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abstract: Abstract Resistive random access memory (RRAM) devices with a nickel top electrode form controllable metal nanofilaments and have robust resistive switching performance. We investigate the Ni/HfO 2 /SiO x /n Si RRAM structure, which forms a Ni rich defect in the silicon underneath the Ni nanofilament in the dielectric layers after a SET process. The formation of these defects may affect the retention of the devices, so we applied a detailed Finite Element Method and Kinetic Monte Carlo approach to simulate the Ni rich defect evolution under different compliance current settings. We confirm that the chemical composition of the defects is metallic NiSi 2 and that their size is determined by the compliance current. These simulation results are supported by in situ STM like experiments inside a transmission electron microscope (TEM) NiSi 2 defects are shaped as truncated square pyramids, and we show that this is due to the low activation energy of Ni migration along the (111) crystal plane of Si. Our results demonstrate that electromigration is the main driving force for Ni migration initially, after which thermal migration and especially stress migration become the dominant mechanism. This work gives a fascinating example of an as grown metal insulator semiconductor (MIS) system that can be controllably converted to a metal insulator metal (MIM) configuration for down scaled RRAM operation.
author_names: Sen Mei

Michel Bosman

Nagarajan Raghavan

Xing Wu

Kin Leong Pey
title: Compliance current dominates evolution of NiSi2 defect size in Ni/dielectric/Si RRAM devices
venue: Microelectron. Reliab.
year: 2016

abstract: Spinel ferrites are an important class of compounds which have a variety of electrical, magnetic, and catalytic applications. Neodymium doped cobalt ferrites with composition CoNdxFe2 xO4 where x is 0.00 0.20 were synthesized by a sol gel method. A sol gel method was preferred because it has good control over stoichiometry, crystallite size, and particle size distribution. An X ray diffraction (XRD) technique was used for structural analysis, and a crystal structure was found to be spinel for all samples. Fourier transform infrared spectroscopy (FTIR) of the samples was also performed for structural information. DC resistivity has been studied, as a function of temperature, by using a two probe method. It showed direct dependence on temperature and inverse dependence on the concentration of Nd dopant. I V characteristics of samples at room temperature were analyzed. The non linear I V curves showed a non ohmic behavior. When voltage was reversed, it did not follow the same values but showed a hysteresis loop type trend. This confirms the presence of resistive switching effect in this voltage range at room temperature. The studied material is a potential candidate for resistive random access memory application.
author_names: S Abbas

Mohammad Saeed

A Munir

Muhammad Anis-Ur-Rehman
title: Study of Transport Properties in Co (Fe Nd) Ferrite Nanoparticles for RRAM Applications
year: 2017

abstract: We report the first demonstration of in situ electrical biasing and resistive switching of TiO2 based RRAM while observing oxygen vacancy migration under the TEM. Specifically, repeated extension and retraction of Wadsley defects due to the migration of oxygen vacancies was observed to be concurrent with the SET and RESET processes (respectively) in the hysteretic I V behavior of single crystal TiO2 devices. Additionally, structural evolution within a 20 nm size grain was observed during SET processes in ALD grown TiO2 devices using STEM HAADF imaging.
author_names: Jonghan Kwon

Yoosuf N Picard

Marek Skowronski

Abhishek A Sharma

James A Bain
title: In situ biasing TEM investigation of resistive switching events in TiO2 based RRAM
venue: 2014 IEEE International Reliability Physics Symposium
year: 2014

abstract: Two dimensional materials are gaining more and more attention in the field of electronic devices because of their unique advantages, such as high crystalline quality and clean and flat contact planes; compared to traditional materials, the use of two dimensional materials as the working layer of a resistive random access memory (RRAM) has the potential to further reduce the device size and enhance its performance. Herein, a black phosphorus (BP) single crystal flake passivated by hafnium oxide is used as the working layer for an RRAM. The devices show a switching on/off ratio of 102 in more than 100 cycles, and others can even be as high as 106. We speculated the working mechanism of the BP RRAM based on the results of serial experiments and transport analysis.
author_names: Xiaoyuan Yan

