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Rainer Waser FZJ Forschungszentrum Jülich & RWTH Aachen University Outline Forschungszentrum Jülich Center of Nanoelectronic Systems for Information.

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Presentation on theme: "Rainer Waser FZJ Forschungszentrum Jülich & RWTH Aachen University Outline Forschungszentrum Jülich Center of Nanoelectronic Systems for Information."— Presentation transcript:

1 Rainer Waser FZJ Forschungszentrum Jülich & RWTH Aachen University Outline Forschungszentrum Jülich Center of Nanoelectronic Systems for Information Technology Scaling Projections for Resistive Switching Memories 1Introduction - Features and Classification of Resistive RAM 2Electronic switching effects - including FTJ (brief) 3Fuse-Antifuse switching effect (brief) 4Ionic switching effects - cation migration redox systems (electrochem. metallization cells) 5Ionic switching effects – anion migration redox systems

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3 Evolution of the memory density – geometry aspects - 3 R. Waser (ed.), Nanoelectronics and Information Technology, 2nd ed. Wiley, 2005

4 Limit: Dielectric Area Challenges Fundamentals: Physics & Chemistry Resistive Superparaelectric Limit Interface & Scaling Effects Ferroelectric DRAM Mega-Bit EraGiga-Bit EraTera-Bit Era Phase Change Electrochem. Metallization Redox-based Oxides Molecular Switches Random Access Memories - 4

5 write-Operation by large voltage pulses (typically with current compliance) read operation by small (sensing) voltage pulses Operation Electrical switching between ON(LRS) and OFF(HRS) state Polarity modes of RRAM I V Read RESET SET I V Read RESET SET Unipolar (symmetrical) - URS Bipolar (antisymmetrical) - BRS Memory cell Two-terminal element between electrodes Active Matrix: F 2, 1 cell / 1 T Passive Matrix: array size e.g. 8 x 8, approx 4 F 2 Basic Definitions of Resistive RAM History many reports since the 1960s mainly binary oxides, mainly unipolar switching CC - 5

6 homogeneously distributed effect? effect confined to filaments? Location of the switching event - In the electrode area along the entire path / in the middle area? at one of the interfaces? Location of the switching event - Between the electrodes asymmetry of the system (e.g. different electrode materials) Requirement of bipolar switching electrical stress as a precondition for switching sometimes: formation during first cycle(s) Forming process - 6

7 Classification of the switching mechanisms Resistive Switching Thermal effect Electronic effect Ionic effect Phase Change Effect (well known) Fuse-Antifuse effect Charge trap – Coulomb Barrier Charge injection – IMT transition effect Ferroelectric Tunneling barrier Cation migration - electrometallization Anion migration - redox effect R. Waser and M. Aono, Nature Materials,

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9 1.Charge trap effects / Coulomb repulsion Trapping at an interface – e. g. Taguchi Sensor effect or at internal traps (compare: Flash) – e.g. polymer - embedded metal nanoclusters 2.Insulator-Metal Transition effects Charge doping change of band structure 3.Ferroelectric tunneling barrier effects tunneling parallel / antiparallel FE polarization modification of the tunnel barrier (thickness or height) Variants General Effect Electronic Switching Mechanisms Charge injection or movement of displacement charges modification of the electrostatic barrier, bipolar switching Geometry Homogeneous current density scaling limits (?) - 9

10 Ferroelectric Tunnel Junction Basic Effect Different Tunneling currents parallel and antiparallel to the ferroelectric polarization (no lateral confinement required) References: Esaki (1968), Tsymbal, Kohlstedt et al, Science (2006) No reliable experimental evidence yet ! Results

11 State-of-knowledge Projections – Ferroelectric Tunnel Junction Memory Scaling limits Speed Energy dissipation Challenges No experimental verification yet; competition by redox-based processes. worse than MTJ, because the ferroelectricity fades below approx. 2 nm thickness (ab-initio theory & HRTEM experiment) no inherent speed limits because polarization reversal in ferroelectrics < 1ns Recharging of a tiny FE capacitor (much smaller than in FeRAM) low energy switching expected 1. realization of a pronounced effect unlikely 2. expected scaling limits not favorable

