Multilevel capability is greatly desired in order to attain high effective memory density. Here, we demonstrate that 3-b per memory cell storage capacity can take place by incorporating small (~3 nm diameter) Pt nanocrystals (NCs) within 45-nm TiO _2-x -based resistive random access memory. Seven low-resistance states as well one high-resistance state were achieved by setting proper compliance current (I _cc ) limit between 2 and 200 μA. In striking contrast, the reference sample without NCs exhibited 2-b multilevel switching at higher operating voltages. The enhanced performance of the Pt NCs embedded sample is ascribed to the preferable creation of small conducting filaments around the NCs with high density of oxygen vacancies, due to the concentrated electric field effect, which improves the uniformity of the switching characteristics regardless of the random nature of switching effect.

The threshold switching effect is considered of outmost importance for a variety of applications ranging from the reliable operation of crossbar architectures to emulating neuromorphic properties with artificial neural networks. This property is strongly believed to be associated with the rich inherit dynamics of a metallic conductive filament (CF) formation and its respective relaxation processes. Understanding the origin of these dynamics is very important in order to control the degree of volatility and design novel electronic devices. Here, we present a synergistic numerical and experimental approach in order to deal with that issue. The distribution of relaxation time is addressed through time-resolved pulse measurements whereas the entire switching behavior is modeled through a 2D dynamical model by taking into account the destructive interference of the drift/diffusion transport mechanisms and the Soret diffusion flux due to the intense local Joule heating. The proposed mechanism interprets successfully both the threshold to bipolar switching transition as well as the self-rectifying effects in SiO ₂ -based memories. The model incorporates the effect of electrode materials on the switching pattern and provides a different perception of the ionic transport processes, shading light into the ultra-small lifetimes of the CF and explaining the different behavior of the silver or copper active materials in a conductive bridge random access memory architecture.

The operating principle of resistive random access memories (RRAMs) relies on the distribution of ionic species and their influence on the electron transport. Taking into account that formation and annihilation of conducting filaments (CFs) is the driving mechanism for the switching effect, it is very important to control the regions where these filaments will evolve. Thus, homolayers of titanium oxide with different oxygen contents were fabricated in order to tune the local electrical and thermal properties of the CFs and narrow down the potential percolation paths. We show that the oxygen content in the top layer of the TiO_2−x/TiO_2−y bilayer memristors can directly influence the morphology of the layers which affect the diffusion barrier and consequently the diffusivity and drift velocity of oxygen vacancies, yielding in important enhancement of switching characteristics, in terms of spatial uniformity (σ/μ < 0.2), enlarged switching ratio (∼10⁴), and synaptic learning. In order to address the experimental data, a physical model was applied, divulging the crucial role of temperature, electric potential and oxygen vacancy density on the switching effect and offering physical insights to the SET/RESET transitions and the analog switching. The forming free nature of all the devices in conjunction with the self-rectifying behavior, should also be regarded as important assets towards RRAM device optimization.

The insertion of an HfO _2-y layer within TiO _2-x /HfO _2-y /TiO _2-x resistive random access memory (RRAM) yields in low set power of 50 nW (10 nA at 5 V), low reset power of 3 nW (1 nA at -3 V), and good cycling variability (σ/μ <; 0.5). In addition, under pulse experiments, fast switching time of 1 μs, good 107 cycling endurance and retention performance at 150 °C, was demonstrated. The confinement of the switching effect into the HfO _2-y film, which has the highest oxygen content and deeper oxygen vacancy energy levels compared with the adjusting two layers of TiO _2-x which act as two series resistances, can explain the low switching energy. The gradual modulation of the resistance permits also the manifestation of longterm potentiation synaptic plasticity, induced by the application of a train of pulses with different repetition intervals. A quantitative model was applied in order to reproduce the analog SET/RESET responses of the trilayer configuration and highlight the role of the local distribution of oxygen vacancies. These effects in conjunction with the room temperature fabrication process used and the forming-free nature of the thin films are considered as an optimization route toward high-density RRAM design.

Although multilevel capability is probably the most important property of resistive random access memory (RRAM) technology, it is vulnerable to reliability issues due to the stochastic nature of conducting filament (CF) creation. As a result, the various resistance states cannot be clearly distinguished, which leads to memory capacity failure. In this work, due to the gradual resistance switching pattern of TiO_2−x-based RRAM devices, we demonstrate at least six resistance states with distinct memory margin and promising temporal variability. It is shown that the formation of small CFs with high density of oxygen vacancies enhances the uniformity of the switching characteristics in spite of the random nature of the switching effect. Insight into the origin of the gradual resistance modulation mechanisms is gained by the application of a trap-assisted-tunneling model together with numerical simulations of the filament formation physical processes.

In this work, we demonstrate that TiO_2−x based Resistive Random Access Memory devices can function without an initial electroforming process and a wide range of switching ratios could be achieved by controlling the oxygen content, the compliance current, the sweep bias amplitude, and the width of the voltage pulse applied on the memory cell. The influence of deposition ambient and more particularly of oxygen flux during thin film sputtering at room temperature to the resistive properties of titanium oxide will be discussed in detail. By controlling the density of oxygen vacancies into the dielectric matrix, we can also improve the repeatability and the operation of the device, in terms of distribution of the SET/RESET voltages. We propose that ultra high density of vacancies deteriorate the switching phenomenon, whereas high vacancy density results in better switching behavior. Moreover, we conclude that the oxygen vacancies density and distribution have a direct impact on the conducting filament diameter, in terms of sensitivity of the conducting paths (high OFF/ON ratio). By increasing the oxygen content, we reduce the size of vacancy based filaments, resulting in a more stable operation of our device. In addition, manipulation of population of oxygen ions into the Ti top electrode enables the creation of multilevel switching states. Switching speed, endurance, and retention performance reveals the excellent functionality of our device as a non-volatile memory element and conduction mechanism analysis demonstrates the manifestation of Poole-Frenkel emission in conjunction with trap-assisted tunneling, which is also deployed in order to interpret the gradual increase of current during SET process.