Practical Approach to Induce Analog Switching Behavior in Memristive Devices: Digital-to-Analog Transformation

The capability of memristor devices to perform weight changes upon electrical pulses mimics the analogous firing mechanism in biological synapses. This capability delivers the potential for neuromorphic computing and pushes renewed interests in fabricating memristor with analog characteristics. Nevertheless, memristors could often exhibit digital switching, either during the set, reset, or both processes that degenerate their synaptic capability, and nanodevice engineers struggle to redesign the device to achieved analog switching. This chapter overviews some important techniques to transform the switching characteristics from digital to analog in valence change and electrochemical metallization types memristors. We cover physical dynamics involving interfacial diffusion, interfacial layer, barrier layer, deposition, and electrode engineering that can induce digital-to-analog switching transformation in memristor devices

Formation of a ternary oxide barrier layer and its role in switching characteristic of ZnO-based conductive bridge random access memory devices

The insertion of a metal layer between an active electrode and a switching layer leads to the formation of a ternary oxide at the interface. The properties of this self-formed oxide are found to be dependent on the Gibbs free energy of oxide formation of the metal (ΔGf°). We investigated the role of various ternary oxides in the switching behavior of conductive bridge random access memory (CBRAM) devices. The ternary oxide acts as a barrier layer that can limit the mobility of metal cations in the cell, promoting stable switching. However, too low (higher negative value) ΔGf° leads to severe trade-offs; the devices require high operation current and voltages to exhibit switching behavior and low memory window (on/off) ratio. We propose that choosing a metal layer having appropriate ΔGf° is crucial in achieving reliable CBRAM devices

Status and Prospects of ZnO-Based Resistive Switching Memory Devices

In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges

Neutral oxygen beam treated ZnO-based resistive switching memory device

The room-temperature oxidation process allows irradiation with neutral oxygen particles onto the resistive layer that leads to the absorption of oxygen by the surface of the ZnO layer. The irradiation is effective in controlling the defect concentrations; thus, the ON and OFF resistances of devices can be significantly increased. These characteristics promote the occurrence of resistive switching at much lower current compliance as well as induce switching behavior in very thin ZnO films with thicknesses of 14–42 nm. The thickness dependence of the transformation from filamentary to homogeneous switching was also studied using the neutral beam technique, and the underlying mechanism is discussed

Influence of rf sputter power on ZnO film characteristics for transparent memristor devices

The impact of the radio-frequency (rf) sputtering power on the switching characteristics of ZnO-based transparent memristor devices hasbeen investigated. Memristor devices made with a high rf power exhibited excellent switching characteristics; meanwhile, decreasing the rfpower led to less-apparent switching behavior and increased likelihood of device breakdown. However, high rf power memristors exhibiteda reduced switching uniformity as the rf power significantly affected the defect concentration as well as the microstructure of the deposited ZnO films, which determine the switching characteristics and performance of memristor devices

Film-nanostructure-controlled inerasable-to-erasable switching transition in ZnO-based transparent memristor devices: sputtering-pressure dependency

We found that the write-once-read-many-times (WORM, inerasable)-to-rewritable (erasable) transition phenomenon results from the different structures of the filament, which is determined by the grain orientations of the deposited films. The conduction mechanism of this switching transition and its impact on the synaptic behavior in various ZnO nanostructures are also discussed. Furthermore, our WORM devices have a programmable physical damage function that can be exploited for use in security systems against data theft, hacking, and unauthorized use of software/hardware. This work proposes ZnO-based nonvolatile memory for invisible electronic applications and gives valuable insight into the design of WORM and rewritable memories

Controlled resistive switching characteristics of ZrO₂-based electrochemical metallization memory devices by modifying the thickness of the metal barrier layer

The effects of varying the thickness of the TiW barrier layer on the switching characteristics of a ZrO2-based electrochemical metallization memory (ECM) device were investigated. The thickness of the TiW barrier layer may alter the resistive switching characteristics of Cu/TiW/ZrO2/TiN ECM devices. Devices made without a TiW barrier layer exhibit unstable cycle-to-cycle behavior. The switching stability of ZrO2 devices improves after inserting a TiW layer. However, the insertion of TiW beyond critical thickness leads to switching degradation. We suggest that an appropriate TiW barrier layer thickness is necessary for achieving good switching performance

Low-power electronic technologies for harsh radiation environments

Electronic technologies that can operate in harsh radiation environments are important in space, nuclear and avionic applications. However, radiation-hardened (rad-hard) integrated circuits often require additional processing and more complex configurations than conventional systems. Here we review the development of low-power, rad-hard electronics, examining the underlying phenomena of radiation-induced electronic failure and the design methodologies available with conventional complementary metal–oxide–semiconductor (CMOS) technologies to mitigate the problem. We also explore the potential use and applications of various emerging memory technologies in rad-hard electronics

Conduction mechanism of Co-doped ZnO transparent memristive devices

The Co dopant substitutes the Zn atomic position in the hexagonal crystal lattice and generates acceptor defects. These defects play significant role in modulating the conduction mechanism of the memristive device. The devices without Co dopant have high concentration of donor defects so that the electron can flow easily through hopping these donor defects; henceforth, only weak filaments can be formed during the set process. Meanwhile, the increase of the acceptor defects in the films enhances the film resistivity. This acceptor defects also contribute to an increase of barrier height at the electrode/dielectric interface where the electrons require higher energy to overcome this barrier and, eventually, induce the formation of strong filaments during the set process

The impact of TiW barrier layer thickness dependent transition from electro-chemical metallization memory to valence change memory in ZrO₂ -based resistive switching random access memory devices

The effect of TiW metal barrier layer thickness on voltage-current characteristics of the Cu/TiW/ZrO2/TiN conductive bridge random access memory device was systematically investigated. The change of reset behavior from abrupt decrease to gradual decrease with increasing TiW thickness was observed. Electronic conduction during the forming process was also analyzed to obtain detailed information about the effect of TiW layer thickness on the nature of the conduction phenomenon. The temperature coefficient of resistance of the conductive filament confirms that an electro-chemical metallization (ECM) based conduction was observed in the devices made with a thinner TiW layer. On the other hand, valence change memory (VCM) based conduction was observed with a thick TiW layer. A conduction mechanism is proposed to explain the ECM to VCM conduction transformation phenomenon