Nonvolatile Resistive Memory Devices Research Articles

Multibit Operation of Cu/Cu-GeTe/W Resistive Memory Device Controlled by Pulse Voltage Magnitude and Width

We report the demonstration of multilevel operation using Cu/Cu-GeTe/W nonvolatile resistive memory devices for enhanced storage density. We incorporated Cu atoms into GeTe solid-electrolyte switching layers in Cu/Cu-GeTe/W nonvolatile resistive memory devices by applying a bias to the sample holder during a radio-frequency sputtering process. By analyzing the dependence of the device current (resistance) on both the pulse input voltage magnitude and width, we achieved four distinct resistance levels that correspond to the 2-bit operation of a single memory cell. Moreover, a model was suggested and discussed to account for the observed multibit operation.

Forming and switching mechanisms of a cation-migration-based oxide resistivememory

We report detailed current–voltage and current–time measurements to reveal the forming and switchingbehaviors of Cu/Ta2O5/Pt nonvolatile resistive memory devices. The devices can be initially SET (fromthe OFF state to the ON state) when a low positive bias voltage is applied tothe Cu electrode. This first SET operation corresponds to the first formationof a metal filament by inhomogeneous nucleation and subsequent growth ofCu on the Pt electrode, based on the migration of Cu ions in the stableTa2O5 matrix. After the forming, the device exhibits bipolar switching behavior(SET at positive bias and RESET (from the ON state to the OFF state)at negative bias) with increasing the ON resistance from a few hundredΩ to afew kΩ. From the measurements of the temperature stability of the ON states, we concludedthat the RESET process consists of the Joule-heating-assisted oxidation of Cuatoms at the thinnest part of the metal filament followed by diffusion and driftof the Cu ions under their own concentration gradient and the applied electricfield, disconnecting the metal filament. With ON resistances of the order of a fewkΩ, the SET and RESET operations are repeated by the inhomogeneous nucleation and theJoule-heating-assisted dissolution of a small filament on a remaining filament. Thisswitching model is applicable to the operation of cation-migration-based resistive memoriesusing other oxide materials.

Switching characteristics of ZnO based transparent resistive random access memory devices grown by pulsed laser deposition

AbstractResistance switching characteristics, observed in metal oxide thin films, has recently attracted a great deal of attention to develop next generation low power, low cost, high speed, rugged and nonvolatile resistive random access memory (RRAM) devices. The memory effect in these materials is realized through the switching of the resistance of their thin films between two states of high and low resistances. Amongst the known metal oxides currently being explored for the development of RRAM, ZnO has been demonstrated to be a potential candidate. ZnO is an n‐type wide bandgap semiconductor and is highly transparent in the visible spectral region. Moreover its conductivity can be tailored in a broad range from metal like to insulator like by suitable impurity doping. Therefore it is possible to develop a fully transparent RRAM entirely based on ZnO. In this paper we report growth of a novel transparent RRAM devices based on ZnO and its variants. We studied the resistive switching characteristics of these devices and associated conduction mechanisms responsible for the switching. The details of this study will be presented in the paper (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Voigt effect measurement on PLD grown NiO thin films

AbstractNiO has great potential applications in spin valves, magnetooptical sensors, optical fibers, solar thermal absorbers, and in nonvolatile resistive random access memory devices. In our study NiMnO and NiMnLiO films have been grown on double‐side polished r‐plane sapphire substrates by pulsed laser deposition (PLD). We measured the complex Voigt angle using the polarized light from a HeCd laser, a Glan Taylor polarizer, a Hinds PEM‐100 [1] and two Lock‐Ins. The Voigt effect is a second order magnetooptic effect [2]. The polarization state of light after transmission through a sample consisting of ca. 1 μm thick, weak ferromagnetic NiO thin on purely diamagnetic r‐plane sapphire substrates has been modelled using the 4×4 matrix formalism [3] in dependence of an external magnetic field applied in‐plane, i.e. in Voigt configuration. The modelling results revealed that for the diamagnetic sapphire substrate the Voigt angle depends parabolically on the external magnetic field and that the weak ferromagnetic NiO thin films change the parabolic dependence of the Voigt angle in the range of ±0.1 T to a flat‐top shape in agreement with the experimentally determined Voigt angle (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonvolatile resistive switching in spinel ZnMn2O4 and ilmenite ZnMnO3