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13 Materials Fuse-Antifuse Switching Mechanism MIM thin film stack with I = transition metal oxide showing a slight conductivity e. g. Pt/NiO/Pt SET process Controlled dielectric breakdown e. g. by thermal runaway formation of a conducting filament RESET process Thermal dissolution of the filament (fuse blow) disconnected filament I. G. Baek et al. (Samsung Electronics), IEDM

14 Temperature profile - Thermal effect assisting other switching types? FEM simulation (Ansys ®) of metallic TiO filament (3 nm) in TiO2 matrix Pt TE Pt BE TiO 2 27 nm 54 nm 2 nm 5 nm Pt TE Pt BE TiO 2 27 nm 54 nm 2 nm 1 filament 3 filaments 390 K 1100 K Toggle between bipolar and unipolar switching has been possible by adjusting the current compliance; demonstrated for TiO2 thin films (Jeong et al. 2006) and Cu:TCNQ (Kever et al. 2006) High current compliance unipolar fuse/antifuse switching Relationship to other switching effects - 14

15 FEM Simulation of the RESET process 160 nm thick NiO film on n-Si with Au top electrodes U. Russo et al., IEDM

16 Current compliance Higher current compliance (~ 3 mA) leads to transition of the switching mode (BRS URS) This transition to URS is irreversible: (URS BRS) Details: see talk H. Schroeder & D.-S. Jeong (F6) and Jeong, Schroeder, Waser, APL89, (2006) Transition from the BRS to the URS mode Pt/TiO2(27nm)/Pt stack, sputter deposited, electroformated at 1mA for BRS operation LRS of a stable URS

17 State-of-knowledge Projections – Fuse-antifuse effect Scaling limits Speed Energy dissipation Challenges - thermo-chemical effect; - filamentary nature confirmed; - nature of ON-state filament still unknown (Ni? NiOx<1? else?) equal or larger than filaments in redox systems (perhaps larger because of the thermal nature of the effect) – 10 nm diameter ?? approx. 10 ns (speculative value) Relatively high, because of thermal nature; no scaling expected because of filamentary nature; material optimization will lead to reduction better understanding required in order to optimize material and processing

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19 Working principle ON-switching: cathode Ag filament formation Ag + + e Ag or OFF-switching: anode Ag Ag + + e or Electrolyte: Ag + ion conductor; Question: is the presence of Ag + ions required? What concentration? Required ingredients Type of cations: M/M z+ in a moderate area of the electrochemical potential series high exchange current density (Butler-Volmer equation) Examples: Ag, Cu, Ni, (and few more) M. Faraday (1834) Cation Migration Redox Systems Names: Electrochemical Metallization Cell (ECM); PMC; CBRAM - 19

20 Electrochemical deposition/dissolution Growth speed of a (cylindrical) nanofilament Faraday´s law Phase generation / dissolution rate Example: Ag filament of 10nm diameter at I = 1 A - 20

21 Ag Pt Ag + Oxidation of top electrode Ion migration Ag Pt Reduction at bottom electrode Electrodeposit formation Ag Pt Non-volatile conductive connection Ag Pt Dissolution of the conductive path under reverse bias Working Principle of the ECM Cell

22 Prototypical Electrochemical Cell – Pt / H 2 O / Ag Ag Pt I-V: bipolar switching Time evolution during switching on: formation of a Ag dendrite tree at the Pt electrode X. Guo, C. Schindler, S. Menzel, R. Waser, APL (2007) - 22

23 Electrolyte: GeSe x (or GeS x ) – dissolution of Ag + by photoassisted or thermally assisted oxidation of adjacent Ag phase; high driving force in case of excess Se in GeSe x (as polyanions). Structure: Ag 2 Se rich nanocrystals in amorphous GeSe x matrix What is special?: stable reservoir of Ag + ions (buffer) high Ag + mobility & activation energy (easy migration path in amorphous matrix?) Example: Ag + electrochemical GeSe x cells (M. Kozicki, 1997) Pt or W GeSe x +Ag 2 Se Ag 50nm GeSe on W - 23

