Alex Ignatiev

Bio: Alex Ignatiev is an academic researcher from Center for Advanced Materials. The author has contributed to research in topics: Thin film & X-ray photoelectron spectroscopy. The author has an hindex of 36, co-authored 309 publications receiving 6464 citations. Previous affiliations of Alex Ignatiev include Texas Center for Superconductivity & Cornell University.

Electric-pulse-induced reversible resistance change effect in magnetoresistive films

Abstract: A large electric-pulse-induced reversible resistance change active at room temperature and under zero magnetic field has been discovered in colossal magnetoresistive (CMR) Pr0.7Ca0.3MnO3 thin films. Electric field-direction-dependent resistance changes of more than 1700% were observed under applied pulses of ∼100 ns duration and as low as ±5 V magnitude. The resistance changes were cumulative with pulse number, were reversible and nonvolatile. This electrically induced effect, observed in CMR materials at room temperature has both the benefit of a discovery in materials properties and the promise of applications for thin film manganites in the electronics arena including high-density nonvolatile memory.

Evidence for an oxygen diffusion model for the electric pulse induced resistance change effect in transition-metal oxides.

Abstract: Electric-pulse induced resistance hysteresis switching loops for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ perovskite oxide films were found to exhibit an additional sharp ``shuttle tail'' peak around the negative pulse maximum for films deposited in an oxygen-deficient ambient. The resistance relaxation in time of this ``shuttle tail'' peak as well as resistance relaxation in the transition regions of the resistance hysteresis loop show evidence of oxygen diffusion under electric pulsing, and support a proposed oxygen diffusion model with oxygen vacancy pileup at the metal electrode interface region as the active process for the nonvolatile resistance switching effect in transition-metal oxides.

Electrically variable multi-state resistance computing

Abstract: An electrically operated, overwritable, multivalued, non-volatile resistive memory element is disclosed. The memory element includes a two terminal non-volatile memory device in which a memory film material is included, and a circuit topological configuration is defined. The memory device relates generally to a unique new electrically induced variable resistance effect, which has been discovered in thin films of colossal magnetoresistive (CMR) oxide materials. The memory material is characterized by: 1) the electrical control of resistance through the application of short duration low voltage electrical pulses at room temperature and with no applied magnetic field; 2) increase of the resistance or decrease of the resistance depending on the polarity of the applied pulses; 3) a large dynamic range of electrical resistance values; and 4) the ability to be set at one of a plurality of resistance values within said dynamic range in response to selected electrical input signals so as to provide said single cell with multibit/multivalued storage capabilities. The memory element includes a circuit topology to construct a ROM/RAM configuration. The features of the memory element circuit are: 1) the ability to set and then measure the resistance of the two terminal multi-valued memory devices with negligible effects of sampling voltage and current; and 2) the ability to step up or down the resistance value, i. e., to set one of multiple number of resistance states, with repeated applications of pulses of varying amplitude.

Interactions of ion beams with surfaces. Reactions of nitrogen with silicon and its oxides

Abstract: Ion beam studies of chemical reactions between nitrogen and surfaces of silicon and its oxides are reported. A spectrometer system designed for these studies which combines the techniques of x‐ray and uv photoelectron spectroscopy, Auger electron spectroscopy, secondary ion mass spectroscopy, low energy electron diffraction, and ion bombardment is described. This work employs XPS and UPS to examine the products induced by 500 eV N+2 beams on targets of elemental Si, SiO, and SiO2. The N+2 ions undergo charge exchange and dissociation at the surface of the target to form hot N atoms. Reaction with Si, produces nitrides which are similar to those of the type Si3N4. Reaction with SiO and SiO2 forms nitrides, with no evidence of nitrate or nitrite formation. The chemical nature of the reaction is suggested by identification of the reaction products through XPS and UPS and energy level shifts. The thickness of the silicon nitride layer on Si(111) formed by 500 eV N+2 bombardment has been determined to be ∼19 A thick by using the film/bulk Si XPS intensity ratio. Estimates obtained by depth‐concentration profiling with 1 keV Ar+ and by using LSS projected ion range calculations agree with this approximate thickness.

Nanoionics-based resistive switching memories

Abstract: Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

Memristive devices for computing

Abstract: Memristive devices are electrical resistance switches that can retain a state of internal resistance based on the history of applied voltage and current. These devices can store and process information, and offer several key performance characteristics that exceed conventional integrated circuit technology. An important class of memristive devices are two-terminal resistance switches based on ionic motion, which are built from a simple conductor/insulator/conductor thin-film stack. These devices were originally conceived in the late 1960s and recent progress has led to fast, low-energy, high-endurance devices that can be scaled down to less than 10 nm and stacked in three dimensions. However, the underlying device mechanisms remain unclear, which is a significant barrier to their widespread application. Here, we review recent progress in the development and understanding of memristive devices. We also examine the performance requirements for computing with memristive devices and detail how the outstanding challenges could be met.

Memristive switching mechanism for metal/oxide/metal nanodevices.

Abstract: Nanoscale 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. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.