Conductance Quantization in Cu/Ta2O5/Pt Resistive Random Access Memory Devices

In this work we investigate the conductance quantization (QC or CQ) phenomenon in the Cu/Ta2O5/Pt resistive random access memory (RRAM) devices. The Ta2O5 film was deposited on Pt-Si substrate using RF magnetron sputtering, followed by patterning of Cu top electrodes. The devices demonstrate robust bipolar resistive switching behavior, with low set and reset voltages (< 1V), and high resistance contrast (~103 ), owing to the electrochemical metallization mechanism. Quantized conduction was observed during the reset operation with well-defined conduction steps. Conductance (G) histograms revealed Gaussian distribution of G/G0 around the integral and half-integral values, where G0 is the fundamental unit of conductance (77.6 μS). The maximum conductance state observed was 310G0. Moreover, the currentvoltage curves of QC reset cycles clearly revealed that multiple conduction states appear at the same current value which opens potential avenues for developing multi-state memory devices based on QC states. The findings in this work contribute to the understanding of quantized conduction in RRAM devices for non-conventional computing applications such as in-memory and neuromorphic computation.  </p

Controllable Conductance Quantization in Electrochemical Metallization Based Tantalum Oxide Crossbar RRAM Devices

In the past decade resistance-based memory devices, or the resistive random access memory Devices (RRAM) have emerged as a potential candidate for multi-state memory storage and non-conventional computing applications. Reports on conduction quantization (QC) have added an interesting layer to the utility of these RRAM devices for ultra-dense memory and neuromorphic computing applications due to the occurrence of integral and half-integral conduction (resistive) states. Since the first reports of QC phenomena in RRAM devices, there have been detailed studies on the nature of the conducting filaments, switching mechanisms, and tunability of the QC states, but there exists a scarcity of studies exploring controllability of QC phenomena in scalable device geometries. In this work, we report compliance current controlled tunable QC phenomena in crossbar RRAM cells based on electrochemical metallization switching mechanism. The devices exhibited robust bipolar resistive switching, with well separated high and low resistance states.  The magnitude and number of the QC states were found to increase from ~2.5 to 3.5 and from 4 to 6, respectively as the IC increased from 50 to 200μA. The Cu/Ta2O5/Pt device structure was chosen to strategically govern the metallic nature of the conduction filament (CF) formation, which helped postulating factors contributing to the tunability of the states via compliance current. We report the lateral dimension variability as the main factor governing the magnitude and number of quantized steps observed in RRAM devices, where we also discuss a numerical method to approximate the diameter of the CFs. The increase in number and magnitude of QC steps with IC was explained considering the fact that thicker CF obtained at higher ICC, when undergoes a gradual rupture during reset process, results in larger number of QC steps compared to a thinner CF.</p