Symposium EM10 : Emerging Materials and Technologies for Nonvolatile Memories

Cation diffusion dynamics, especially the sensitivity to the delay between spikes, contribute to the learning functions of biological synapses. Nonvolatile electronic memristors have been a leading candidate to simulate/emulate synapse function for neuromorphic computing. However, many memristors that are used for data storage and memory are active only during the application of a voltage pulse, and thus do not display the delayed temporal response of an actual synapse. These dynamics can be incorporated into a second memristor, a diffusive memristor, that has a similar physical mechanism to its bio-counterpart based on metal atom diffusion that persists after the power to the device is switched off. The combination of these two types of memristors in series provides a direct functional emulation of both short-term and long-term plasticity of synapses to yield power efficient learning for neuromorphic computing, which is just one of many applications enabled by the diffusive memristor.

Vanadium dioxide (VO₂) is a prototypical correlated oxide system that possesses a monoclinic insulating state at room temperature and transforms to metallic state with tetragonal rutile structure when heated to 67°C in pristine form. The voltage-driven phase change is considered a threshold switch, since removal of the voltage leads to recovery of the insulating state by thermal relaxation in two-terminal devices. However, when point defects are introduced into the oxide, non-volatile resistance changes can be realized. This is because of the intricate interplay between disorder, orbital overlap and electronic structure in correlated oxides. We will present a comparison of vanadium dioxide films grown by RF-sputtering and atomic layer deposition with respect to their electrical properties and switching characteristics. Introduction of oxygen vacancies by low pressure annealing leads to substantial suppression of the insulating phase and can be mimicked by electrolyte gating in three-terminal devices. Such memory devices enable homogenous switching of the phase in a non-volatile manner and are of growing interest to a variety of memory technologies in electronics (resistance switching) and photonics (programmable metasurfaces). We will then compare crystalline, defective vanadium dioxide with properties of amorphous vanadium oxide films to illustrate how crystallinity influences the ionic versus electronic nature of resistance switching and endurance.

Abstract—Niobium oxide (NbOx) memristor devices have gained a surge of recent attention due to their application as selectors in storage-class resistive random access memory (ReRAM). The underlying principle of these devices is the local activity exhibited by NbOx upon an energy stimulus, often seen as a region of negative differential resistance (NDR) in a plot of current and voltage.
In this work we investigated the physics behind the NDR behavior in NbOx devices by performing scanning transmission synchroton x-ray spectromicroscopy experiments. In order to explore this technique, similarly as we did with TaOx material [1], the devices need to be fabricated on Si₃N₄ membranes. A thin materials stack of ~60 nm is required by this technique to enable soft x-ray transmission in the O K-edge spectral region (500-600 eV). Recently, NbOx based devices were reported with strong NDR behavior. However, the total thickness of these devices was ~100 nm.

We fabricated crossbar devices with 1.5 µm electrodes on membranes with a thickness of ~ 50 nm. The device size was dictated by scanning transmission synchroton x-ray spectromicroscopy limitation. The spatial resolution of this characterization method is 30 nm, furthermore the smallest devices seen by this technique range between 0.5 µm and 5 µm.
Here we experimented with various growth conditions for reactively sputtered NbOx films. We explored films grown from a NbO₂ and Nb₂O₅ targets. We built devices with a variety of film stacks, especially with the inclusion of a reactive electrode material, TiN. We correlated these variations with their electrical characteristics and identified growth parameters and the resulting film’s compositional and structural characteristics that allowed the devices to exhibit NDR. We found that the presence of a TiN electrode was necessary for NDR. There have been several recent efforts to model the origins of the NDR behavior based on multiple conduction mechanisms.

Reference:
[1] K. Suhas, C. Graves, J. P. Strachan, E. Merced, D. Kilcoyne, T. Tyliszczak, J. N. Weker, Y. Nishi and R. S. Williams, “Direct Observation of Localized Radial Oxygen Migration in Functioning Tantalum Oxide Memristors,” Advanced Materials, 28, 14 (2016), pp. 2772-2776.

Vanadium oxide (VO_x) thin films were grown by atomic layer deposition (ALD) and treated with hydrogen plasma to control the stoichiometry-related properties for nonvolatile memory applications. Instead of relying mainly on precursor and temperature conditions, hydrogen plasma was pulsed during the ALD cycle which provides an additional ability to finely tune properties correlated to the amount of oxygen deficiency in the films. Such capabilities were used to control the transport properties and enhance the performance of metal/VO_x/metal memristive devices, where non-stoichiometric defects such as oxygen vacancies play a critical role on the device performance. Various hydrogen plasma processing conditions and the effects on stoichiometry, morphology, crystallinity, and electrical properties will be addressed. The mechanisms involved in the enhanced performance of the working devices will also be discussed in this presentation.

Resistive Random-Access Memories (ReRAMs, or memristors), are attracting industry attention as a new technology to replace Flash nonvolatile memories. Further, the impedance value can be tuned by the adjustment of both resistance and capacitance values giving the MEM-Impedance (MEM-Z).In a recent work on HfO₂ MEM-Z [1], we demonstrated that the imaginary part of Resistive Random-Access Memories could be switched by external bias.
Thus, such devices could be used in tunable electronic circuits , such as reconfugurable impedance matching network, reconfigurable amplifiers, programmable filters and oscillators. In order to make possible these applications, a good control of the impedance value and the possibility to change the impedance range of use by an external excitation will be necessary.
In this work, we first present new capabilities of HfO₂ MEM-Z structures elaborated by atomic layer deposition (ALD). For that, various external excitations were tested as the amplitude of the alternating voltage, the temperature (from 25°C to 125°C), the frequency of the signal. Results will be discussed as a function of these excitations and their combination on the value of resistance and capacitance and on the range of set – reset.
In a complementary study, we developed thin ALD TiO₂-HfO₂ stacks to create MEM-Z structures. The peculiar migration of defects which control impedance transition in such structures allows obtaining new performances for electronic applications.





[1]. T. Wakrim, C. Vallée, P. Gonon, C. Mannequin, and A. Sylvestre “From MEMRISTOR to MEMImpedance device”. Applied Physics Letters 108, 053502 (2016);

Resistive switching memories based on electrochemical systems (ReRAMs) are intensively studied in aspects of applications ranging from non-volatile memories to selector devices, logic operations and neuromorphic functionalities.
Detailed studies in recent years have reported details on the device kinetics and charge transport including in situ, ex situ experimental analysis and modeling for both ECM and VCM type cells.
In this talk the latest achievements on fundamental aspects in the field will be discussed. The focus will be set on the redox reactions preceding and influencing the resistive switching and the effects of moisture, strongly tuning the electrochemical behavior and cell/device performance. The processes of (electro)chemical oxidation/passivation of metals at Me/MeOx interfaces (Me = Ta, Hf, Al, Ti etc.) and its influence on the SET/RESET mechanism in VCM will be highlighted.
The role of the host cation motion in oxides such as TaOx, HfOx and TiOx will also be shown as bridging the ECM and VCM mechanisms.

