Rainer Bruchhaus

Integrated Complementary Resistive Switches for Passive High-Density Nanocrossbar Arrays

Recently, the sneak-path obstacle in passive crossbar arrays has been overcome by the invention of complementary resistive switches (CRSs) consisting of two bipolar antiserially connected memristive elements. Here, we demonstrate the vertical integration of CRS cells based on Cu/SiO2/Pt bipolar resistive switches. CRS cells were fabricated and electrically characterized, showing high resistance ratios (Roff/Ron >; 1500) and fast switching speed (<; 120 μs). The results are one step further toward the realization of high-density passive nanocrossbar-array-based gigabit memory devices.

Thermochemical resistive switching: materials, mechanisms, and scaling projections

In this article, resistive switching based on the thermochemical mechanism (TCM) is reviewed. This mechanism is observed when thermochemical redox processes dominate over electrochemical processes. As the switching is based on thermal effects, it is inherently unipolar, i.e., the transitions between the resistive states can be induced by the same bias voltage polarity. NiO has emerged as a “model material” for resistive switching based on the TCM effect and the discussion of the resistance states and the switching processes are focused on this material with the appropriate electrodes, mainly Pt. Unipolar switching is unambiguously filamentary. Conductive filaments are formed during the electroforming process needed prior to memory switching. The SET operation is interpreted as a sequence of threshold switching and subsequent Joule heating which triggers local redox reactions in which oxygen deficient NiO and, if the amount of released oxygen exceeds a certain amount, also metallic Ni will form. The RESET transition can be described as a thermally activated solid-state process resulting in a local decrease of the metallic Ni species. In terms of operation and reliability, a trade-off between RESET current reduction and retention was experimentally found. This is due to the decreasing long-term stability of the filaments with decreasing size. In addition, the scaling projection of a TCM-based memory technology with NiO is directly related to RESET currents and the availability of appropriate select devices.

Crossbar Logic Using Bipolar and Complementary Resistive Switches

Memristive switches are promising devices for future nonvolatile nanocrossbar memory devices. In particular, complementary resistive switches (CRSs) are the key enabler for passive crossbar array implementation solving the sneak path obstacle. To provide logic along with memory functionality, “material implication” (IMP) was suggested as the basic logic operation for bipolar resistive switches. Here, we show that every bipolar resistive switch as well as CRSs can be considered as an elementary IMP logic unit and can systematically be understood in terms of finite-state machines, i.e., either a Moore or a Mealy machine. We prove our assumptions by measurements, which make the IMP capability evident. Local fusion of logic and memory functions in crossbar arrays becomes feasible for CRS arrays, particularly for the suggested stacked topology, which offers even more common Boolean logic operations such as and and nor .

Capacity based nondestructive readout for complementary resistive switches.

Complementary resistive switches (CRS) were recently suggested to solve the sneak path problem of larger passive memory arrays. CRS cells consist of an antiserial setup of two bipolar resistive switching cells. The conventional destructive readout for CRS cells is based on a current measurement which makes a considerable call on the switching endurance. Here, we report a new approach for a nondestructive readout (NDRO) based on a capacity measurement. We suggest a concept of an alternative setup of a CRS cell in which both resistive switching cells have similar switching properties but are distinguishable by different capacities. The new approach has the potential of an energy saving and fast readout procedure without decreasing cycling performance and is not limited by the switching kinetics for integrated passive memory arrays.

Analysis of Transient Currents During Ultrafast Switching of

In this letter, bipolar fast-pulse switching in TiO2 -based nanocrossbar devices was investigated. A dedicated measurement setup was used to measure the transient currents during 5-ns resistive switching. Transient peak currents for the set and reset processes were as high as 200 and 230 μA, respectively. The currents observed during fast-pulse switching are explained and simulated by Joule heating, which is needed for fast oxygen-vacancy movement. The measured transient currents enable a further optimization of resistive switches based on TiO2.

\hbox{TiO}_{2}

In this letter, bipolar resistive switching in TiO2-based memory elements deposited on CMOS-compatible W-plugs is examined. By comparison with a Pt/TiO2/W resistive switch, it is demonstrated that the use of a 5-nm-thin Ti or W interlayer between the Pt top electrode and the 25-nm TiO2 film is the key step for the release from the necessity of electroforming, which is usually required in redox-based resistive switching elements. This is explained by an intentional barrier lowering between the oxide and the electrode materials. The forming-free characteristics on W-plugs make the devices very attractive for future nonvolatile memory applications.

Nanocrossbar Devices

Complementary Resistive Switches (CRS) alleviate size limitations for passive crossbar array memory devices by the elimination of sneak paths. Since CRS cells consist of two anti-serially connected bipolar resistive elements, e.g. electro-chemical metallization (ECM) elements, it is straightforward to use their corresponding memristive models for circuit simulation. Here we show that simple linear memristive models, which are often used in literature, are inapplicable. Therefore, we apply a physics based nonlinear model for ECM elements which is capable of simulating correct CRS behavior for anti-serially combined elements. Interconnecting memristive element models in CRS configuration is an advantageous way to check for memristive model consistency.

Forming-Free

Resistance change Random Access Memory (RRAM) devices in which at least two resistance states are used are a top candidate for future nonvolatile data storage. Simple Metal-Insulator-Metal (MIM) structures form the memory element which can be easily incorporated in large arrays. In particular, in the so-called Valence Change Memories (VCM) the drift of anions, typically oxygen, is considered as the key step to explain the bistable resistive switching behavior. A first-order classification of the observed material changes is related to the geometrical location. In “filamentary” type switching the formation and rupture of a thin filament is responsible for the resistance change. In the “distributed” systems the switching can be traced back to modifications at interfaces. Oxygen ion migration into thin tunnel oxides in high electric fields and Schottky barrier engineering with metals and complex perovskites are two mechanisms under discussion for the distributed systems. In the filamentary type of switching fast oxygen ion transport along extended defects is demonstrated to be the key step for the formation of the conducting filaments. The bistable resistance characteristics with switching induced by voltage pulses is a promising approach for future nonvolatile memory technologies. Excellent scaling behavior to sizes below 20 nm has been demonstrated.

\hbox{TiO}_{2}

Switchable metal-insulator-metal (MIM) structures are the key elements for future non-volatile resistive RAM (RRAM) devices. Recently this type of memory device has attracted considerable interest due to the prospect of non-volatile data storage combined with low power consumption, excellent scalability and very fast write/read operation. However the physical processes responsible for the fast resistive switching are still under investigation. In this work we will present the replacement of the time consuming quasi-static current driven electroforming process by short voltage pulse induced electroforming. Furthermore resistive switching was measured with voltage pulses down to 5 ns pulse width with an in-situ recording of the current response. The high-frequency measurements provide a deeper insight into the physical background of fast data operation.