La2NiO4+d, a Mixed Ionic-Electronic Conductor for Interface-Type Valence Change Memories

Abstract : This thesis is focused on the understanding and development of novel materials for valence-change memories (VCMs), a type of resistive switching memories in which the memory storage mechanism is based on internal redox reactions. VCMs are in essence electrochemical systems. Their implementation in integrated electronic circuits relies on a voltage (or current) to measure and operate the memory, but their functionality is highly dependent on the chemical properties of the materials constituting the memory. In this work we present how the mixed ionic-electronic conducting La2NiO4+δ compound offers an interesting playground for VCM applications due to its intrinsic bulk oxygen-ion conducting properties. We have successfully prepared La2NiO4+δ in the form of highly oriented thin films on SrTiO3 single crystal substrates using pulsed-injection chemical vapour deposition (PiMOCVD). Post-annealing treatments in oxidizing/reducing atmospheres allow tuning the oxygen content and the p-type semiconducting properties of La2NiO4+δ due to a self-doping mechanism. The obtained oxygen over-stoichiometry in the 0 ≤ δ ≤ 0.08 range induced a variation of the film resistivity between 5.7 Ω.cm and 5.3x10-3 Ω.cm for hydrogen or oxygen-annealed samples, respectively. The optimized La2NiO4+δ thin films have been used as a base for the microfabrication of metal/La2NiO4+δ/metal heterostructures. The important role of the metal/oxide junction in interface-type VCMs is discussed in detail. In particular, an ohmic contact is obtained with La2NiO4+δ when using a high work function metal such as Pt, while rectifying contact properties are obtained when using Ti due to the presence of a spontaneously-formed TiOx interlayer (~8 nm) at the Ti/L2NO4 interface. An asymmetric Pt/La2NiO4+δ/Ti heterojunction has been selected as a first prototype to assess the memory capabilities of a La2NiO4+δ-based memristive device. A continuous bipolar analogue-type memory behaviour has been measured, together with strong multilevel programing capabilities when operated in pulsed mode. In addition, the promising results offered by this prototypical device have been extended for the first time to La2NiO4+δ/LaNiO3 bilayers, showing memory relaxation properties, which are potentially interesting for short-term memory and filtering applications in neuromorphic-based computational hardware.
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Klaasjan Maas. La2NiO4+d, a Mixed Ionic-Electronic Conductor for Interface-Type Valence Change Memories. Electric power. Université Grenoble Alpes, 2019. English. ⟨NNT : 2019GREAI012⟩. ⟨tel-02160021⟩

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