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Synergistic Resistive Switching of Perovskite and Silicon Carbide materials for Advanced ReRAM micro Devices

Periodic Reporting for period 1 - PerovSiC (Synergistic Resistive Switching of Perovskite and Silicon Carbide materials for Advanced ReRAM micro Devices)

Reporting period: 2017-07-01 to 2019-06-30

The massive proliferation of connected devices and digital services is generating amazing amounts of new data, already known as the ‘oil’ of 21st century. How to quickly convert these data into useful information and store them efficiently is the current concern. Emerging Non-Volatile Memory (NVM) devices beyond flash memories are receiving substantial attention to cover the current data storage capacity gap, because they combine the non-volatility with a low latency, a high integrated density and low power consumption. In the last few years, NVM devices based on the electrically switchable resistance phenomenon have generated significant interest both in the industry and in the scientific community. Among ReRAM, metal oxide thin films are one of the most promising family of materials due to their compatibility with silicon mainstream fabrication technology, their simple metal-insulator-metal (MIM) structure and the good results of resistive switching (RS) they have already shown. A huge variety of binary metal oxides showing hysteretic RS, such as NiO, TiO2 or HfO2, have been extensively studied. Recently, particular attention has been drawn to the development of new ReRAM multilevel (ML) memories to significantly enhance the storage density, i.e. multiple distinguished states in the metal oxide can be used to store data, and enable new computing paradigms such as the neuromorphic computation. Although binary metal oxides present desirable working performance, their filamentary and stochastic nature (i.e. formation and breakdown of conductive filament paths of oxygen vacancies in the material) leads to a very strong variability, which severely limits their integration in a large matrix and disable their multilevel programming.
Perovskite oxides appear as strong candidates for the development of novel multilevel ReRAM switching devices, since they tolerate large variations in the oxygen content while maintaining their crystal structure. Currently, the ML storage properties of a number of perovskite materials and heterostructures has been successfully demonstrated. Despite the significant advances in perovskite-based ReRAM devices, the underlying RS mechanisms remain unclear and thus constitute a significant barrier to their widespread application . How to tune at best a perovskite resistive switch response or how to precisely control the size/resistance of the nanoscale defects causing the resistance change is still a critical challenge.
One of the major issues that this technology faces is to improve the reliability and stability of the MIM structure. The synthesis of perovskites demands high temperatures (typically above 500°C), which are incompatible with the thermal stability of metals. Metals suffer from quick thermal degradation, known as dewetting, in which agglomeration starts. The morphology of the metal evolves to a discontinuous and rough layer and finally breaks down into isolated particles, losing its functionality (breakdown of in-plane percolation) and leading to serious problems of device reproducibility and reliability. Up to know, the most adapted alternative is the use of an additional conductive oxide. However, this extra oxide film complicates enormously the study of the RS mechanisms, as it can contribute or modify the RS process through the mobility of O2- (or VO..). It is noteworthy to mention that TiN, which is also commonly used as conductive material, is not compatible with the synthesis of perovskites because it easily oxidizes from 450°C . Therefore, the development of new electrode-perovskite-electrode (EPE) structures which are thermally and chemically more stable, and which show an effective blocking to oxygen, is of great interest to control the RS of the functional layer and to be able to deeply investigate the RS mechanisms in perovskites.

The overall aim of the project is to move from fundamental studies of new EPE heterostructures based on perovskite thin films to their direct application as ML ReRAM devices, by:

1) Understanding of the nanoscale mechanisms governing the RS process.
2) Control, tuning and reproducibility of the RS behaviour.
3) Optimization of the RS response.
4) Demonstrating the effectiveness and application of the optimized EPE structures by designing and fabricating a reliable perovskite-based ML ReRAM device as a proof-of-concept.
The project has focused on evaluating the lanthanum manganite, LaMnO3 (LMO) in the thin film form, as a promising memristive material. LMO is able to accommodate a wide range of oxygen stoichiometry, which leads to changes in its electrical and mass transport properties. Different growth strategies by pulsed injection metal organic chemical vapour deposition (PI-MOCVD) have developed to fabricate a feasible LMO-based heterostructure compatible with mainstream microelectronics industry. The physicochemical properties of LMO films have investigated using standard, such as XRD, SEM, TEM, Raman, and EDX. After optimizing the deposition parameters, a set of samples combining LMO films with tuned oxygen content and several metals as top electrodes were micro fabricated using platinized-based silicon substrates. The main steps included were: (i) design the top electrodes in terms of geometry, size and material; (ii) optimize the microfabrication process, and (iii) electrical performance.
These devices were electrical characterized to validate the resistive switching. A filamentary behavior was found, which can be tuned to work in interfacial mode when applying low voltages and using active metallic top electrodes towards oxidation, such as Ti. Next, the physicochemical properties at the different resistance levels of LMO were investigated to understand the switching mechanisms, using cutting-edge techniques, such as ToF-SIMS, XANES, c-AFM and XPEEM. The movement of oxygen combined with the changes in the oxidation state of manganese were determined as drivers of the switching phenomenon. Based on all the findings, a comprehensive model was proposed describing the resistive switching mechanism at the nanoscale and linking both, the electronic and physicochemical changes occurring during the phenomenon. Finally, an optimized configuration was designed, fabricated, and tested as a stable multilevel ReRAM device prototype.
The results of the project has led to unexpected breakthroughs for neuromorphic and memories devices field. The results have widely disseminated via oral communications and publications in top journals. The advanced lead in material science using cutting-edge techniques for RS and novel EPE heterostructures will bring high citation rates as the RS field grows and the technology comes out into the market. To maximize the impact of the project, all the collaborators have acted as “ambassadors” distributing the knowledge and fruitful results among different audiences and disciplines by participating in important national and international conferences. The results produced have also disseminated through a seminar to other researchers that are working in the same or a closely related topic.
Finally, the project was promoted in the Science Festival, a national festival that takes place every year during a week in France. Finally, to enable a successful dissemination of the project a website was created.
Resistive switching of LMO thin films on platinized substrates