Resistive-switching
random access memory (ReRAM) technologies are
nowadays a good candidate to overcome the bottleneck of Von Neumann
architectures, taking advantage of their logic-in-memory capability
and the ability to mimic biological synapse behavior. Although it
has been proven that ReRAMs can memorize multibit information by the
storage of multiple internal resistance states, the precise control
of the multistates, their nonvolatility, and the cycle-to-cycle reliability
are still open challenges. In this study, the analog resistance modulation
of Pt/HfO2/Ti/TiN devices is obtained and studied in response
to different programming stimuli, linking the electrical response
to the internal dynamics of the ReRAM cells. The resistance modulation
during RESET operation is explained by the progressive dissolution
of the conducting filament, whose switching kinetics is inspected
in detail, describing the filament evolution during voltage sweep
measurements and under the effect of 1 μs pulses. Exploiting
the gradual nature of the RESET process, which is an intrinsic property
of our devices, a linear resistance modulation over the wide operating
window of 103 is obtained by negative pulse ramping. The
intermediate resistance states are characterized by small spatial
and temporal variability and stable retention over time. To explore
the synaptic long-term plasticity properties, the resistance variation
over 102 consecutive depression–potentiation cycles
is presented and up to 15 discrete distinguishable states are defined
through the evaluation of the maximum step-to-step variability. The
linear resistance modulation over a wide resistance window coupled
with the stable retention of intermediate states represents a fundamental
step forward to enhance HfO2 ReRAM performance in neuromorphic
applications.
Resistive switching (RS) devices are considered as the most promising alternative to conventional random access memories. They interestingly offer effective properties in terms of device scalability, low power-consumption, fast read/write operations, high endurance and state retention. Moreover, neuromorphic circuits and synapse-like devices are envisaged with RS modeled as memristors, opening the route toward beyond-Von Neumann computing architectures and intelligent systems. This work investigates how the RS properties of zinc oxide thin films are related to both sputtering deposition process and device configuration, i.e. valence change memory and electrochemical metallization memory (ECM). Different devices, with an oxide thickness ranging from 50-250 nm, are fabricated and deeply characterized. The electrical characterization evidences that, differently from typical nanoscale amorphous oxides employed for resistive RAMs (HfO x , WO x , etc), sub-micrometric thicknesses of polycrystalline ZnO layers with ECM configuration are needed to achieve the most reliable devices. The obtained results are deeply discussed, correlating the RS mechanism to material nanostructure.
Tungsten (W) is one of the most promising materials to be used in resistive random‐access memory electrodes due to its low work function and compatibility with semiconductors, which raises the possibility of device integration, scalability, and low power consumption. However, W has multiple oxidation states that affect device reliability, due to the formation of semistable oxides at the switching interface. W chemical interaction is modulated through the insertion of Al2O3 or Ti interfacial layers. The time‐dependent switching kinetics are investigated in transient Set/Reset operations. It is observed that a compact and stoichiometric atomic‐layer‐deposited Al2O3 barrier layer completely prevents W oxidation, resulting in a sharp current transient. The use of a sputtered Ti buffer layer allows a partial W oxidation, defining a tunable high‐resistance state by pulse rise time control. Notable improvements in endurance, power consumption, resistance state stabilization, and cycle‐to‐cycle and device‐to‐device variability are reported. Switching kinetics and conductive nanofilament evolution are studied in detail to understand the microscopic effect of the interface modifications. The tunability of multi‐HRS states by pulse timing control in Pt/HfO2/Ti/W is in the interest of network and brain‐inspired computing applications, adding a degree of freedom in the modulation of its resistance.
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