Xueting Wang

Boran Xing

Yingxia Yu

Jiadong Yao

Xinyue Niu

Mengge Li

Jian Sha

Yewu Wang
title: Resistive memory based on single crystalline black phosphorus flake/HfOx structure
year: 2020

abstract: Resistive random access memory (RRAM) utilizes the resistive switching behavior to store information. Compared to charge based memory devices, the merits of RRAM devices include multi bit capability, smaller cell size, and energy per bit ~fJ/bit) In this chapter, we review different perovskite material based resistive random access memories (RRAMs) We first introduce the history of RRAM development and operational mechanism of conduction, followed by a review of two types of materials with perovskite crystal structure. One is conventional perovskite oxides (PCMO, a LCMO, etc. and the other is perovskite halides (organic inorganic hybrid perovskites and inorganic perovskites) that have recently emerged as novel materials in optoelectronic fields. Our goal is to give a comprehensive review of perovskite based RRAM materials that can be used for neuromorphic computing and to help further ongoing development in the field.
author_names: Jiaqi Zhang

Wubo Li
title: Perovskite Materials for Resistive Random Access Memories
year: 2019

abstract: Abstract Multiferroic B i F e O 3 (BFO) and B i 0.97 Y 0.03 F e 0.95 S c 0.05 O 3 (BYFSO) films were fabricated on FTO coated glass substrate using sol gel spin coating technique. Y Sc co doping induces the structural distortion without changing the crystal structure (rhombohedral: R3c) of BFO. The electrical and magnetic properties of co doped film may tuned due to increase in grain and particles size. The high dielectric constant e r 290 and low loss t a n d 0.084 indicates the improved insulating properties in BYFSO film. Increase in oxygen vacancy concentration in BYFSO film is explained with leakage current density, which follows Ohmic SCLC conduction mechanism. Switching of 180 0 domains into non 180 0 (maximum 71 0 and 109 0 type) ensures improved ferrolectric and piezoelectric properties in BYFSO film. The magnetic properties enhance significantly with saturation magnetization M s remanent magnetization M r are 9.45 e m u c m 3 6.154 e m u c m 3 respectively at coercive H c field of 0.822 kOe. Room temperature multiferroic BYFSO film with improved insulating and magnetic properties may be useful for non volatile RRAM memory application.
author_names: Ashis Kumar Jena

J Arout Chelvane

J Mohanty
title: Simultaneous improvement of piezoelectric and magnetic properties in diamagnetic ion modified BiFeO3 film
venue: Journal of Alloys and Compounds
year: 2019

abstract: Formation of metal rich conductive filaments and their rearrangements determine the switching characteristics in HfO2 based resistive random access memory (RRAM) devices. The initiation of a filament formation process may occur either via aggregation of pre existing vacancies randomly distributed in the oxide or via generation of new oxygen vacancies close to the pre existing ones. We evaluate the feasibility of vacancy aggregation processes by calculating the structures and binding energies of oxygen vacancy aggregates consisting of 2, 3 and 4 vacancies in bulk monoclinic (m) HfO2 using density functional theory (DFT) We demonstrate that formation of neutral oxygen vacancy aggregates is accompanied by small energy gain, which depends on the size and shape of the aggregate. In the most strongly bound configurations, vacancies are unscreened by Hf cations and form voids within the crystal, with the larger aggregates having larger binding energy per vacancy 0.11 to 0.18 eV) The negatively charged di vacancy was found to have similar binding energies to the neutral one, while the positively charged di vacancy was found to be unstable. Thus aggregation process of either neutral or negatively charged oxygen vacancies is energetically feasible.
author_names: Samuel R Bradley

Gennadi Bersuker

Alexander L Shluger
title: Modelling of oxygen vacancy aggregates in monoclinic HfO2: can they contribute to conductive filament formation?
venue: Journal of physics. Condensed matter an Institute of Physics journal
year: 2015