We report that spinel ZnMn2O4 and ilmenite ZnMnO3 show excellent unipolar resistive switching behaviors, with ON/OFF ratios larger than 104. For both oxides, retention of more than 10 h and good endurance are achieved. Conduction of the OFF state is dominated by the space-charge-limited conduction mechanism, while the Ohmic behavior dictates the ON state, which suggests a filamentary conduction mechanism. Our study introduces two promising materials candidates for nonvolatile resistive random access memory devices, and furthermore it suggests that formation and rupture of conducting filaments are universal in certain ternary oxides even though they may possess distinct crystalline structures.

Statistical characterization of the memory effect in polyfluorene based non-volatile resistive memory devices

Non-volatile resistive memory devices based on a polyfluorene layer sandwiched between two electrodes were studied. The working mechanism was ascribed to the formation of metallic filaments. We characterized the switching probability of multiple devices and the consistency of the switching capability within the same device. Together with cross-section scanning electron microscope images and finite element simulation of electric field distributions, a model was developed to describe the resistive switching phenomenon and explain the variations between devices as well as between switching cycles.

Nonvolatile resistive switching characteristics of HfO2 with Cu doping

AbstractIn this work, resistive switching characteristics of hafnium oxide (HfO2) with Cu doping prepared by electron beam evaporation are investigated for nonvolatile memory applications. The top metal electrode/ hafnium oxide doped with Cu/n+ Si structure shows two distinct resistance states (high-resistance and low-resistance) in DC sweep mode. By applying a proper bias, resistance switching from one state to the other state can be achieved. Though the ratio of high/low resistance is less than an order, the switching behavior is very stable and uniform with nearly 100% device yield. No data loss is found upon continuous readout for more than 104 s. The role of the intentionally introduced Cu impurities in the resistive switching behavior is investigated. HfO2 films with Cu doping are promising to be used in the nonvolatile resistive switching memory devices.

Nonvolatile resistive switching memory utilizing gold nanocrystals embedded in zirconium oxide

Resistive switching characteristics of ZrO2 films containing gold nanocrystals (nc-Au) are investigated for nonvolatile memory applications. The sandwiched top electrode/ZrO2 (with nc-Au embedded)/n+ Si structure exhibits two stable resistance states (high-resistance state and low-resistance state). By applying proper voltage bias, resistive switching from one state to the other state can be achieved. This resistive switching behavior is reproducible and the ratio between the high and low resistances can be as high as two orders. The intentionally introduced nc-Au in ZrO2 films can improve the device yield greatly. ZrO2 films with gold nanocrystals embedded are promising to be used in the nonvolatile resistive switching memory devices.

1-D simulation of a novel nonvolatile resistive random access memory device

The operation of a novel, nonvolatile memory device based on a conductive ferroelectric/semiconductor thin film multilayer stack is simulated numerically. The simulation involves the self-consistent steady-state solution of the transport equation for electrons assuming a drift-diffusion transport mechanism and the Poisson equation. Special emphasis is put on the screening of the spontaneous polarization by conduction electrons as a function of the applied voltage. Depending on the orientation of the polarization in the ferroelectric layer, a high and a low resistive state are found, giving rise to a hysteretic I-V characteristic. The switching ratio, ranging from > 50% to several orders of magnitude, is calculated as a function of the dopant content. The suggested model provides one possible physical explanation of the I-V hysteresis observed for single-layer ferroelectric devices, if interfacial layers are taken into consideration. The approach will allow one to develop guidelines to improve the performance of these devices.