24 K. Terabe, M. Aono, et al., Nature (2005) Special case – the Atomic Switch Quantized conductance atomic switch – tunneling gap as an electrode - 24

25 Aono group: Cu/Cu 2 S X-bar Qimonda: Mbit demonstrator - 25

26 Ag/Ag-Ge-Se/Pt cells fabricated in vias of a Si3N4 layer. Via diameter 2.5 m by photolithogrpahy. Low-Write Currents in Ag + electrochemical GeSe x cells I-V curves with current compliance set to 1 nA (C. Schindler, R. Waser, et al., NVMTS 07) Latest results (unpublished): 10 pA ! - 26

27 Prospect of Multi-Bit Data Storage The write current determines the ON resistance and erase current. By varying the write current over 5 orders of magnitude, the ON state is variable over the same range increasing the opportunity for multi-bit data storage. 2.5 µm via C. Schindler et al., NVMTS 07

28 Cell Size Dependence The ON state is cell size-independent. Electrodeposition starts at the point of highest E-field. Tip of the growing metallic path is the most probable site for further electrodeposition. Pt Ag Although the cross sectional area varies by a factor of 400, the ON resistance does not vary by a factor of more than 5 between the smallest and the largest cell. C. Schindler et al., NVMTS 07

29 Similar switching characteristics as observed in Ag-Ge-Se! V on = 400 mVV off = -100 mV I write = 25 µAI erase = -12 µA-100 mV < V read < 400 mV Ag-Ge-Se Unconventional electrolyte: SiO2 Sputtered SiO2 thin film For comparison C. Schindler, M. Kozicki, R.Waser, submitted

30 No Yes Modeling: MD approach to Browian Dynamics 40 nm S. Menzel et al., to be published

31 State-of-knowledge Projections – ECM effect Scaling limits Speed Energy dissipation Challenges - reasonably well understood; - filamentary nature confirmed; - some open questions with respect to nanoclusters in GeSe & OFF switching Very thin filaments conceivable (< 2 nm ??); i. e. min. F = 5 nm ?? < 30 ns ? (perhaps even faster; compare to fuse-antifuse) ON switching: electrochem. reduction of Ag+ within one filament (or fraction of) + migration energy (Faraday transfer number = 1 assumed) OFF switching: ?? improvement of the voltage – speed bargain;

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33 Capacitor-like structure with Cr-doped SrZrO 3 thin films (Ba,Sr)TiO 3 thin films SrTiO 3 Single crystals as resistive element 1 A. Beck, J. G. Bednorz, Ch. Gerber, C. Rossel and D. Widmer, Appl. Phys. Lett. 77, 139 (2000). SrZrO 3 (0.2 %Cr) SrTiO 3 (100) Pt SrRuO nm Examples of Resistive Switching in Oxides Characteristics after forming process: reversible switching between stable impedance states non-volatile non-destructive read-out multilevel memory Redox-based resistive switching type - 33

34 Example: SrTiO 3 :1at% La, T=1100°C High temperature defect equilibria Schottky equilibrium (for SrO lattice) Redox equilibrium law of mass action local electroneutrality homogeneous material Preconditions [D] - 34

35 0nm 25nm 5 m NC-AFM of etched (100) surface of strontium titanate (estimated density of dislocations of 4*10 9 /cm 2 ) c-AFM mapping of local conductivity of (100) surface of thermally reduced strontium titanate -> hot spots density of strong hot spots ~5*10 10 /cm 2 density of weak hot spots ~5*10 11 /cm 2 8nA 4nA 2nA 8nA 7pA U=3.5V Electrical characteristics of dislocations: Preferential conductivity paths in SrTiO 3 single crystals K. Szot, 1999,