abstract: DOI: 10.1002/aelm.201700608 gap structure cannot be able to integrate in vertical direction. In 2008, Salahuddin et al.[8] proposed negative capacitance transistor using the polarization change induced abrupt switch with steep slope (Figure 1b) But at present, the integration of the ferroelectric materials with smaller hysteresis window is remained to be studied. A critical question that can be considered is if there is another mechanism that can be applied to build novel transistor with steep slope. Here, we propose a new kind of steep slope transistor named "filament transistor" with SS down to sub 10 mV dec 1. The abrupt filament set/reset induced by gate can enable the abrupt current change with steep slope. Conventional resistive random access memory (RRAM) is usually a simply two terminal device with a metal oxide layer sandwiched by two metal electrodes. Due to the high carrier densities (above 1020 cm 3) of metal electrode, the gate E field is screened by the bottom electrode without any gate tunability. In this work, the bilayer grapheme (BLG) is used as bottom electrode with low carrier densities[20] (around 1012 cm 2) connecting to the source terminal, which can let the gate E field effective penetrate through the graphene. The channel material above graphene is AlOx with 5 nm thickness in vertical direction, so that the gate E field can effectively control the filament formation/rupture inside the AlOx. The top electrode Al above the AlOx is connected to drain terminal. The schematic device structure and operation mechanism for the filament transistor at on state and off state are shown in Figure 1c. At positive gate voltage condition, the positive E field from gate can attract oxygen ions moving toward the bottom surface. In this case, the effective oxygen ions density is low, which is named as the "oxygen ions depleted" state. As a result, it requires higher set voltage at drain terminal. When negative gate voltage is applied, the negative E filed from gate can repel the oxygen ions to top electrode, which is named as "oxygen ions accumulated" state. The "oxygen ions accumulated" state can promote the filament formation, which means a lower set voltage at drain terminal can be sufficient for the filament formation. The device was fabricated as following: the single crystal bilayer graphene was obtained via chemical vapor deposition (CVD) method or mechanical exfoliation. For CVD method, first one single layer graphene (SLG) with 50 mm size was grown and then through the center defect, another SLG was grown followed by oxygen patterning. The BLG was transferred/exfoliated to a SiO2 (300 nm)/Si substrate with a marker. This was There is a growing trend of developing steep slope transistors with subthreshold swing (SS) below 60 mV dec 1. At present, there are mainly three kinds of steep slope transistors: tunneling transistor, negative capacitance transistor, and nano electromechanical relay. Here, a new kind of steep slope transistor named "filament transistor" is proposed with SS down to sub 10 mV dec 1. Due to the gate E filed can penetrate through the graphene, the gate can effectively control the filament formation/rupture inside the 5 nm AlOx with very steep slope. In reverse sweep, the gate can induce the filament formation with SS down to 4.6 mV dec 1. In forward sweep, the gate can break the filament with SS down to 12.2 mV dec 1. Taking the advantages of nanofilament, the filament transistor holds a great potential to scaling down to 5 nm without performance changing.
author_names: He Tian

Xuefeng Wang

Haiming Zhao

Wentian Mi

Yi Yang

Po-Wen Chiu

Tian-ling Ren
title: A Graphene Based Filament Transistor with Sub 10 mVdec 1 Subthreshold Swing
year: 2018

abstract: Resistance random access memory, short RRAM, which employs two or more resistive states of a material for data storage, has attracted considerable attention as a highly scalable future non volatile memory concept. 1 2 These memory cells that can also be described as so called memristors are particularly interesting when multilevel resistance values or even analogue values should be stored and processed. 3 5 A large variety of binary and ternary oxides exhibit resistive switching phenomena, however, the details of the complex microscopic mechanisms are rarely understood and depend strongly on the specifi c material combination. In the search for promising oxide materials for future non volatile memories, special attention has to be paid to their scaling capabilities. The issue of scaling is strongly linked to the question of, whether the switching current is distributed homogeneously across the device area or localized to one or a few conducting fi laments. While in the former case the scaling limit will be connected to the minimum device area, that guarantees suffi cient switching currents for a reliable circuit operation, in the latter case, scaling might suffer from too large fi lament dimensions or their insuffi cient density and regularity within the material. Complex transition metal oxides, e.g. manganites, 6 9 titanates and zirconates, 10 11 usually exhibit different resistance states at opposite polarities of electrical stimulation. It has become widely accepted that this so called bipolar resistive switching is connected with a voltage driven oxygen vacancy movement and a resulting redox process. 12 Both, fi lamentary as well as homogenous switching has been reported in the literature. For thermally reduced SrTiO 3 single crystals it has been clearly demonstrated by conductive tip atomic force microscopy (conductive AFM) that resistive switching at free surfaces occurs along conducting fi laments which can be identifi ed with the exits of dislocations. 13 For crystalline SrTiO 3 thin fi lm samples as well as amorphous TiO 2 capped between macroscopic electrodes, the indispensable electroforming process results in the formation of a single m m size fi lament. 14 18
author_names: Ruth Muenstermann

