Soutenance de Thèse d'Alejandro Schulman; Mémoires résistives non volatiles à base de jonctions métal-oxyde complexe

Mémoires résistives non volatiles à base de jonctions métal-oxyde complexe Keywords : resistive switching, memristor, oxygen diffusion, time relaxation, fatigue effects. Mots clés

Thèse d'Alejandro Schulmann
Directeurs de thèse : Michel BOUDARD (
LMGP) et  Carlos ACHA (Professeur à l’Université de Buenos Aires)
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Abstract : Mémoires résistives non volatiles à base de jonctions métal-oxyde complexe

Resistive Random Access Memories (RRAM) have attracted significant attention recently, as it is considered as one of the most promising candidates for the next generation of non-volatile memory devices. This is due to its low power consumption, fast switching speed and the ability to become a high density memory compatible with the conventional CMOS processes.
The working principle of this kind of memories is the resistive switching (RS) which is simply the controlled reversible change in the resistivity of a junction generated by an external electric field. It has been proposed that the RS is coupled with the migration of oxygen vacancies generating a reversible conduction path inside the oxide. Many experiments have been done to address the switching mechanism during the last decade without any conclusive answer of what is the physical mechanism beneath the RS.
The main goal of the present work it’s to understand the physical mechanism that controls the RS and to point out which are the key parameters that can help improve the performance of the memory devices from a technological point of view.
In this dissertation we report on the studies of the RS in different interfaces metal/oxide where we have utilized gold, silver and platinum as metal andand as complex oxides: YBa2Cu3O7–δ (YBCO), La0.67Sr0.33MnO3 (LSMO) and La0.7Sr0.3CoO3 (LSCO). This oxides have been chosen because all of them are strongly correlated compounds with physical properties strongly dependent of their oxygen stoichiometry. They also have a similar crystalline structure (perovskite type) and a high oxygen mobility.
We realized the proof of concept for each type of junction successfully and effect and explained the RS utilizing an electric assisted diffusion of oxygen vacancies model. We characterized then the conduction mechanism of the junctions with a conduction dominated by the Poole-Frenkel effect in the YBCO and by the SCLC mechanism in the LSCO. The feasibility of the memory devices in this junctions have been tested reaching high repeatability with optimize power consumption with more than 103 successful switching events.
We have also studied the effects of accumulating cyclic electrical pulses of increasing amplitude on the non-volatile resistance state of the junctions. We have found a relation between the RS amplitude and the number of applied pulses, at a fixed amplitude and temperature. This relation remains very similar to the Basquin equation use to describe the stress-fatigue lifetime curves in mechanical tests. This points out to the similarity between the physics of the RS and the propagation of defects in materials subjected to repeated mechanical stress. This relation can be used as the basis to build an error correction scheme.
Finally, we have analyzed the time evolution of the remnant resistive state in the oxide-metal interfaces. The time relaxation can be described by a stretched exponential law that is characterized by a power exponent close to 0.5. We found that the characteristic time increases with increasing temperature and applied power which means that this is not a standard thermally activated process. The results are a clear evidence of the relation between RS and the diffusion of oxygen vacancies on a surface with a temperature-dependent density of trapping centers, which may correspond, physically, to the diffusion along grain boundaries.

M. Pablo Ignacio FIERENS
Professeur à lnsituto Tecnológico del Buenos Aires (Président)
M. Adrián Néstor FAIGÓN
Professeur à l’Université de Buenos Aires (Membre)
M. Rodolfo Alberto BORZI
Professeur à l’Université de La Plata (Membre)
M. Mariano Horacio QUINTERO
Chargé de recherche, CONICET (Membre)
Directeur de recherche CNRS à l'Institut Néel, Grenoble (Membre)
Chargé de recherche CNRS, LMGP- Grenoble (Directeur de thèse)
M. Carlos ACHA Professeur à l’Université de Buenos Aires (Directeur de thèse)