The development of high permittivity oxides (high k) for advanced metal-insulator-metal (MIM) structures is a key for advanced microelectronics to achieve high density capacitors. Titanium dioxide (TiO₂) is one of the most attractive high k candidates due to its high dielectric constant. TiO₂ can be deposited, in anatase or rutile phase, by ALD using an organometallic precursor of titanium and an oxygen source. Dielectric constant of rutile TiO₂ is higher but rutile is only possible at high deposition temperature. Rutile phase at low temperature can be obtained by depositing TiO₂ on (rutile) RuO₂ thanks to the small lattice mismatch between these two materials. However, many experimental results point out the role of oxygen source. Indeed, it has been shown that using H₂O as oxygen source leads to an anatse phase TiO₂, even when deposited on RuO₂ layer.
In the first part of this paper, we show that the dependance of TiO₂ crystalline phase on the oxidant species can be explained by the nature of the interfacial chemical reactions taking place at the first stages of TiO₂ growth. Indeed, X-ray Photoelectron Spectroscopy (XPS) and depth-resolved XPS analysis indicated the presence of Ru-Ti bonds and the Ti₂O₃ titanium oxide bonds when H₂O is used as oxygen source. Thanks to these results, a chemical reactional scheme is proposed to explain the pathways that lead to anatase or rutile TiO₂ phase, depending on the oxidant species. In the first stage of the ALD process, titanium precursor reduces the RuO₂ to produce Ru-Ti-O that is re-oxidized to give RuO₂ and TiO₂ when O₂ plasma is used. Rutile TiO₂ is thus grown thanks to the RuO₂ template. When H₂O is used as oxidant, the Ru-Ti-O is furher reduced to give Ru-Ti bond and Ti₂O₃ monolayer. Subsequent ALD cycles lead to progressive transformation of RuO₂ into metallic Ru and to the growth of TiO₂, but on a Ti₂O₃ monolayer. In this case, anatse TiO₂ is obtained.
In the second part of this paper, we study the influence of the RuO₂ layer composition on the electrical properties of the deposited TiO₂ layer. Indeed, RuO₂ is obtained by O₂ plasma oxidation at 400°C at different durations. When oxidation time increases, RuO₂ chemical bonds are modified. It will be shown that stoichiometric RuO₂ leads to the growth of TiO₂ exhibiting better electrical properties in term of dielectric constant.

Metal oxide memristors, or resistive random access memory (ReRAM), are gaining academic and commercial interest due to their potential to provide next-generation storage-class memory, while there is much to be understood in terms of the nanoscale physico-chemical nature of their operation. Building a compact predictive circuit model of their behavior is critical in their successful commercial deployment. One of the bottlenecks in coming up with such a comprehensive model is the lack of complete understanding of the nanoscale mechanisms involved in formation and dissolution of conduction channel during resistance switching in memristors. In particular, the presence and sign (direction) of thermophoresis (temperature-gradient-driven force) of oxygen atoms has been under controversy. Using nanoscale x-ray spectromicroscopy, we recently observed radial migration of oxygen perpendicular to the direction of current flow upon resistance switching of micrometer-sized hafnium oxide memristors. We identified a stable sub-100 nm cylindrical channel of high conductivity rich in oxygen vacancies, surrounded by a sub-500 nm concentric region of relatively lower conductivity rich in oxygen interstitials. This result is consistent with our previous study on tantalum oxide memrsistors [Advanced Materials, 28, 2772 (2016)], which indicates the universality of this phenomenon in metal oxide memristors.

Using a combination of mainly Soret effect (thermophoresis) driven by temperature gradients and Fick diffusion driven by concentration gradients, we show that we are able to successfully make a 3-dimensional model of the formation and dissolution of the oxygen rings. Here we used multiple coupled domains: temporal evolution of temperature profile using the heat equation, Soret and Fick diffusions acting on the oxygen defects, joule heating from the device current, convective and conductive cooling within different materials and mechanical deformation of electrodes. In addition, we show that the formation and dissolution of the oxygen rings are modeled by significantly different diffusion parameters, highlighting an important asymmetry in memristor operation. We argue that this critical insight originates from the fact that during conduction channel formation and stabilization, the oxygen defects (vacancies and interstitials) cluster by overcoming the columbic repulsions, which have to be broken down in order to dissolve the ring. Such clustering does not exist in an as-grown film, which explain the asymmetry in the diffusion parameters.

The development of intrinsic resistive RAM (ReRAM) technologies is a stage toward the fabrication of CMOS compatible, post FLASH memory arrays. However, the reliability of these devices is uncertain due gaps in the understanding of the electrically driven transformations occurring during switching. Our test devices are composed of thin, amorphous silicon suboxide layers sandwiched between various metallic electrodes. We have previously demonstrated three-dimensional profiling of the conductive channels that govern switching, often referred to as filaments. Additionally, we have shown that oxygen is lost from devices during switching. This loss and the corresponding formation of a conductive region must be better understood for ReRAM to be optimised and integrated into consumer electronics. Here, using a combination of conductance tomography and Raman spectroscopy we present results that allow us to begin spatially correlating the electronic, structural and compositional changes. We demonstrate that the conductive regions of the switching layer correspond to a reduction in structural disorder and an increase in silicon-silicon coordination. These results support the model of oxygen reliance in which the conductivity of the active layer is increased as oxygen is removed.

Programmable metallization memory cells operating on the filamentary switching mechanism have demonstrated promising performance in areas of cycle endurance, area scaling, retention, and power use. However, the underlying redox processes and ion migration that lead to the formation and disruption of a conductive filament under high electric field, and their dependence on the structure of the oxide dielectric layer, are not fully understood, leading to large inter-device and intra-device variability. In particular, the nano-scaled nature of the conductive filament has posed a major experimental challenge for traditional morphological and chemical thin film analysis techniques, so that basic questions regarding the shape, volume, and multiplicity of conductive filaments and the dependence of these features on oxide microstructure remain unknown. In this work, we have applied a serial scribing technique, utilizing conductive atomic force microscopy (CAFM) to locate conductive filaments and controllably etch through the oxide layer so that a nanometer resolution 3-D tomographic image of conductive regions within the insulating oxide layer is obtained. The morphology of conductive filaments was studied in p+Si/HfO₂/Cu and TiN/HfO₂/Cu structures after chemical removal of the copper top contact. The microstructure of the HfO₂ switching layer was varied between amorphous, textured, and randomly oriented polycrystalline under variable atomic layer growth temperature, and post-deposition annealing temperature, and the morphology of conductive regions was correlated with the initial microstructure of the oxide dielectric. Hence, a comparison between oxide layer microstructure and morphology and distribution of the initially formed filaments will be presented. Device IV behavior, as well as reactive molecular dynamics simulations of ion migration within the oxide dielectric are used to interpret the resulting filament morphology. Here, the implementation of a non-standard CAFM technique offers unique insight into the dependence of conductive filament morphology on oxide microstructure, and the possible impacts on device variability.

Resistive Random Access Memories (ReRAMs) is one of the promising simple devices for new-generation nonvolatile memories. A resistive switching (RS) phenomenon, namely reversible transitions between the low and high resistance states after forming process, is believed to be caused by the formation and rupture of a conductive filament. We reported that filaments including a Quantum Point Contact (QPC) in Pt/NiO/Pt stack structures were formed by semi-forming, the first step of the forming under voltage sweeping [1]. Although quantized conductance steps have been reported in other ReRAM cells [2, 3], the appearance condition of the quantized conductance remains unclear. In this study, we examine microscopic structures and the appearance condition of QPC in Pt/NiO/Pt cells.
Pt as a bottom electrode was deposited by sputtering, and subsequently a NiO thin film was deposited by reactive radio-frequency sputtering. Cross-sectional Transmission Electron Microscopy (TEM) revealed that a NiO layer exhibits a columnar polycrystalline structure with a grain diameter of tens of nm. When the NiO layer was sputtered at a selected O₂ flow rate, two different modes of the forming were observed in a cell. After semi-forming, the cell exhibited several discrete conductance values characterized by integer multiples of G₀=2e²/h. Note that RS behaviors were confirmed after both semi-forming and second forming. The results suggest that semi-forming occurs by the formation of filaments including a QPC and second forming occurs by the formation of a new filament.
When the O₂ flow rate during sputtering was relatively high, the cells showed single (conventional) forming. At a relatively low O₂ flow rate, on the other hand, the cell did not exhibit forming because of low resistance. In High-angle Annular Dark Field Scanning TEM analyses of NiO layers sputtered at lower O₂ flow rate, more bright spots were randomly observed at grain-boundary triple-points in NiO layers. Energy Dispersive X-ray spectroscopy revealed that the average composition ratio of O to Ni along the thickness direction at triple-points is smaller than the value within grains. These results indicate that conductive filaments are formed along Ni- or Vo-rich grain boundaries, and that the composition ratios (O/Ni) both equivalent to and larger than a critical value are required to create a filament including a QPC and another filament formed by second forming. Furthermore, it is confirmed that cells with a smaller size tend to exhibit larger semi-forming voltage or single forming and that cells with a larger size show lower resistance. This cell size dependence suggests that defects (Ni or Vo) which act as the source of a filament including a QPC by semi-forming are randomly distributed in a NiO layer according to Poisson statistics.
[1] H. Sasakura, et al, Appl. Phys. Lett. 107, 233510 (2015).
[2] T. Tsuruoka, et al., Nanotechnol. 21, 425205 (2010).
[3] S. Long, et al., Sci. Rep. 3, 2929 (2013).

