novel devices circuits computing - ucsbstrukov/ece594bbwinter2013/veiwgraphs/...winter 2013 lecture...
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Novel Devices and Circuits for Computing
UCSB 594BBWinter 2013
Lecture 6 : TCM cell
Class OutlineClass Outline
• TCM = ThermoChemical Memory• TCM = ThermoChemical Memory– General features
RESET F i d SET h i d– RESET, Forming and SET mechanisms and modeling
Bi Pi t• Big Picture
LargeLarge choice of materials
Typical I‐V and Switching Mechanism Cartoon for TCM DevicesCartoon for TCM Devices
Z O TiO F 2O3 C O C O‐ ZrOx, TiOx, Fe2O3, CoO, CuO, NiO, Al2O3, SiO2(highly resistive virgin film due to deep traps / neutral p p /defects)
‐ Works with symmetric electrodesLinear ON state; high ON/OFF‐ Linear ON state; high ON/OFF
‐ ON state is controlled by current compliance
‐ Current compliance for SET switching is essential otherwise could lead to permanent breakdown
Filamentary Conduction in l / /Planar Pt/CuO/Pt
‐ SEM image‐ XRAY absorption
‐ PEEM
XAS shows oxidation states of Cu: 0 in Cu; and +1 in Cu2O in a channel as opposed to +2 in CuO elsewhere
Typical Temperature DependenceTypical Temperature Dependence
‐ Metallic ON states‐ Thermally activated OFF and virgin state with 0.1 … 0.3 eV (Schottky and or
Poole Frenkel)‐ Some samples shows metallicSome samples shows metallic behavior in OFF state when cooled down
Switching Mechanism: Forming and SET
Energy of Formation for Transition Metalfor Transition Metal
Oxides
‐ Oxides with lower valance of cations are more stable for increasing T at given PO2more stable for increasing T at given PO2
Switching Mechanism: Forming and SET
‐ Two step process:
(1) Electrical breakdown(2) Vacancy migration
‐ Some devices shows both unipolarand bipolar behaviour
‐ Neutral vacancies ‐ Charged vacanciesNeutral vacancies Charged vacancies
1D SimulationsSET RESET
(a) Continuous equation for Fick‐Soret(b) Steady heat profile(c) Ad‐hoc metal to insulator condition(c) Ad hoc metal to insulator condition(overlap of Bohr radius ~ good approximation for Magnelli phases)
Experimental TestExperimental Test
Main observation = change of sweeping rate changes direction of switching
More Accurate Modeling of RESET Transition
More Accurate Modeling of RESET Transition
Scaling and Application ProspectsScaling and Application Prospects
• Heating is EssentialHeating is Essential– Scaling of current
Thermal Crosstalk– Thermal Crosstalk
• Unipolar devices allows for diode integration
Big Picture: Resistive Switching By Main Driving Forces and Types of CellsMain Driving Forces and Types of Cells
Field dominating thermal dominatingField + thermal Concentrationgradient
Vn
-10
-5
0
5
10
Cur
rent
(mA
)
-4
-2
0
2
4
Cur
rent
(mA
)
-4
-2
0
2
4
Cur
rent
(mA
)
x x+a
U A
El h i l t lli ti (ECM) ll
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Voltage (V)
-1.0 -0.5 0.0 0.5 1.0Voltage (V)
-1.0 -0.5 0.0 0.5 1.0Voltage (V) Field
gradient
• Elecrochemical metallization (ECM) cells – Oxidation, drift and reduction of electrochemically active
electrode (such as Ag)• Valence change memories (VCM) cell
Temperature gradientT
Eaq
– Migration of anions (oxygen ions) leads to change in stochiometry (and hence redox of cations) with changes in resistance
• Thermochemical memory (TCM) cellTak
B
y ( )– Thermophoresis induced change in stochiometry (valence)
A More Complete Picture: Few Other h dd dMechanisms Added