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