Resolving voltage-time dilemma using an atomic-scale lever of subpicosecond electron-phonon interaction

Evidence of soft bound behaviour in analogue memristive devices for neuromorphic computing

The development of devices that can modulate their conductance under the application of electrical stimuli constitutes a fundamental step towards the realization of synaptic connectivity in neural networks. Optimization of synaptic functionality requires the understanding of the analogue conductance update under different programming conditions. Moreover, properties of physical devices such as bounded conductance values and state-dependent modulation should be considered as they affect storage capacity and performance of the network. This work provides a study of the conductance dynamics produced by identical pulses as a function of the programming parameters in an HfO2 memristive device. The application of a phenomenological model that considers a soft approach to the conductance boundaries allows the identification of different operation regimes and to quantify conductance modulation in the analogue region. Device non-linear switching kinetics is recognized as the physical origin of the transition between different dynamics and motivates the crucial trade-off between degree of analog modulation and memory window. Different kinetics for the processes of conductance increase and decrease account for device programming asymmetry. The identification of programming trade-off together with an evaluation of device variations provide a guideline for the optimization of the analogue programming in view of hardware implementation of neural networks.

Scalability of voltage-controlled filamentary and nanometallic resistance memory devices

Much effort has been devoted to device and materials engineering to realize nanoscale resistance random access memory (RRAM) for practical applications, but a rational physical basis to be relied on to design scalable devices spanning many length scales is still lacking. In particular, there is no clear criterion for switching control in those RRAM devices in which resistance changes are limited to localized nanoscale filaments that experience concentrated heat, electric current and field. Here, we demonstrate voltage-controlled resistance switching, always at a constant characteristic critical voltage, for macro and nanodevices in both filamentary RRAM and nanometallic RRAM, and the latter switches uniformly and does not require a forming process. As a result, area-scalability can be achieved under a device-area-proportional current compliance for the low resistance state of the filamentary RRAM, and for both the low and high resistance states of the nanometallic RRAM. This finding will help design area-scalable RRAM at the nanoscale. It also establishes an analogy between RRAM and synapses, in which signal transmission is also voltage-controlled.

Distinguishing uniform switching from filamentary switching in resistance memory using a fracture test

Resistance random access memory (RRAM) is a rapidly developing emergent nanotechnology. For practical applications and basic understanding, it is important to ascertain whether RRAM undergoes uniform or filamentary switching, but on this point previous area-scaling studies have often shown ambiguous and conflicting findings. Here we demonstrate a simple test-physically breaking the device into two and studying their characteristics individually-can make a definitive determination. Our experiment on two prototypical RRAMs found that one (the nanometallic memory) switches and conducts uniformly while the other (the filamentary memory) does not. It also probes the statistics of nanofilaments: the resistance statistics of the filamentary memory reveals for the first time a large population of partially developed filaments in addition to the filament that dominates switching. Remarkably, the filamentary memory can also be stress-switched to a lower resistance state during fracture, which is reminiscent of stress-switching of the nanometallic memory and may be taken as direct evidence of electron-phonon interaction in the filaments.

Nanofilament Dynamics in Resistance Memory: Model and Validation

Filamentary resistive random-access memory (ReRAM) employs a single nanoscale event to trigger a macroscopic state change. While fundamentally it involves a gradual electrochemical evolution in a nanoscale filament that culminates in an abrupt change in filament's resistance, understanding over many length and time scales from the filament level to the device level is needed to inform the device behavior. Here, we demonstrate the nanoscale elements have corresponding elements in an empirical equivalent circuit. Specifically, the filament contains a variable resistor and capacitor that switch at a critical voltage. This simple model explains several observations widely reported on disparate filamentary ReRAMs. In particular, its collective system dynamics incorporating the power-law time-relaxation of the variable capacitance can accurately account for the responses of variously sized single-filament HfOx ReRAMs to DC/quasi-static and pulse electrical stimulation, exhibiting Avrami-like switching kinetics and a pulse-rate dependence in on/off voltages.

Forming-free and self-rectifying resistive switching of the simple Pt/TaOx/n-Si structure for access device-free high-density memory application

The search for self-rectifying resistive memories has aroused great attention due to their potential in high-density memory applications without additional access devices. Here we report the forming-free and self-rectifying bipolar resistive switching behavior of a simple Pt/TaOx/n-Si tri-layer structure. The forming-free phenomenon is attributed to the generation of a large amount of oxygen vacancies, in a TaOx region that is in close proximity to the TaOx/n-Si interface, via out-diffusion of oxygen ions from TaOx to n-Si. A maximum rectification ratio of ∼6 × 10(2) is obtained when the Pt/TaOx/n-Si devices stay in a low resistance state, which originates from the existence of a Schottky barrier between the formed oxygen vacancy filament and the n-Si electrode. More importantly, numerical simulation reveals that the self-rectifying behavior itself can guarantee a maximum crossbar size of 212 × 212 (∼44 kbit) on the premise of 10% read margin. Moreover, satisfactory switching uniformity and retention performance are observed based on this simple tri-layer structure. All of these results demonstrate the great potential of this simple Pt/TaOx/n-Si tri-layer structure for access device-free high-density memory applications.