Resistive Random-Access Memories (ReRAMs, or memristors), are attracting industry attention as a new technology to replace Flash nonvolatile memories. ReRAMs make use of resistance switching in metal-insulator-metal (MIM) structures which are subjected to voltage biasing. Up to now, attention was focused on dc resistance transition. Less attention has been paid to the impedance variation, i.e. to the influence of voltage biasing on ac conductance and capacitance in ReRAM devices.
In a recent work on HfO₂ ReRAMs [1], we demonstrated that the capacitance, and more generally the imaginary part of RRAM devices, can also be switched by external bias. This paves the way for the realization of capacitive switching memories, or memcapacitors. Besides digital memory applications, memcapacitors could also be used in a broad range of electronic applications, such as reconfigurable RF circuits. From the fundamental point of view, impedance studies can provide information about conduction mechanisms and related switching phenomena.
This work aims to better understand the mechanisms of ac conductance and capacitance variation in ReRAM devices. Studied devices are HfO₂ MIM structures elaborated by atomic layer deposition. Devices are subjected to a constant dc voltage bias, and the impedance (real and imaginary parts) is recorded as a function of time. This allows us to follow kinetics of impedance switching, i.e. of both the ac conductance and the capacitance transition. Measurements are carried out at different frequencies and at different temperatures. The frequency and temperature dependence of the impedance in the high and low resistive states is discussed along ac conduction and polarization models implying intrinsic and bias-induced defects. Study of the capacitance switching time, as a function of bias, brings further information about the generation rate of defects which control impedance transition.






[1]. T. Wakrim, C. Vallée, P. Gonon, C. Mannequin, and A. Sylvestre “From MEMRISTOR to MEMImpedance device”. Applied Physics Letters 108, 053502 (2016);

Oxygen defects can have profound effects on the properties of transition metal oxides. For instance, electromigration of oxygen vacancies provides a viable mechanism for the formation, rupture and reconstruction of conducting filaments in insulating oxides [1, 2], an effect used in nanoscale resistive switching devices. In complex oxides where magnetic, ferroelectric and superconducting phases emerge from strong correlations between localized transition metal valence electrons, oxygen vacancies can radically alter a plurality of intrinsic properties via valance changes or structural phase transitions [3]. The ability to reversibly control the concentration and profile of oxygen vacancies in oxide nanostructures would open comprehensive prospects for new functional ionic devices. Advancements in this direction require experimental techniques that allow for simultaneous measurements of oxygen vacancy dynamics, atomic-scale structural transitions and macroscopic physical properties.
Here, we use in situ transmission electron microscopy (TEM) to demonstrate reversible switching between distinctive resistance states in functional transition metal oxide films. The switching events are not based on dielectric breakdown via the formation of filaments in an insulating oxide. Instead, simultaneous high-resolution imaging and resistance probing reveal that the resistive switching is directly correlated to the nucleation and growth of uniform structural phases. Reversible migration of oxygen vacancies between the Nb-doped SrTiO₃ substrate and the conducting oxide films, induced using combined effects of Joule heating and bias voltage, triggers the structural and resistive transitions. Our experimental findings, which are reproduced by numerical electro-thermal simulations, open clear paths to active ionic control of other properties of transition metal oxides, including magnetism, ferroelectricity, and superconductivity. Transfer of this ionotronics concept to lithographically defined oxide nanostructures is anticipated to promote the emergence of new devices with ionically controlled functions.
[1] R. Waser and M. Aono, Nature Mater. 6, 833 (2007).
[2] J.J. Yang, D.B. Strukov, and D.R. Stewart, Nature Nanotech. 8, 13 (2013).
[3] S. Kalinin and N.A. Spaldin, Science 341, 858 (2013).

Redox-based resistive switching devices (ReRAM) are a potential candidate for non-volatile high-density memory and oxide-based devices have been highly investigated in the recent years. Switching from a high resistive state (HRS) to a low resistive state (LRS) is called SET, the reverse process requires the application of voltages with opposite polarity and is named RESET. In oxide-based devices, also called valence change mechanism memory (VCM) and consisting of a metal-oxide-metal structure, switching is commonly believed to rely on the formation and dissolution of an oxygen-deficient conductive filament.
A better understanding of the interplay of the involved processes is crucial for future application. Therefore, an experimental study of the SET kinetics of 100 x 100 nm² Pt/8 nm Ti rich STO/TiN devices was conducted covering 13 orders of magnitude in switching time. The current transients are thoroughly analyzed revealing that SET switching is a two-part process irrespective of the applied voltage. Before the abrupt switching event a slow degradation process with small increase in current exists. This degradation process is characterized by a non-volatile change in resistance and the resulting slope depends on the applied pulse magnitude. Also the transition time defined as characteristic for the abrupt switching event is found to rely on the voltage.
The experimental results are motivated by simulations with a physics-based compact model. Using an average oxygen vacancy concentration as state variable, SET switching is described by the temperature and field dependent change of the ion concentration near the Pt/STO-interface. The simulation results show that the slow change in resistance before the abrupt switching event is related to a non-volatile change in oxygen vacancy concentration. Also the voltage dependent different slopes of the change can be explained with the same underlying physics. The abrupt switching event is characterized by a thermal runaway resulting from a positive feedback between change in ion concentration, temperature rise, and current increase. Further, the influence of electric field and temperature on the transition time is studied separately.

Among several emerging technologies for the nonvolatile memory, resistive switching memory based on metal ion transport in a thin ionic conductor film is one of the most attractive candidates, because of their high ON/OFF resistance ratio, excellent scalability, and high speed switching time. From the similarity of the operation mechanism of a ‘gap-type atomic switch’ [1], metal-ion-transport-based resistive memories are referred to as a ‘gapless-type atomic switch’ [2].
We have investigated the operation mechanism of Cu,Ag/Ta₂O₅/Pt atomic switch cells as a model system and have demonstrated their unique functions, which cannot be realized by conventional semiconductor switches. Ta₂O₅-based atomic switch exhibits bipolar resistive switching behavior under bias voltage sweeping, in which SET and RESET processes are attributed to the formation of a metal filament by nucleation on Pt and the dissolution of the filament due to thermochemical reactions, respectively [3]. It was found that moisture absorption of the matrix oxide plays a crucial role in redox reactions of Cu (Ag) and resistive switching behavior [4], in relation to the film morphology [5,6]. The switch also exhibits conductance quantization and synaptic/memristive behaviors, indicating its potential for use in neural computing systems [7].
We have also demonstrated that an atomic switch can be realized using a solid polymer electrolyte (SPE) [8]. Ag/SPE/Pt atomic switches, fabricated with a mixture of poly(ethylene oxide) (PEO) and Ag salt, showed bipolar resistive switching under bias voltage sweeping, similar to oxide-based atomic switches. SPE switches inkjet-printed on a plastic substrate showed stable switching behavior upon substrate bending, indicating their great potential for flexible switch/memory applications [9]. Moreover, we succeeded to observe the filament growth behavior in a planar structure [10] and found kinetic factors determining the growth processes in SPE-based switches [11]. The quantized conductance can also be observed in this system [12].

[1] Terabe et al., Nature 433 (2005) 47, [2] Hasegawa et al., MRA Bull. 34 (2009) 929, [3] Tsuruoka et al., Nanotechnology 21 (2010) 425205; 22 (2011) 254013, [4] Tsuruoka et al., Adv. Funct. Mater. 22 (2012) 70, [5] Tsuruoka et al., Adv. Funct. Mater. 25 (2015) 6347; Jpn. J. Appl. Phys. 55 (2016) 06GK02, [6] C. Mannequin et al., Jpn. J. Appl. Phys. 55 (2016) 06GJ09; Nanotechnology (submitted), [7] Tsuruoka et al., Nanotechnology 23 (2012) 435705, [8] S. Wu, Adv. Funct. Mater. 21 (2011) 93, [9] S. Mohapatra et al., AIP Adv. 2 (2012) 022144, [10] K. Krishnan et al., Adv. Mater. 28 (2016) 640; Jpn. J. Appl. Phys. 55 (2016) 06GK02, [11] K. Krishnan et al., Nanoscale (published online), [12] K. Krishnan et al., (submitted).

