Branden Long

Switching Characteristics of

We report the switching characteristics of RRAM devices consisting of Ru/HfO2/TiO2-x/Ru stacks with and without an external access device. In addition to bistable switching, we also achieved an analog reconfiguration of resistance by controlling the compliance current or the reset voltage to achieve a low-resistance state (LRS) or a high-resistance state (HRS), respectively. All intermediate states were nonvolatile in nature. The transport studies using temperature-dependent I-V measurement indicated the mechanism of conduction to be ionic in LRS and Frenkel-Poole in both HRS and virgin resistance state of a device.

\hbox{Ru/HfO}_{2} \hbox{/TiO}_{2-x}\hbox{/Ru}

We report the switching dynamics and charge transport studies on Ru/HfO2/TiOx/Ru resistive random access memory devices in low resistance state (LRS), high resistance state (HRS), and virgin resistance state (VRS). The charge transport in LRS is governed by Ohmic conduction of electrons through local filamentary paths while it is governed by a combination of Frenkel-Poole emission and trap assisted tunneling process in HRS and VRS. The area of the filament in LRS is extracted and related to the compliance current. The thickness of the re-oxidized filament is extracted and related to the reset voltage in HRS. The energy consumed during the reset process was analyzed on the time-scale to experimentally demonstrate joule-heating mediated oxidation dynamics of filament during device reset.

RRAM Devices for Digital and Analog Nonvolatile Memory Applications

Memory is an integral and important component of both general-purpose and embedded systems. It is widely acknowledged that energy of the memory structure is a major contributor in overall system energy. Recent advances with emerging non-volatile memory (NVM) technologies can potentially alleviate the issue of memory leakage power. However, they introduce new challenges and opportunities for dynamic power management in memory. In this paper, we consider resistive random access memory (RRAM), a promising NVM technology, and observe that a specific feature of the memory, namely, its multi-level cell (MLC) structure, can be used to significantly reduce its read access energy. Unlike conventional CMOS static random access memory (SRAM), the read access energy in RRAM largely depend on the stored content. Based on this observation, we present an efficient encoding technique for improving the energy efficiency for multi-level cell RRAM. Our simulation results with benchmark applications demonstrate an order-of-magnitude energy reduction with modest area overhead.

Switching dynamics and charge transport studies of resistive random access memory devices

We report the switching characteristics of Mg-doped HfO2-based ReRAM devices consisting of Ru/ Mg:HfO2/TiN/W stacks. The concentration of the Mg dopant was varied from 0% to 20% for four samples to show the impact on the forming voltage. In addition to reducing the forming voltage from 2.8 to 1.7 V, Mg doping in HfO2 also improved the repeatability in the initial device characteristics, switching characteristics, and retention. The mechanism of conduction was identified as Frenkel-Poole emission in doped and undoped samples in virgin resistance state (VRS). Further analysis showed that the increased conductance in the doped samples in VRS was due to higher carrier concentration.

Content-aware encoding for improving energy efficiency in multi-level cell resistive random access memory

This report presents the charge transport mechanisms in the virgin resistance state (VRS) and breakdown resistance state (BRS) of transition metal oxide memristor devices with tungsten (W) electrodes. The devices behaved as reconfigurable diodes up to ±3.2 V in VRS without the need of any intentional electroforming. The mechanism of conduction in VRS was observed to be governed by tunneling at low temperatures and Frenkel-Poole (F-P) conduction at high temperatures. The BRS was achieved by applying sweep voltages above ±3.2 V after which the device failed to reset. The mechanism of charge transport in BRS was governed by ohmic conduction through defect-assisted localized conduction channels. The barrier height for F-P conduction in VRS and activation energy of defects for ohmic conduction in BRS were experimentally measured.

Effects of Mg-Doping on

In this paper, we report multi-level cell (MLC) switching characteristics of resistive random access memory devices with a W/Zr/HfO2/TiN stack. A multi-step forming technique was implemented in this work which efficiently suppressed the forming current overshoot and allowed device switching at a low set/reset voltage and current. Four distinct resistance states, achieved by controlling the reset stop voltages, showed excellent endurance. Write/read/erase energy values for different states were also calculated. Amongst four MLC states, it was found that the lowest resistance state of three distinct high-resistance states was prone to failing over time under constant voltage stress.

{\rm HfO}_{2}

In this work we introduce a two-terminal memristive device using Mn doped HfO2. The devices can emulate synaptic behavior based on their transient characteristics. These properties can be exploited to show spike-timing based learning in a network of neurons and synapses. We use the device characteristics to simulate a 4×4 crossbar array of synapses and observe the evolution of the weights over time. The effect of device variability on the performance of synaptic weight update has been examined based on different test conditions of initial randomness and variation in percentage change of strength during spike-timing based updates. Some inferences have been drawn regarding the need of additional circuits for improving reliability of the cross-bar arrays. We believe this study is critical in assessing the design constraints and requirements necessary for integrating memristive devices in crossbars for spike based computations.

-Based ReRAM Device Switching Characteristics

This paper discusses the impact of the bidirectional diode parameters on the read failures in 1ReRAM 1Diode (1D1R) crossbar array memory architectures. Our studies show that while a diode is integral for the successful read operation, the maximum achievable crossbar memory capacity is a strong function of the reverse saturation current of the diode. An acceptable reverse saturation current target for diodes should be 0.1 A/cm2 for 10 nm × 10 nm ReRAM cell for high density memory. For multi-level-cell (MLC) operation, read failure for multiple high resistance states is limited by the reverse saturation current of diode while the line resistance of crossbar arrays plays significant role for read failure of multiple low resistance states.

Understanding the Charge Transport Mechanism in VRS and BRS States of Transition Metal Oxide Nanoelectronic Memristor Devices

We report the synaptic characteristics of novel two-terminal reconfigurable devices fabricated using a doped transition metal oxide. These devices demonstrate short-term plasticity, frequency-dependent synaptic augmentation, long-term potentiation, and long-term depression, and have a potential to show spike timing-dependent plasticity that are macroscopically similar to a biological synapse. The underlying mechanism behind the observed synaptic characteristics was studied using charge transport characterization. Based on this study, a fundamental correlation between the governing device physics and the synaptic characteristic has been established. We believe that by carefully engineering the dopants, the synaptic transmission of these devices can be modulated, which will provide a viable route to replicate the functional diversity of a biological neural system on chip.

Switching characteristics of W/Zr/HfO2/TiN ReRAM devices for multi-level cell non-volatile memory applications

We report an experimental study to understand the reduction in the forming voltage with slow electro-forming of HfO2 based ReRAM devices. Using a combination of capacitance-voltage and current-voltage measurements, we captured the change in capacitance due to dielectric polarization as function of voltage sweep rates. The dielectric polarization was attributed to the charge trapping or internal redistribution of charged centers under electric field. Retention testing showed that this change in capacitance was volatile and decayed to initial values over time after the removal of bias indicating a dielectric relaxation. The dielectric polarization was significantly higher when voltage-sweep rate was slow which causes electro-forming of the dielectric at lower forming voltages.