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Novel Devices and Circuits for Computing
UCSB 594BBWinter 2013
Lecture 1: Phenomenology and Basics of Resistive Switching
Course OutlineCourse Outline
Mostly focus on resistive switching memories (Devices, Circuits and y g ( ,Applications)
Course work ) d1) Papers to read
2) Project
GradingGrading 1) Paper presentation 2) Class project
Web site: www.ece.ucsb.edu/~strukov/
Textbook (not required): Nanoelectronics and Information Technology , 3rd editionISBN: 978 3 527 40927 3ISBN: 978‐3‐527‐40927‐3
Tentative Topics on Resistive Switching PhenomenaPhenomena
• Devices– Phenomenology, basics and performance aspects– Switching mechanism for ECM, VCM, TCM cells and some others
• Circuitsb d– Xbar and CMOL
• Applications– Memories (passive crossbar, complimentary resistive switch …)
L i (FPGA li ti ifi i li ti l i li ti i– Logic (FPGA, application specific, implication logic, applications in security and redesigning memory hierarchy …)
– Neuromorphic (STDP, STP and LTD, Pavlov’s dog, classification and pattern recognition, Hopfield networks, …)
Comparison to other competitive emerging devices and circuits whenever appropriate
Very Small Subset of Review Literature on Resistive Switching Eff t d th i A li tiEffects and their Applications
– Nanoelectronics and Information Technology , 2nd editionh id “ i i i ”• Chapter 24 G Snider, “Cognitive computing”
• Chapter 23 DBS and KK Likharev, “Reconfigurable nano‐crossbar architectures”
• Chapter 30: R Waser, “Redox‐based resistive switching memories”– MRS Bulletin, February 2012, vol. 37 (2)
• DBS and H Kohlstedt, “Resistive switching phenomena in thin films, “Materials, devices, and applications”
• W Lu et al, “Electrochemical metallization cells – blending nanoionics into l ”nanoelectronics”
• J Yang et al, “Metal oxide memories based on thermochemical and valence change mechanisms”
• T Lee et al, “Organic resistive nonvolatile memory materials” – J Yang, DBS and D Stewart, Memristive devices for computing,
Nature Nano, January 2013– DS Jeong et al, “Emerging memories: Resistive switching mechanisms
and current status”a d cu e t status
Resistive Switching is Hot TopicResistive Switching is Hot Topic
Basics and phenomenology ofBasics and phenomenology of resistive switching
Basics and Phenomenology
S MetalS
A
V
Metal
Insulator
Metal
• Metal‐Insulator‐Metal Structure
A Metal
insulator = ion conducting or mixed ionic‐electronicoxides, chalcognedes, ionic solids
possibly two different metals• Controlled breakdown• Based on internal redox reactions ‐ electrochemical and
thermochemical effects• First observation in 1960th but forgotten because of Si‐
based memories (EEPROM and DRAM)• Many different names RRAM ReRAM memristor or• Many different names – RRAM, ReRAM, memristor or
memristive devices, electroresistive memories, CBRAM• Pinched hysteresis I‐V loop
Current and resistance response to triangular or sinusoidal voltage sweep for bipolar cell
Basics and Phenomenology
• Often require initial electroforming stepRESET tti th d i t hi h i ti• RESET = setting the device to high resistive state (also called erase or programming to the OFF state)SET is the oppositepp
• Above certain threshold voltage Vset and Vreset the cell changes resistance rapidly
• Dotted line = current compliance• Intermediate states (multilevel switching) by
different current compliance• The state can be read with relatively small
VreadVread• Current excitation with recording voltage is
also plausible• Characteristics of I‐V strongly depends onCharacteristics of I V strongly depends on
the material system – e.g. unipolar devices Current and resistance response to triangular or sinusoidal voltage sweep for bipolar cell
Basics and Phenomenology• For bipolar switching in different polarities,
for unipolar in the same quadrant• Typically reset at higher voltage but lower
current• Bipolar switching due to assymetry in MIM
structure (i.e. M’ and M’’) or dedicated voltage polarity during forming
• Interplay between electrochemical and thermochemical redox processes determinesthermochemical redox processes determines type of switching
• Both types have been shown in the same device
• Main application is memoryMain application is memory• Crude requirements (to be discussed more in
details):• <30 ns write (to beat DRAM)• Iread > 100 nAIread > 100 nA• Roff/Ron > 10 • High endurance (10^3 – 10^7 for
flash)• High retention > 10 years (at
Bipolar and unipolarswitching
High retention > 10 years (at potnetially high temperatures and certain small DC bias i.e. Vread)
Switching Kinetics Requirements
• Vwrite should force changes in time Twriteg• Vread should not disturb the state, i.e. ensure Tretention
/ /• Vwrite/Vread ~ 10 Tretention /Twrite > 10^15 ! (with say Vwrite – 3 V Vread = 0 3 V Tretention(with, say, Vwrite – 3 V, Vread = 0.3 V, Tretention= 10 years, Twrite = 30 ns)How is it achieved?