One of interface-type resistive switching is considered to originate from the movements of oxygen vacancies near the electrode/oxide interface, showing also bipolar switching. However, there remains an essential issue that the interface-type switching in various perovskite oxides has not been investigated in detail. In this work, we have tried to increase oxygen vacancies in MOD-made BaTiO₃ (BT) thin film by nitrogen annealing especially near the interface, thereafter evaluated the effect on the endurance properties of BT thin film diode.

MOD solution with a stoichiometric composition of BT was spin-coated on the Pt(111)/Ti/SiO₂/Si substrates at 500 rpm-3 s/4,000 rpm-30 s. After the spin coating, the film was dried on a hot plate. Thereafter, in air or N₂ (gas flow rate: 0.1 L/min), it was pre-annealed at 450 degree C -5 min at every coating, and final-annealed 800 degree C -5 min using an RTA apparatus. These processes were repeated triple to obtain about 120 nm-thickness. After a final annealing, Au top electrode was deposited by evaporation.

The annealing steps were composed of pre-/final-annealing in air/air, air/N₂, and N₂/ N₂, respectively. After the final annealing, we analyzed the O 1s spectrum of BT thin film near the surface by XPS. The curve was deconvoluted to three peaks that are assigned to O^2- bound to Ti⁴⁺, O^2- bound to Ti³⁺, which is one of the origins for the formation of oxygen vacancies, and the adsorbed oxygen. From the obtained XPS results, the increase in oxygen vacancies near the surface by N₂ annealing was implied by the increase in the magnitude of the O^2- bound to Ti³⁺ peak intensity.
The BT film showed little relation with a conduction path as a filament-type because, in each annealing condition, we confirmed the resistance hysteresis forminglessly from the first I-V measurement with no step-like current change. Also, the BT diode current decreased monotonously by reducing the electrode area, indicating the diode is not filament-type but interface one.
BT thin films with both pre- and final-annealing in N₂ showed ON/OFF ratio of 1 digit or more in resistance hysteresis until 10⁶ endurance cycles, although in air/N₂ and air/air annealing, they were less than 10³ and 10⁰. This also suggests that pre-annealing in N₂ is intrinsically effective to improve the endurance behavior, thus implying that not only the interface but also the middle part of the film would contribute the interface-type mechanism.

BT film with pre- and final-annealed in N₂ showed the endurance properties of 10⁶ switching cycles. It was suggested that oxygen vacancies near the electrode/oxide interface are increased by N₂ annealing and enhanced the interface-type resistive switching. Pre-annealing in N₂ was also found to be very effective to improve endurance properties, implying that not only the interface but also the middle part of the film would contribute the interface-type mechanism.

Resistive switching devices, where the resistance states of capacitor-like metal-insulator-metal structures are reversibly switched by an applied electric field, are promising for a wide range of applications including non-volatile random access memories and bio-inspired neuromorphic computing. However, the underlying atomic mechanisms governing the change in resistance are still unclear, which prevents their widespread application. In-situ X-ray investigations of devices at real operation conditions with chemical specificity and structural sensitivity are extremely valuable for clarifying the mechanisms. Here we report on an in-situ study of resistive switching processes in epitaxial WO₃ thin films by synchrotron X-ray multimodal imaging at the Advanced Photon Source.

The mechanisms behind resistive switching of oxide thin films are believed to be redox reactions that create or annihilate oxygen vacancies and migration of those oxygen vacancies under an electric field. In-situ X-ray characterization has the capability to test this hypothesis. The valence state change of tungsten ions during redox reactions is shown by micro-fluorescence imaging around tungsten absorption edge. At the same time, the lattice distortions and changes in oxygen content/octahedral tilt during the formation and migration of oxygen vacancies are monitored by changes to the integer-order and half-order diffraction peaks. The length scale of real space features during resistive switching is on the order of a few mm, which is well resolved with a sub-micron focused X-ray beams. The insights gained from these in-situ experiments will be discussed in the context of improved performance and new device design.

ransparent electronics is an emerging technology that involves ‘invisible’ electronic circuitry and optoelectronic devices, such as building-integrated photovoltaics and touch panel displays. In the sheer electronics communication, transparent nonvolatile memory devices are required. Al₂O₃ is promising as a resistive switching material in addition to its wide band gap (~7 eV), large dielectric strength (~5 MV/cm), high permittivity (~8), and good thermal stability. Among a variety of 2-D nanomaterials, graphene has motivated wide-ranging scientific and engineering studies because of its excellent mechanical, thermal, and electronic properties. Graphene is an emerging nanomaterial for the next generation of faster and smaller electronic devices, such as solar cells, light emitting diodes, and photodetectors. It is of great interest for various nano-electronic device applications especially due to its low sheet resistance and high optical transparency.
With this motivation, we studied the resistive switching characteristics of Graphene/Al₂O₃/ITO heterostructures for transparent resistive memory applications. Amorphous Al₂O₃ thin films of ~50 nm thickness was deposited by remote-plasma enhanced ALD in a self-limiting vapor-phase chemisorption window on ITO coated glass substrates kept a temperature of 200 ^oC using trimethyl-aluminum precursor and oxygen plasma. Bilayer graphene synthesized by hot filament chemical vapor deposition method was transferred onto the thin film of Al₂O₃ as a top metal electrode by standard poly(methyl methacrylate) assisted wet-transfer method. Transmission spectroscopy studies of the Graphene/Al₂O₃/ITO-coated-glass heterostructure showed a high average transmittance of ~80 % within the visible wavelength region from 400 to 800 nm. The above non-volatile memory structures exhibited a typical bipolar, reliable, and reproducible resistive switching behavior, with ON/OFF resistance ratio (>20), narrow spread of set and reset voltages, better switching endurance up to 10³ cycles, and longer data retention (>10³ s) in a temperature range of 200–400 K. The possible resistance switching and current conduction mechanisms in these potential transparent trilayer memory devices will be presented.

The emergence of room-temperature ferroelectric polarization in ultra-thin films of otherwise nonferroelectric SrTiO₃ was recently demonstrated and attributed to the electrically induced alignment of polar nanoregions that can naturally form due to the presence of intrinsic defects in SrTiO₃ [1, 2]. This effect can enable a considerable increase in the storage capacity of nonvolatile ferroelectric memories by using low-dimensional ferroelectric structures. Here we aim to establish a better atomic-scale understanding of the influence that various intrinsic defects can have on polarization behavior of bulk and thin-film SrTiO₃ by means of density-functional-theory based calculations. Specifically, our calculations show that titanium antisite defects (Ti_Sr) should result in high polarization and low coercive field crucial for nonvolatile device applications, but the presence of oxygen vacancies bound to Ti_Sr leads to a non-insulating state and makes the polarization switching irreversible due to much higher switching barriers between polarization states. The impact of other intrinsic defects and defect clusters in bulk and thin films of SrTiO₃ will also be discussed in the context of polarization switching mechanism.

References
[1] M Choi, F Oba, and I Tanaka. Role of Ti antisite-like defects in SrTiO3. Physical Review Letters, 103(18):185502, 2009.
[2] D Lee, H Lu, Y Gu, S-Y Choi, S-D Li, S Ryu, TR Paudel, K Song, E Mikheev, S Lee, et al. Emergence of room-temperature ferroelectricity at reduced dimensions. Science, 349(6254):1314-1317, 2015.

For practical use of Resistive Random Access Memory (ReRAM), understanding of the resistive switching (RS) mechanism in transition metal oxides (TMO) is important. Some papers predict its mechanism by using first-principles (FP) calculation; for example, TMO become conductive by introducing oxygen vacancies (Vo) in bulk single crystalline TMO [1]. However, most of ReRAM samples have polycrystalline structures [2]. We have constructed a grain model, a triangle pole structure with a trigonal pyramid on it, based on our experimental data and suggested that the RS is caused on the side surfaces of the grain, SSG, (i.e. grain boundaries) of polycrystalline NiO films [3].

