Buried graphene heterostructures for electrostatic doping of low-dimensional materials.

The fabrication and characterization of steep slope transistor devices based on low-dimensional materials requires precise electrostatic doping profiles with steep spatial gradients in order to maintain maximum control over the channel. In this proof-of-concept study we present a versatile graphene heterostructure platform with three buried individually addressable gate electrodes. The platform is based on a vertical stack of embedded titanium and graphene separated by an intermediate oxide to provide an almost planar surface. We demonstrate the functionality and advantages of the platform by exploring transfer and output characteristics at different temperatures of carbon nanotube field-effect transistors with different electrostatic doping configurations. Furthermore, we back up the concept with finite element simulations to investigate the surface potential. The presented heterostructure is an ideal platform for analysis of electrostatic doping of low-dimensional materials for novel low-power transistor devices.

Memristively programmable transistors.

When designing the gate-dielectric of a floating-gate-transistor, one must make a tradeoff between the necessity of providing an ultra-small leakage current behavior for long state retention, and a moderate to high tunneling-rate for fast programming speed. Here we report on a memristively programmable transistor that overcomes this tradeoff. The operation principle is comparable to floating-gate-transistors, but the advantage of the analyzed concept is that ions instead of electrons are used for programming. Since the mass of ions is significantly larger than the effective mass of electrons, gate-dielectrics with higher leakage current levels can be used. We demonstrate the practical feasibility of the device using a proof-of-concept study based on a micrometer-sized thin-film transistor and LT-Spice simulations of 32 nm transistors. Memristively programmable transistors have the potential of high programming endurance and retention times, fast programming speeds, and high scalability.

Quantum conductance and switching kinetics of AgI-based microcrossbar cells.

Microcrossbar structured electrochemical metallization (ECM) cells based on silver iodide (AgI) solid electrolyte were fabricated and analyzed in terms of the resistive switching effect. The switching behavior implies the existence of quantized conductance higher than 78 µS which can be identified as a multiple of the single atomic point contact conductivity. The nonlinearity of the switching kinetics has been analyzed in detail. Fast switching in at least 50 ns was observed for short pulse measurements.

Beyond von Neumann--logic operations in passive crossbar arrays alongside memory operations.

The realization of logic operations within passive crossbar memory arrays is a promising approach to expand the fields of application of such architectures. Material implication was recently suggested as the basic function of memristive crossbar junctions, and single bipolar resistive switches (BRS) as well as complementary resistive switches (CRS) were shown to be capable of realizing this logical functionality. Based on a systematic analysis of the Boolean functions, we demonstrate here that 14 of 16 Boolean functions can be realized with a single BRS or CRS cell in at most three sequential cycles. Since the read-out step is independent of the logic operation steps, the result of the logic operation is directly stored to memory, making logic-in-memory applications feasible.

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.

Embedded nanoparticle dynamics and their influence on switching behaviour of resistive memory devices.

Resistively switching Conductive Bridge Random Access Memories (CBRAMs) rely on the controlled formation and dissolution of metallic filaments within a solid insulator, and are emerging building blocks for beyond von Neumann computing architectures and neuromorphic computing. A lack of understanding of the underlying switching mechanisms currently prevents further device utilisation and optimisation. We present a study of lateral and vertical CBRAM model devices that allow us to systematically relate important switching properties and their statistics to a direct characterisation of their critical switching region by scanning and transmission electron microscopy, i.e. to the physical nature of metal filaments and inclusions on all relevant length scales. We find that filaments are composed of metallic clusters and show how filament dynamics link to migration effects of embedded nanoparticles under voltage bias stress. The formation of metal clusters is promoted by a dynamic interplay of cation mobility and redox rate during switching. These clusters are not completely removed from the switching material matrix upon RESET and appear to grow by consumption of smaller clusters. We discuss in detail the interfacial stress of the nanoparticles in the context of their interaction with the switching material and ambient atmosphere. This allows us to consistently interpret previous literature and to suggest future device improvements.

Nanobatteries in redox-based resistive switches require extension of memristor theory.

Redox-based nanoionic resistive memory cells are one of the most promising emerging nanodevices for future information technology with applications for memory, logic and neuromorphic computing. Recently, the serendipitous discovery of the link between redox-based nanoionic-resistive memory cells and memristors and memristive devices has further intensified the research in this field. Here we show on both a theoretical and an experimental level that nanoionic-type memristive elements are inherently controlled by non-equilibrium states resulting in a nanobattery. As a result, the memristor theory must be extended to fit the observed non-zero-crossing I-V characteristics. The initial electromotive force of the nanobattery depends on the chemistry and the transport properties of the materials system but can also be introduced during redox-based nanoionic-resistive memory cell operations. The emf has a strong impact on the dynamic behaviour of nanoscale memories, and thus, its control is one of the key factors for future device development and accurate modelling.