Tobias Menke

Regina Dittmann

Rainer Waser
title: Coexistence of filamentary and homogeneous resistive switching in Fe doped SrTiO3 thin film memristive devices.
venue: Advanced materials
year: 2010

abstract: The antimony atom in the crystal structure of [ammonium (rram cyclohexane 1,2diaminetetraacetato)antimonate(III) [ammonium chloride] which is (4O 2N) chelated by the diaminetetracarboxylato group [Sb 0= 2.216(5) 2.433(6) Sb N 2.325(5) A] exists in a ps pentagonal bipyramidal geometry in which the lone pair occupies an equatorial site. The (cyclohexane 1,2diaminetetraacetato)antimonate(III) anion lies on a two fold axis, and is linked to two adjacent anions through the carboxyl oxygen atom that forms the longer Sb 0 bond to result in a zig zag chain. If these weak interactions [Sb O 3.153(6) A; O Sbl 0 3 108.1(2)deg] are regarded as formal bonds, the eightcoordinate antimony atom shows distorted dodecahedral coordination. Lone pair stereochemistry is discussed quantitatively in relation to the structures of the previously reported polyaminocarboxylate complexes of antimony(III) and bismuth(III) Crystal data: CuFLeN tOgClSb, FW 535.59, monoclinic, C2/c, a 12.534(2) b 23.570(3) c 8.708(1) A, b 133.22(1)deg, Dx.ray 1.898 g cm" m 13.77 mm" F(000) 1080 for Z 4. Introduction The VSEPR model [1] incorrectly predicts the molecular geometries for a number of the heavier congeners of subvalent fourth group metal compounds owing to the stereochemical inactivity of the lone pair of electrons [2] Stereochemical inertness is also noted for some fifth group metal compounds; in particular, the bismuth(IIl) complexes of polyaminocarboxylates display no stereochemical activity [3] as seen from the cubic environment found in the di(nitrilotriacetato)bismuthate(III) trianion [4] Owing to the smaller size of the antimony atom compared with the bismuth atom, lone pair effects dominate, these deforming the polyhedra of antimony(III) compounds. When a diaminepolycarboxylato ligand chelates in a hexadentate (4O 2N) manner, the resulting polyhedron assumes a distorted ps pentagonal bipyramid and the lone pair will be either axially or equatorially positioned [5] The triaminepolycarboxylato ligand is able to give rise to ps pentagonal bipyramidal as well as the ps monocapped square antiprismatic geometries [6] For such antimony(III) complexes, however, the correct geometry should assigned only when all weaker or secondary interactions have been included [7] these interactions are typically intermediate between normal covalent bond lengths and van der Waals distances. The (cyclohexane 1,2diaminetetraacetato)antimonate(III) anion has been characterized as its hydrated sodium salt; the three water molecules engage in extensive hydrogen bonding, and appear to be necessary to consolidate the structure [8] In the present study, the sodium cation is replaced by the ammonium counterion, but the synthesis gave the ammonium antimonate(III) as a 1/1 complex with ammonium chloride (Scheme I) Its crystal structure and the polyhedral distortion are discussed in terms of the displacement of the antimony atom from the 'best' centroid of the coordination polyhedron [9] Experimental Sodium antimony 1 tartarate (3.5 g, 5.7 mmol) prepared by treating antimony oxide with racemic tartaric acid and sodium hydroxide [10] was dissolved in water (25 mL) and the solution was acidified with concentrated hydrochloric acid (2 mL) The filtered solution was added dropwise to a soltuion of iram cyclohexane 1,2 diaminetetraacetic acid (2.0 g, 5.7 mmol) dissolved in a mixture of water (128 mL) and IN ammonia (22 mL) The mixture was heated under reflux for 5 h. The solution was concentrated to about 5 mL; the colorless crystals that precipitated have no definite melting point, but begin to decompose
author_names: Shengzhi Hu

Yizhi Fu

Lisbeth E Toennessen,

Ruven L Davidovich

Seik Weng Ng,
title: LONE PAIR STEREOCHEMISTRY OF ANTIMONY(III) POLYAMINOCARBOXYLATES. CRYSTAL STRUCTURE OF [AMMONIUM (trans CYCLOHEXANE 1,2 DIAMINETETRAACETATO) ANTIMONATE][AMMONIUM CHLORIDE]
year: 1998