In this study, we checked the adequacy and universality of our model by new experimental method and FP calculations [4] for various MgO surfaces, MgO which has the same crystalline structure with NiO.

A Pt top electrode (EL) was deposited on NiO/Pt structure by sputtering method (SD-EL: sputter-deposited electrode). We also used a soft-probe [5] with which we can form an electric contact without physically damaging the surface of samples. By contacting the Pt soft-probe to the surface of the NiO/Pt film, a Pt/NiO/Pt structure is formed in the contact area (SC-EL: a soft-probe contact electrode). The forming, V_form, set, V_set, and reset voltage, V_reset, were extracted from I-V characteristics.

V_form 's of SC-EL samples (9.0 ~ 13.0 V) are much higher than those of SD-EL samples (2.0 ~ 5.0 V), whereas V_set's and V_reset's have no clear difference between the SC- and SD-EL samples. Conductivities of the SD-EL samples strongly depends on the surface conductivities of the SSG because the Pt SD-EL contacts to the rough polycrystalline NiO surface burying the concaves. Accordingly, the electric current flows from the SD-EL to the SSG without flowing through the {001} surfaces of the trigonal pyramid. For SC-EL samples, on the other hands, the Pt SC-EL contacts only on tops of the trigonal pyramids (i.e. insulating {001} surfaces) and the electric current must flow through there. Accordingly, V_form of SC-EL samples are higher than those of SD-EL samples.

This experimental results supports our RS mechanism [3], V_o exists on {11-2} surfaces, (i.e. grain boundaries) and its electronic states become insulating (metallic) by disruption (cohesion) of V_o's.

Our FP calculations for MgO surfaces indicate that the band gap of MgO depends on surface orientations. This tendency is also observed in NiO and suggests that the RS is caused with the same mechanism of NiO regardless of magnetism.

[1] H. D. Lee, B. M-. Kope and Y. Nishi, Phys. Rev. B 81, 193202 (2010).
[2] S. Seo et al., Appl. Phys. Lett. 85, 5655 (2004).
[3] T. Moriyama et al., MRS Adv. Accepted.
[4] T. Ohno et al., SC'07 Proceedings of the 2007 ACM/IEEE conference on Supercomputing, Article No. 57.
[5] M. Yoshitake, S. Yagyu and T. Chikyow, e-J. Surf. Sci. Nanotech. 13, 307 (2015).

Semiconductor memories, both volatile and non-volatile, are mostly integrated using standard processes and standard architectures. This means that the standard silicon memory devices, such as flash memories, are Rad-tolerant but not Rad-Hard. Therefore, for hard radiation applications a new approach is required to avoid such radiation-related failures.

Resistive Random Access Memories (RRAM) are intrinsically radiation tolerant, thus a proper candidate to achieve the mentioned target. Nowadays, RRAM based on HfO₂ is one of the most promising technology candidates because its full compatibility with CMOS processes. Its behavior is based on the possibility of electrically modifying the conductance of a Metal-Insulator-Metal (MIM) stack: the Set operation moves the cell into a Low Resistive State (LRS), whereas Reset operation brings the cell back to a High Resistive State (HRS). That switching effect of the RRAM devices is determined by the formation and modification of conductive filaments composed of oxygen vacancies, which are controlled through the motion of oxygen vacancies by an applied electric field.

Nevertheless, the 1T-1R structure of the memory array consists of NMOS access transistors, which are sensitive to radiation. In standard NMOS devices, ionizing radiation may generate holes trapped in the gate oxide, and the trapped holes could induce leakage paths from the drain to the source region. A suitable approach to eliminate the leakage path in NMOS transistors is to adopt a gate-enclosed layout. Therefore, the 1T-1R devices are based on the combination of an Enclosed Layout Transistor (ELT) and a TiN/HfO₂/Ti/TiN based resistor.

In order to show the affordability of this approach, a complete electrical characterization was performed in AC mode through the Incremental Step Pulse with Verify Algorithm (ISPVA). This technique consists of a sequence of increasing voltage pulses on the Drain terminal during Forming/Set operation, whereas this sequence of pulses is applied on the Source terminal during Reset operation. In each operation, when the Read current reaches a certain target value this operation is stopped.

Resistive RAM (ReRAM or RRAM) is a new non-volatile memory technology that holds promise as a replacement for flash memory. ReRAM operates via the generation of defects (oxygen vacancies or metal migration) within a transition metal oxide, which form a conducting filament through the dielectric under sufficient applied voltage. The active stack of a typical ReRAM device consists of a transition metal oxide (TMO) (e.g., HfO_x, TaO_x, TiO_x) layer sandwiched between conducting electrodes such as Pt or Ta. Precise control of stoichiometry, microstructure and interfaces is critical for device performance.

We have investigated the feasibility of inert ion beam etch (IBE) for the patterning of ReRAM-type structures, in conjunction with existing methodologies. IBE is the process-of-record for patterning magnetic tunnel junctions (MTJs) in magnetic read heads that have similar M-I-M (metal-insulator-metal) geometry as ReRAM devices. IBE has several advantages for the patterning of narrow-pitch devices of this type: an inert chemical environment for avoidance of chemical damage or residue; tight control of ion energy; independent control of the incident angle of the etching ions; and good selectivity of electrode materials to typical etch mask materials.

We report on the role of the angle-dependent ion beam etch rates in device area control and the minimization of sidewall re-deposition. The etch rates of key ReRAM materials are presented versus incidence angle and ion beam energy. As the ion beam voltage is increased, we demonstrate a significant enhancement in the relative etch rate at glancing incidence (for example, by a factor of 2 for HfO₂). Since the feature sidewall is typically exposed to glancing incidence, this energy-dependence plays a role in optimization of the feature shape and in sidewall re-deposition removal. We utilize the measured etch rates in simulating the IBE patterning of tight-pitched ReRAM features, and generate etched feature shapes and composition.

We present results of SRIM simulations to estimate depth of ion-bombardment damage to the TMO sidewall.[1] Damage is minimized by minimizing ion energy; its depth can be reduced by roughly a factor of 5 over typical IBE energy ranges. For example, ion energies of less than ~300eV are indicated to maintain damage below ~2nm. Multi-angle and multi-energy etch schemes are proposed to maximize sidewall angle and minimize damage, while eliminating re-deposition across the TMO.

[1] SRIM: The Stopping and Range of Ions in Matter, J. Ziegler, 1998, available at http://www.srim.org/

Vanadium dioxide, which shows a sharp (3-5 orders of magnitude) resistance change across its phase transition from room-temperature insulating phase to high-temperature metallic phase [1], has been proposed as an emerging material promising for non-volatile memory applications. For any kinds of vanadium dioxide applications for electronics devices including memories, it is indispensable to study and understand the metal contacts behavior on top of it. This study is generally simple for metals on most of semiconductor materials, however, it is challenging and still an open area for vanadium dioxide due to its distinct physical properties of two different phases. In this presentation, we report our recent studies of various metal contacts on vanadium dioxide thin films and nanowires. The metal contacts on the vanadium dioxide thin films show significant different contact resistances at various temperatures, especially the regime across the phase transition, shown several orders of magnitude contact resistance change. In addition to study on thin films, the contact resistance of metal electrodes deposited on vanadium dioxide nanowires were for the first time measured by transmission line method at different temperatures as well.

Typical non-volatile memory, such as Flash, is approaching its scalability limit, leading to a need for novel technologies. Intrinsic filamentary oxide based Resistive RAM (ReRAM) devices show promising characteristics thanks to scalability, CMOS compatibility, retention and low power operation. Nevertheless, low device endurance remains an issue to be solved before commercialization. We studied our TiN/SiO_x/TiN (x<2) devices, which can develop unwanted insulating surface bubbles on the anode under high-fileld/current operation. We provide positive correlation of these deformations with the release and incorporation of oxygen to and from atmosphere, leading to “breathing” devices. We detected O₂ and O₂^- emission through Secondary Ion Mass Spectroscopy (SIMS) and Residual Gas Analyzer (RGA) analyses during in situ switching in high vacuum (2x10^-6 mbar). Cycling in an isotopically labeled oxygen atmosphere with ¹⁸O allowed us to detect incorporated oxygen through SIMS and Time of Flight (ToF)-SIMS depth profiling; this sheds light on how filamentation may occur. Additionally, we report the dependence of device electrical behavior on the ambient atmosphere. These findings provide evidence useful in improving endurance of future devices through the use of oxygen sinks and sources..

