In recent years manganites with tailored functional properties have attracted special attention for their use in miniaturized devices such as resistive switching (RS) memories, spintronic sensors, micro solar cells and micro solid oxide fuel cells [1]. When prepared in the form of thin films (from a few to around a hundred nanometers) their functional properties can largely vary in comparison to their intrinsic bulk properties. There is thus a large interest in understanding and controlling the influence of different parameters, such as epitaxy, substrate-induced strain and grain boundary effects, for the envisioned use of manganites in applied functional devices, and in particular for non-volatile resistive random access memories (ReRAM).

In this work we have focused on the study of LaMnO_3±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) [2]. 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 LaMnO_3+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. V_O^..). The best RS response is obtained for LaMnO_3+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.

 References

[1] D. Pla, C. Jimenez and M. Burriel, Adv. Mat. Interfaces (2017), DOI: 10.1002/admi.201600974, in press

[2] Töpfer J, Goodenough JB, J Solid State Chem (1997) 130 (1) 117–28

Figure 1 a) Transmision electron microscopy cross-section image of a LaMnO_3+δ film deposited on a platinized Si substrate; b) Raman spectra obtained for an as deposited LaMnO_3+δ film and for the rhombohedral and orthorhombic crystal structures obtained after anneal treatments