36 I/V 1 I/V Applied bias (V) metallic semicond. insulating I/V 1 I/V nm 1nm I ~ 1.2 nA I ~0.009nA Current (nA) (nA) Thermal preformation by reduction annealing: conductive Tip AFM Mapping – types of I-V Characteristics SrTiO 3 s.c. thermally reduced at 850 C, pO2 ~ bar K. Szot, W.Speier, G. Bihlmeyer, R. Waser, Nature Materials,

37 a b 50nm 0nm 3nm 50nm 0nm 3nm Applied bias (V) n1n1 n2n2 n3n3 n4n4 n5n5 n6n6 n7n n 15 Current (nA) 1000 Resistance (Ω) Ω Ω Distance (nm) c d non-metallic metallic on off K. Szot et al., Nature Materials,

38 Edge dislocations in SrTiO 3 crystal (stacking fault) Jia et al PRL.(2006) TiO 2 SrO/SrO SrO TiO 2 SrO TiO 2 /TiO 2

39 Formation of localized metallically conducting sub-oxides by electroreduction N V O cm -3 A-B Ti +2 Ti +3 Ti +4 Ti +2 Ti +3 Ti +4 AB Extended defects after reduction - 39

40 Redox Reactions at the Electrodes - Interconnected network of extended defects - Switching ON – oxygen vacancy accumulation near the surface; conduction through the Ti (4-x)+ sublattice - 40

41 Learning from lateral cells SrTiO 3 K. Szot, etc. Nature Materials, 2006 M. Janousch et al. Adv. Mater. 19, 2232 (2007) Optical micrograph and CAFM (above) and Cr K-edge XANES (right) mapping

42 I(mA) Φ1Φ1 Φ2Φ2 Φ3Φ3 Φ4Φ4 Φ 5 & Φ 6 V(V) Switching of SrTiO 3 (100), Potential distribution RT, p=10 -8 mbar Interface I Interface II I(mA) V(V) Φ 1 – Φ 2 Φ 2 – Φ 3 Φ 3 – Φ 4 Φ 4 – Φ 5 Φ 5 – Φ 6 E max >10 4 V/cm E max ~30V/cm E max ~20V/cm E max >10 4 V/cm Φ1Φ1 Φ2Φ2 Φ3Φ3 Φ4Φ4 Φ6Φ6 Φ5Φ5 I-Source Generator Electrometer SrTiO 3 crystal 3mm K. Szot, to be published

43 K. Szot, R. Dittmann, R. Waser, rrl-pss, 2007 LC-AFM characterization of epi-STO (10nm) / SRO / STO - 43

44 Write / erase patterning LC-AFM write/erase processes on epi-STO (10nm) / SRO / STO K. Szot et al., rrl-pss,

45 K. Szot, R. Dittmann, et al., rrl-pss, 2007 Temperature dependence - 45

46 Local redox equilibria Continuity equation Poisson equation Concentration profiles, field profiles, space charge profiles N V O cm -3 A-B Ti +2 Ti +3 Ti +4 Ti +2 Ti +3 Ti +4 Switching model Formation and resistive switching of redox-active dislocations 3-D network of extended defects? Nanoscale effects ? Phase formation or frozen kinetics? Electronic charge injection mode (Schottky emission, FN ?) Fast transport tracks ? Current questions K. Szot,

47 ΔxΔx i = 1 i = ni = 2 Switching model – former results T. Baiatu, K. H. Härdtl, R. Waser, J. Am. Cer. Soc. (1990)

48 Switching model – recent results D.-S. Jeong et al., to be published Pt/TiO2(27nm)/Pt stack, sputter deposited, electroformated at 1mA for BRS operation

49 State-of-knowledge Projections – redox-type memory effect Scaling limits Speed Energy dissipation Challenges - redox mechanism suggested during formation; - details of the switching mechanism need to be studied (thermal assisted?) - nature of ON-state filament: conducting (sub-)oxide? estimated filament diameter: 2 – 10 nm ? perhaps similar to ECM; possibly thermal assisted, i.e. additional dissipation Much better understanding required in order to optimize material and processing (and to make more precise projections) < 30 ns ? (perhaps even faster; compare to fuse-antifuse); literature reports of values of approx. 10 ns


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