Novel Devices and Circuits for Computing

Transcription

1 Novel Devices and Circuits for Computing UCSB 594BB Winter 2013 Lecture 3: ECM cell

2 Class Outline ECM General features Forming and SET process RESET Variants and scaling prospects Equivalent model

3 Electrochemical Cell Other names: Solid state electrolyte memory Conductive bridge memory Programmable metallization cell Active Electrode (e.g., Ag, Cu) Solid state electrolyte with host ions M Z+ ions (e.g. Ag 2 S, Cu 2 S, RbAg 4 I 5 ) or insulator doped with M Z+ (e.g. SiO 2, WO 2, GeS, GeSe) typically amorphous or crystalline Counter Electrode (e.g., Pt, Ir, Au, W)

4 Electrochemical Cell

5 Typical I V and Process Cartoon (A) OFF state (could be thought of half cell) (B, C) SET process: (i) Ag oxidized to Ag + ; (ii) drift in electric field ; (iii) reduced and electro crystallized (D) SET process: complete bridging (compliancerelated) (E) RESET process: opposite of SET Ag Ge Se

6 SET and Electroforming Process (a) Anodic oxidation and dissolution of M: M M z+ +ze (b) Migration of M z+ across thin film (migrations is strongly enhanced by extended df defects) (c) Reduction and electrocrystallization of M on the surface of CE: M z+ +ze M Formation of filament growing (typically) in the direction of the active electrode. The growth is limited by current compliance 2D system: Radial ion motion

7 Radial Transport Ag Ge S Ag Ag H 2 O Pt Pt H 2 O

8 Filamentary Nature of ON state Many filaments but typically only one champion In lateral devices filament grows at the surface less stress Growth in bulk has to overcome mechanical stress typically happens along extended defects Ag/As 2 S 3 :Ag/Au lateral device Optical micrograph

9 Joule heating (if large current >1 ua) Field and temperature enhanced filament dissolution and redeposition before rapture Purely electrochemical dissolution after filament rapture (?) Complete dissolution of the filament (?) Growth and dissolution by Faraday law: r J Faraday law: r J Mz+ RESET Process

10 Nature of Nonlinearity electrode electrode ion hoping e oxidation z + z + + V e reduction equilibrium Butler Volmer equation for oxidation reductions Exponential for high overpotentials Energy profile fl of an electron transfer reaction between a metal ion M at the surface of the metal electrode and a corresponding cation M Z+ Overpotential applied to interface

11 Nonlinear Kinetics Ag/Ag Ge S/W g / Good paper for presentation

12 Some Controversy: Rate limiting step and fully dissolved filament? Waser theory: Forming is bulk limited Forming create a easy diffusion channel Reset fully dissolve filament and set/reset are redox limited Another explanation is that the filament does not dissolve completely and gap is thickness independent Note very low set/reset currents Cu/SiO 2 ECM cell

13 Some Controversy: Filament Growth and Dissolution

14 Some Controversy: Filament Growth and Dissolution

15 Variants : Bi Layer ECM Similar mechanism Highly conductive MIEC (mixed ionic/electronic conductor) Fulfills the role of AE, i.e. supply of ions No voltage drop ion motion by diffusion Some of the best devices more robust natural nonlinearity Homogeneous transport in some cases reported(?) Cu/Cu:TCNQ (tetracyanoquinodimethane)/al 2 O 3 /Al Cu:CuTe x Cu:GeS x

16 Variants: gap type ECM Vacuum nanogap Reverse direction of the growth Similar structure: STM tip as top electrode

17 Scaling Prospects and Multilevel Capabilities Barrier shape modulation (including Shottky effect) as a result of removal of ions)

18 Multilevel Capabilities tunneling (nonlinear ON) galvanic (linear ON) Huge dynamic range! 10-3 Cu urrent (A A) V A Ag a-si Voltage (V) Pt Resistive switching in electrochemical metallization memory cells, PhD Thesis by Christina Schindler

19 Equivalent Circuit Model (no battery effects) Z f faraday impedance Ci,Ag and Ci,Pt interface capacitance Re and Ce capacitance of electrolyte Rp electronic leakage