conditions and anneal treatments, either the orthorhombic or the rhombohedral structure could be obtained.
The resistive switching behavior of the films was tuned as a function of the metallic nature of the top electrode, showing very promising results and a large resistance window for operation. Three metals were investigated as top electrodes, i.e. Ag, Ti and Au. In addition, with the aim of relating the functional properties of the films to their structural characteristics, a fundamental study was carried out combining in-situ X-ray diffraction and Raman spectroscopy under different oxygen partial pressures. The structural, compositional and electronic properties of LMO films showed a remarkable influence of the type of defects (cation or anion vacancies), while the La/Mn ratio is kept close to 1. A rhombohedral phase is obtained for films treated under O2, pointing out to a 3+d composition (i.e. cation vacancies) whereas films treated under Ar showed an orthorhombic phase, linked to a 3-d oxygen content (i.e. VO..). The best RS response is obtained for LaMnO3+δ(oxygen over-stoichiometric), which shows a reproducible switching behaviour with a high programming window (up to 3 orders of magnitude) using low set/reset voltages for all three top metal electrode configurations.
 D. Pla, C. Jimenez and M. Burriel, Adv. Mat. Interfaces (2017), DOI: 10.1002/admi.201600974, in press
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Figure 1 a) Transmision electron microscopy cross-section image of a LaMnO3+δ film deposited on a platinized Si substrate; b) Raman spectra obtained for an as deposited LaMnO3+δ film and for the rhombohedral and orthorhombic crystal structures obtained after anneal treatments