• May not be true for some applications (i.e. with destructive read operation)
Forming Process and Geometry AspectsAspects
• Very often need electroforming step• Electroforming produce filament • Single crystal require 10 – 100 V and long time (hours to break), thin films of the orderSingle crystal require 10 100 V and long time (hours to break), thin films of the order
of few volts and seconds• Forming voltage is proportional to film thickness of I layer (field dependence) which is
not the same for subsequent switching• Subsequent switching takes place in the localized are of the filament (either at interfaceSubsequent switching takes place in the localized are of the filament (either at interface
of in the bulk) and hence switching parameters (switching voltage) is typically independent of film thickness
• Electrical breakdown local Joule heating in localized channel Morphological and redox changesg
• Current compliance to control the size of the channel
metallicinsulating
E
gap
forming
E
onintermediateoffvirgin
gap
Forming Process and Geometry Aspects
• Proofs for filamentary conduction?
Aspects
1) Cut electrodes after forming (only one specific place is changed)
2) Current in ON state independent of electrode area (but electrodeof electrode area (but electrode resistance should be always considered)
3) Pressure‐Modulated Conducting3) Pressure Modulated Conducting Map with nonconductive AFM tip
• There are some devices with uniform conductance (likely to consider later)
• In reality could be several filamentsfilaments
Experiment scheme and resistance map
Generic Aspect of Resistive Switching• M’ and M’’ carry electronic current only• I layer is denoted MX may carry
electronic and ionic currents– Anions X(‐)– Anions X(‐)– Cations M(+) or M’(+)
• Ionic current in MX leads to electrochemical reaction (oxidation at the anode and reduction a the cathode)anode and reduction a the cathode)
• Contribution of anions and cationscurrents and specific reaction is determined by ReRAM
• One of the interfaces could be ionOne of the interfaces could be ion blocking leading to accumulation of ions
• Ad‐atom diffusion
• Main driving forces:• Main driving forces:– Gradient in electochemical potential – Gradient in temperature– Could be also gradient in stress
All conceivable processes related to electroforming and switching in MIM under load
• State of matter is changed by redoxreactions reversibly SET and RESET switching
Redox ProcessRedox Process
• Redox = Coupled reduction (gain of electron)Redox = Coupled reduction (gain of electron) and oxidation (loss of electron) reaction– H + F → 2 HF (hydrogen is oxidezed and flourine– H2 + F2 → 2 HF (hydrogen is oxidezed and flourineis reduced)
– Trapping/detrapping is also redoxTrapping/detrapping is also redox
• Redox processes are typically accompanied by transport of atoms and ionstransport of atoms and ions
Resistive Switching By Driving Forces and Types of Cells Concentrationand Types of Cells
Field dominating thermal dominatingField + thermal
Concentrationgradient
Vn
U A
-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
Field gradient
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)