Future technological developments in non-volatile resistance switching random access memory (ReRAM) applications [1] exceeding the limitations of present-day flash devices are expected to comply with the basic requirements of a small size for a high data storage density as well as fast read and write operations performed at reasonably low voltages and easily detectable current levels. Moreover, it is also a key issue that such passive circuit elements exhibit a strongly non-linear response function, so that the device is stable against low-level read-out signals and, at the same time, responds quickly to write operations carried out at higher biases. Tunable, nanometer scale junctions formed between metallic electrodes by reversible solid state electrochemical reactions represent extremely promising candidates to satisfy the above criteria [2]. The resistive state of such a memory element, called memristor [3] is altered by biasing the device above its writing threshold (V_th). Readout is performed at lower signal levels which preserve the stored information.

We studied the dynamics of the resistive switchings in Ag–Ag₂S–PtIr nanojunctions [4]. We showed that the resistance change simultaneously exhibits multiple time scales ranging from a nanosecond to seconds upon a switching voltage pulse. The resulting non-exponential transition between the OFF and ON states as well as the achievable, technologically convenient R_OFF/R_ON = 2–10 ratios are largely affected by the amplitude and frequency of the biasing signals. This fundamental, inherent property of the Ag₂S ionic conductor provides the unique opportunity for combination of GHz write/erase operations [5] performed at bias levels of a few Volts, non-volatile read-out with slower signals of a few 10 mV and robust information storage at zero bias in a two-terminal, nanometer scale analog memory device.

References
[1] J.J. Yang et al., Nature Nanotechnology, 8, (2013) 13.
[2] R. Waser et al., Adv. Mater., 21, (2009) 2632.
[3] L. Chua, IEEE Trans. Circuit Theory, 18, (1971) 507.
[4] A. Gubicza, M. Csontos, A. Halbritter, G. Mihály, Nanoscale, 7 (2015) 4394.
[5] A. Geresdi, M. Csontos, A. Gubicza, A. Halbritter, G. Mihály, Nanoscale, 6 (2014) 2613.

Metal nanostructures exhibiting resistive switching phenomena demonstrate significant promise as next generation functional memory storage devices due to simple, highly scalable structures, low power consumption, fast response times, and multi-state logic potential. In this work we report on the resistive switching character of individual nanoribbons comprising single-crystalline sub-10 nm strontium titanate nanocubes (STONCs). The single-crystalline STONCs served as the precursor component in fabricating highly ordered nanoribbons using a modified doctor-blade approach. This method, flexible blade deposition (FBD), utilizes the flow of a colloidal suspension under a flexible polyethylene terephthalate (PET) blade at a fixed height, which significantly restricts the geometry as the STONCs assemble on a Nb doped SrTiO₃ substrate surface. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the topography and the cross-sectional profile, respectively. Conductive AFM places a Ir/Pt coated conductive tip in direct contact with individual nanoribbons, thus creating a Pt/Ti/Ir(tip) /STO/NSTO/Ti test structure. The semi-log I-V characterization shows bipolar resistive switching behavior and relatively robust endurance properties. Notably, we observe “eightwise” (counterclockwise) resistive switching response, indicative of homogeneous switching, and opposite of the “counter-eightwise” response recently observed in 10-nm thick epitaxial intrinsic STO thin films,¹ which attributes switching to the presence of dislocations. Moreover, we demonstrate the transfer of the nanoribbons from the original (hard) substrate onto a second, flexible substrate while retaining the switching character, posing significant implications for tailoring the resistive response type through substrate selection.
(1) K. Szot, R. Dittmann, W. Speier, R. Waser, Phys. Status Solidi Rapid Res. Lett. 2007, 1, R86.

Organic-inorganic hybrid materials can promise both the superior carrier mobility of inorganic semiconductors and the processability of organic materials. Perovskite alkyl ammonium lead iodide has the strong potential to serve as the high efficient solar sensitizer for next-generation solar cells. Also (CH₃NH₃)PbI₃ has similar band gap of silicon, thus it is attracting huge attention for various applications.

In this study, we report that electrical (I-V) characteristics of (CH₃NH₃)PbI₃. Even though electrical properties of polycrystalline (CH₃NH₃)PbI₃ films synthesized with solution process largely vary with the grain size, we could get uniform (CH₃NH₃)PbI₃ thin films with precise thicknesses and reproducible crystallinity by spin coating. Our experimental results reveal that I-V characteristics of (CH₃NH₃)PbI₃ films show bipolar memristive behavior. In positive bias voltage, current increases rapidly, but the films are like insulator in negative bias voltage. From these bipolar memristive behavior, it is possible to analyze switching and conduction mechanism in (CH₃NH₃)PbI₃ films. We reveal that the resistive switching and conduction mechanism from the change of metal for active electrodes and counter electrodes with conductive atomic force microscopy. Also, It is suggested that interplay between Schottky emission and Ohmic conduction are a key for unveiling switching mechanism for high performance transistors based on perovskite alkyl ammonium lead halides.

A considerable amount of research has been carried out on Ni-Zn ferrites because of their innumerable applications in non-resonant devices due to their high magnetization .Similarly, Al doped SrFe_12-XAl_XO₁₉ has their distinct magnetic properties such as their high T_c, high coercive force, have made them popular for industrial application such as microwave device in recording media.
Energy product as a combination of magnetization and coercivity essentially determines the quality of the permanent magnets. But unfortunately, magnetic materials tend naturally to have one rather both; soft magnetic materials usually have relatively high magnetization, while most of hard magnets have high coercivity, but quite low magnetization. Exchange spring principle can identify a route to provide the possibility for improving remanence and energy product of the permanent magnets by making the composite of suitable material. The present study is to obtain Pure phase exchange-coupled nanocomposites of hard-soft magnetic oxides, (hard) SrFe_12-XAl_xO₁₉ - (soft) Y Ni_0.5Zn_0.5Fe₂O₄ with X=0, 0.5, 1, 1.5 and 2 and Y=wt.10%, wt.20%, wt.30%, wt.40% and wt.50% were prepared via autocombustion method. The X-ray powder diffraction (XRD) patterns of as prepared nanocomposite peaks are in good agreement with the hexagonal and cubic phases and their peaks are broadened due to their nanometer sizes. VSM measurement report that a _~19% increase in Ms value for nanocomposite with x=0 (69.5 emu/g) for hard and 50 Wt. % of the soft phase. A linear increase in Mr /Ms with soft-phase content indicates the presence of enhanced exchange-coupling between hard and soft phases of the nanocomposite. The highest Mr /Ms ratio of 0.60 was obtained for nanocomposite containing wt. 10.% of the soft-phase with X=1.5 Al doped sample which is significantly larger than the value of 0.5 predicted for non-interacting permanent magnet. This indicates the existence of intergrain exchange coupling between hard and soft phases. The observed reduction in coercieve field values of the nanocomposite with increase in soft-phase content is explained on the basis of competition between exchange and dipolar interaction between hard-soft and soft–soft phases of the nanocomposite.

Floating gate single Poly non-volatile memory arrays featuring single cells with enhanced sensitivity to ionizing radiation were developed and investigated. Radiation sensors based on CMOS technology and integrated monolithically on a single chip with the readout circuitry allow fabrication of low power consuming dosimeters for various emerging applications, like Internet of Things and environmental monitoring (background radiation and pollution). They are also promising for personnel wearable dosimeters and absorbed dose control in medicine and security systems. We report on ultra-low power consuming radiation sensors implemented in a standard CMOS technology without using additional masks. The investigated array-type devices allow registration of absorbed radiation with record spatial resolution and pronounced spectral response, making them suitable for X-ray and Gamma imaging and spectroscopy. The sensors were exposed to ionizing radiation from various types of sources (Co-60, Cs-137, X-ray tubes, medical accelerators, ion beams) . Physical mechanisms responsible for sensitivity and devices reliability performance were studied in detail. The practical value of the obtained results is confirmed by the developed demonstrators for the in-vivo dosimetry in radiotherapy and safety applications.

