Gilberto Medeiros-Ribeiro

Memristor-CMOS hybrid integrated circuits for reconfigurable logic

Hybrid reconfigurable logic circuits were fabricated by integrating memristor-based crossbars onto a foundry-built CMOS (complementary metal-oxide-semiconductor) platform using nanoimprint lithography, as well as materials and processes that were compatible with the CMOS. Titanium dioxide thin-film memristors served as the configuration bits and switches in a data routing network and were connected to gate-level CMOS components that acted as logic elements, in a manner similar to a field programmable gate array. We analyzed the chips using a purpose-built testing system, and demonstrated the ability to configure individual devices, use them to wire up various logic gates and a flip-flop, and then reconfigure devices.

High switching endurance in TaOx memristive devices

We demonstrate over 1×1010 open-loop switching cycles from a simple memristive device stack of Pt/TaOx/Ta. We compare this system to a similar device stack based on titanium oxides to obtain insight into the solid-state thermodynamic and kinetic factors that influence endurance in metal-oxide memristors.

A scalable neuristor built with Mott memristors

The Hodgkin-Huxley model for action potential generation in biological axons is central for understanding the computational capability of the nervous system and emulating its functionality. Owing to the historical success of silicon complementary metal-oxide-semiconductors, spike-based computing is primarily confined to software simulations and specialized analogue metal-oxide-semiconductor field-effect transistor circuits. However, there is interest in constructing physical systems that emulate biological functionality more directly, with the goal of improving efficiency and scale. The neuristor was proposed as an electronic device with properties similar to the Hodgkin-Huxley axon, but previous implementations were not scalable. Here we demonstrate a neuristor built using two nanoscale Mott memristors, dynamical devices that exhibit transient memory and negative differential resistance arising from an insulating-to-conducting phase transition driven by Joule heating. This neuristor exhibits the important neural functions of all-or-nothing spiking with signal gain and diverse periodic spiking, using materials and structures that are amenable to extremely high-density integration with or without silicon transistors.

Anatomy of a Nanoscale Conduction Channel Reveals the Mechanism of a High‐Performance Memristor

By employing a precise method for locating and directly imaging the active switching region in a resistive random access memory (RRAM) device, a nanoscale conducting channel consisting of an amorphous Ta(O) solid solution surrounded by nearly stoichiometric Ta(2) O(5) is observed. Structural and chemical analysis of the channel combined with temperature-dependent transport measurements indicate a unique resistance switching mechanism.

Direct Identification of the Conducting Channels in a Functioning Memristive Device

Structures composed of transition metal oxides can display a rich variety of electronic and magnetic properties including superconductivity, multiferroic behavior, and colossal magnetoresistance. [ 1 ] An additional property of technological relevance is the bipolar resistance switching phenomenon [ 2–4 ] seen in many perovskites [ 5–7 ] and binary oxides [ 8 ] when arranged in metal/insulator/metal (MIM) structures. These devices exhibit electrically driven switching of the resistance by 1000x or greater and have recently been identifi ed [ 9 ] as memristive systems, the fourth fundamental passive circuit element. [ 10 , 11 ] A full understanding of the atomic-scale mechanism and identifi cation of the material changes within the oxide remains an important goal. [ 12 ] Here, we probe within a functioning TiO 2 memristor using synchrotron-based x-ray absorption spectromicroscopy and transmission electron microscopy (TEM). We observed that electroforming of the device generated an ordered Ti 4 O 7 Magnéli phase within the initially deposited TiO 2 matrix. In a memristive system, [ 11 ] the fl ow of charge dynamically changes the material conductivity, which is “remembered” even with the removal of bias. While bipolar resistance switching of metal oxides has been observed since the 1960s, [ 2 , 4 ] only recently has the connection to the analytical theory of the memristor been made. [ 9 ] In an attempt to describe microscopically the source of the resistance change, many physical models have been put forth, including generation and dissolution of conductive channels, [ 3 , 6 ] electronic trapping and space-charge current limiting effects, [ 13 ] strongly correlated electron effects such as a metal-insulator transition, [ 14 ] and changes localized to the interface. [ 15 ] Identifying the correct model and quantifying its physical parameters has been diffi cult using primarily electrical characterization. Meanwhile, direct physical characterization [ 7 ]

Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001)

By choosing a material that has an appropriate asymmetric lattice mismatch to the host substrate, in this case ErSi2 on Si(001), it is possible to grow one-dimensional epitaxial crystals. ErSi2 nanowires are less than one nanometer high, a few nanometers wide, close to a micron long, crystallographically aligned to 〈110〉Si directions, straight, and atomically regular.

EVOLUTION OF GE ISLANDS ON SI(001) DURING ANNEALING

The evolution of the shape and size distributions of Ge islands on Si(001) during annealing after deposition has been studied at different temperatures and effective coverages. The initial distributions of square-based pyramids, elongated “hut” structures, faceted “dome-shaped” islands, and much larger “superdomes” depends on the deposition conditions. During annealing after deposition, the islands coarsen over a limited range of times and temperatures. Those pyramidal-shaped islands that grow transform to faceted, dome-shaped islands as they become larger. Initially dome-shaped islands that dissolve transform to a pyramidal shape as they become smaller during the process of dissolving. Outside of this coarsening regime, the islands can achieve a relatively stable, steady-state configuration, especially at lower temperatures. At higher temperatures, intermixing of Si into the Ge islands dominates, decreasing the strain energy and allowing larger islands to form. At lower and intermediate temperatures, the...

Engineering nonlinearity into memristors for passive crossbar applications

Although TaOx memristors have demonstrated encouraging write/erase endurance and nanosecond switching speeds, the linear current-voltage (I-V) characteristic in the low resistance state limits their applications in large passive crossbar arrays. We demonstrate here that a TiO2-x/TaOx oxide heterostructure incorporated into a 50 nm× 50 nm memristor displays a very large nonlinearity such that I(V/2) ≈ I(V)/100 for V ≈ 1 volt, which is caused by current-controlled negative differential resistance in the device.

The switching location of a bipolar memristor: chemical, thermal and structural mapping

Memristors are memory resistors promising a rapid integration into future memory technologies. However, progress is still critically limited by a lack of understanding of the physical processes occurring at the nanoscale. Here we correlate device electrical characteristics with local atomic structure, chemistry and temperature. We resolved a single conducting channel that is made up of a reduced phase of the as-deposited titanium oxide. Moreover, we observed sufficient Joule heating to induce a crystallization of the oxide surrounding the channel, with a peculiar pattern that finite element simulations correlated with the existence of a hot spot close to the bottom electrode, thus identifying the switching location. This work reports direct observations in all three dimensions of the internal structure of titanium oxide memristors.

Continuous Electrical Tuning of the Chemical Composition of TaOx-Based Memristors

TaO(x)-based memristors have recently demonstrated both subnanosecond resistance switching speeds and very high write/erase switching endurance. Here we show that the physical state variable that enables these properties is the oxygen concentration in a conduction channel, based on the measurement of the thermal coefficient of resistance of different TaO(x) memristor states and a set of reference Ta-O films of known composition. The continuous electrical tunability of the oxygen concentration in the channel, with a resolution of a few percent, was demonstrated by controlling the write currents with a one transistor-one memristor (1T1M) circuit. This study demonstrates that solid-state chemical kinetics is important for the determination of the electrical characteristics of this relatively new class of device.