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Rainer Waser JARA-FIT @ FZJ Forschungszentrum Jlich & RWTH Aachen University Outline Forschungszentrum Jlich 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 Slide 2 - 2 Slide 3 Evolution of the memory density geometry aspects - 3 R. Waser (ed.), Nanoelectronics and Information Technology, 2nd ed. Wiley, 2005 Slide 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 Slide 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: 6...10 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 Slide 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 Slide 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, 2007 - 7 Slide 8 - 8 Slide 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 Slide 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 Slide 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 Slide 12 - 12 Slide 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 2004 - 13 Slide 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 Slide 15 FEM Simulation of the RESET process 160 nm thick NiO film on n-Si with Au top electrodes U. Russo et al., IEDM 2007 - 15 Slide 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, 082909 (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 Slide 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 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, 2004 - 35 Slide 36 I/V 1 I/V 2 -0.5 0.0 0.5 Applied bias (V) 0.01 0.001 100 10 1 1000 0.1 metallic semicond. insulating I/V 1 I/V 2 1.0 0.1 10 0.01 50nm 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 ~ 10 -20 bar K. Szot, W.Speier, G. Bihlmeyer, R. Waser, Nature Materials, 2006 - 36 Slide 37 a b 50nm 0nm 3nm 50nm 0nm 3nm Applied bias (V) 0 1234 5 n1n1 n2n2 n3n3 n4n4 n5n5 n6n6 n7n7 800 600 400 200 0 n 15 Current (nA) 1000 Resistance () 1.4 10 10 10 6 10 10 8 3.2 10 6 80 0 40 Distance (nm) 10 12 c d non-metallic metallic on off K. Szot et al., Nature Materials, 2006 - 37 Slide 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 Slide 39 Formation of localized metallically conducting sub-oxides by electroreduction N V O 10 22 cm -3 A-B Ti +2 Ti +3 Ti +4 Ti +2 Ti +3 Ti +4 AB Extended defects after reduction - 39 Slide 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 Slide 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 Slide 42 I(mA) 11 22 33 44 5 & 6 V(V) Switching of SrTiO 3 (100), Potential distribution RT, p=10 -8 mbar Interface I Interface II I(mA) V(V) 1 2 2 3 3 4 4 5 5 6 E max >10 4 V/cm E max ~30V/cm E max ~20V/cm E max >10 4 V/cm 11 22 33 44 66 55 I-Source Generator Electrometer SrTiO 3 crystal 3mm K. Szot, to be published Slide 43 K. Szot, R. Dittmann, R. Waser, rrl-pss, 2007 LC-AFM characterization of epi-STO (10nm) / SRO / STO - 43 Slide 44 Write / erase patterning LC-AFM write/erase processes on epi-STO (10nm) / SRO / STO K. Szot et al., rrl-pss, 2007 - 44 Slide 45 K. Szot, R. Dittmann, et al., rrl-pss, 2007 Temperature dependence - 45 Slide 46 Local redox equilibria Continuity equation Poisson equation Concentration profiles, field profiles, space charge profiles N V O 10 22 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, 2004 - 46 Slide 47 xx i = 1 i = ni = 2 Switching model former results T. Baiatu, K. H. Hrdtl, R. Waser, J. Am. Cer. Soc. (1990) Slide 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 Slide 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 Slide 50