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems, that can further lead to the manipulation of their spin-polarized properties for technological purposes.

Superconductors permit the flow of Charge in the absence of Ohmic dissipation, but since the Cooper pairs of electrons have antiparallel spins, charge currents cannot carry a net spin. Furthermore, since such singlet pairs are easily disrupted by magnetism, the coupling of superconductivity and ferromagnetism might appear useless for applications in spintronics. However, during the past few years a series of discoveries have shown that, not only can magnetism and superconductivity be made to cooperate, but in carefully engineered superconductor/magnet systems new functionality can be created in which spin, charge and superconducting phase coherence can work together (1). By combining these different degrees of freedom a whole new spectrum of exciting predictions is waiting to be explored, and the field of superconducting spintronics has emerged.
In the first part of this talk I will review the state-of-the-art in superconducting spintronics (superspin) with an emphasis on the generation of spin-polarized (spin triplet) Cooper pairs. In the second part, I will discuss my group’s recent results including manipulation of superconductivity in superconducting spin-valves (2), superconductor / magnet bilayers (3,4), triplet proximity effects in half-metallic ferromagnets, and the triggering of exotic superconductivity in single layer graphene.

1. J. Linder and J.W.A. Robinson. Nature Physics 11, 307 (2015)
2. N Banerjee, C Smiet, R Smits, A Ozaeta, S Bergeret, M Blamire, JWA Robinson. Nature Com. 5, 3048 (2014)
3. Y Gu, GB Halász, JWA Robinson, MG Blamire, Physical Review Letters 115, 067201 (2015)
4. A Di Bernardo, S Diesch, Y Gu, J Linder, M Blamire, E Scheer, JWA Robinson. Nature Com 6, 8053 (2015)
5. A Di Bernardo, Z Salman, X Wang, M Amado, M Egilmez, M Flokstra, A Suter, S Lee, J Zhao, T Prokscha, E Morenzoni, MG Blamire, J Linder, JWA Robinson, Phys. Rev. X 5, 041021 (2015)
6. Y Kalcheim, O Millo*, A Di Bernardo, A Pal, JWA Robinson, Physical Review B 92, 060501(R) (2015)

The interplay of spin, charge, orbital, and lattice degrees of freedom in complex oxides has exhibited a vast variety of phases and physical phenomena. Recently, a long-range ordered ferroelectric vortex phase, in which electrical polarization continuously rotates about its core and forms an ordered vortex-antivortex arrays, was reported by engineering PbTiO₃/SrTiO₃ superlattice under optimal strain. [ref. 1] However, the dynamic response of this exotic phase under electrical and thermal excitation is unexplored. By employing a pump-probe method, with optical Second Harmonic Generation (SHG) as probe [ref. 2] and controlling the arrival of the 400 nm optical pulse, we achieved a capability of understanding polarization states with ~200 fs time resolution.

SHG pump-probe study was performed on three vortex system with (PbTiO₃)_n/(SrTiO₃)_n superlattice periodicity of n=6,12,16. An unusual enhancement (reaching 30%) of SHG signal was observed at time zero, when the sample was probed right after pumping by 400 nm. Detailed polarimetry measurement showed no change of polarization configuration under such a fast time scale, indicating a pure electronic effect which enhances SHG activity. However, at longer time scale, a suppression of SHG was observed, with minimum at t=14 ps, 29 ps, 39 ps and scaling linearly with superlattice periodicity of n=6,12 and 16. Considering 400 nm optical pulse is above band gap of PbTiO₃ and below band gap of SrTiO₃, a thermal dissipation from PbTiO₃ layer to SrTiO₃ layer is expected along with the suppression process. Under a pumping fluence of 50 mJ/cm², a permanent polarimetry change was observed, indicating a new phase with different polarization configuration was written in the vortex area. This has potential application in optical nonvolatile memory materials.


1. A.K.Yadav, et al., Nature 530, 198-201 (2016)
2. Sava A. Denev, et al., J. Am. Ceram. Soc., 94 [9] 2699–2727 (2011)

Phase change memory has recently been the focus of next-generation memories because of their advantages such as low fabrication cost, non-volatility, and commercial scalability for sub 20 nm cell designs. Previous studies have shown that the currently used Ge2Sb2Te5 has shown several disadvantages, including low crystallization temperature, long crystallization time, and high power consumption. As an alternative, Ge_xSb_1-x is suggested for phase change memory applications owing to its very short crystallization times, which, however, still shows low crystallization temperatures at the low Ge concentrations.
In this study, we investigated the effect of selenium (Se) doping on electrical properties, thermal stability, and data retention to enhance the reliability of Ge_xSb_1-x. When selenium was incorporated into the Ge_xSb_1-x films, both the crystalline and amorphous sheet resistance increased, translating to a higher R_set/R_reset ratio. Simultaneous Thermal Analyzer analysis showed that the crystallization temperature and activation energy of Se-doped Ge_xSb_1-x films are higher than pure- Ge_xSb_1-x films. According to isothermal temperature-resistance measurements, the failure time and data retention increased when more Se was incorporated. As the Se concentration increased, the threshold current of the amorphous state decreased and the threshold voltage increased. A Se-doped Ge_xSb_1-x cell, which is operated by voltage pulse, exhibited a lower Set/Reset operation voltage than pure Ge_xSb_1-x films. In addition, as the Se concentration increased, the programing window likewise increased. On the basis of our results, we conclude that Se-doped Ge_xSb_1-x films have the potentials to be used as phase change materials for next-generation PRAM applications due to their high thermal stability and large window for reliability as well as possibility of operation power reduction.

Silicon Nitride based charge trap devices have been studied for more than four decades for applications in non-volatile memories. Although the initial studies were reported on devices with metal gate, the Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) stack as the non-volatile memory gate stack has been the focus since the 1990s. Several enhancements in SONOS layer materials have been invented to reduce the programming voltage and improve the reliability of the SONOS memory cell. Some of the key enhancements include band gap engineered SONOS (BE-SONOS) stack and scaling down of layer thicknesses without sacrificing uniformity. With these enhancements, SONOS memories which can be programmed at voltages as low as 7.5V and can meet the 10 year retention at 100°C ambient temperature are now commercially available. With the advent of high K – metal gate technology for CMOS transistors, it was logical that high K dielectrics would be evaluated for the charge trap layer or blocking layer of the SONOS stack. The same is true for FinFETs which has led to research on SONOS based FinFETs. Furthermore, with the advent of 3D non-volatile memories, SONOS devices have been invented for 3D memory cells and these are now in manufacturing. SONOS has also been very successful in embedded memories where the cost of integration into an existing CMOS baseline process is most critical. Here the embedded SONOS has proven to be much more economical than the traditional Floating Gate technology for many applications.
This talk will review the early years of SONOS and then highlight the various innovations that have enhanced SONOS memory performance, reliability and low cost of manufacture. Topics that will be covered include various improvements in the ONO stack such as Band gap engineering, High K – Metal Gate for SONOS, 3D SONOS, SONOS FinFETs and embedded SONOS.

Due to their ability to threshold switch within nanoseconds, amorphous chalcogenides show great promise as selector devices for RRAM crossbar arrays. The mechanism that drives initiation, holding and decay of the current filament in these materials has long been asserted to be primarily electronic in nature. However, there is little work that has explored the extent to which Joule heating of the material can affect the switching dynamics, despite the fact that these materials exhibit a strong dependence of conductivity on temperature. We investigate this question by fabricating identical 3x3µm TiN/GeTe₆/TiN crossbar devices on silicon substrates that have been coated to produce two different thermal environments. The coatings are 100nm of AlN and 1µm of SiO₂, which have thermal conductivities that differ by approximately two orders of magnitude and produce different switching characteristics. At room temperature, the DC threshold voltage for devices fabricated on AlN/Si was 0.4V higher than for devices on SiO₂/Si (3.2V versus 2.8V) while the threshold currents remain roughly the same (around 12 µA). Since more power dissipation is required to switch devices that have better heat sinking, this suggests that a minimum temperature rise must occur before the threshold event. At several volts above the DC threshold value, devices on both substrates can switch in less than 10ns; however, at lower voltages approaching the DC threshold voltage, the incubation time required to switch devices on AlN/Si is almost an order of magnitude greater than for devices on SiO₂/Si. Finite element thermoelectrical simulations of devices prior to switching show that the temperature rise vs. time within both devices is similar up until 10ns. At that point the temperature rises diverge, with devices on SiO₂ heating to higher temperatures at a faster rate than on AlN. The temperature rise could explain the observed behavior of both the DC and transient device characteristics, and indicates that device self-heating prior to switching may play a critical role in the switching initiation process.