Temperature
Eaq
• Elecrochemical metallization (ECM) cells – Oxidation, drift and reduction of electrochemically active
electrode (such as Ag)• Valence change memories (VCM) cell
Temperature gradientT
– Migration of anions (oxygen ions) leads to change in stochiometry (and hence redox of cations) with changes in resistance
• Thermochemical memory (TCM) cell
TakB
y ( )– Thermophoresis induced change in stochiometry (valence)
Nonlinear switching kinetics
effective barrier modulation due to:
heating
electric field
1
2 ion hopping
e‐
ion hoping
z+z+e‐
electrodeelectrode
UA
~Eaq/2
~ kB∆T
initial profile
2
1
eoxidation reduction‐+ v
Eaq/2
energy a∆UA
h t iti d ti3
3
2
hop distance
position
phase transition or redox reaction3
Speed vs. retention
linear ionic transport linear ionic transport pp
TI
I
write
store ~)()0(
VV
DV
Vvv
nonnonlinearlinear effect due to temperature and/or electric field
)(~ writeB
A
storeB
A
store TkU
TkU
eeVV
e.g. temperature only:
Twrite V
Typical Switching Kinetics in TiO2
RESET: R =Rd
setvoltage initialize to R0FF
10
100
RESET: R0=RON
SET: R0=ROFF
reset
read
time initialize to R0N
1
10
R/R
0
‐ Small pulse amp = finer state change butmay require exp long time
‐ Large pulse amp faster but at cruder step
1E 8
0.11E-4
-0.9VmV
(A) -0.5V to -0.8V
1E-81E-6
1E-40.01
1
-1.5-1.0-0.5
0.00.5
1.01 5 Tim
e (s)
Pulse voltage (1E-5
-1.0V
-1.1V
-1.2V
-1.3V
Cur
rent
@ -2
00
1.5 Timge (V)0 1x10-5 2x10-5
Time (s)
Empirical Set and Reset Characteristics• SET: Ron = Von/Icc
as a reminder compliance due 1) series resistance1) series resistance, 2) transistor in series, or 3) equipment (param.analaz.)
• Why voltageWhy voltage redistribution + strong nonlinearity
• Ireset = Vreset/Ron = A*Icc (but typically for linear ON state devices)linear ON state devices)
• Dependence is somehwat universal d i d d fand independent of
physical mechanism
Technology and Device Aspects
Most important issues:Most important issues:
• Reliability (device to device and cycle to cycle variance)variance)
• Endurance (limited number of times the cell b i h d)can be switched)
• Retention
Reliability • Problems due to
– Random walk of ions
Weilbull distribution of Ron
– Irregular atomic structure
and Roff values in 1 kbit 1T1RRu/Ta2O5/TiO2/Ru VCM cells
b) (c)50
30
40
Devic
es
(d
0
10
20
# of
D
2.92.6 3.2 3.5 3.8 4.1 4.40
Vth1 (V)…threshold variations in Ag/p‐Si/Pt ECMYield and …
Endurance
• How many write cycles can be performed before it fallsbe performed before it falls out of predefined acceptance window
1013 • Limited endurance due to
109
1011
10
SAIT
ycle
s)
HP Labs
– Morphological changes• Gradual growth or dissolution of phases
105
107
10
Fujitsu Labs
Panasonic Corp.
drua
nce
(cy
– Oxydation and/or drift of electrode material
– Leakage of oxygen
2006 2007 2008 2009 2010 2011103
10
En
Year
several groups
g yg
Retention
• One or all states are thermodynamicallythermodynamically metastable
High activation energy for hopping but with possible tradeoff to endurance
Retention in Ag/GeSx/W ECM cell
tradeoff to endurance• Accelerated life time test by heating and
l hextrapolation with Arrhenius plot
Ir/TaOx/TaN VCM cell