Yang, Jianhua
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Yang
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Jianhua
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Publication A Family of Electronically Reconfigurable Nanodevices(2009-01) Yang, Jianhua; Borghetti, Julien; Murphy, David; Stewart, Duncan; Williams, R StanleyAFM image of 17 nanodevices with a zoom-in cartoon schematically shows an individual crosspoint device consisting of two Pt metal electrodes separated by a TiO2 bi-layer memristive material. By applying an electric field across the memristive material, oxygen vacancies can drift up and down, leading to four current-transport end-states. The switching between these end-states results in a family of nanodevices.Publication High switching endurance in TaOx memristive devices(2010-12) Yang, Jianhua; Zhang, M; Strachan, John; Miao, Feng; Pickett, Matthew; Kelley, RonaldWe demonstrate over 1×1010 open-loop switching cycles from a simple memristive device stack of Pt/TaO��/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.Publication Memristive switching mechanism for matal/oxide/metal nano-devices(2008-06-15) Yang, Jianhua; Pickett, Matthew; Li, Xuema; Ohlberg, Douglas; Stewart, Duncan; Williams, R. StanleyNanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron –ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance.Publication Diffusion of Adhesion Layer Metals Controls Nanoscale Memristive Switching(2010-07-30) Yang, Jianhua; Strachan, John; Xia, Qiangfei; Ohlberg, Douglas; Kuekes, Philip; Kelley, Ronald; Stickle, William; Stewart, Duncan; Medeiros-Ribeiro,, Gilberto; Williams, R StanleyThermal diffusion of Ti through Pt electrode forms Ti atom channels of 1 nm diameter along Pt grain boundaries, seeding switching centers and controlling nanoscale memristive switching. The image shows EFTEM maps of Ti overlaid on HRTEM images for a Si/SiO2 100 nm/Ti 5nm/Pt 15 nm sample in-situ annealed in ultrahigh vacuum at 250 °C for 1 hour.Publication The mechanism of electroforming of metal oxide memristive switches(2009-05-05) Yang, Jianhua; Miao, Feng; Pickett, Matthew; Ohlberg, Douglas; Stewart, Duncan; Lau, Chun Ning; Williams, R StanleyMetal and semiconductor oxides are ubiquitous electronic materials. Normally insulating, oxides can change behavior under high electric fields—through ‘electroforming’ or ‘breakdown’—critically affecting CMOS (complementary metal–oxide–semiconductor) logic,DRAM (dynamic random access memory) and flash memory, and tunnel barrier oxides. An initial irreversible electroforming process has been invariably required for obtaining metal oxide resistance switches, which may open urgently needed new avenues for advanced computer memory and logic circuits including ultra-dense non-volatile random access memory (NVRAM) and adaptive neuromorphic logic circuits. This electrical switching arises from the coupled motion of electrons and ions within the oxide material, as one of the first recognized examples of a memristor (memory–resistor) device, the fourth fundamental passive circuit elementoriginally predicted in 1971 by Chua. A lack of device repeatability has limited technological implementation of oxide switches, however. Here we explain the nature of the oxide electroforming as an electro-reduction and vacancy creation process caused by high electricfields and enhanced by electrical Joule heating with direct experimental evidence. Oxygen vacancies are created and drift towards the cathode, forming localized conducting channels inthe oxide. Simultaneously, O2− ions drift towards the anode where they evolve O2 gas, causing physical deformation of the junction. The problematic gas eruption and physical deformationare mitigated by shrinking to the nanoscale and controlling the electroforming voltage polarity. Better yet, electroforming problems can be largely eliminated by engineering the devicestructure to remove ‘bulk’ oxide effects in favor of interface-controlled electronic switching.Publication ‘Memristive’ switches enable ‘stateful’ logic operations via material implication(2010-04-08) Borghetti, Julien; Snider, Gregory; Kuekes, Philip; Yang, Jianhua; Stewart, Duncan; Williams, R StanleyThe authors of the International Technology Roadmap for Semiconductors1 —the industry consensus set of goals established for advancing silicon integrated circuit technology—have challenged the computing research community to find new physical state variables (other than charge or voltage), new devices, and new architectures that offer memory and logic functions1–6 beyond those available with standard transistors. Recently, ultra-dense resistive memory arrays built from various two-terminal semiconductor or insulator thin film devices have been demonstrated7–12. Among these, bipolar voltage-actuated switches have been identified as physical realizations of ‘memristors’ or memristive devices, combining the electrical properties of a memory element and a resistor13,14. Such devices were first hypothesized by Chua in 1971 (ref. 15), and are characterized by one or more state variables16 that define the resistance of the switch depending upon its voltage history. Here we show that this family of nonlinear dynamical memory devices can also be used for logic operations: we demonstrate that they can execute material implication (IMP), which is a fundamental Boolean logic operation on two variables p and q such that pIMPq is equivalent to (NOTp)ORq. Incorporated within an appropriate circuit17,18, memristive switches can thus perform ‘stateful’ logic operations for which the same devices serve simultaneously as gates (logic) and latches19 (memory) that use resistance instead of voltage or charge as the physical state variable.Publication Engineering Nonlinearity into Memristors for Passive Crossbar Applications(2012-03) Yang, Jianhua; Zhang, M; Pickett, Matthew; Williams, StanleyAlthough 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.Publication Anatomy of a Nanoscale Conduction Channel Reveals the Mechanism of a High-Performance Memristor(2011-11-08) Miao, Feng; Strachan, John; Yang, Jianhua; Zhang, M; Goldfarb, Ilan; Torrezan, Antonio; Eschbach, Peter; Kelley, Ronald; Medeiros-Ribeiro, Gilberto; Williams, R StanleyBy employing a precise method for locating and directlyimaging 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 Ta2O5 is observed. Structural and chemical analysis of the channel combined with temperature-dependent transport measurements indicate a unique resistance switching mechanism.