Phase change memory (PCM) is one of the most promising candidates for high performance Storage Class Memory (SCM). Endurance of PCM, which is likely to exhibit greater than flash memory by several orders of magnitude (for example 10⁷ to 10⁸), is one of the key aspects that would enable dynamic random access memory (DRAM)-like applications for PCM. To achieve high endurance, understanding failure mechanisms of PCM is critical. One of the main failure mechanisms of PCM devices is void formation which leads to inability to switch between the two resistance states of the PCM cell. Void formation is also related to the change in mass density of the active component accompanied with phase transformation, induced during continued switching cycles [1].
A novel confined PCM cell with a metallic liner can improve write-endurance and optimize multi-level cell (MLC) performance [2]. In this work, in-situ transmission electron microscopic (TEM) procedure was developed to track phase changes and possible void formation of the new PCM cell during the operation in order to study failure mechanisms in detail. We observed void formation as well as void movement, which are correlated to resistance states of the cell.
We show that cells that have been cycled up to 10⁹ times can have large voids within the confined phase change material volume, leading to electrical fail stuck in the RESET mode. Further investigation by energy dispersive X-ray spectroscopy (EDS) analysis has confirmed composition segregation. In-situ TEM also reveals void accumulation in which voids move in a hopping fashion. Finally, we show that these voids can self-heal during a set-reset pulse operation of the cell which might provide an insight for improving PCM endurance.

[1] B. Rajendran et al., VLSI Tech. Dig., p. 96, (2008)
[2] S. Kim et al., IEDM Tech. Dig., p. 762, (2013)

Phase-change memory utilizes a chalcogenide material, for example, Ge₂Sb₂Te₅, that reversibly transforms between amorphous and crystalline states. Amorphization (melt and quench) by a short high power electrical pulse, ~ 20 ns, is much faster than crystallization (annealing) by a long moderate power pulse, ~ 100 ns. In this reason, the crystallization kinetics, represented by nucleation rate [cm^-3 s^-1] and growth velocity [cm s^-1], determines the operational speed of the device. In addition, the amorphous phase spontaneously becomes the stable crystalline phase, which is the root cause of the data retention failure. Furthermore some statistical variation is induced due to the stochastic nature of the nucleation. In this work, our crystallization model that calculates nucleation and growth in mesoscale phase-field method is presented. Then, the scaling trends of the set speed and its statistical variation in terms of nucleation rate, growth velocity and memory cell dimensions are extracted at different temperatures using the model. Finally, the cell design for achieving the gradual decrease of the resistance by repetitive pulses that is crucial for synaptic devices in a neuromorphic computing system is discussed.

The storage elements of STT-MRAM are magnetic tunnel junctions (MTJs) in which the magnetization of the magnetic electrodes are lying out-of-plane thanks to a strong interfacial anisotropy arising at the CoFeB/MgO interface. This anisotropy is often assumed to be uniaxial i.e. of the form E=–K₁cos²θ where θ is the angle between magnetization and normal to the plane of the layers. By performing detailed hard-axis magnetoresistance measurements between 5K and 300K on such MTJs of diameter ranging between 50nm and 150nm, it was observed that the anisotropy can also contain higher order terms of the form -K₂cos⁴θ. This higher order contribution can exist both in the free and reference layers. When the -K₂/K₁ ratio exceeds 0.5, the magnetic anisotropy changes from “easy-axis” to “easy-cone”. This means that in its ground state, the magnetization of the corresponding layer lies along a cone and can rotate in a very small field around the cone axis. The cone angle is directly related to the -K₂/K₁ ratio. The easy-cone regime has clear signatures in the shape of the hard-axis magnetoresistance loops. The existence of this higher order anisotropy was also confirmed by ferromagnetic resonance experiments on FeCoB/MgO sheet films. It is of interfacial nature and is believed to be due to spatial fluctuations at the nanoscale of the first order anisotropy parameter at the FeCoB/MgO interface. Such second order anisotropy of the storage layer can have several beneficial influences in STT-MRAM working principle. To trigger the magnetization reversal due to spin transfer torque, a more reproducible initial tilt angle is given to the storage layer magnetization by this easy-cone anisotropy than the one provided by random thermal fluctuations as in conventional STT-MRAM. This reduces the stochasticity of the switching and therefore the width of the distribution of switching voltage. It also allows much faster switching rate (to ns range). It also allows increasing the Figure of merit D/Ic where D is the thermal stability factor and Ic the switching current. It has however a drawback: it reduces the thermal stability factor by more than a factor 2. For applications requiring short retention (a few hours), the easy-cone anisotropy can be usable with existing materials. For applications requiring longer retention (e.g. 10 years), higher anisotropy materials would be needed to benefit from this easy cone anisotropy.

CoFeB/MgO based magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy are promising candidates for high density spin-transfer torque (STT) MRAM owing to their high tunneling magnetoresistance (TMR) and large interfacial magnetic anisotropy. As the MTJ shrinks in size, the MgO barrier must be thin enough to keep the junction resistance reasonably small. This may cause reliability problems such as electric shorts and barrier breakdowns.
Typically, the MgO film is deposited by rf-sputtering, but the sputtered MgO film contains a large amount of oxygen vacancy defects, which attributed to the leakage current of MTJs [1]. In this work, we have examined the effect of in-situ post oxidation (PO) of the MgO barrier with the intent to reduce the density of oxygen defects and improve the reliability of the MgO barrier.
On a 300-mm wafer, we fabricated a top-pinned MTJ characterized by the following stacked structure: bottom electrode/CoPt (6)/Ru (3)/Ta (1)/CoFeB (0.8)/ MgO barrier/CoFeB (1.7)/Ta (0.4)/ CoPt (6)/Ru (1)/CoPt (10)/top electrode (in nm). Two types of MgO barriers with resistance area product (RA) of 7 Ωμm² were compared; one is MgO with PO, prepared by sputtering a 0.86-nm-thick MgO layer followed by an in-situ 5s natural oxidation, and the other is MgO without PO, which consists of a 0.93-nm-thick sputtered MgO film. The MTJ size is about 50 nm in diameter.
MR loops of more than 700 devices were measured for both MTJs, with and without PO, and the MR ratio and resistance in the parallel state were extracted. It was found that the electrical shorting of the MTJs drastically reduced from 11.6% to 0.3% after PO process. Perhaps because of this, the average MR ratio of the MTJ with PO exhibits about 20% higher value than that of the MTJ without PO.
Furthermore, we conducted write endurance tests under bipolar pulse voltages of +1.05/-1.15 V using a time-dependent dielectric breakdown method [2]. The pulses had a constant width of 50 ns. This test was repeated until barrier breakdown occurred and the number of cycles at breakdown (N_BD) was determined. The N_BDs of both MTJs, with and without PO, were fitted by a Weibull distribution. The value at 63.2% of N_BD distribution (representing the number of cycles that causes 63.2% of devices to fail) was estimated to be 3.1×10⁹ cycles for the MTJs with PO: this number is two orders of magnitude bigger than that determined for the MTJs without PO.
To understand the improved reliability of the MTJs with PO, atom diffusion in CoFeB/MgO MTJ was investigated using electron energy-loss spectroscopy (EELS). The study revealed a clear suppression of Fe diffusion into the MgO layer, which might be of key importance for the high-reliable MgO barrier.
This work was partly funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).
[1] D. J. Kim et al., Appl. Phys. Lett. 97 (2010) 263502.
[2] A. Amara-Dababi et al., Appl. Phys. Lett. 99 (2011) 083501.